Low-cost, high-purity vacuum pumps and systems

Abstract

A reduced contamination pumping system is disclosed. The vacuum pumping system includes a mechanical vacuum pumping mechanism within a sealed pump housing that hermetically isolates the pumping mechanism from ambient air. The pump inlet is hermetically sealed within the sealed pump housing. The pump outlet is hermetically sealed at one end to the sealed pump housing and at the other end to an inlet of a Peclet Seal tube. The vacuum pumping system generates a vacuum within a vacuum processing chamber. A sweep gas source injects a sweep gas into at least one of (i) the sealed pump housing and (ii) the inlet of the Peclet Seal tube. The sweep gas and process gas flow through the Peclet Seal tube, substantially isolating a counterflow of ambient air through the Peclet Seal tube.

Description

[Technical Field]

[0001] Reference to earlier application This application claims the benefit of U.S. Provisional Patent Application No. 63 / 403,569, entitled "System and Method for Ultra-High Purity Sintering," filed September 2, 2022. This application also incorporates by reference U.S. Patent Application No. PCT / US2021 / 020347, filed March 1, 2021, the contents of which are incorporated herein by reference in their entirety. BACKGROUND

[0002] Various processing systems and laboratory equipment require high purity yet operate at medium or rough vacuums. This is particularly true for high-temperature processes such as metalworking and sintering, but also for a wide range of systems and technologies outside of metalworking. High levels of purity, such as parts per million (ppm) or parts per billion (ppb), are often desired, meaning, for example, that there is an average of only one contaminant molecule per billion molecules of ambient gas. These high levels of purity are also desirable when the vacuum level is relatively modest and would be considered "medium," "low," or "rough" vacuum in other industries, such as semiconductor manufacturing. For example, it may be possible to sinter metals with 300 Torr of process gas, such as pure argon (which is considered a rough vacuum), while exotic metals, such as certain titanium alloys, may require purity levels as low as 0.1 ppb in nearly inert process gases. In this case, there would be one contaminant molecule per 10 billion molecules of argon.

[0003] Vacuum pumps, including mechanical pumps such as piston pumps, diaphragm pumps, scroll pumps, screw pumps, rotary vane pumps, and other displacement pumps, may be configured to evacuate vacuum processing chambers to suitable medium or rough pressures, but may be unable to produce extremely high purity (e.g., ppm or ppb) chamber atmospheres due to the backflow of air, contaminants, and / or pump lubrication.

[0004] One conventional approach to achieving high purity at medium or rough vacuum involves employing a relatively expensive pumping system, such as one containing multiple pumps staged in series, and purchasing very expensive, best-in-class pumps. Other conventional applications pursue high purity through aggressive methods, such as providing excess gas flow to at least somewhat suppress backflow. Excessive gas flow refers, for example, to a gas flow rate greater than would be required if the system were to exhibit superior purity. Aggressive approaches typically result in crude compromises that are expensive to operate and fall short of the truly desired purity level. Many such compromises are routinely adopted and may provide a suitable compromise that is considered "good enough" given the potentially high costs associated with further improving purity levels. Operators may simply accept the compromise because a better option is unavailable. Summary of the Invention

[0005] Disclosed are systems and methods for increasing purity within vacuum processing chambers by using what is called Peclet sealing, which in most embodiments involves a long length-to-cross-sectional area tube combined with a sweep gas flow through the tube that prevents backflow of contaminants and ambient air through the tube.

[0006] In one embodiment, the pump system includes a sealed pump housing that is hermetically sealed from ambient air. The pump system is hermetically connected to a vacuum processing chamber and generates a vacuum. The pump system outputs to a Peclet Seal tube. By injecting a sweep gas through the Peclet Seal tube, the Peclet Seal tube prevents backflow of contaminants and ambient air, providing isolation to the pump system and enabling high purity levels within the vacuum processing chamber.

[0007] In an embodiment of a furnace system for debinding and sintering a part, the vacuum processing chamber has a pumping tube for venting process gases and contaminants to the outside. The pumping system generates a vacuum within the vacuum processing chamber. The pumping tube is heated during at least the debinding step to reduce condensation of contaminants within the pumping tube, including debinding by-products vented to the outside during the debinding cycle, to a predetermined standard. The process gas source is configured to inject a sweep gas into the vacuum processing chamber during at least the sintering cycle so that the pumping tube provides peck sealing during sintering. The pumping system employed may be the pumping system described above.

[0008] In another embodiment of the furnace system, a dual pumping system is employed. A pumping tube from the vacuum processing chamber is used to vent to the outside and is connected to first and second valves. The pumping tube and valves are heated at least during the debinding process to prevent condensation of contaminants. The first valve is utilized during the debinding process, allowing the first pumping system to create a vacuum within the vacuum processing chamber. The second valve is utilized during the subsequent sintering process, allowing a second vacuum to be created within the vacuum processing chamber. The second vacuum system utilizes a Peclet seal tube and sweep gas and is isolated during the sintering process. The first pumping system is isolated from the vacuum processing chamber during the sintering process. Thus, the first pumping system can be a "dirty" pump, contaminated by the debinding process, without affecting the purity achieved during the sintering process.

[0009] By utilizing the above-described systems and associated methods, significant purity can be achieved without the use of costly pumps. Applicant has utilized these systems to sinter aluminum and other metals that have historically been difficult or impossible to successfully sinter.

[0010] Various other embodiments are disclosed in detail below. [Brief explanation of the drawings]

[0011] 1A-B show a prior art pump system.

[0012] FIG. 2 shows a second prior art pump system and contaminant embodiment.

[0013] 3A-B show a third prior art pump system and contaminant embodiment.

[0014] FIG. 4 shows a fourth prior art pump system and contaminant embodiment.

[0015] FIG. 5 shows a pump system with contaminant reducing sealing.

[0016] 6A-C show three embodiments of the contaminant-reducing pump system.

[0017] Figure 7 shows a description of a Peclet seal tube.

[0018] FIG. 8 is a plot showing the relationship between Peclet number and normalized concentration.

[0019] FIG. 9 shows another embodiment of the pump system.

[0020] FIG. 10 shows another embodiment of the pump system.

[0021] FIG. 11 shows a plot of the temperature over time during the debinding step and the subsequent sintering step.

[0022] FIG. 12 shows an embodiment of a furnace with a reduced contaminant pumping system.

[0023] FIG. 13 shows another embodiment of a furnace with a reduced contaminant pumping system.

[0024] Figure 14 shows the furnace employing the double seal system in a closed state.

[0025] FIG. 15 shows the furnace of FIG. 14 in an open position.

[0026] FIG. 16 shows a perspective view of the dual seal system of FIGS. 14-15.

[0027] FIG. 17 shows a plan view of the dual seal system of FIGS. 14-15.

[0028] FIG. 18 shows a plan view of another dual seal system.

[0029] 19A-H show an embodiment of a dual seal system.

[0030] 20A-D show an embodiment of sealing in a tube furnace.

[0031] FIG. 21 shows an embodiment system having a first stage pump connected in series with the embodiment pump system.

[0032] 22A-B show an embodiment of the furnace where the retort has a Peclet seal tube to reduce contaminants.

[0033] 23A-I show an embodiment of the furnace adapter arrangement using a pecre sealing ring to reduce contaminants.

[0034] FIG. 24-C shows an embodiment of the furnace adapter arrangement using a pecre sealing ring to reduce contaminants.

[0035] FIG. 25 shows an embodiment furnace adapter arrangement that includes a bellows and enlarged tube to help prevent clogging while the vented gas cools before entering the binder trap.

[0036] Figures 26A-C show various schematic diagrams of furnace configurations.

[0037] The present disclosure can provide relatively low-cost systems and methods for achieving ppm or ppb purity, or even better than ppb, without introducing expensive ultra-high vacuum pumps or stages and without excessive gas flows. At least some embodiments described herein can be configured to achieve sealing and outlet-inlet isolation from ambient air at parts per million (ppm), parts per billion (ppb), or even better than ppb, at medium and / or rough vacuums using extremely robust and heavy-duty pumps that are less expensive than conventional pumps. As used herein, "outlet-inlet isolation" can refer to the separation of the air from the outlet of a pumping system and its inlet, and the term "sealing" can refer to more traditional seals (e.g., gaskets, O-rings) between the interior and exterior of a chamber, tubing, or pumping system.

[0038] The present disclosure may relate to vacuum chambers and pumps that operate at medium or rough vacuum and require sufficient sealing and inlet-outlet separation to achieve high purity from ppm to ppb, or even better than ppb, at least against outside air intrusion and / or leakage.

[0039] For purposes of this disclosure, medium vacuum corresponds to 3E-4 Torr or greater, and may even include 759 Torr. Technically, definitions may vary depending on the field. For example, the term "crude vacuum" in one field may correspond to hard vacuum in another. For example, a molecular beam epitaxy (MBE) equipment operator may consider 10E-6 Torr to be a rough vacuum, while a sintering furnace operator may consider 10E-6 Torr to be a deep or "hard" vacuum. For purposes of this disclosure, hard vacuum corresponds to less than 1E-4 Torr, medium vacuum corresponds to 1E-4 to 100 Torr, and rough vacuum corresponds to 101 to 759 Torr. (Note that atmospheric pressure is approximately 760 Torr.)

[0040] Purity levels can be characterized as "parts per N," where parts is the number of molecules of the contaminant in the pure gas and N is the mass of the pure gas molecules. For example, a sample of highly pure argon containing 1 billion ppb of oxygen would have roughly one oxygen molecule contaminated for every billion argon molecules and could be considered highly pure except for extreme applications. As with vacuum, technical terms for atmospheric purity can vary by field. As explained here, high purity can correspond to 100 parts per million (ppm) or more (more pure). Moderate purity can correspond to 100 ppm to 1 part per thousand (ppt), and poor purity can correspond to less than 1 part per thousand (ppt) (less pure).

[0041] When utilizing vacuum, many different disciplines tend to rely on the same catalogs and vendors, which may focus on the most stringent vacuum requirements. For example, manufacturers of chambers, seals, pumps, and vacuum gauges (e.g., MDC Kurt Lesker, and Ideal Vac) tend to focus on relatively expensive, similar products for achieving high vacuums of 10E-6 Torr or higher. Achieving high purity with this technology may be relatively easy. Manufacturers and users of sintering furnaces may tend to rely on vacuum equipment manufactured and sold for such high-vacuum operation, at least because the technology is well-known and widely available.

[0042] Furthermore, vacuum industry vendors and salespeople may have an incentive to encourage designers and users to rely on high-vacuum equipment due to the lack of available alternatives. Therefore, those seeking to achieve high purity typically tend to employ standard, commercially available high-vacuum equipment.

[0043] FIG. 1A is a schematic diagram of an exemplary existing high-vacuum system designed to use a vacuum pump for operation at high vacuums below 1E-4 Torr. The vacuum system for material processing can include a process gas flow 1001 that can be injected into a high-vacuum processing chamber 1002 by a mass flow controller (MFC) 1003 fed by a supply of high-purity process gas 1004. Such a system may require a multi-stage turbo-mechanical and / or thermo-mechanical pumping system as depicted in FIG. 1A. The system may consist of a vacuum processing chamber as the high-vacuum chamber 1002, which is hermetically sealed to prevent air leakage from the outside, and a mechanical high-vacuum pump 1005, such as a turbomolecular pump, a thermo-mechanical diffusion pump, or a turbomolecular drag pump. Each of these high vacuum mechanical pumps may require a secondary "roughing" pump 1006 in series to pump at the outlet of the high vacuum pump. High outlet-inlet separation 1007 can be achieved by the overall series pump arrangement.

[0044] Diffusion pumps are described herein as "thermo-mechanical" because the mechanism for pumping gas molecules can involve generating high-velocity oil droplets that collide with the gas molecules to mechanically promote gas flow in a manner similar to the action of a turbomolecular pump, where pump blades collide with the gas molecules. Note that non-mechanical high-vacuum pumps, such as ion pumps and cryopumps, are generally only used at very high vacuums, generally below 1E-6 Torr, whereas diffusion pumps and turbopumps tend to be used in chambers at the high pressure end of the "hard vacuum" range, and may even be operated at medium vacuum pressures.

[0045] 1A shows an exemplary technique of using a multi-stage pumping system consisting of a high vacuum pump 1005 (e.g., a thermo-mechanical pump or a turbo-mechanical pump) pumping in series with a medium vacuum "roughing" pump 1006 on a high vacuum processing chamber 1002 as the mechanism for achieving high vacuum, and also shows high outlet-inlet separation 1007. Vacuum sintering furnaces can often include other components and / or features borrowed from vacuum systems, particularly pumps, valves, gauges, and chambers.

[0046] FIG. 1B schematically illustrates a typical medium-vacuum system including a vacuum processing chamber 1008 and a roughing pump 1009, such as a mechanical pump, configured to receive process gas and pumped by at least one mechanical vacuum pump. As discussed below, relatively low-cost “roughing” pumps, unlike some relatively expensive, best-in-class, mechanical pumps, tend to allow a significant amount of air backflow from the pump exhaust to the pump inlet. Also, contaminants and / or vapor pump lubricant can backflow from the interior of the roughing pump 1009 to the pump inlet 1010. In some cases, this backflow can be somewhat mitigated by increasing the process gas flow, introducing various forms of traps 1011 (such as cryogenic traps and / or molecular sieve traps), or by adding multiple pump stages in series. However, these mitigation strategies can be expensive and / or unsatisfactory, or at least compromised. Even when a medium-vacuum chamber is hermetically sealed to the latest technology levels (e.g., similar to those used in ultra-high vacuum systems), the purity of the process atmosphere can still be limited by the limitations of backflow and mitigation techniques. Users and designers often solve this problem by employing expensive pumping systems with high initial and operating costs. For example, it may be common to employ a Roots blower pump in series with a best-in-class rotary vane pump. However, even with this configuration, it may be necessary to include a cryogenic inlet trap (e.g., a liquid nitrogen trap) to reduce backflow of pump oil into the chamber.

[0047] Figure 2 is a schematic diagram of backflow mechanisms in typical high-, medium-, and low-cost roughing pumps, such as piston pumps, diaphragm pumps, rotary vane pumps, or other displacement pumps. As previously mentioned, regardless of cost, each of these pumps can exhibit significant backflow of ambient air 2001 from the pump exhaust to the pump inlet, preventing them from providing ppm, let alone ppb, separation at the pump inlet. Inlet purity can be further degraded by air diffusion 2002 through the pump housing itself, including housing leaks and leaks from shaft seals or faulty gaskets. Pumps that can achieve ppm separation relative to air tend to use oil, which can introduce backflow of contaminants and / or lubricant 2003 from within the pump, as shown in Figure 2. For example, even best-in-class rotary vane pumps can exhibit backflow of oil and other hydrocarbon contaminants to the extent that sufficient purity with respect to oil and hydrocarbons is difficult to achieve. Various traps, including cryotraps, are often used to mitigate at least some of the oil mist. In such cases, a moderate process gas flow (e.g., 1 slm) and a relatively thin pumping tube (e.g., 1 meter long, 1 / 2-inch diameter tube) between the medium-vacuum processing chamber and the pump can improve purity within the chamber compared to the pump inlet. However, these approaches can result in performance issues such as higher than desired pressures or require larger pumps to achieve the desired pressures with higher gas flows, leading to significantly higher costs. In other words, "brute-forcing" the use of larger gas flows can increase costs not only in terms of operation but also in terms of equipment. Also, as previously mentioned, cryogenic inlet traps and other traps can be used, but these approaches add cost, complexity, and other compromises.

[0048] Figures 3A and 3B show the basic pump mechanism of a piston pump with inlet and outlet valves (allowing inlet and outlet flow, respectively) and a reciprocating piston. A piston pump is described here for illustrative purposes; it should be understood that the issues described may also apply to other types of displacement pumps. As shown in Figure 3A, inlet valve 3001 may be open during at least a portion of the intake stroke (Figure 3A) so that piston 3003 displaces volume from the inlet into the piston, and outlet valve 3002 may be closed during most (or all) of the intake stroke. As seen in Figure 3A, some backflow from the pump housing to the pump inlet typically occurs. During the exhaust stroke, inlet valve 3001 may be closed, and outlet valve 3002 may be open during most (or all) of the exhaust stroke so that the contents of the piston are displaced outward. As seen in Figure 3B, some backflow 3004 from ambient air into the pump housing typically occurs.

[0049] As shown in Figure 4, various imperfections, such as imperfect seals, leaks, and imperfect geometric fit and tolerances, can each contribute to the presence of some degree of backflow, even in relatively expensive, best-in-class pumps. This backflow can reduce the pump's outlet-inlet separation. Housing leaks, gasket leaks, and shaft seal leaks can also affect the pump's base pressure. Additionally, displacement pumps may contain a finite amount of excess inert or "dead" volume within the piston chamber that cannot be removed with each exhaust stroke. This dead volume can capture a significant amount of residual air molecules from the ambient atmosphere, contributing to limiting base pressure and backflow. Backflow tendency can be causally related to a quantifiable performance specification known as "base pressure." The base pressure of a given pump can be defined as the measured inlet pressure (pressure at the inlet) when the pump inlet is sealed during operation. Because base pressure is often limited by backflow, relatively low-cost, low-precision pumps tend to exhibit more backflow and therefore have inferior (higher) base pressure. For example, a best-in-class rotary vane, piston, or diaphragm pump (examples might be manufactured by Edwards, Varian, or Kinney) might cost several thousand dollars and exhibit a base pressure of 0.001 Torr of external air, while a relatively low-cost piston or diaphragm pump used in pneumatic applications might exhibit base pressures of 0.001 Torr to 1 Torr, 1-10 Torr, 10-100 Torr, 100-300 Torr, and 300-750 Torr of external air. The inlet base pressure can be caused by backflow, which can be significant in low-cost pumps, so the air base pressure can become the limit of the pump's outlet-inlet separation.

[0050] 4, the piston 4001 and drive mechanism 4002 may be housed within a sealed pump housing 4003 having leaks including a relatively leaky shaft seal 4004 (where the motor shaft enters the housing), gasket leaks at static seals 4005 (such as bonded face seals or gaskets that seal two separate parts of the pump housing together), and housing leaks 4006 through porous housing materials such as plastic and / or cast metal. Some or all of these leaks are considered particularly acceptable as long as the sum of all of these leaks introduces an amount of air that is similar to or less than the amount of air introduced through backflow 4007.

[0051] Applicants further recognize that, from a pump cost perspective, for low cost and / or moderate performance pumps, it may not be necessary to provide a truly hermetic shaft seal, static seal, and / or impermeable material configured to block air significantly better than the pump itself. In other words, the base pressure due to backflow may constitute a meaningful restriction, and from a cost perspective, it may not be necessary to provide a pump housing and shaft seal that generates leakage significantly less than the leakage due to backflow.

[0052] Furthermore, it may be unnecessary to include a pump seal capable of sealing to an extent that exceeds the outlet-inlet separation developed by a given pumping mechanism. For example, a relatively low-cost permeable housing formed from cast aluminum may be suitable, and thus, the use of a somewhat more expensive housing such as machined aluminum may not be necessary if the pump itself is configured to provide a base pressure of ambient air, e.g., 0.01 Torr. Note that with non-hermetic pumps, residual air molecules may be introduced either through the pump outlet or various housing leaks. Again, while Figures 3 and 4 illustrate piston pumps, it should be understood that these figures are included to clarify various principles that tend to apply, at least generally, to other types of displacement pumps.

[0053] 5 illustrates a pump 5001 having a hermetically sealed pump housing 5002 constructed of an impermeable housing material such as non-porous steel or aluminum, a hermetic static seal 5003 such as an O-ring, and an exemplary drive mechanism 5004 (for converting the rotary motion of the motor into the linear motion of the piston) that does not have a shaft seal and therefore does not result in shaft seal leakage. For purposes of this disclosure, when referring to a hermetically sealed pump housing, it should be understood that leakage through the housing is at least one order of magnitude lower than the outlet and inlet backflow exhibited by the pump. Applicant routinely manufactures hermetically sealed pumps that exhibit leakage (through the housing, shaft, and static seal) that is 3-6 orders of magnitude lower than the outlet and inlet backflow.

[0049] Figure 6A illustrates a pumping system 6001 that can utilize a mechanical vacuum pumping mechanism 6002 within a sealed pump housing 6003 that hermetically isolates the mechanical vacuum pumping mechanism, achieving sufficient hermeticity and inlet-outlet isolation throughout the system, and enhancing the purity of the vacuum process chamber to ppm, ppb, and better levels, achieving relatively low cost pumps, including low cost pumping mechanisms that exhibit relatively low base pressures (PB), for example, in the ranges of 0.001 Torr to 1 Torr, 1-10 Torr, 10-100 Torr, 100-300 Torr, and 300-750 Torr. Figure 6A illustrates a piston-type pumping mechanism of the type shown in Figure 5 having a sealed pump housing 6003 with impermeable housing walls and gas-tight pump housing seals at all joints within the housing to hermetically isolate the mechanical vacuum pumping mechanism 6002 from the outside atmosphere. In this embodiment, the motor 6010 may be partially housed within a sealed pump housing to avoid the use of a potentially leaky shaft seal. A pump inlet 6004 is hermetically sealed to the sealed pump housing 6003 and serves as an inlet path to the vacuum pumping mechanism 6002. A pump outlet 6005 is hermetically sealed to the sealed pump housing 6003 and serves as an outlet path from the mechanical vacuum pumping mechanism 6002. The vacuum pumping system 6001 generates a vacuum within the vacuum processing chamber 6006. A process gas 6007 may be injected into the vacuum processing chamber. A Peclet seal tube 6008 has a Peclet seal tube inlet 6009 hermetically sealed to the pump outlet 6005. Upon operation of the pumping system 6001, the process gas flows from the Peclet seal tube inlet toward the Peclet seal tube outlet 6011, substantially isolating it from the counterflow of ambient air through the Peclet seal tube 6008. The Peclet Seal tube 6008 can optionally include a ballast volume 6010 disposed in gaseous communication with the Peclet Seal tube inlet 6009 so that the ballast volume can reduce pressure fluctuations caused by pump pressure ripples. The mechanical vacuum pump mechanism 6002 can be a displacement pump. Examples of suitable displacement pumps include, but are not limited to, piston pumps, diaphragm pumps, and scroll pumps. The Peclet Seal tube 6008 is preferably constructed from a material that resists condensation of contaminants. In certain embodiments, the Peclet Seal tube is constructed from metal.

[0054] The pump system of FIG. 6A can provide sufficient inlet-outlet isolation and sufficient seal to achieve ppm or ppb chamber purity (relative to ambient air) with a relatively low-cost pump, e.g., a pump exhibiting a relatively poor base pressure (PB) in the range of PB = 0.01 Torr to 300 Torr. The thin Peclet seal tube can be, for example, a 1 / 8-inch diameter (e.g., 1 / 8-inch inner diameter) metal tube 0.5 meters to several meters long. With sufficient process gas flow to generate laminar flow within the Peclet seal tube, and with the Peclet seal tube being sufficiently long and thin, there may be no theoretical limit to the degree of Peclet seal isolation achievable at the outlet of the Peclet seal tube relative to ambient air. (However, there may be practical limitations and considerations, such as off-gassing of contaminants from the inner walls of the Peclet seal tube and Peclet seal design and performance considerations discussed below.) While Peclet seal tubes may not form a "tight seal" in the traditional sense, the tubes may nevertheless be referred to as Peclet "seal" tubes to emphasize the relatively high degree of outlet-inlet separation, e.g., of ambient air, that may be achieved between the tube's outlet and inlet. While Peclet seal tubes may provide ppm, ppb, or even better separation than ppb, the term "tight seal" is appropriate in the sense that it prevents air flow or diffusion from the outlet from reaching the inlet. For pure gas flow rates greater than 0.05 slm, achieving ppm and ppb separation of the Peclet tube inlet relative to the Peclet tube outlet may be straightforward. Flow rates less than 0.05 slm through the chamber and into the pump inlet are more challenging, but can be achieved with the techniques described herein.

[0055] The overall system and method may have the advantage of achieving relatively high purity at relatively low cost. This may be achieved in part because the method and system at least generally decouples the issues of base pressure and purity, in the sense that the pump no longer needs to do all the work of achieving both vacuum and isolation. This is because conventional pumping systems typically rely on both vacuum isolation between the inlet and outlet. Unlike conventional systems that primarily rely on the pump to achieve high isolation (often characterized in vacuum engineering terms as compression ratio), the systems and methods described herein may include a Peclet Seal tube to establish isolation between the outlet and inlet of the pumping system, while the pump may be primarily relied upon to generate the desired vacuum. Additional outlet-inlet isolation against backflow achieved by the pump may be beneficial, but not necessarily required. Furthermore, even if the pump does not exhibit excellent isolation against backflow, with respect to air at the outlet of the Peclet Seal tube, the pump can still provide a vacuum, and the tube can provide much, or even the majority, of the seal and outlet-inlet isolation. It should be understood that even with a relatively low-cost, low-performance pump mechanism, the seal of the pump housing may be airtight, particularly with respect to the embodiment shown in FIG. 6A. However, sealing the pump housing need not be expensive, even when a very high degree of tightness is required. In general, static seals can be relatively simple and cost-effective if properly designed and implemented in accordance with well-known vacuum sealing techniques. Relaxing the specifications for the base pressure and compression ratio of the internal displacement pump mechanism may allow for a relatively low-cost and / or robust pump mechanism configured to provide the required vacuum pressure while the Péclet tube provides high purity.

[0056] FIG. 6B illustrates an exemplary embodiment that may facilitate the use of an unprocessed, non-hermetic pump that does not require hermetic sealing of the pump body. In this embodiment, the pump may be housed in an external hermetic pump housing 6011 configured as a container with hermetic tubing feedthroughs at the inlet and outlet of the hermetic pump housing. In this embodiment, there may be a hermetic sealed pumping tube 6012 at the pump inlet 6013, and a similar pump outlet 6014 may optionally be included. This pump outlet 6014 can prevent contamination of the hermetic pump housing, but in the absence of contamination, the system can function as intended without this tubing. For example, the pump may vent into a container, and the Peclet Seal tubing 6015 can continue to provide outlet-inlet separation, similar to when the pump outlet is hermetic sealed to the Peclet Seal tubing inlet. A sweep gas source 6015 injects a quantity of sweep gas into the hermetic pump housing, providing a sweep gas flow through the Peclet Seal tubing, similar to the process gas in FIG. 6A. As with the embodiment of Figure 6A, in the embodiment of Figure 6B, a relatively low-cost pump can be used to provide the necessary vacuum pressure, while the Péclet tube 6015 can provide ultra-high purity. To further clarify, ppm purity at the pump inlet can be achieved in accordance with Figure 6B by operating with a relatively low-cost piston or diaphragm pump (e.g., a KNF or Welch brand diaphragm pump) that is typically used in low-cost, low-performance pneumatic applications and typically cannot provide even parts per thousand (ppt) of outlet-to-inlet separation. In this case, the pump alone may perform 100 times worse, or even 10 times worse, or even worse.

[0057] 6C shows a further embodiment in which a motor 6016 outside the hermetic pump housing drives a mechanical vacuum pump mechanism 6017 via a hermetic rotary coupler 6018. In certain embodiments, the hermetic rotary coupler is a magnetic rotary coupler.

[0058] For clarity of explanation, it is useful to reiterate two different mechanisms by which displacement pumps provide sealing and isolation. In one mechanism, a sealed pump housing may provide a seal between the interior of the pump and the air outside the pump. This seal can be thought of as a housing seal; for pumps with hermetically sealed housings, the integrity of the housing seal can be very high; for example, a sealed pump housing can provide a leak rate through the housing in the range of 1E-6 Torr-liters per second (TL / S) to less than 1E-9 TL / S. Another form of isolation can be described as outlet-inlet isolation between the pump's outlet and inlet; pumps with less backflow generally provide better isolation in this regard. The "compression ratio" of a pump corresponds to its "separation," and in many cases, compression ratio and base pressure are related to each other. For example, a pump with a compression ratio of 1E6 will pump to air and have a base pressure of approximately 0.001 Torr. Mechanical pumps with low base pressures often offer high inlet-outlet separation, coupled with high compression ratios. Compression ratios such as 1E6 may be easily achieved with expensive, best-in-class displacement pumps. In contrast, lower-cost pumps, such as diaphragm pumps and low-cost dry piston pumps, may only achieve compression ratios of 10, 100, 1000, or 10,000, and the cost of a given pump tends to decrease with lower compression ratios.

[0059] In at least some embodiments of the present disclosure, a relatively low-cost pump having a sealed pump housing and a relatively modest base pressure of 0.01 Torr-100 Torr can be hermetically sealed to a Peclet Seal tube at the pump outlet such that the pump system and Peclet Seal tube cooperate to provide ppm to 0.1 ppb isolation at the pump inlet from the outside air.

[0060] 6A and / or 6B, a system with a gas flow of at least 0.1 slm, a pump inlet purity of 1 ppm (relative to ambient air) can be achieved with a low-cost pump with a compression ratio of 10 sealed in a Peclet-sealed tube with a Peclet separation of 10 ppm relative to ambient air. The pump system can be configured to provide approximately 10 times the inlet-outlet separation in addition to that of the Peclet tube seal.

[0061] 6A and / or 6B, at a gas flow of at least 0.1 slm, a pump inlet purity of 1 ppm can be achieved with a low-cost pump having a compression ratio of 100 sealed in a Peclet-sealed tube with a Peclet separation of 100 ppm relative to ambient air. The pump can be configured to provide approximately 100 times the inlet-outlet separation in addition to that of the Peclet tube seal.

[0062] In at least some embodiments of the present disclosure, based on FIGS. 6A and / or 6B , a pump inlet purity of 1 ppm can be achieved with a low-cost pump having a compression ratio of 1,000, hermetically sealed in a Peclet-sealed tube with a Peclet separation of 1,000 ppm relative to ambient air, at a gas flow of at least 0.1 slm. The pump can be configured to provide approximately a 1,000-fold increase in inlet-outlet separation in addition to the Peclet tube seal. While an exemplary embodiment (using a pump with a base pressure of 0.001 Torr) is described for completeness, using a pump with a compression ratio of 1,000 may be unnecessary, perhaps even excessive. In fact, as discussed below, there may even be a disadvantage to using a pump with an excessively high compression ratio (and low base pressure), at least because such a pump may be more sensitive to contaminants and more difficult to decontaminate compared to lower-cost designs more suited to the approach described herein. Therefore, it may be preferable to use a pump that has a high enough compression and a low enough base pressure to provide the desired vacuum, but not a very strong vacuum.

[0063] In at least some embodiments of the present disclosure, based on Figures 6A and / or 6B, with a process gas flow (e.g., pure argon) of at least 0.1 slm, a pump inlet purity of 1 ppb (relative to ambient air) may be achieved with a low-cost pump having a compression ratio of 10 hermetically sealed in a Peclet-sealed tube with a Peclet separation of 10 ppb relative to ambient air.

[0064] 6A and / or 6B, at a gas flow of at least 0.1 slm, a pump inlet purity of 1 ppb can be achieved with a low-cost pump having a compression ratio of 100 hermetically sealed in a Peclet seal tube with a Peclet separation of 100 ppb relative to ambient air. The pump system can be configured to provide approximately 100 times the inlet-outlet separation in addition to that of the Peclet tube seal.

[0065] 6A and / or 6B, at a gas flow of at least 0.1 slm, a pump inlet purity of 1 ppb can be achieved with a low-cost pump having a compression ratio of 1,000 that is hermetically sealed in a Peclet seal tube with a Peclet separation of 1,000 ppb relative to ambient air. The pump system can be configured to provide an outlet-inlet separation of approximately 1,000 times that of the Peclet tube seal.

[0066] The outlet-inlet separation can be quantified as the unitless ratio of the amount of extraneous air at the pump inlet divided by the amount of air outside the pump and at the pump outlet.

[0067] Applicant recognizes that, in general, commercially available mechanical vacuum pumps (i.e., roughing pumps) are not intended to provide significant isolation from ambient air. For example, dry pumps such as piston, scroll, and diaphragm pumps only exhibit compression ratios insofar as they reduce pressure, but do not provide any degree of isolation in the absence of process gas flow because the entire base pressure under no-flow conditions is comprised of external air. As an exception, wet rotary vane pumps are available that use pump oil as a sealant. However, such pumps tend to introduce hydrocarbon gases. Furthermore, low-contamination oil pumps can have short-lived cleanliness when exposed to contaminants and can be relatively expensive.

[0068] Applicant further recognizes that in conventional vacuum systems, a high performance vacuum pump, such as a turbomolecular pump with a high compression ratio (>1E^), may be relied upon to provide vacuum pressure and separation between the pump inlet and the air outside the pump and / or at the pump's exhaust. However, in the exemplary approach described herein, the functions of the pump and Peclet Seal tube can be distributed such that (i) the pump system is relied upon to provide vacuum pressure at the pump inlet while contributing little to the separation, and (ii) the Peclet Seal tube, while not completely contributing to the vacuum, can provide the majority of the separation between the pump inlet and the ambient air outside the pump and / or at the Peclet Seal tube's outlet.

[0069] As noted above, the Peclet tube has been described only to the extent necessary for the purpose of including it in a pump system. Note that Peclet seals can tolerate numerous dimensional variations, and it is practical considerations and characteristics that tend to determine their actual practical performance. This section describes the basic operating principles of Peclet tube seals, as well as details regarding their design and practical application. An exemplary equation for designing a Peclet tube seal, shown in Figure 7, is as follows: definition L = length of Peclet tube (m) A = cross-sectional area of ​​the tube (m 2 ) V = average velocity of sweep gas in the tube (m / s) D=diffusivity s(m 2 / s) Pe = dimensionless Peclet number I = separation (no units) Q = volumetric flow rate of sweep gas through the tube (m 3 / s) Here, diffusivity refers to the rate at which one gas diffuses through another gas at a given temperature and pressure. For example, at room temperature and atmospheric pressure, the diffusivity of oxygen in argon is approximately D(O-Ar) = 0.3 cm 2 / s=3E-5m 2 / s. More complex calculations can be performed when multiple species are tracked. Furthermore, those skilled in the art with access to the literature on diffusivities can easily account for various levels of complexity, including consideration of temperature effects, pressure dependence, and nonlinear effects such as turbulence. For example, one of many potentially useful sources in the literature is R.B. Bird, W.E. Stewart, and E.N. Lightfoot, Transport Phenomena, 2nd ed., New York: John Wiley & Sons, 2002. The dimensionless Peclet constant is the dimensionless ratio Eqn 1. Pe=V*L / D is. The Peclet number can be useful in that a higher Peclet number indicates a better seal according to the following equation: 2. I=exp(-Pe) Equation 1 is the flow rate: Eqn 3. Pe=Q*L / (A*D) Note that this can be rewritten to include

[0070] Armed with these equations and concepts as disclosed herein, one skilled in the art will recognize that these insights can be used to generate numerous embodiments and examples of Peclet tube sealing, and that this one-dimensional model and approximation reveals that Peclet tube sealing is an effective technology, allowing for the generation of a wide variety of solutions with a large amount of theoretical margin. For example, 0.1 slm of argon (0.1 m at stp) 3 Even given very small process and / or sweep gas flows of 1 / s, a 3 mm diameter tube approximately 6 mm long can provide separation between the inlet and outlet of I-2E-31 that is 20 orders of magnitude better than 0.01 ppb, according to Equations 1 and 2. Figure 8 shows the curves that can be calculated based on the above equations, which represent the normalized ratios of concentrations in a log-log plot at the inlet to the outlet of the tube for a given Peclet number, and at the inlet to the outlet of the Peclet tube for a given Peclet number.

[0071] It should be understood herein and throughout the present application that process gases are useful not only as sweep gas flow contributors but also as contributors to specific processes. Peclet sweep gases can be injected through the inlet of a sealed pump housing or Peclet seal tubing. Alternatively, process gases can function as sweep gases when injected into a vacuum processing chamber.

[0072] Note that the above analysis is applicable to flow channels with many different cross-sectional geometries. For example, the above concepts and equations apply to the planar flow of a fluid in a gap defined by parallel plates with a gap height G and width W, defining a cross-sectional area G*W. For a flow channel of length L, the cross-sectional gap area can be used in Equation 1 as V = Q / (G*W). Furthermore, note that the equation applies very well at vacuum pressures, as long as the flow remains laminar. However, the diffusivity D depends on pressure, approximately inversely proportional to the pressure. To calculate the performance of a Péclet seal against vacuum pressure, we relied on the pressure- and temperature-dependent equation for D found on page 48 of "Vacuum Technology" by A. Roth, second edition, published by North Holland in 1988. Our model using this equation has been rigorously validated through extensive empirical studies.

[0073] Applicant recognizes that the above equation corresponds to a relatively simple, one-dimensional model. However, this model may generally correspond to a physical pumping system configured to achieve ppb and even greater separations using relatively short, even relatively large-diameter tubing. Furthermore, for rational tubing design, it may be desirable to evaluate various practical considerations beyond the theoretical design of the Péclet tube seal. Such practical considerations tend to govern performance limitations and may include consideration of contaminant off-gassing from within the walls of the Péclet tube and leakage from various seals, such as O-rings and metal gasket seals sealing the pump outlet and Péclet tube inlet. To achieve ppm and ppb purity in a low-cost, practical manner, it may be advantageous to take into account various considerations, such as those described below.

[0074] One exemplary approach is to use 1- or 2-meter-long tubes, setting the tube diameter so that the tube does not limit or otherwise clog the pump and / or sweep gas displacement rate. Therefore, it may be desirable to make the tube as small as possible without significantly impacting the pump. Following this approach, tubes with inner diameters of 2 mm to 10 mm may be suitable for process gas flows between 0.1 slm and 10 slm, respectively. In such cases, predictions based on the one-dimensional model described above may yield performance orders of magnitude greater than necessary. For example, tubes approximately 1 m, 2 m, or 3 m long, with diameters ranging from 1 / 8 inch to 0.59 inches (1.5 cm), are readily practical and are not overly restrictive. In fact, theoretical designs of tubes according to the present disclosure may tend to provide theoretical Peclet separation at least 10 orders of magnitude better than required, without significantly limiting pumping behavior. In such cases, other practical aspects tend to determine the limits of system performance.

[0075] Based on the aforementioned approach, important practical aspects and / or considerations include the following, which are illustrated schematically in FIGS. 8 and 9: (1) Use impermeable tubes such as metal tubes. (2) Use tubing that is easily cleaned and / or replaced, such as stainless steel tubing, and replace and / or clean the tubing if it becomes contaminated. In some cases, this may include cleaning and reinstalling the tubing after each run (e.g., after each sintering cycle). In some cases, tubing cleaning may be done in-situ by running a solvent through the tubing. Note that pumps may also be cleaned in-situ, if necessary. 10 shows a relatively clean inlet valve 10004 and a clean outlet valve 10005 suitable for flowing cleaning solvent through a pump and / or Peclet tube seal. In various embodiments, the pump inlet valve and Peclet tube outlet valve may remain closed while the cleaning solvent is washed through the pump and / or Peclet tube. (3) Use sealed tube fittings at the sealed pump housing, Peclet seal tube outlets, and other parts of the system's sealed envelope. a.Swagelok fittings. b O-ring and KF or ASA flange. c. Copper gasket seal with conflat flange. (4) When the chamber is not in use, even during an operating cycle, the interior of the Pecletseal tube is hermetically separated from the exterior (described below). a. This can be achieved by providing a hermetically sealed valve at the outlet of the tube. b. This can be achieved by continuously flowing a pure inert sweep gas through the Peclet seal tubing while the system is not in use. (5) Maintain smooth laminar flow through at least a portion of the tubing to compensate for the pulsating action of the pump. a. This can be achieved with a ballast volume at the inlet of the tube, shown as an option in FIG. 9 as ballast volume 9009. b. This can be achieved by lengthening the tubing, allowing the tubing itself to smooth out pressure fluctuations along the gas flow. It may not be necessary to make the tubing long (e.g., a length of 1 meter may be 10 or even 100 times longer than theoretically desirable, at least in some applications). (If a longer length is desired, the tubing can be coiled so that it does not take up excessive space.) c. The previous technique, for example, a and b, may be combined.

[0076] With metallic, hermetically sealed tubing, such as stainless steel tubing and Swagelok fittings, contamination and off-gassing of the Pecleseal tubing itself, especially in the section closest to the pump, can be a major practical problem. It can take hours or even days for moisture and other contaminants to be flushed out by argon. This process can be accelerated by using low-cost heaters, such as nichrome wire heaters, to "burn out" the contaminants. The use of hermetically sealed valves may be effective in maintaining cleanliness between runs.

[0077] It may be desirable to provide high tightness of connections and Peclet tube seals (e.g., helium leak rates less than 1E-10 Torr liters / second TL / S), so the need to frequently replace the tubes, even between runs, may not be a burden in most applications.

[0078] FIG. 9 illustrates another embodiment of a pumping system. A vacuum processing system is connected to the vacuum pumping system via a pumping tube 9009 separated from a pump inlet 9004 via a valve 9003. A pump outlet 9005 is hermetically sealed in a Peclet Seal tube 9006. A sweep gas source 9007 is configured to inject sweep gas into the Peclet Seal tube 9006 such that the sweep gas flows through the Peclet Seal tube from an inlet 9010 of the Peclet Seal tube 9006 toward an outlet 9011 of the Peclet Seal tube 9006, substantially isolating the Peclet Seal tube from counterflow of ambient air. A ballast 9012, as previously described, may be employed. A valve 9008, located at the outlet 9011 of the Peclet Seal tube, may be used to seal the Peclet Seal tube from ambient air when sweep gas is not being injected, as described above.

[0079] As shown in Figure 9, by injecting gas at the inlet of the Peclet-sealed tube, the systems and methods of the present disclosure can be implemented even in medium-vacuum systems with little or no process gas flow. Furthermore, the sweep gas contribution within the Peclet-sealed tube can be further provided from the process gas and / or the sweep gas injected into the pump housing. As previously mentioned, the systems and methods described herein can achieve relatively high purity without the use of relatively expensive high-purity pumps. These systems and methods can enable the use of pumping mechanisms that are sufficient to provide the desired vacuum but may not necessarily provide the necessary isolation. Thus, the role of the pump can be significantly reduced to maintaining the vacuum. In the case of Figure 9, the injection of gas after the pumping may not reduce the role of the pump if the pressure at the injection point is only slightly above atmospheric pressure, which may be relatively easily achieved due to the robustness of the Peclet-sealed mechanism.

[0080] The systems and methods described herein may offer additional advantages over the use of serial stages and / or multiple pumps connected in series. As discussed above, the pump itself tends to limit purity as contaminants accumulate within the pump mechanism. Often, stacking pumps in series does little to overcome contaminants in a pump directly connected to a chamber. Furthermore, best-in-class pumps tend to be relatively sensitive to contaminants and can be difficult to clean in practice. In contrast, the requirements for pumps described herein may be relatively minimal (e.g., orders of magnitude lower than those of conventional approaches). Thus, the pump systems described herein can incorporate pumps that are relatively simple, resistant to contamination, easily cleaned in place, and even self-cleaning. For example, for a system operating at 10 Torr, it is possible to employ a relatively simple Teflon-coated, oil-free piston pump that is relatively small in size and capable of self-cleaning by circulating alcohol through the pump, as shown in Figure 10.

[0081] FIG. 10 shows another embodiment pump system similar in many respects to FIG. 6A, and the disclosure related to FIG. 6A generally applies to FIG. 10. Valve 10001 controls flow from a vacuum processing chamber (not shown) to a sealed pump housing 10007. Valve 10003 controls flow from the sealed pump housing 10007 to a Peclet seal tube 10008. Pump cleaning heater 10005 and tube cleaning heater 10006 can be activated both during and between runs to drive out moisture and other contaminants. Relaxed performance specifications (e.g., relaxed base pressure and / or compression ratio requirements) can increase and enhance the flexibility to design and / or obtain pumps that tend to remain clean and / or can be easily cleaned in place. For example, suitable pump designs may operate at 50-100 degrees, 100-200 degrees, 200-300 degrees, or even above 300 degrees, and these challenging pump designs may be achievable, at least in part, due to relaxed specifications on base pressure, which may allow for larger gaps and looser mechanical tolerances incompatible with typical high performance, high compression pumps. Techniques such as in-situ solvent flushing and in-situ heating tend not to be practical with high performance displacement pumps such as rotary vane pumps, scroll pumps, and Roots blowers.

[0082] FIG. 11 shows a graph of process chamber temperature (vertical axis) versus time (horizontal axis) in the hot zone within a vacuum processing chamber for a typical two-stage debinding and vacuum sintering cycle that may be performed within the vacuum processing chamber for debinding and sintering powder metal parts. The graph illustrates the increase in process chamber temperature 11001 from an initial temperature (e.g., room temperature) to a debinding temperature 11002. In a two-stage debinding and sintering process, parts can be debound during the debinding cycle for a residence time DT at the debinding temperature sufficient to remove the binder from one or more parts within the processing chamber, during which binder by-products can off-gas from the parts. The part processing temperature 11003 can then be increased to a sintering temperature 11004, which can be maintained during the sintering cycle for a sintering time ST before cooling 11005 is initiated by controllably reducing power and / or deactivating the furnace heater. It should be emphasized that this disclosure generally relates to lower costs relative to vacuum atmospheres, rather than atmospheric pressure. With respect to discussions of metal sintering, it should be emphasized that high purity is often desired during the sintering cycle but may not be critical during debinding. With respect to debinding (as opposed to sintering), it should be understood that the multi-stage sintering systems and methods disclosed herein can be configured to operate during the debinding cycle at any pressure, including vacuum, atmospheric pressure, or slight positive pressure. Emphasis throughout these discussions is placed on sintering systems and methods that minimize or eliminate the presence of oxygen and debinding by-products during sintering and optionally exclude oxygen (or other contaminants) during debinding. For example, with certain steels, a chamber atmosphere with a high oxygen content may be tolerated during debinding but not during sintering. On the other hand, with sintering of titanium and / or aluminum, maintaining ultra-low oxygen levels during debinding and sintering may be important.Furthermore, in all cases describing powder metal sintering within this disclosure, it should be understood that the described debinding systems and methods are configured to minimize and / or prevent condensation of debinding by-products within any portion or extension of the vacuum processing chamber, including the vacuum chamber and inlet and outlet tubes. While the foregoing description focuses on a two-step process, it should be understood that many variations are possible, including multiple steps separated over multiple time spans, where debinding is performed prior to sintering and debinding by-products are eliminated or minimized below a predetermined threshold. The foregoing description focuses on a simple example, and it should be understood that in addition to multiple steps, temperatures may be controlled to vary continuously within a predetermined range throughout a predetermined time span, e.g., corresponding to open-loop and / or closed-loop process control. For example, the debinding temperature may be feedback-controlled to vary within a predetermined range in response to continuously measured variations in pressure rise due to debinding. Often, the predetermined threshold requires that there be no observable or measurable residue of debinding products within the chamber or tube for at least the duration of the sintering cycle. Applicants routinely achieve this threshold using the systems and methods described herein. It is again further emphasized that the systems and methods described herein for achieving ultra-high purity and reducing the condensation of various contaminants can be applied to non-sintering applications and processes, including semiconductor processing and other vacuum processing steps unrelated to metal sintering.

[0083] FIG. 12 includes a schematic embodiment of a vacuum processing system including a vacuum processing chamber 12001 in which parts can be processed, a furnace (or oven) heater 12002, and insulation 12003. The vacuum processing chamber 12001 includes a pumping tube 12004 having a pumping tube inlet 12005 and a pumping tube outlet 12006. The pumping tube 12004 can be optionally heated with a heater system 12007, which may be a tube heater, and optionally insulated with tube insulation 12009 to remove and / or reduce to a predetermined threshold the condensation within the pumping tube 12004 of contaminants, including, but not limited to, debinding by-products. In many cases, the predetermined threshold is simply that no accumulation of residue remains within the chamber or tube that is detectable by eye or odor or otherwise observable to humans. Applicants routinely achieve this significant threshold result, and further, in many cases, the appreciable presence or effect of debinding by-products within the processed part cannot be measured chemically or microscopically. Applicant is unaware of any other sintering furnace apparatus that can achieve such a low threshold of condensation simultaneously in all portions of the processing chamber and in the pumping tube during and following the debinding process. The vacuum processing chamber 12001 also can include an inlet tube 12010, which can be heated by an inlet tube heating system 12011 and optionally insulated by inlet tube insulation 12012. The inlet tube 12010 can be used to inject process gas, which can function as a Peclet sealing sweep gas either or both: (i) when exhausted through the pumping tube 12004; and / or (ii) when contributing to the sweep gas flow of the Peclet tube seal at the outlet of a seal pump (not shown), such as the pumping systems of FIGS. 6A-6C.

[0084] The embodiment of FIG. 12 can be operated according to many different processes for many different purposes and applications where high purity and low condensation are desired. As described above, process gas can be injected into the vacuum processing chamber 12001 through one or more inlet tubes 12010. Depending on the vacuum pressure and the diameter of the pumping tube 12004, the process gas can act as a sweep gas within the pumping tube 12004, providing at least some degree of Peclet sealing. In many cases, as described above, this Peclet sealing can achieve ppm or even ppb separation or better between the outlet 12006 and inlet 12005 of the pumping tube 12004. For example, Applicant routinely operates 1 / 8-inch to 3 / 8-inch diameter pumping tubes in 8-inch lengths with process gas flows of 1-3 slm, achieving ppm and ppb levels of purity. For various combinations of process gas flow, tube length, tube diameter, and vacuum pressure, the Pumping Tube 12004 can provide excellent Peclé sealing, exceeding parts per million and even parts per billion. For example, in a 10-liter chamber, using an 8-inch long, 3 / 8-inch internal diameter pumping tube and a process gas flow of 0.5-5 slm as the Peclé sweep gas, we have routinely demonstrated relative Peclé sealing between the inlet and outlet in the ppm to ppb range at chamber vacuum pressures of 5 torr to 100 torr. As discussed above with reference to Figures 7 and 8, it is readily possible to estimate Peclé sealing across these pressure ranges as long as laminar flow conditions are maintained.

[0085] While the embodiment of FIG. 12 can be applied to many applications, Applicant has recognized that it can provide particularly significant advantages in the context of two-stage debinding and sintering applications, including, for example, aluminum and titanium sintering. In various ways during debinding, the chamber maintains the debinding temperature while the pumping tube 12004 and / or inlet tube 12010 can be heated to a temperature somewhat below, at, or above the debinding temperature to prevent or reduce condensation of binder within the inlet tube 12010 and pumping tube 12004. For a given binder material, Applicant often empirically establishes a condensation threshold temperature to avoid human-observable (i.e., by sight, touch, and smell) condensation, sometimes lower than the actual debinding temperature. In such cases, Applicant often controls one or more of the tube heaters to ensure that the tube temperature remains above the empirically established condensation threshold temperature. For example, for a particular binder, Applicant conducted the aforementioned laboratory tests to determine that the condensation threshold temperature was in the range of 300-400°C, and then routinely debound various bonded powder metal parts at debinding temperatures of 400-500°C using pumping tubes heated to temperatures of 300-400°C. In these cases, Applicant has yet to detect any evidence of condensation. In other cases, the design threshold for overheating the tube connectors is above 500°C, and Applicant employs air debinding at approximately 300°C while maintaining the tubes above this temperature to thoroughly prevent condensation there within the empirically established threshold. This has enabled Applicant to provide vacuum sintering of metals highly sensitive to oxygen and binder contaminants, including even the sintering of high-quality aluminum alloys. Surprisingly, Applicant has achieved excellent powder aluminum sintering at pressures between 10 Torr and 400 Torr using the 8-inch pumping tube described above.Aluminum alloys are generally considered to be the most delicate and difficult metals to sinter because they are prone to oxidation, and even ppm levels of oxygen tend to inhibit sintering. The successful sintering of aluminum alloys in these systems using these methods can be seen as a testament to their significant advantages. It should be noted that in many cases, it is possible to pump from the outlet of the pumping tube using low-cost, low-performance, or even highly contaminated vacuum pumps, and yet by following the guidelines described above with respect to Figures 7 and 8, it is possible to achieve ppm or ppb Peclet separations or even higher at vacuum pressures of 10-100 torr or even higher. When lower pressures or larger diameter tubes are necessary or desired, other embodiments, such as Figure 13, can be employed.

[0086] In some embodiments, the system of FIG. 12 operates as a furnace system for reduced-contamination powder metallurgy. The vacuum processing chamber 12001 is configured to perform a debinding cycle at a debinding temperature sufficient to debind at least one part, such that debinding by-products are off-gassed from the at least one part. The debinding cycle can be followed by a sintering cycle at a sintering temperature higher than the debinding temperature. The vacuum processing chamber 12001 has a pumping tube 12004 having an inlet end 12005 sealed to the vacuum processing chamber 12001 and an outlet end 12006 separated from the vacuum processing chamber 12001 by the pumping tube 12004. The heating system 12008 includes at least one heater configured to heat the pumping tube 12004, at least during the debinding cycle, to a temperature sufficient to reduce, to a predetermined threshold, condensation of contaminants in the pumping tube 12004, including debinding by-products that are outgassed from the vacuum processing chamber 12001 during the debinding cycle. The pumping system 12013 is sealed to the outlet end 12006 of the pumping tube 12004 and configured to create a vacuum within the vacuum processing chamber 12001. A process gas source (not shown in FIG. 12, but shown in FIG. 6A) is configured to inject a sweep gas into the vacuum processing chamber 12001 during at least the sintering cycle such that the pumping tube 12004 provides a certain amount of pneumatic sealing during sintering.

[0087] FIG. 13 illustrates an embodiment similar to FIG. 12 that may provide additional benefits for multi-stage processing, particularly in the context of a multi-stage process such as a debinding sintering furnace. In this system, the outlet of the pumping tube 13002 is sealed to a first heated debinding valve 13005, which is sealed to the inlet of a debinding pump 13008, and a heated sintering valve 13006, which may be sealed to the inlet of a sintering pump 13009, including, but not limited to, a low-cost, high-purity pumping system such as those previously described with reference to FIGS. 6A and 6B and elsewhere in this application. In this embodiment, the high-temperature hot valves 13005 and 13006 may be heated by the same heater system 13007 used to heat the pumping tube 13002. During debinding, heating the pumping tube 13002 and the high-temperature valves 13005 and 13006 substantially reduces and / or prevents condensation of binder by-products inside the pumping tube 13002 and the valves 13005 and 13006. Many variations are possible within the scope of this disclosure, for example, two pumping tubes could be employed with a debinding hot valve sealed to the outlet end of the first pumping tube and a sintering hot valve sealed to the end of the second pumping tube.

[0088] This embodiment can be advantageous in many multi-stage processing applications. For example, it can be configured to operate as a furnace system for reduced-contamination metal powder metallurgy. The vacuum processing chamber 13001 is configured to perform a debinding cycle at a debinding temperature sufficient to debind the part, such that debinding by-products are off-gassed from the part. The debinding cycle can also be followed by a sintering cycle at a sintering temperature higher than the debinding temperature. The vacuum processing chamber 13001 has a pumping tube 13002 having an inlet end 13003 sealed to the vacuum processing chamber 13001 and an outlet end 13004 separated from the vacuum processing chamber 13001 by the pumping tube 13002. A first valve 13005, configured as a debinding valve that opens during debinding, and a second valve 13006, configured as a sintering valve that opens during sintering, are each sealed to the outlet 13004 of the pumping tube 13002. The heating system 13007 includes at least one heater configured to heat the pumping tube 13004, the first valve 13005, and the second valve 13006 at least during the debinding cycle to a temperature sufficient to reduce condensation of contaminants in the pumping tube 13002 and the first valve 13005 and the second valve 13006 to a predetermined threshold, including debinding by-products outgassed from the vacuum processing chamber 13001 during the debinding cycle. The first vacuum pump system 13008 (debinding pump) is arranged as a debinding pump for pumping during debinding and is connected to the first valve 13005 (debinding valve). The first vacuum pump 13008 is for pumping through the pumping tube 13002 via the first valve 13005 into the vacuum processing chamber 13001 during debinding. A second vacuum pump system 13009 (sintering pump) is connected to a second valve 13006 (sintering valve). The second vacuum pump 13009 is for pumping into the vacuum processing chamber 13001 during sintering through a pumping tube 13002 by means of the second valve 13006.The second vacuum pumping system 13009 may include a second mechanical vacuum pumping mechanism within a sealed pump housing configured to hermetically isolate the second mechanical vacuum pumping mechanism from ambient air outside the sealed pump housing. The second vacuum pumping system 13009 may include a second pump inlet 13010 connected to a second valve 13006 and a second pump outlet 13011. The second pump outlet 13011 may be hermetically sealed to the inlet of the Peclet Seal tube, for example, as described above for FIGS. 6A-C. A sweep gas source may be configured to inject a sweep gas into the second sealed pump housing and / or the inlet of the Peclet Seal tube (as shown in FIGS. 6B and 9). A process gas source may be configured to inject a process gas into the vacuum processing chamber 13001 (as shown in FIG. 6A). The sweep gas flows through the Peclet Seal tube from the inlet of the Peclet Seal tube toward the outlet of the Peclet Seal tube, and is substantially isolated from counterflow of ambient air through the Peclet Seal tube. The controller can be configured to, during at least a portion of the debinding process, place the first valve 13005 in an open position, place the second valve 13006 in a closed position, and activate the first mechanical vacuum pump 13008 to create a vacuum in the vacuum processing chamber 13001. During at least a portion of the sintering step, the controller is configured to place the first valve 13005 in a closed position, place the second valve 13006 in an open position, and activate the second mechanical vacuum 13009 to create a vacuum in the vacuum processing chamber 13001.

[0089] Applicant does not intend the foregoing embodiments (systems and methods) to be limiting, and many variations are possible, such as using air debinding to "burn off" the binder during atmospheric debinding, which provides the same functional benefits, including condensation prevention and Peclet separation during sintering. One skilled in the art, with the benefit of this description, can be expected to design many modifications to allow for a variety of debinding cycles, while maintaining the range for high purity and low condensates during sintering. Furthermore, as will be described below, for example with reference to FIG. 22, the above-described combination of a heated pumping tube and multiple heated valves and pumps can provide a wide range of benefits when applied to a wide range of furnaces and vacuum processing systems.

[0090] Figures 14 and 15 show details regarding one embodiment applicable to the systems of Figures 12 and 13. Furnace 400 includes heater 112 and insulation 22. Furnace 400 can include an optional protective cover 404, which can be simply a mechanical shield, but can also be arranged to be at least somewhat hermetically sealed to contain a somewhat pure, somewhat oxygen-free gas in the manner of a glove box, and possibly as a somewhat hermetically sealed vacuum chamber. Hot zone 28 heats vacuum retort 406, which includes retort body 410, retort base 408, and retort seal 412.

[0091] The system is shown in a closed, sealed position in FIG. 14 and in an open position for loading and / or unloading parts in FIG. 15. The system includes a vacuum processing chamber 15001, to which an inlet tube 15002 and an extreme-temperature pumping tube 15003 are sealed together, either by welding (in the case of a metal chamber) or by monolith bonding and / or molding in the case of a ceramic chamber. In this embodiment, the pumping tube can be considered an "extreme-temperature" tube insofar as the inlet end of the tube is presumably capable of operating at sintering temperatures well above the debinding temperature. As will be described further below, during debinding, it is routine to operate the entire length of the inlet and pumping tubes at temperatures well above 200°C, typically between 300°C and 500°C, to prevent condensation and contaminants from forming inside the tubes. During sintering, portions of both tubes operate at temperatures significantly higher than the debinding temperature. Furthermore, as described in detail in the appropriate sections of this disclosure, we have designed an extreme-temperature pumping tube 15003 with a sufficiently long length and small enough inner diameter to achieve ppm and better Peclet separation of gaseous contaminants from the pumping tube inlet and outlet. The use of extreme temperatures during Peclet sealing, debinding, and sintering of the pumping tube 15003, combined with the main retort seal described immediately below, contributes to the successful routine production of ultra-high purity atmospheres by utilizing a non-porous sintered SiC chamber according to the design illustrated herein, which is routinely operated at temperatures up to 1500°C. Applicant has successfully sintered aluminum alloys with the low-cost embodiments of Figures 14 and 15, using low-cost ceramics and / or low-cost high-temperature steels for the chamber. It is emphasized that the processing chamber 150001 shown in Figures 14 and 15 can be configured to function as a vacuum chamber in the absence of another external vacuum chamber. In these embodiments, it may be advantageous for the chamber material to be non-porous and impermeable to gases, particularly the outside air that, in some embodiments, immediately surrounds the vacuum processing chamber 150001.Other embodiments, such as those described below with reference to FIG. 22, may utilize the same or similar structure as the retort within the external vacuum chamber, allowing the retort material to be somewhat porous and permeable within acceptable limits. In such embodiments, although not yet pursued, the retort could function as a partial vacuum chamber if some pressure differential could be created, intentionally or otherwise, between the interior and exterior of the retort. In still other embodiments, the chamber could function as a vacuum chamber and be surrounded by atmospheric gas, with the outer chamber acting as a glove box to at least block oxygen from the outside air. Generally, we have not found a need, or any application where a glove box would be required around the insulation, and typically operate with the external ambient air immediately surrounding the process chamber 150001.

[0092] In other embodiments at high sintering temperatures above 800°C, Applicant routinely sinters high-quality titanium in the embodiments of FIGS. 14 and 15 using a SiC chamber (or retort) 406 with a SiC heater and high-grade high-temperature insulation suitable for operation up to 1500°C. In particular, Applicant has constructed multiple embodiments using sintered alpha-phase SiC with sintering chamber sizes exceeding 1.5 cubic feet. Applicant is currently preparing to purchase a system with a sintered alpha-phase SiC chamber (or retort) according to the design of FIGS. 14 and 15, with a volume of 4 cubic feet therein, as a vacuum processing chamber. Applicant has successfully pursued the purchase of such a chamber at a reasonable cost, despite numerous recommendations that such components are, and would be, unavailable except at prohibitive and commercially impractical prices. In this regard, Applicant considers it both surprising and remarkable to demonstrate a vacuum sintering furnace according to these descriptions operating at 1500°C with a volume exceeding 1.5 cubic feet. Furthermore, such furnaces, constructed according to Figures 14 and 15, for example, with a volume of a few cubic feet, are expected to offer significant technical and commercial advantages over conventional industrial sintering furnaces.

[0093] FIG. 16 illustrates a main retort seal that can be utilized in various embodiments herein, including those of FIGS. 14 and 15. In the context of these descriptions, the terms retort and chamber are interchangeable, and applicants note that in many applications, the retort will routinely refer to the retort as a vacuum chamber with no external vacuum chamber other than the retort itself, such that the retort is surrounded by external ambient air. In such a configuration, the retort functions as a vacuum chamber, and as depicted in FIGS. 12 and 13, functions as a processing chamber 12001 and 13001. Note that the system of FIGS. 14 and 15 can operate as one embodiment of the system and method shown generally in FIGS. 12 and 13. A retort and / or vacuum chamber body 410 has a main retort seal system 412 in a retort base 408. The retort and / or chamber seal 412 includes an internal seal 430, such as a high-temperature gasket, and an external seal 416, such as the illustrated Peclet gap seal. The high-temperature gasket may be offset against a gasket ledge 434 that can protrude upward to allow easier access when grinding the gasket sealing surface 433. Note that the gasket ledge need not protrude upward, as long as there is a (preferably precision-ground) gasket surface to support the gasket as it is compressed against the gasket sealing. For example, for metal sintering using a non-porous sintered αSiC chamber, Applicant routinely utilizes Grafoil at temperatures up to 1500°C and sometimes even higher. Grafoil gaskets tend to leak more easily than traditional elastomeric vacuum O-ring gaskets, and Applicant has found it difficult to identify a high-temperature and extreme-temperature gasket that is cost-effective and will operate at temperatures above 400°C without detectable and unacceptable leakage rates.Applicant has discovered that by placing a double seal, including an outer Peclet gap seal 416 that separates the inner seal from the outside air, there can be no detectable air leakage through the gasket into the chamber, and the outside air can be isolated within the groove, separating it from the gasket, at ppm or even ppb levels, making leakage insignificant to the purity within the chamber. In other words, if there is no detectable outside air within the groove, leakage from the groove into the chamber may be insignificant. The Peclet gap seal operates according to the principles described above with reference to Figures 7 and 8. The sweep gas tube 426 can inject a sweep gas 422 into the Peclet gap 418 formed between the defining surfaces 436 and 438, allowing the sweep gas to flow freely into the Peclet gap. These surfaces can be considered Peclet sealing surfaces facing each other with a gap G between them. This ensures high purity within the groove 444, so that gasket leakage does not affect the process, at least because the leakage is composed only of high-purity, oxygen-free process gas. For example, with a gap thickness 418 of 0.005 inches and a sweep gas 422 flow rate of 2 slm of argon, applicants have routinely observed ppm and even ppb resolution at gap widths of approximately ½ inch, as described in more detail with reference to FIG. 17.

[0094] FIG. 17 is a schematic diagram of a previously described high-temperature chamber and / or retort seal arrangement including an internal non-hermetic gasket seal 414 sandwiched between gasket sealing surfaces 433 and 433′ and an external Péclet gap seal 416 having a gap size G and gap length L such that the cross-sectional area A of the Péclet seal can be estimated as the product of the perimeter (or circumference, for a circular chamber) of groove 444 and the gap height G. Sweep gas 422 is introduced through a hermetically sealed sweep gas supply tube 426, and the sweep gas flows into groove 444 and then through the Péclet gap, providing isolation from an external atmosphere, such as ambient air. The principles and equations described for Péclet seals (FIGS. 7 and 8) still apply, with area A being the cross-sectional area of ​​the gap perpendicular to the sweep gas flow direction Q (gap size G multiplied by the circumference of groove 444). As noted in the discussion of FIG. 7, the gap here is defined by parallel Péclet sealing surfaces 436 and 438. Note that groove 444 can be positioned to have a cross-sectional area sufficient to allow the sweep gas to enter the Peclet gap with a nearly uniform pressure all around (note that for gaskets thinner than 0.030 inches, this condition is generally not met without a groove). One skilled in the art with this disclosure in hand can readily design a groove that provides a very uniform supply pressure across the entire Peclet gap.

[0095] In general, applicants recognize that at temperatures above 300 degrees Celsius where elastomers tend to degrade, and especially above 400 degrees Celsius where even expensive, state-of-the-art metal vacuum seals begin to fail, obtaining or employing a single conventional vacuum seal gasket for high temperature operation may be difficult, costly, and / or impractical. However, Applicants recognized that imperfect "leaky" gaskets such as Grafoil are readily available at low cost, and Applicants developed the above-described high-performance dual seal arrangement to achieve ppm and even ppb-level isolation between the chamber interior and the ambient atmosphere, including but not limited to the external ambient air, even while using a leaky gasket. For chambers with volumes between 0.25 cubic feet and 4 cubic feet, Applicants can easily achieve ppm and even ppb-level isolation of the chamber relative to the external ambient air using between 1 and 5 slm of Peclet sweep gas and a gap size through the outer Peclet gap seal of 0.003-0.012 inches.

[0096] FIG. 18 schematically illustrates another embodiment of a seal arrangement between the retort and / or chamber body 204 and a high-temperature double seal 258 including an inner gasket seal 264 and an outer gasket seal 265. The space 18001 between the inner gasket seal 264 and the outer gasket seal 265 can be used to sweep out at least a portion of any ambient air, contaminants, or gases that leak into the gap from the outer gasket. For example, 1 slm of sweep gas (such as argon or nitrogen) can be introduced through the sealing tube 214 and delivered through another tube (not shown) on the opposite side of the chamber. In another embodiment, one or more tubes can be used to vacuum out the gap to evacuate at least a portion of any ambient air, contaminants, or gases that leak into the gap. Note that the inner and outer seals of FIG. 18 can be arranged in embodiments similar to those of FIGS. 16 and 7.

[0097] 19A-19E are cross-sectional views of a portion of an exemplary retort and / or vacuum chamber configuration 200 illustrating an embodiment of a double seal that may be implemented in conjunction with a vacuum processing chamber to seal the retort body 204 to the base 202. In each of FIGS. 19A-19E, the left side illustrates the outside of the chamber, which may be any environment immediately surrounding the retort and / or vacuum processing chamber. In each of FIGS. 19A-19E, the seals on the right represent the inner seals (902A, 902B, 902C), and the seals on the left represent the outer seals (904A, 904B, 904C). In each of the exemplary configurations illustrated in FIGS. 19A-19E and FIG. 18, the chamber may include grooves (shown dotted) between the seals (to allow sufficient conductivity for the flow of sweep gas or a vacuum pump), and / or the gasket may be thick enough (e.g., from about 0.05 inches to about 0.1 inches) to form a space between the seals so that a groove is not required. Contact seals, often referred to as "lap seals," may be formed by opposing surfaces in direct contact with one another. Lap seals are generally formed by contact between surfaces that have been machined and / or polished to a relatively high degree of flatness. For example, for metal chambers such as SiC chambers, the flatness may be from about 0.001 inch to about 0.0005 inch, from about 0.001 inch to about 0.002 inch, etc. For SiC or other ceramic retort materials, the flatness of the lap seal or lap joint may be from about 0.0001 inch to about 0.0005 inch, or from about 0.0005 inch to about 0.0015 inch. It is emphasized that in all cases of Figures 19A-19E, a sweep gas or vacuum pump may be applied, as described with reference to Figure 18.

[0098] As shown in Figure 19A, the inner seal 902A and the outer seal 904A may each be a gasket seal. Referring to Figure 19B, the inner gasket seal 902A may be combined with an outer lap seal 904B. Figure 19C shows the inner lap seal 902B positioned internally relative to the outer gasket seal 904A.

[0099] FIG. 19D shows an internal gasket seal 902A positioned internally relative to an external Peclet gap seal having a Peclet gap 904C according to the Peclet seal described above (e.g., with respect to FIGS. 14-17) with a gap thickness G and a gap length L. FIG. 19E illustrates an internal lap seal 902C positioned internally relative to the external Peclet gap 904C. For the configurations of FIGS. 9D and 19E, Peclet sweep gas may be applied within the groove or space according to the previous description of Peclet sealing. In each configuration including a gasket (e.g., gaskets 902A, 904A), the gasket may be a grafoil gasket or other suitable high-temperature gasket, such as ceramic felt or fiber. While not shown here, one or more additional external seals may be included to form third, fourth (or more) internal and / or external seals.

[0100] While techniques have been described for producing hermetic seals at extreme temperatures, e.g., in the range above 800°C and as high as 300°C to less than 800°C, it should be noted that the high-temperature sealing techniques described above and below with respect to high-performance tube furnaces can also be used to provide ceramic tube-to-metal seals and / or metal tube-to-metal seals, for example, at the outlet end of pumping tubes. Applicant routinely employs reduced-diameter designs to seal the ends of both inlet and outlet tubes, as well as the outer end of Péclet feed tubes. The same designs and principles have been found to be easily scaled down and miniaturized; for example, using an internal Grafoil seal and an external Péclet gap seal, 1-inch to 2-inch diameter tube seals have been routinely and successfully produced. Note that the amount of sweep gas required to achieve ppm or ppb performance and above tends to be very small compared to the chamber seal sweep gas requirements, and Applicant routinely provides state-of-the-art leak-free joints passing helium leak rates of <1E-10 torr liters / second, and Applicant routinely does so using as little as 0.1 slm of sweep gas. Applicant further notes that Applicant routinely manufactures high-temperature valves of all-metal and ceramic construction (e.g., the high-temperature valve of Figure 13). Commercially available high-temperature valves typically avoid the use of elastomers in and around the valve seat, and often include very long valve stems with elastomer seals that are spatially separated from the high-temperature valve seat and operate at temperatures below 300°C. Such high-temperature valves are readily available, and custom designs are possible for those skilled in the art of valve design and manufacturing.

[0101] Applicants recognize that the above-described techniques can be utilized to great advantage by modifying various conventional furnaces to add the features described herein. For example, as described below, significant performance advantages can be obtained by applying these teachings to conventional tube furnaces.

[0102] FIG. 20A illustrates an embodiment of an advanced, high-performance, high-purity processing chamber based in part on tubular furnace technology, useful for many applications, including, but not limited to, two-stage debinding and sintering applications (i.e., FIG. 11). The system includes a vacuum processing chamber 20001 within a ceramic or metal tube 20002 spanning a central tube section 20003 surrounded by an extreme-temperature furnace heater 20004 and extreme-temperature furnace insulation 20005 capable of withstanding sintering temperatures, and two-way chamber extensions 20006 (in the case of a double-ended tubular furnace) having the same or similar cross-sectional shape and area as the vacuum processing chamber 20001. It should be understood that the central and extended sections of the tube may all be part of a single tube, and nevertheless, different terms may be used to designate these sections of the tube for purposes of description. In the case of a tubular furnace, the cross-sectional area and shape are substantially the same, regardless of the degree of distortion, intentional or otherwise, in the tube.

[0103] Applicant has operated such a tube furnace with one or two additional high-temperature extension heater systems 20007 surrounding one or both ends of the processing chamber that can heat the chamber extension 20006 at one or both ends, at least during debinding, to prevent or at least reduce binder by-product contamination within the chamber extension, including one or more sealed high-temperature end caps 20008. Based on the principles previously described with respect to the tube heater for heating the pumping tube, the extension heaters 20007 can heat the extensions 20006 and end caps 20008 to prevent or at least minimize condensation of debinding by-products therein below a predetermined threshold. These measures can ensure the cleanliness of the atmosphere within the vacuum processing chamber after debinding and during sintering, at least with respect to debinding by-products and other contaminants. 12 and 13, the furnace may include a high temperature pumping tube 20009 configured in accordance with the teachings above, such that it may be heated by a high temperature pumping tube heater 20010 and optionally surrounded by high temperature tube insulation 20012 to prevent or reduce contamination of the binder product during debinding. Additionally, as was the case in these embodiments, the high temperature pumping tube 20009 may be configured with a sufficiently small diameter and a sufficiently long length to provide at least some predetermined degree of Peclet sealing (based at least on the principles and teachings of FIGS. 7 and 8, and described in more detail below), provided that a sufficient flow of process gas 20011 is injected into the inlet tube 20013 of the system.In the context of the preceding discussion (e.g., Figures 12 and 13), the arrangement of Figure 20A can be considered a furnace system having a processing chamber (in this embodiment, the center of the tube as central portion 20003) capable of extreme temperature operation, for example, during sintering, transitioning to a chamber extension 20006 (in this case, an inlet and an outlet) having the same or similar cross-sectional area as the central tube portion 20003. The chamber extension 20006 can be heated by a high-temperature heating system 20007 configured to heat at least during debinding to prevent condensation therein, including within the cap. While extreme-temperature conventional tube furnaces are routinely employed for powder metallurgy, including debinding and sintering, commercially available tube furnaces are generally susceptible to contamination by air and binder by-products. Furthermore, conventional tube furnaces often rely on non-hermetic elastomeric seals, at least insofar as they allow significant diffusion of oxygen, limiting the purity of the sintering atmosphere within the tube. In contrast, the systems and methods described herein have been demonstrated to provide tube furnaces with sintering atmospheres orders of magnitude purer than tube furnaces sealed with conventional, non-hermetic elastomeric seals. In general, the systems and methods described herein can provide sufficiently high atmospheric purity that conventional elastomeric O-rings and gaskets are considered non-hermetic. We recognize that porous materials, such as grafoil, can be so permeable that vacuum experts consider them to exhibit significant leakage, far below the standard standard of hermeticity. On the other hand, those same experts may refer to elastomeric seals as hermetic in some contexts. For example, this is reasonable in medium-vacuum applications, where the purity is moderate relative to air. However, this is not the case in ultra-high-purity, ultra-high-vacuum systems. For the remainder of this application, we will generally refer to all elastomeric seals as "non-hermetic," especially in the context of high-temperature hermetic tube adapters employing Peclet seals.Applicant has shown that the configuration of Figure 20A can be configured with respect to pumping tubes and / or using multiple hot valves with separate debinding and sintering pumps, and provides many of the same significant advantages as previously described with respect to furnace embodiments configured in accordance with Figures 6A-C, 9-10, and 14-17, including, but not limited to, a high purity atmosphere and low oxygen content, and minimal binder condensation, despite the use of low-cost vacuum pumping systems and / or mechanisms. For example, the extended heater system 20007 and tube heater system 20010 can be maintained at, near, or above the debinding temperature to prevent or reduce binder by-product condensation, and the pumping tube 20009 can be configured to provide a predetermined degree of pneumatic sealing to achieve ppm or ppb purity or better. As with the previous embodiment, the central tube portion 20003 can be controlled to operate at a much higher temperature during sintering than the inlet tube 20013 and extension 20006 as pumping tube 20009. It is again emphasized that pumping tube 20009 can be configured and operated with a sufficient amount of process gas flow therethrough to act as a Peclet seal, isolating contaminants at the pumping tube outlet 20025 from impinging on the inlet of that pumping tube 20024.

[0104] In one method, power to extension heater 20007 and tube heater 20010 can be deactivated or controllably reduced after debinding as central tube section 20003 is ramped up to sintering temperature, so that extension 20006 and the tube are maintained at or below the temperature maintained during debinding. Such high performance tubular furnaces can demonstrate significant utility when employed as low-cost process development furnaces.

[0105] 20B shows an embodiment of an advanced, high performance tube furnace which may be a tube furnace in which the furnace chamber extension 20013 may be heated with an extension heater 20014 with optional insulation 20015 surrounding it and a high temperature resistant all-metal and / or metal and ceramic valve 20016 leading to a first vacuum pumping system 20019 and a second vacuum pumping system 20020 (which may be separate pumps for debinding and sintering, as previously described). Note that the high temperature valve 20016 may be sealed at the inlet end, the outlet end of the pumping tube 20017, or at various points between the inlet and outlet of the pumping tube 20017. The pumping tube 20017 can be heated, at least during debinding, by a tube heater 20010. Applicant recognizes that the inclusion of the processing chamber extension 20013 allows one or more end caps to be utilized at a much lower temperature compared to the processing chamber, and thus the valve 20016 can be integral with or adjacent to the end cap. Figure 20C shows an embodiment in which the valve 20018 is located toward the outlet end of the pumping tube for clarity of illustration.

[0106] The above-described embodiments of the advanced tube furnace are illustrative and not limiting. These systems and methods can be applied to provide advanced, high-performance tube furnaces based on many variations, including, for example, vertically oriented single-ended tube furnaces. With reference to FIG. 20B, it should be understood that not all tube furnaces are double-ended, nor are they always horizontally oriented. For example, a tube furnace may be a single-ended construction with only one end cap, and the opposite end of the tube may be closed and adjacent to or completely housed in the process chamber insulation, with the loose end forming part of the process chamber. Applicant recognizes that a single-ended tube furnace can be configured to operate in any orientation, including vertical, horizontal, or otherwise, fully in accordance with the teachings herein, for example, by utilizing one of the tubes in FIG. 20B as an inlet tube and the other as a pumping tube.

[0107] FIG. 20D illustrates an end cap 20019 that can be configured with a high-temperature dual seal to provide a high-temperature seal that exceeds the maximum temperature limitations of typical commercially available elastomeric seals. Herein, the term "high temperature" as used throughout this specification with respect to the seals, end cap flanges, and tubes can be interpreted as operable at or somewhat above debinding temperatures, as opposed to the extreme temperatures associated with sintering. An internal high-temperature gasket seal 20020, such as a grafoil gasket seal, can be combined with an external Péclet gap seal 20021, according to the principles described with reference to FIGS. 16 and 17. A sweep gas 20022 can be delivered into the end cap using a supply tube 214 to supply the Péclet gap seal 20021. Hereinafter, at various points, the supply tube 214 may be referred to interchangeably as an injection channel 214, a sweep gas tube 214, or similar language. Various other high-temperature dual seals can be implemented in the end cap, including, but not limited to, the variations described in FIGS. 19A-19E. It should be understood that the dual seals need not be coplanar. For example, any internal seal can sealably engage the tube face, or the inner or outer surface of the tube, and any external seal can sealably face and / or engage the end face or outer surface of the tube end. As shown in the figures, a gasket can be sandwiched between gasket sealing surfaces 433 and 433'. For example, internal seal 20020 in FIG. 20D is a gasket that faces and sealably engages the end face (first gasket sealing surface) of tube 20023, and external seal 20020 is a Peclet gap seal that faces the outer surface of the tube end; each of the dual seal embodiments of FIGS. 19A-19E can be oriented accordingly.

[0108] With continued reference to FIG. 20D, it is noted again that the high-temperature sealing techniques described immediately above with respect to high-performance tubular furnaces can also be used to provide ceramic tube-to-metal seals and / or metal tube-to-metal seals, for example, at the outlet end of pumping tubes. Reduced-diameter designs based on the foregoing diagrams are routinely used to seal metal tubes and / or valves at the ends of both inlet and outlet tubes, and at the outer ends of sweep gas supply tubes. The same designs and principles have been found advantageously reduced to routinely successfully produce 1- to 2-inch diameter tube seals, for example, using an inner Grafoil seal and an outer Peclet gap seal.

[0109] With respect to the above description and embodiments, applicants understand that in applications requiring ultra-high purity, particularly at ppm or ppb levels or higher, those skilled in the art typically use expensive, high-performance ultra-high vacuum pumps. For example, typical systems designed for high-purity processing (ppm, ppb, or higher) often use expensive, high-compression turbomolecular pumps, diffusion pumps, ion pumps, or cryopumps with oxygen compression ratios (i.e., compression C > 1E6, or even C > 1E8). Such high-vacuum pumps generally have significantly higher costs compared to the high-purity pumps and processing systems and methods described herein. Applicants have discovered that, in some cases, contrary to conventional intuition and rules of thumb, the use of high or ultra-high vacuum can further create unexpected and even surprising problems that result in inferior and / or compromised processes compared to the systems and methods described herein. For example, various practitioners have attempted to sinter titanium using ultra-high vacuum, but in some cases, the presence of residual contaminants increases the diffusion rate within the system, creating process challenges and compromises. In other words, high-vacuum pumps can significantly increase trace contaminant levels within the system compared to the systems described herein. Thus, contrary to conventional wisdom and intuition of those skilled in the art, surprisingly good performance can sometimes be achieved at pressures higher than those typically associated with high-vacuum techniques and products when high purity is required. For example, Applicant has found significant advantages in sintering high-purity aluminum and titanium at pressures of 1 torr or greater, while other practitioners often advocate processing these materials at pressures of 0.001 torr or even lower. Applicant has discovered that the systems and methods described above, including but not limited to the embodiments of Figures 12, 13, 14, 15, and 20A-D, offer overwhelming advantages over high-vacuum, low-pressure (<0.1 torr) sintering of aluminum, aluminum alloys, titanium, high-carbon steel alloys, and many other sensitive and difficult-to-sinter metals and alloys. In particular, Applicant routinely sinters aluminum and titanium alloys in furnaces configured according to all of these embodiments.

[0110] FIG. 21 illustrates an embodiment of a vacuum processing system utilizing a two-stage pumping system 21001 for achieving ultra-high purity at low cost. This system may be used in a variety of applications, including, but not limited to, semiconductor processing systems, including, but not limited to, sputtering and etch plasma processing systems. In this embodiment, a low-cost, low-performance, and / or extremely rugged, yet low-cost turbomolecular pump 21002 having an unusually low compression ratio may be positioned between the vacuum processing chamber 21003 and a low-cost, high-purity mechanical pump 21004, as previously described, including a hermetically sealed mechanical pump with a Peclet seal at the outlet through which the sweep gas flows. This embodiment achieves ultra-high purity at a lower cost than conventional multi-stage systems, at least because the system can achieve ultra-high purities of parts per billion or greater even with very low turbopump compression ratios. The use of the low-cost, high-purity mechanical pumping system 21004 allows for the use of a low-cost, "non-rated" turbomolecular pump 21002 having a compression ratio of less than 1E6, less than 1E5, less than 1E4, or less than 1E3. For example, this embodiment can be configured as a sputtering system operating at IE-4 torr, achieving ppb purity even when the low-compression turbopump exhibits a compression ratio of only 1000 or 100 to oxygen. Conventional turbopumps are readily available with compression ratios of 10E8, 10E9, and even higher, and Applicants recognize that such high compression ratios, while resulting in significant costs, are considered desirable for achieving ultra-high purity. Applicants further recognize that non-rated turbopumps have less mechanical precision in their internal mechanisms compared to state-of-the-art high-compression pumps, allowing for designs with superior durability and reliability. Note that process gas may optionally be introduced into vacuum processing chamber 21003 via process gas source 21005 and contribute to the Péclet sweep gas as described above. Note also that sweep gas may be injected into the pump housing and / or the inlet of the Péclet tube, similar to the previous discussion regarding low-cost, high-purity mechanical pumping systems.

[0111] As noted above, the systems and methods described herein can be adapted to provide overwhelming advantages even when retrofitted into many furnace embodiments. FIG. 22 illustrates a vacuum sintering furnace 100 having inner and outer insulation 24 and 26 within the vacuum chamber wall 32. The furnace 100 can include an outer external heater 298 and / or an outer heater system 296 that can be embedded in the insulation configured to heat the outer insulation 26. Both outer heater options are included herein, and applicants have had success with both. The furnace 100 includes an inner heater 112, an inlet tube 78, and a pumping tube 73. The steel chamber includes high-temperature insulation surrounding a furnace heater positioned to heat a hermetically or semi-hermetically sealed component retort, such as a non-porous or porous ceramic, refractory metal, or graphite component. Note that in the context of FIG. 22, the retort generally need not function as a vacuum chamber, provided that at least the outer chamber 32 serves that purpose. In some embodiments, the inlet tube 78 may be used to inject process gas, and the sealed, or semi-sealed and / or semi-porous retort 22001 may include a retort pump tube 22002 that receives at least a portion of the process gas flow for pumping the retort 22001 and can provide at least some degree of pneumatic sealing between the exterior and interior of the retort. This pneumatic sealing by the retort pump tube can provide some degree of isolation from the intrusion of air or other contaminants that may be present within the steel chamber and outside the retort. The system may include an additional outer heater system 296 and / or 298, including a heater 296 embedded in the outer layer of insulation, or may be located as a heater 298 outside, or even external to, the insulation. In some embodiments, the outer heater system 298 may be located just outside the vacuum chamber. These outer heaters may operate at least during debinding of part 22003 to maintain the outer insulation 26 at a sufficiently high temperature to reduce or prevent binder condensation on the insulation and within the vacuum chamber.Additionally, the vacuum chamber pumping tube 73 can be heated by a pumping tube heater 22004 and insulated with optional tube insulation 22005, at least during debinding. Applicant has recognized that furnaces with insulation within a sealed vacuum chamber but no external heater, as shown in FIG. 22, are prone to significant binder condensation outside the insulation and inside the vacuum chamber. Applicant has installed external heaters in various embodiments (embedded within the insulation, outside the insulation, either inside or outside the vacuum chamber wall). In various approaches, the external heater and / or pumping tube heater are controlled in conjunction with the furnace heater so that the external insulation and / or pumping tube are heated during debinding sufficiently to significantly reduce condensation of debinding by-products during debinding. This use of an external heater results in substantially improved part quality. Applicant has successfully utilized the valve, pump, and pumping tube of FIG. 13 in combination with the chamber embodiment of FIG. 22. Various combinations of the combined features and methods provided overwhelming advantages for air purity in the retort 22001. Applicant has implemented this combination in the context of many systems, including vacuum sintering furnaces employing water cooling during sintering of the chamber walls 32. In these configurations, Applicant purged the water cooling lines before and during debinding so that the chamber walls were heated during debinding to reduce or eliminate condensation of binder by-products. In some embodiments, Applicant installed a conventional water-cooled sintering furnace, retrofitted the system with a heated pumping tube, semi-enclosed retort 22001, and retort pumping tube 22002 (FIG. 12), and operated the furnace during sintering with sufficient process gas flow to achieve a high degree of peculiar sealing with the retort pumping tube 22002. In this combination, Applicant was able to achieve significantly higher air purity compared to conventional sintering furnace operation.In another embodiment, Applicant further modified the furnace to include a heated pumping tube and two heated valves, as in the embodiment of Figure 13, and used two separate pumps during debinding and sintering, thereby achieving the additional advantage of allowing Applicant to sinter parts with superior metallurgical properties compared to parts sintered in an as-installed conventional furnace. By combining the chamber of Figure 22 and the features therein with the vacuum manifold of Figure 13, including a heated pumping tube, two heated valves, and two separate pumps, significant advantages were demonstrated, particularly with regard to preventing condensation of binder by-products.

[0112] In addition to sophisticated systems and methods for vacuum pumps, the application discloses numerous vacuum furnace systems for debinding and / or sintering, with a general emphasis on systems and methods for avoiding the accumulation of contaminants during debinding as one of several factors that may contribute to achieving unprecedented ultra-high vacuum purity, particularly during sintering. Note that these systems are sometimes referred to by the technical term "batch furnace." Batch sintering furnaces are sometimes so named based on their method of use, which involves loading a single batch of parts into the furnace per run for debinding and / or sintering in a single chamber, according to a time and temperature cycle, for example, as shown in FIG. 11 . The term batch furnace is often intended to distinguish such systems from so-called continuous or conveyor furnaces, which move parts through a long chamber with different temperature zones arranged along the path of travel. Applicant is unaware of other batch sintering furnace systems and methods capable of providing the levels of sintering atmosphere purity routinely achieved using the various described embodiments, e.g., PPM, parts per million, parts per billion, or PPB, or even higher levels as needed. All of these purity levels have been routinely demonstrated using these methods and embodiments after debinding and during sintering. Furthermore, we believe that the aforementioned methods and embodiments, generally and generally described in FIG. 12 and specific embodiments and aspects described in FIGS. 14, 15, 16, 17, and 19, provide superior air purity compared to conventional systems and methods in the state of the art. For example, even when used solely for sintering fully and separately debound parts, contaminants entering the furnace from the atmosphere or from previous runs tend to collect on the vacuum chamber walls and insulation of conventional furnaces, continually contaminating the sintering atmosphere during sintering. The systems described herein and throughout this disclosure are free of cold spots where contaminants can easily condense during debinding and sintering.

[0113] It should be noted that many of our systems and methods employ binder traps to capture binder vapors and prevent them from impinging on the vacuum pump. We have developed advanced systems and methods for preventing backflow of binder vapors from the binder trap back into a given sintering chamber. This includes, but is not limited to, the advanced use of Péclet isolation in the pumping tube during and, in some cases, after debinding to completely isolate the inlet of the vacuum pumping tube, which is in gaseous communication with the chamber, from backflow of binder from the trap and / or the pumping tube. This also includes heating the Péclet tube to a sufficiently high temperature to prevent detectable buildup therein. Regarding this improvement, outlet tube heating, especially when properly performed during debinding, has benefits beyond simply preventing vacuum tube clogging, and we routinely demonstrate that this technique, when combined with Péclet sealing, can prevent observable or detectable buildup within the inlet section of a given vacuum pump tube. To emphasize this point, the pumping tube will hereinafter be referred to as a high-temperature vacuum pumping tube or an extreme-temperature vacuum pumping tube, where the term "high-temperature" refers to temperatures equal to or greater than debinding temperatures, typically well below 800°C, and the term "extreme" refers to temperatures above 800°C, such as those required for sintering.

[0114] In the context of conventional batch furnaces and methods, the insulation arrangement of conventional vacuum chambers and furnaces (particularly the outer portions of the insulation and the inner walls of the chamber) is very commonly the primary or dominant source of binder contamination during sintering, even when a separate debinding is performed externally before charging the batch of already debound parts. To our knowledge, conventional chamber pumping tubes tend to be larger than ours (presumably to allow accumulation without clogging) and are often not heated, particularly during debinding, so little attention is paid to reducing, let alone removing, contaminants. In conventional batch sintering furnaces, the chamber (particularly the inner walls of the vacuum chamber and the insulation therein) tends to become highly contaminated, so backflow from the inlet of a conventional pumping tube can be significant during sintering, but nevertheless contributes little or no significant amount to the much greater degree of contamination of conventional sintering atmospheres coming from the vacuum chamber and furnace insulation. As previously mentioned, conventional systems and methods often tend to configure the system with large outlet tubes to avoid clogging due to the accumulation of condensed binder material.

[0115] Therefore, we believe that in conventional furnace design and operation, little attention has been paid to removing the relatively small amount of contaminants in the pumping tube that may backflow into the chamber from the pumping tube and / or binder trap—at least because backflowing contaminants typically contribute only to a minor contamination of an already highly contaminated chamber and atmosphere. In our embodiment, on the other hand, contamination in the sintering chamber tends to be sufficiently negligible that we have focused on the additional step of reducing or completely eliminating the outlet tube as a detectable contamination source. Heating the vacuum pumping tube during debinding contributes to this goal by reducing or completely preventing binder condensation within the tube during debinding. As a given part processing cycle progresses from debinding to sintering, binder contaminants are largely or completely eliminated except at the outlet end of the tube, and Pecle flow from the tube inlet to the outlet can completely suppress backflow into the sintering chamber.

[0116] Applicant has recognized that, when properly implemented, Peclet sealing literally prevents backflow. This may seem counterintuitive to those skilled in the art, and even top experts often lose intuitive understanding of this truly amazing element of applied physics. One metaphorical image that helps those skilled in the art intuitively understand why Peclet sealing (when properly implemented) is so effective is to imagine a school of fish swimming upstream through a large water pipe. Given the average maximum speed at which any one fish can travel, if the velocity of the water flow significantly (e.g., by an order of magnitude) exceeds this speed, one would expect that no fish would be able to make it from the outlet to the inlet of the pipe, even if the pipe were relatively modest in length. For example, if a large pipe, say 3 feet in diameter and 10 feet long, were to be subjected to a 100 MPH water flow, it would be reasonable to expect that no typical small freshwater fish would be able to pass through the pipe, regardless of the size of the school of fish. The same is true for gas molecules, given the right conditions. This analogy is somewhat oversimplified, since at least the laminar flow at the pipe wall will be slower, and if one fish is mentally intelligent enough to stick close to the wall, it will have a better chance and will certainly travel farther than the less intelligent members of the school. Fortunately for the metal sintering industry, gas molecules are not endowed with mental intelligence and travel only by random diffusive motion. As long as the Péclet tube is much longer than its diameter (or the gap length L is much longer than the gap size G), the statistical probability of a molecule remaining close to the wall throughout its entire journey becomes negligible. Extensive laboratory testing has been conducted in our facility to fully verify the claimed performance, and our model has been amply proven to function as described.

[0117] Here and throughout this specification, we describe systems and methods that are free of detectable impurities. This means that we were unable to detect contaminants, either by instrumentation (such as modern spectrometers or helium leak detectors) or by indirect evidence, such as contamination within the components. For example, we routinely sinter aluminum and titanium using these systems and methods, and subsequent metallurgical studies have failed to detect any discernible contaminants that would indicate direct or indirect contamination of our hermetic seals. Generally, when our systems are functioning properly, any contaminants we detect or any effects we observe are due to either or both: (i) contamination within the process gas from an initial source, such as a liquid argon dewar, and (ii) contamination within the powder metal components themselves, such as when the raw powder contains trace impurities. As such, we routinely manufacture components that exhibit no measurable degradation as a result of leaks from the main seals, tube furnace seals, or high-temperature tube adapters described and claimed herein.

[0118] In addition to the above considerations, the powder metallurgy industry in general is moving towards an increased emphasis on low cost, space-saving compactness, lower power usage, and reduced maintenance costs and greater ease of use, all factors that are adequately addressed in the disclosed embodiments.

[0119] Referring to the figure, a sintering furnace system can include a sealable vacuum chamber for vacuum-heating and debinding a part comprised of powder bound into the part shape with at least one organic binder containing carbon to a predetermined chamber temperature range, including a thermal debinding temperature range high enough to induce off-gassing of organic binder vapors from the part for debinding purposes. The furnace can be heated to temperatures well above 250°C, even above 450°C, during debinding and can be evacuated through a vacuum pumping tube sealed in gaseous communication with the vacuum chamber, whereby a furnace exhaust stream contaminated with binder vapors can be diverted during debinding. A tube heater can be provided and configured to maintain at least a portion of the length of the vacuum pumping tube within a tube temperature range high enough to prevent clogging and, in fact, suppress directly and / or indirectly observable buildup and / or condensation therein, thereby limiting the buildup of binder material to prevent contamination, particularly within the initial inlet length of the vacuum pumping tube, where that portion is in close gaseous communication with the chamber. The system may include a vacuum pump capable of contributing to the outflow of at least said furnace exhaust while simultaneously providing pumping action to provide a vacuum within the chamber within a predetermined vacuum pressure range.

[0120] Regarding contamination levels in the pumping tube, the tube temperature should be maintained at a high enough temperature (i.e., 250-600°C) that direct or indirect detection of contamination, particularly including carbon and carbon-based molecules due to binder evaporation, is impossible or at least very difficult. As previously mentioned, indirect detection may contain evidence of contamination from parts being processed in the absence of contamination measured with gas analysis equipment. Materials scientists will be able to identify a variety of sensitive techniques for directly and / or indirectly detecting contamination. For example, initial runs at insufficient purity can be intentionally performed by intentionally introducing carbon and / or oxygen-based impurities, followed by experimental sweeps in which parts are inspected in a materials lab and measurable amounts of carbon and / or oxygen contaminated by carbon are detected. This is straightforward for ultrasensitive sample materials like titanium. For subsequent runs, the pumping tube temperature, at least during debinding, can be increased with each run until a temperature is established at which no detectable carbon and / or oxygen contamination is observed in sintered test parts. This is one of many ways to reach a measurable temperature that can be considered indirectly free of detectable contamination in the pumping tube. All materials, including high-purity stainless steel, contain a certain amount of atomic and / or molecular contamination, so at the very least, the concept of zero contamination is considered non-physical. While a "theoretical" definition of zero contamination is certainly non-physical, the concept of zero contamination is intended for practical and functional purposes and should be considered broadly to mean one or more of the following: (i) Oxygen or carbon contamination cannot be detected directly (using state-of-the-art absolute gas analyzers sampling the sintering atmosphere) or indirectly (using metallurgical laboratory equipment to characterize parts such as titanium components). Again, this should be seen as a practical and functional statement, rather than an endless theoretical conceptualization of ultimate non-existence. (ii) As another functional concept, our system can be considered practically free of impurity contributions to the extent that the amount of impurities introduced directly by the ultra-high purity process gas injected through the inlet is greater than the amount introduced (or condensed and then reintroduced) by our system. For example, if there is a difference of an order of magnitude or more, then the system should be practically considered free of unintended or unexpected contamination. (iii) It is well known to vacuum scientists that, when operating at pressures below 10E-9 torr, typical vacuum systems of atmospheric purity tend to be limited by contaminants physically arising from deep within the actual steel vacuum chamber via solid-state diffusion. This is by no means a species analogy; purities of parts per billion or better can be achieved, if necessary, when operating an empty or nearly empty chamber at 1 torr sintering pressure. It should be understood that a high-vacuum system operating at 10E-9 Torr would be expected to contain the same number of impurities per cubic centimeter that we appear to experience when operating at 1 Torr chamber pressure and PPB purity. While it is difficult to pinpoint our performance to this extent, we believe this is the likely condition experienced by our system and is in full agreement with all direct and indirect measurements. For example, contamination at the inlet of our steel pumping tube is likely limited to this effect (solid-state diffusion) rather than due to oxygen diffusing up the pumping tube or leaving trace binders on its surface.

[0121] In various embodiments, the hermetically sealed vacuum chamber can be in a hot zone that includes extreme temperature furnace heaters both within the insulation and outside the chamber and is surrounded by ambient air, as shown in Figure 14. In other embodiments, the chamber can be a single-ended or double-ended tubular furnace, as described with reference to Figures 20A-D, 26B-26C, and supplemental implementation details described with reference to Figures 24A-24B and 24D, as well as additional techniques described with reference to Figures 23A-23D.

[0122] Focusing on the former, a hermetically sealed vacuum chamber may require a main chamber opening that maintains a highly isolated seal (with respect to the air and / or gases surrounding the outside of the sealed chamber) even at the often extreme sintering temperatures. Until the development of the physical system embodiment depicted in Figures 14, 15, 16, and 17, no one else had developed and / or demonstrated a hermetic door seal that could be used at temperatures exceeding 800°C. In this configuration, the vacuum pump tube can extend from inside the high-temperature zone defined by the heater and furnace insulation, through at least the inner portion of the insulation, and toward the outside of the insulation. In such cases, the vacuum pump tube can be constructed of an extreme-temperature material that is hermetically impermeable to gas diffusion and can withstand extreme temperatures (i.e., sintering temperatures). The furnace insulation arrangement (operable at extreme temperatures together with the furnace heater disposed therein) can be configured to heat the vacuum chamber therein to temperatures exceeding 800°C for at least the sintering portion of the furnace's heating cycle for cycles that include sintering. Note that in this configuration (Figures 14, 15, 16, and 17), the vacuum chamber can be surrounded by ambient air (containing oxygen and other contaminants) while maintaining an ultra-high purity, oxygen-free sintering atmosphere within it. The chamber can (and often must) be constructed of materials that are hermetically impermeable to the diffusion of ambient gases and can withstand vacuum pressures from the ambient air (i.e., 15 PSI) throughout the furnace heating cycle. When the chamber is closed (e.g., during debinding and / or sintering), the interior volume is vacuum-sealed from the ambient air by a main hermetic retort seal located within the hot zone, and the seal is capable of blocking leakage of ambient air into the interior volume sufficiently to prevent adverse effects of ambient air on part quality.

[0123] In other embodiments, the sealed vacuum chamber may be a tube furnace as described with reference to FIG. 20A, with the vacuum exhaust tube 20009 positioned and operated according to the aspects of the following description. In such embodiments, the end plates of the tube furnace may be metal flanges capable of withstanding high temperatures, including debinding temperatures. The inlet and / or vacuum pumping tubes that can be connected to these flanges may be hermetically connected to and / or fabricated with the flanges and constructed of high-temperature tubing material capable of withstanding debinding temperatures. In various systems and methods, the sintering process gas flow enters the tube furnace through the inlet tube, and a balanced amount of process gas may simultaneously exit the vacuum pumping tube to provide Péclet separation of any contaminants impinging on the outlet of the vacuum pumping tube. Such contaminants include binder vapors emanating from binder traps, vacuum pumps, and other downstream components of the vacuum manifold. In such applications, when an appropriate flow rate of process gas is provided, the flow of this process gas from the inlet to the outlet of the pumping tube is relied upon to provide a Peclet seal that separates the outlet of the pumping tube from the inlet as well as the chamber. With respect to the tubular furnaces described herein, the vacuum pumping tube is described herein as being capable of operating at "high temperatures" to emphasize that the vacuum pumping tube can be designed to withstand debinding temperatures that are beyond the range of typical room temperature vacuum components such as elastomeric O-rings.

[0124] In various embodiments, including at least some of those described above, heating of the vacuum pumping tube can be provided indirectly by the same furnace heater located within the furnace insulation. In many embodiments, it can be provided by a separate tube heater in thermal contact with the vacuum pumping tube. In the former case (when heat is provided by a furnace heater), the thermal conductivity of the vacuum pumping tube must be sufficient to transport sufficient heat along the tube. Regardless of the two configurations described above, the high-temperature sealed tube adapter (described below) can support and include an additional heater that can heat the adapter and simultaneously contribute to maintaining high tube temperatures (i.e., above 300°C, typically 450°-500°C) along the vacuum pumping tube, including during debinding.

[0125] The vacuum pumping tube can extend through the furnace insulation from the inside to the outside, providing insulation for the chamber and furnace heater as well as for maintaining the high temperatures of the extreme-temperature pumping tube, including during debinding. For embodiments such as those shown in Figures 14 and 15, at least a portion along the inlet end of the pumping tube must be capable of operating at sintering temperatures, often exceeding 800°C. For conventional vacuum tubes and seals, most commercially available hardware is constructed of stainless steel with copper gaskets that cannot operate above several hundred degrees Celsius. In a tubular furnace, the furnace tube can be considered to have a main section that operates as the sintering section and at least one end section that serves as a tube extension, extending from the high-temperature zone where sintering is achieved to a less extreme heat zone where a high-temperature sealing flange (e.g., a steel flange) is connected and capable of operating at high temperatures. On the one hand, this section of the furnace tube can be considered a vacuum pumping tube, even without relying on Pecle sealing. In this case, the second, smaller-diameter tube can also be considered a vacuum pumping tube, capable of Pecle sealing. The flange in this case can be described as a high temperature tube adapter that can sealingly fit from an extension of the furnace tube onto a smaller pumping tube, for example, as shown in Figure 24A. Consistent with discussions throughout this disclosure, it can be highly advantageous to maintain high temperatures (i.e., debinding temperatures) at all three of: (i) the end of the tubular furnace, (ii) the flange acting as an end plate, and (iii) the high temperature vacuum pumping tube.

[0126] As previously mentioned, the vacuum chamber and / or tubular furnace chamber can be configured to include an inlet tube for injecting, at least during sintering, a sintering process gas, which may be highly purified and may include inert gases such as argon and / or nitrogen, at a flow rate in the range Qmin-Qmax. In some cases, Qmin can be as low as 0.01 slm per cubic foot of chamber volume and as high as 10 slm per cubic foot of chamber volume. Many of our implementations are limited to between 0.05-2 slm per cubic foot of chamber volume. The vacuum pumping tube can have inlet and outlet ends sealed to the vacuum chamber, and the vacuum pumping tube can be positioned to utilize the process gas flow to provide a "Pecle seal" that isolates the inlet end of the vacuum pumping tube (and therefore the chamber) from contaminants that may be present at the outlet end of the vacuum pumping tube. With this separation mode, at least a portion of the pumping tube (e.g., the initial inlet portion of the tube length) can be characterized as having a cross-sectional area A and a length L, where A and L are selected such that, for a given range of process gas flows at at least a given vacuum pressure, Pe(Q, A, L, D) is less than 10E-6 relative to the D(air) of oxygen and D(CX) of binder vapor molecules in the process gas flow for the given pressure. While a tube with a uniform shape and cross-section is employed here, it is possible to vary A along the tube, which would make the calculations more complex, but would maintain the functionality and principles as described throughout this disclosure.

[0127] Generally, this configuration works very well at vacuum pressures, providing very effective Peclet sealing for vacuum pressures where laminar flow is maintained. As pressures drop below the laminar flow regime, Peclet separation tends to be impaired at vacuum pressures low enough that transitional flow occurs, and can be significantly impaired as pressures drop to submilliliter pressures. It should be noted that the proprietary pumps in the incorporated references are designed for applications requiring deeper vacuums. Many products can operate at pressures of 1 torr or greater, with Peclet separation from properly heated and properly designed vacuum pumping tubes maintaining ppm or better purity at the inlet to the vacuum pumping tube.

[0128] In summary regarding vacuum pumping tube design and dimensions, for a given range of vacuum pressures and process gas flow rates, the vacuum pumping tube can be designed with a sufficiently long and sufficiently small inside diameter (ID) such that, within a given range of process gas flow rates (0.05 slm to 10 slm), the flow of process gas (during sintering) through the vacuum pumping tube, acting as a sweep gas for the tubular Peclet seal, provides better than ppm (often much better), and often better than ppb, separation. Note that details of such tube design considerations and equations are discussed in detail with reference to Figures 7 and 8 and at various locations herein, including paragraphs 56-67 and paragraphs 77 and 78.

[0129] A vacuum chamber equipped with the above parameters can be operated at a vacuum pressure of 0.1 torr to 759 torr at the inlet of the vacuum pumping tube, i.e., inside the chamber. We routinely operate it at a pressure of 1 torr to 500 torr while maintaining purity of ppm or better.

[0130] In one embodiment of the furnace system (illustrated in Figures 22A-B), the sealed chamber may be outside the furnace insulation, as is often found in conventional batch sintering furnaces. As mentioned above, we are not aware of any sintering furnaces with heatable vacuum pumping tubes sized and operated to employ pure process gas and / or sweep gas during sintering to provide a pneumatic seal through the vacuum pumping tube to separate the vacuum pumping tube inlet from contaminants present at the vacuum pumping tube outlet, as shown in Figures 22A-B. The pumping tubes in conventional systems are generally very large and tend to lack tube heaters. As mentioned above, experience with conventional furnaces indicates that the inner walls of the vacuum chamber and the furnace insulation release large amounts of contaminants during sintering, including, but not limited to, binder vapors that condense on and within them, especially during debinding. Furthermore, the amount of gas release tends to be so high, especially in the early stages of sintering, that a perfectly designed and implemented vacuum pumping tube (providing separation better than 10E-6) would make little difference in terms of part contamination. However, by incorporating a heater arrangement inside the chamber, either external to the insulation or embedded in the outer portion of the insulation, as shown and described in Figures 22A-B, we have achieved significant advantages. In one use, this "external heater" is operated at approximately the same temperature as the bonding temperature provided to the part by the furnace heater. This technique significantly reduces, and in some cases completely prevents, binder condensation within the insulation and chamber walls during debinding. We have found that combining this setup and process with the design and operation of a Peclet-sealed, heated vacuum pumping tube in accordance with the disclosure above provides overwhelming advantages, allowing us to increase purity levels by orders of magnitude over conventional furnaces at relatively low cost.

[0131] 22A-B, the sintering furnace system can include a sealable vacuum chamber for vacuum-heating and debinding parts under vacuum to a range of predetermined chamber temperatures, including a range of thermal debinding temperatures high enough to induce off-gassing of organic binder vapors from the parts. The vacuum chamber can include furnace insulation and a furnace heater that define a hot zone into which parts can be loaded and unloaded through a furnace door (not shown). The furnace is sealed to the vacuum chamber and evacuated through a heatable vacuum pumping tube in gaseous communication with the vacuum chamber, configured such that the evacuation induces the flow of binder vapor-contaminated furnace exhaust stream therein during debinding. An outer chamber heater may be provided (shown in the figures as being at or on the outer portion of the chamber interior and furnace insulation, but alternatively may be located outside the vacuum chamber (see FIG. 22B)) and may be activated and operated during debinding to reduce and / or remove binder condensation on and within the insulation and within the interior surfaces of the vacuum chamber. It may be activated early enough and for a long enough time to ensure that the entire thickness of the insulation is at or near the debinding temperature, at least during the final portion of debinding and / or the beginning of sintering. To maximize this benefit and provide even greater purity during sintering, a heater may be provided and configured to maintain at least a portion of the length of a heatable vacuum pumping tube at a range of pumping tube temperatures high enough to inhibit buildup and / or condensation therein, thereby limiting binder buildup to prevent contamination, particularly within the initial inlet length of the tube in close gaseous communication with the chamber. The system may include a vacuum pump that directs the outflow of the furnace exhaust and simultaneously provides a vacuum within the chamber within a predetermined vacuum pressure range.

[0132] 22A-B , at least during sintering, the vacuum chamber can be configured to include an inlet tube for injecting a sintering process gas, which may optionally be highly purified and include an inert gas such as argon and / or nitrogen, at a flow rate in the range Qmin-Qmax. In some cases, Qmin can be as low as 0.01 slm per cubic foot of chamber volume and as high as 10 slm per cubic foot of chamber volume. The vacuum pumping tube can have inlet and outlet ends that can be sealed to the vacuum chamber, and the vacuum pumping tube can be positioned to utilize the outflow of process gas to provide a "Péclet seal" that separates the inlet end of the vacuum pumping tube (and therefore the chamber) from contaminants that may be present at the outlet end of the Péclet tube. For this separation mode, at least a portion of the pumping tube (e.g., the initial inlet portion of the tube length) can be characterized as having a cross-sectional area A and a length L, where A and L are selected so that, for a given range of process gas flows at at least a given vacuum pressure, Pe(Q, A, L, D) is less than 10E-6 relative to the D(air) of oxygen and D(CX) of binder vapor molecules in the process gas flow for a given pressure. It is also possible to vary A along the tube, in which case the calculations become more complex, but the functionality and principles remain as described throughout this disclosure. In general, this configuration is highly effective for any pressure at which laminar flow is maintained within the pumping tube; separation tends to be compromised only when the vacuum pressure is low enough to cause transitional flow, and can be quite severe when pressures drop into the submilliliter range. It should be noted that for applications requiring deeper vacuums, proprietary pumps are available, as referenced in the incorporated text. Our flagship product can operate at pressures of approximately 1 torr or higher.

[0133] In summary regarding the design and dimensions of the vacuum pumping tube, for a given range of vacuum pressures and process gas flow rates, the vacuum pumping tube can be designed to be long enough and have a small enough internal diameter so that, within a given range of process gas flow rates (0.05 slm to 10 slm), the flow of process gas (during sintering) through the vacuum pumping tube acts as a sweep gas for the tubular Peclet seal, thereby providing better (often much better) ppm separation of the vacuum pumping tube inlet from gaseous contaminants present at the vacuum pumping tube outlet. Note that the design details of such tubes are discussed in detail with reference to Figures 7 and 8 and at various locations herein, including paragraphs 56-67 and 77 and 78.

[0134] With the above-described embodiment and operating conditions, the vacuum chamber can be operated at a vacuum pressure of 0.1 torr to 759 torr at the inlet of the vacuum pumping tube, and therefore at the chamber itself. We routinely operate the chamber at pressures of 1 torr to 500 torr, while maintaining purity levels of ppm or better, and in some cases ppb or better.

[0135] To extend the operating temperature range of vacuum pumping tubes, particularly for ceramic tubes, we have developed a high-temperature sealed tube adapter for at least the inlet tube 15002, which can be constructed from ceramic and a flange that can be formed from a refractory material such as high-temperature steel or refractory metal. In the systems shown in Figures 14 and 15, the flange of the adapter assembly is constructed from high-temperature 300 series steel or a similar material, producing a hermetically sealed tube adapter that can operate over the entire temperature range that steel can withstand. While the assembly described below uses 300 series steel, higher-temperature refractory metals can also be used if higher temperatures are required. Typically, a typical supply gas manifold can be manufactured or purchased, delivering process gas through hermetically sealed metal tubing, valves, and manifolds. In the context of ultra-high-purity sintering, metal tubing that delivers process gas to a ceramic inlet tube (e.g., SiC ceramic) tends to require a tightly sealed tube adapter to avoid contamination of the process gas by ambient air, including use at temperatures above 200°C, up to 600°C, or the maximum operating temperature of typical 300 series steel. We identified tube adapter seal requirements for the inlet tube adapter to be similar to those typically associated with high-vacuum systems and hardware. This means that any leakage or diffusion from the adapter during vacuum operation should be undetectable by a conventional helium leak checker with a sensitivity of 10E-9, and preferably even better. This result was achieved using a robust, low-cost, manufacturable, and reliable method described below and applied to the inlet tube, vacuum pumping tube, and sweep gas injection tube 426 attached to the process chamber base plate in Figure 14. Because the helium leakage rate for the inlet tube is much lower than IE-9, the same technique is also applied to the vacuum pumping tube adapter and Peclé sweep tube adapter, as well as the main sweep gas seal 412 described in Figures 16 and 17.

[0136] Shown in Figures 23A and 24A, respectively, are embodiments of a high temperature vacuum tube adapter arrangement configured to mechanically fit between a high temperature or extreme temperature tube 2301 (having an interior 2302) and a conventional metal tube 2303 (having an interior 2304) and provide a sealing arrangement in a manner that substantially prevents leakage and / or diffusion of outside air into the tubes while allowing some leakage of high purity sweep gas into the tubes.

[0137] 23A-I, there is a tube 2301 having a hollow interior 2302. There is a high temperature metal tube 2303 having a hollow interior 2304. A non-hermetic gasket 2305 acts as a non-hermetic vacuum seal between the first gasket sealing surface 433' and the second gasket sealing surface 433.

[0138] Reference numeral 2306 denotes a groove between the exposed inner periphery 2307 of the Peclet seal (with its outer periphery 2308 in direct gas communication with the outside air) and the separated outer seal periphery of the gasket seal 2309 (with its inner periphery 2310 in direct gas communication with the hollow interior of the tube 2302). Reference numeral 2311 denotes the outer seal (i.e., the gap used as the Peclet seal, shown in FIG. 23B as having a flow path length L and a Peclet gap size G). Reference numeral 2312 denotes an injection channel for sweep gas (often configured as a tube). Reference numeral 2313 denotes the sweep gas flow. Reference numeral 2314 denotes a bolt for securing the assembly. Reference numeral 2315 denotes a heater cartridge for optionally applying heat to the adapter arrangement 2316, for example, during debinding. Reference numeral 2316 denotes an adapter arrangement that may include a flange 2317. Reference number 2318 denotes the outside air (puffy cloud) surrounding the assembly. Reference number 2319 denotes the rigid retainer body.

[0139] 23B, adapter array 2316 includes a retainer latch 2320 that extends into indentation 2321 and projects partially past the exterior of tube 2301. The retainer latch may be a split ring. Reference numeral 2322 designates a retainer into which an extension of the retainer latch extends when in place. Reference numeral 2323 designates a resilient element, such as a wave spring.

[0140] Referring now to Figure 23C, which shows an expanded view of parts of the embodiment of Figures 23A-B, reference numeral 2324 denotes a dust shield, reference numeral 2325 denotes a thermocouple, and reference numeral 2326 denotes a power supply to the heater cartridge 2315.

[0141] The tube may have one or both of (i) a flat end face surrounding the hollow interior of the tube and (ii) a sealing surface around the exterior and end of the tube. The adapter includes one or both of a flat opposing surface facing the flat end face and a sealing surface closely surrounding the exterior end of the tube. The tube and adapter flanges are all constructed of a hermetically impermeable solid material.

[0142] The seal includes a non-hermetic vacuum seal surrounded by an outer Peclet seal, the Peclet seal being interposed between the inner seal and the external ambient environment (to separate the inner seal from outside air containing oxygen, moisture, or other gaseous contaminants), such that to reach the non-hermetic vacuum seal, traces of outside air (or gaseous contaminants therein) must first diffuse, leak, or otherwise pass through the outer Peclet seal before impinging on the inner seal.

[0143] Non-hermetic seals utilize non-hermetic vacuum-sealing materials that are somewhat permeable. The gasket material may be a porous material such as graphite or grafoil, or an elastomeric material, including silicone, Buna, or Viton®, all of which are significantly prone to gas diffusion and, for the purposes of this discussion, may be considered non-hermetic. For reference, it should be noted that copper gaskets used in ultra-high vacuum systems may be considered gas-tight, but typically do not operate at the high temperatures (e.g., above 400°C) required by many of the systems and methods described herein.

[0144] The non-hermetic gasket can be sandwiched or otherwise positioned between the gasket-opposed internal sealing surface of the tube and the corresponding internal gasket sealing surface of the adapter flange. The adapter flange and vacuum pumping tube can be formed of a metal (e.g., 300 series steel or nickel) that can withstand temperatures exceeding 300°C. When a vacuum is applied and the tube is surrounded by air, the gasket may exhibit gasket leakage, at least in part due to permeability, which would be unacceptably large if the internal seal were the only seal. Applicant has found that various gasket materials, such as Grafoil, perform well enough to allow the system to achieve the base pressure of the vacuum pump, but are insufficient to prevent contamination if the non-hermetic seal were surrounded by ambient air without a Peclet seal. As with the main retort seals in Figures 14-17, the internal non-hermetic seal described here allows us to operate within the desired vacuum pressure range, with the external seal providing the majority of the ambient air protection. In other words, the inner seal "does part of the work" by allowing the pump to pump the system down to the base pressure of the roughing pump (rotary vane pump), and the outer seal "does the rest" by keeping out oxygen and / or any outside air down to ppm levels of impurities in the tubing, or better than ppb levels. The adapter flange and / or ceramic tubing can include a groove located between the entire outer periphery of the non-hermetic seal and the inner periphery of the Peclet seal. The flange of the adapter arrangement includes an injection channel for receiving a high-purity inert sweep gas and injecting the sweep gas into the groove so that the entire exposed outer periphery of the inner seal is completely surrounded by the sweep gas, the outer seal is a Peclet seal formed as a gap between a Peclet sealing surface portion of the tube and an opposing Peclet sealing surface portion of the adapter body, the sweep gas sweeps through the gap from the groove to the external ambient air at a sufficient velocity and with a sufficient flow path length L to suppress diffusion of external air into the groove, so that the groove is not sufficiently contaminated by the outside, and leakage of the sweep gas surrounding the inner seal gas through the inner seal into the tube is tolerated.Separation can be ppm or better, parts per million or better, parts per billion or better, or even PPB or better. The sweep gas is typically high-purity argon, although other inert gases may be used. In our systems, the Peclet gap G is typically 0.001 to 0.010 inches, and the injection flow rate is typically 0.01 slm to 0.2 slm per 2-3 inches of the separated perimeter of the Peclet seal. We often use a liquid argon container as the source, as the gas tends to be very clean unless there is a significant air leak in the liquid argon dewar. Those skilled in the art, familiar with gas transport and diffusion phenomena and armed with this disclosure, including the above discussion of Figures 6-8, can readily determine the parameters to achieve adequate sealing (air separation) in the above context.

[0145] It should be emphasized that the above description of the sealing mechanism of the adapter of Figure 23 also applies to the main retort seal (of Figures 14 and 15), and that the principles of design and operation are equivalent. Surprisingly, in both cases, the isolation of ambient air from entering the chamber and tube, respectively, is similar and, in some cases, superior to the isolation provided by milled copper gaskets in the context of ultra-high vacuum systems. However, applicants remind the reader that some leakage occurs through the internal seals, and note that in both cases, the leakage rates (of pure sweep gas from the grooves to the interior of the chamber and / or tube) would typically be considered significant and completely unacceptable in the context of high-vacuum systems (e.g., systems operating at IE-6 Torr or below). If a non-hermetic gasket were surrounded by ambient air rather than a high-purity inert sweep gas, leakage would similarly be unacceptable in the applications described herein (e.g., metal sintering).

[0146] In one embodiment shown in Figure 23, the high and / or extreme temperature pumping tube can have a flat gasket sealing surface, an inner portion of which can function as an internal non-hermetic gasket sealing surface and an outer portion of which can function, for example, as an external pec sealing surface. Similarly, the adapter body includes an opposing flat sealing surface (in opposing relation to the flat surface of the tube), an inner portion of which can function, for example, as an internal gasket sealing surface of the adapter, on a flange, and an outer portion of which can function, for example, as an external pec sealing surface of the adapter.

[0147] As shown in Figures 23A, 23B, and 23C, the flange of the adapter arrangement can function as part of an overall sealed tube adapter assembly that includes a retainer arrangement having a main retainer body supporting bolt holes for receiving bolts (either through-bolt holes or tapped holes) that can be used as shown to sandwich the gasket with a clamping force between the gasket surface of the main adapter body and the gasket surface of the vacuum pumping tube. In the illustrated embodiment, one or more retainer latch indents can be recessed into the outer surface of the vacuum pumping tube to receive one or more corresponding high-strength retainer latch objects. In this illustrated embodiment, there is one continuous latch indent continuously circumferentially around the tube, and one corresponding hollow retainer latch ring (one latch object) extends into the corresponding retainer latch indent, with a latch ring extending radially outward of the indent and supported thereby such that the outer periphery of the latch ring is displaced radially outward from the adjacent outer surface of the vacuum pumping tube. The retainer lock can be configured to slide at least partially over the protruding port of the latching object, surrounding and contacting at least a portion of the protruding portion, thereby inhibiting and thus locking (preventing) the latch from disengaging from the latch indent as long as the retainer lock is in place. The main retainer body is forcibly biased against the retainer lock by the bolt force transmitted through the retainer lock, providing the latch with the restraining force necessary to hold the latch in place. The bolt reaction force provides a clamping force on the gasket sealing surface of the adapter body.

[0148] In this embodiment, the retainer latch is a split ring that can expand during installation to slide and / or engage around the tube to allow insertion into a corresponding retainer latch indent. Another embodiment employs two or more latch objects, such as ball bearings and / or short rollers, each of which can be received in a single continuous latch indent (as shown) or multiple indents with one or more latches per indent. Applicant understands that there are many variations of the latch objects and corresponding indents that can share the basic feature that each latch object can have an inner portion that protrudes into and is supported by the corresponding indent, and an outer portion that protrudes from the indent. This allows the latch to be restrained from exiting the indent by a retainer lock with a locking feature that receives, contacts, and surrounds the protruding portion, and can be forcibly biased into the restrained position by a bolt extending from and / or through the adapter body, using variations of the technique shown in Figures 23A-C.

[0149] Applicant understands that implementing complex, precise, and / or ground features in ceramic tubes may be expensive and / or impractical, and further understands that the mechanism of Figures 23A-C should be considered a highly advantageous approach, at least because it does not require grinding or other complex or high-precision features in the ceramic. For example, in our product, we "green machine" the latch indents that hold the latches in SiC in its green (unsintered) state, without grinding or shaping after sintering the SiC, and this low-precision, low-cost feature is fit for purpose because the overall design is very tolerant of small shape distortions.

[0150] Note that the embodiment of Figures 23A-C includes a stiff wave spring stack interposed between the main retainer body and the retainer lock. The bolt force is transmitted from the main retainer body to the retainer lock through the wave spring, which further transmits the bolt force to the retainer latch, as described above. This wave spring feature is included to act as a fit between the latching object and the retention arrangement to provide sufficient compliance to maintain the bias force through the expected range of thermal expansion and contraction that occurs with each cycle. This fit is also advantageous in reducing and / or preventing stress concentrations that may arise due to mechanical variations in tolerances or non-uniform features. The wave spring used in the above description is one of many ways that fit can be introduced; those skilled in the mechanical arts will recognize many other ways to introduce fit into the clamping force. For example, springs and washers can be used on the bolts, or Belleville washers can be stacked on each bolt. Additionally, flexure elements can be integrally incorporated into the retainer body and / or ring.

[0151] Since it would be impractical to itemize every possible design approach, attention is instead drawn to Figure 23D, which omits details of various arrangements and focuses conceptually on concepts common to all of the embodiments described herein. In addition to extreme temperature tube 2301 and flange 2317, most or all of the embodiments described herein share common features, including, but not limited to: (i) Non-sealing gaskets 2305 requiring compression. (ii) Rigid retainer body 2319 (iii) A protrusion which can be a separate retainer latch 2320 as shown here or integral with the tube as shown in subsequent figures. (iv) Clamp arrangement 2327.

[0152] We recognize that to accommodate thermally mismatched materials, it may be advantageous for the clamping arrangement 2327 to exhibit sufficient conformance, at least in the axial direction defined by the tube, to accommodate thermal mismatches and possible mechanical tolerance variations. As noted above, specifying every possible variation is beyond the reasonable scope of this specification, and applicants believe that a skilled mechanical engineer can conceive of an infinite variety of variations that embody the concepts depicted in this figure. Continued reference to FIG. 23D draws attention to the fact that no features providing conformance have been explicitly added. In this regard, it is assumed that the clamping arrangement 2327 exhibits conformance through one or both of: (i) an inherent bending conformance with whatever bolt or clamp is utilized; and (ii) the explicit use of one or more conforming elements, such as springs, wave springs, or Belleville washers.

[0153] Referring to FIG. 23D, Applicant has recognized that this conceptual approach is particularly advantageous when used in conjunction with ceramic tubing, at least because the overall adapter arrangement can be easily configured to ensure that the dominant internal stresses within the ceramic are compressive and / or shear, as opposed to tensile. Because ceramics generally tend to have extremely high compressive strength, bolts could be fully torqued to their specified torque limits without damaging the ceramic. In fact, during stress testing, bolts have been sheared without damaging the SiC. In general, this approach has proven to be very robust and sturdy, meeting the standards typical users would expect from industrial hardware in their factories. We believe this approach will prove useful in a wide range of ceramic-to-metal sealing applications, and furthermore, in many applications where a thermally and mechanically robust connection between metal and any ceramic rod is desired.

[0154] Figure 23E shows one variation in which a compliant element 2323 is interposed between the rigid retainer body 2319 and the upper surface 2328 of a protruding flange 2329 that projects radially outward from the tube and is co-machined with the tube. A rigid nut 2330 secures each bolt 2314. All other features of this sealing adapter are consistent with Figures 23A-C. Note that a compliant washer (such as a Belleville washer), not shown, can be used under each nut. The flanges can be configured in a variety of ways, as shown in Figures 23F and 23G, which depict tubes terminated with flanges having various contoured upper surfaces.

[0155] Figure 23H shows yet another variation in which the insertable latch of 23A is omitted and the tube is surrounded by a protruding portion. The overall concept is similar to the flange arrangement just described and can be operated in a similar manner, with the retainer body pressing against the protruding section in the same manner as the radially protruding flange. The bolts can be tightened with nuts 2801 and can include a Belleville washer located under each head of each bolt. As one of many potential variations that can be envisioned by one skilled in the art, Figure 23I illustrates (via a top view) that the engagement surface surrounding the tube need not necessarily surround the tube continuously. Two or more engagement features 2050, having either protruding portions or recessed portions, or both, can be distributed around the circumference of the tube and engaged by a matching retainer arrangement.

[0156] It should be noted again that the main adapter body, as shown in Figure 23, can include one or more cartridge heaters inserted into the adapter body. As previously mentioned, the adapter body heating can provide heating energy that can be conducted into and up the vacuum pumping tube. This additional heating, in conjunction with heating power from the furnace's high-temperature zone, ensures useful high temperatures throughout the entire length of the vacuum pumping tube, particularly during debinding. It should be noted again that the high temperatures maintained along the vacuum pumping tube can accomplish much more than simply preventing clogging. When used in conjunction with a pecre sealing, it can prevent the accumulation of contaminants that could be off-gassed back into the atmosphere during sintering, preventing even trace buildup, thereby facilitating the maintenance of an ultra-high-purity atmosphere within the inlet section of the extreme pumping tube and contributing to the purity of the atmosphere within the chamber. While clogging can typically be avoided by maintaining tube temperatures well below 200°C, preventing trace buildup and contamination of the tube typically requires temperatures well above 200°C, often exceeding 300°C or even 400°C depending on the binder. To avoid stressing the metal components (300 series steel) used in the adapter assembly, we generally stay below 600°C at least. We believe that even operating temperatures below 500°C would be advantageous for the adapter assembly to tolerate higher temperatures, at least because it provides a margin of safety in case the temperature inadvertently rises too high, for example, during sintering. While not wanting to be bound or restricted by precedent, we believe that for the most frequently used binder systems today, it is best practice to operate the pumping tube and adapter at the peak debinding temperature (currently 450°C).

[0157] When used in the vacuum retort furnace of Figure 14-17, the performance of the tube adapter is considered critical for the inlet tube and the sweep gas tube 426, which delivers the sweep gas to the main retort seal. In the former case, leakage must be avoided because it would directly contaminate the process gas. In the latter case, air leaking into the sweep gas tube could leak through the somewhat porous internal gasket seal. The same adapter is also used at the outlet of pump tube 15003, but this use case is not as critical because as long as the pump tube functions as a Peclet seal and the inlet and outlet are separated, some oxygen contamination at the pumping tube outlet is not a problem.

[0158] Furthermore, as with the main retort seal, it should be noted that the double seal variations listed with respect to FIG. 19 all constitute examples of different arrangements that may be employed with tube adapters having different degrees of performance.

[0159] It should be understood that the above description can be readily extended to larger sized tubes and applied to tube furnaces such as those of Figures 20A-20D. In particular, the above techniques can be applied to seal end caps 20008 to tubes 20002. Figure 24 illustrates various adaptations of the above-described approaches that can be employed to provide a high temperature seal connecting a furnace tube to an end cap, such as a flange.

[0160] FIG. 24A shows another embodiment of a high-temperature tube adapter that can be utilized and adapted for small tubing, such as high-temperature or extreme-temperature pumping tubes, sweep gas supply tubes, and inlet tubes. This embodiment is well-suited for use in ultra-clean tubular furnaces, such as those shown in FIGS. 20A-20D, and can maintain a high-performance seal against the end cap at temperatures much higher than those tolerated by elastomeric seals. Referring to FIGS. 20A-D, 24A-C, and 26B and C, the tubular furnace end caps can be surrounded by insulation and / or heaters and function as adapter flanges that can be sealably bonded and / or machined to high-temperature vacuum pumping tubes, which can be utilized in accordance with the above description of high-temperature and / or extreme-temperature vacuum-heated tubes. The end caps of FIG. 24A can be operated at high temperatures during debinding, minimizing and / or preventing binder contamination, thereby enabling and maintaining an ultra-high-purity sintering atmosphere free of binder contamination. We have employed variations of this technology to realize very low-cost tube furnaces for sintering titanium, aluminum, and other alloys that benefit from an ultra-high purity sintering atmosphere. In other words, flanges and vacuum pumping tubes in this context (tubular furnaces) can be employed to prevent condensation, similar to what is done in the systems of Figures 14-17. In these tube furnace embodiments, the entire tube, end caps, and pumping tube can all be maintained at the debinding temperature during debinding. For all binders, this successfully prevents even trace amounts of binder contamination, maintaining an ultra-high purity atmosphere during sintering. Reference numeral 2401 may be a second compatible element, as previously mentioned.

[0161] FIG. 24B shows a variation that allows for easier removal for inserting and removing parts from the tube furnace. As previously discussed with reference to FIG. 23D, there are numerous ways to employ clamping, and compliance can be employed. In this illustration, a semi-permanent, rigid retainer body 2415 can be split into two halves and fastened together using screws or bolts to secure the retainer body over and around the protruding portion of the latch 2416, preventing any relative axial movement and preventing it from slipping off while a user removes and replaces the end plate. This is believed to be a more user-friendly configuration, allowing users to open and close the furnace more easily, compared to some previous embodiments. For example, two or more pivotable clamping mechanisms 2417 could be positioned around the door, each pivoting about a hinge axis 2418 to release and re-clamp the sealing tube adapter. A compliant element 2411 could also be introduced that compresses in a compliant manner as the clamp rotates into its final position. Alternatively, the pivotable clamping mechanism could include a flex section to provide the proper fit. The clamping mechanism includes an engagement surface 2419 that makes sliding contact with the retainer body and / or mating element, which surface can be tapered in various ways (taper not shown) as needed to facilitate smooth engagement and disengagement.

[0162] FIG. 24C shows yet another embodiment for installing a door on a tube furnace for user access. It includes a high-temperature, sealed, tube adapter fixed adapter flange, e.g., installed and sealed according to the techniques of 23A-C with an internal gasket seal and an external Peclet seal. The fixed adapter flange remains in place during loading and unloading, but the latchable door also includes a dual seal, including a non-sealing door internal gasket seal and an external Peclet seal. This configuration includes a door groove 2420 through which additional sweep gas 2421 can be injected, surrounded by yet another Peclet seal 2422. Two or more user-operable door clamps 2417 can be positioned around the door. These can be user-accessible bolts (not shown) or pivotable clamps the same or similar to those described immediately above. Applicant is unaware of any other furnaces in which a user-operable furnace door can remain sealed at temperatures ranging from 350°C to 600°C, as readily achieved in this embodiment.

[0163] As previously mentioned, vacuum pumping tubes can be configured to support ultra-high purity sintering atmospheres in part by preventing contaminants from condensing within the tube during debinding. Furthermore, high-temperature vacuum pumping tubes can be configured to use process or sweep gases to achieve Peclet sealing / separation between their inlet and outlet ends, from ppm to ppb or even higher, as needed, facilitating ultra-high purity sintering. It should be appreciated that preventing binder condensation within the pumping tube can effectively eliminate clogging, allowing for smaller tube inner diameters than would be feasible in the presence of binder tube buildup. Applicant recognizes that this freedom to use smaller tubes is unexpected, at least because Peclet separation along the tube length requires smaller gas flows as the tube diameter decreases. Meanwhile, employing binder traps that operate at lower temperatures (e.g., room temperature) can also be beneficial. Furthermore, low-temperature binder trapping typically requires significantly slower gas flow rates compared to the high rates desired within the pumping tube. We have developed a system and method for transferring pump flow from the high temperature adapter to a moderate temperature, larger diameter exhaust tubing that can be sealed to conventional vacuum hardware that uses elastomeric seals to connect to binder traps and other vacuum hardware such as valves, tubing, pumps, and manifolds. Applicant has learned through operating experience that it can be important to provide an exhaust design that transitions from high temperature and small diameter (which prevents binder condensation and promotes Peclet sealing) to a lower "trapping temperature" in the binder trap, to a lower temperature and larger diameter exhaust tube.

[0164] One embodiment of such a transition from a high-temperature tube adapter to a low-temperature, large-diameter steel exhaust tube 2501, corresponding to FIGS. 23A-C, is shown in FIG. 25. Referring to FIG. 25, a high-temperature or extreme-temperature pumping tube sealably disposed in gaseous communication with a high-temperature tube adapter arrangement (e.g., as depicted in FIGS. 23A-B and reference numeral 2316) is in turn connected to a high-temperature flexible bellows 2502, a flange cartridge employed to control and maintain the flange at a high temperature, at least during debinding. The flexible bellows terminates in a tapered flange 2503 containing a controllable secondary heater cartridge 2504 to prevent excessive cooling of the lower end of the flexible bellows and, consequently, clogging thereat. The tapered flange is aggressively tapered to a fairly wide diameter over the shortest possible length. This large diameter is empirically and repeatedly designed to be large enough to prevent clogging. The exhaust tube 2501 has an inner diameter large enough to prevent clogging and can optionally include an exhaust tube heater 2505 that operates the tube at a much lower temperature than the debinding, e.g., 50-80°C, but is thermally compatible with the elastomeric seal. At points 2506 and 2507, the exhaust is at a temperature such as 450°C, while at point 2508, the exhaust is below 130°C.

[0148] Having described numerous systems and methods for promoting an ultra-high purity atmosphere during sintering of parts, in the more detailed description that follows, attention is returned to Figure 12, which illustrates, in a simplified manner, various basic concepts embodied in a wide variety of different embodiments. To summarize the key concepts discussed thus far:

[0165] Applicants believe that furnace insulation is one of the more detrimental sources of contamination during sintering in conventional furnaces. One reason for this is that the very nature of furnace insulation—keeping the high-temperature zone separate from the outer, lower-temperature zone—tends to cause binder to collect by condensation in the lower-temperature zone (in the outer insulation and surrounding vacuum chamber) during debinding, which then tends to be re-emitted during sintering. Several approaches to eliminating furnace insulation are described herein. In general, in the absence of insulation within the chamber, maintaining debinding temperatures (e.g., 250–500°C) throughout all parts of the vacuum system in direct gaseous communication with the process chamber, including the inlet tube, chamber, chamber extension, and pumping chamber—in other words, all parts of the system in close gaseous communication with the sintering chamber—can prevent even minor contamination (e.g., from binder contamination) throughout the entire system. After preventing contamination of the critical vacuum jacket (inlet tube, chamber, chamber extension, pumping tube), the next concept is to prevent backflow from the vacuum pump, binder trap, valves, and other components of the vacuum manifold, at least during sintering, which can be achieved by pecre-sealing the pumping tube.

[0166] In one embodiment of a critical vacuum jacket without insulation, the vacuum chamber containing the main opening (main retort seal) is located within the furnace's high-temperature zone, surrounded by the furnace's insulation and heater, with the inlet and outlet tubes extending through the insulation. To achieve a pure sintering atmosphere, the main seal is configured as a double seal, with a leak-resistant, ultra-high-temperature inner seal (grafoil gasket) surrounded and separated by a high-performance outer seal (Pecle seal). In such an embodiment, the critical vacuum jacket can be viewed as a chamber defined by the inlet tube, pumping tube, and retort dome and base plate. The pumping tube is configured to operate at extreme temperatures along at least a major portion of its length and can be designed with an appropriate diameter and length to act as a Péclet seal, isolating its inlet (and therefore the process chamber) from gas contamination at the outlet, for a predetermined range of process gas flow (e.g., 0.05-10 slm). If the retort base and tubing are ceramic, the inlet tubing supplying the sweep gas to the main retort seal, the extreme temperature pumping tubing, and the sweep gas tubing can all terminate in high-temperature sealed tubing adapters, one of which adapts the pumping tubing to a heatable metal exhaust tubing designed to operate at progressively lower temperatures as it transports exhaust toward the pump and / or optional binder trap. Note that the exhaust tubing is not considered part of the critical vacuum jacket and, in this configuration, can be highly contaminated without detrimental effects to the components. Note that the tubing adapters effective herein operate according to the same basic principles as the main retort seal.

[0167] In another embodiment, where the insulation is omitted from the critical vacuum jacket, the vacuum chamber can be a tubular furnace. In this case, the critical vacuum jacket can be considered to include an inlet tube, a furnace tube, and one or more furnace end plates. The furnace tube can consist of one processing section, where the part is sintered at full sintering temperature, and one or two chamber extensions that operate at progressively lower temperatures as they extend toward and ultimately terminate at the corresponding end plates. To achieve a pure sintering atmosphere, the end plates are configured with a double seal, with a leaky, ultra-high-temperature inner seal (a grafoil gasket) surrounded and separated by a high-performance outer seal (a Peclet seal). The pumping tube is sealed to the end plate, configured to operate at high temperatures, along at least a major portion of its length. It can be designed with an appropriate diameter and length to act as a Peclet seal, isolating its inlet (and therefore the process chamber) from gas contamination at the outlet, at a predetermined range of process gas flow (e.g., 0.05-10 slm). The chamber extensions and end plates that are part of the critical vacuum jacket can be controllably heated with separate heaters during debinding and can be insulated with their own insulation separate from the main furnace insulation.

[0168] The remainder of this section (titled "Exclusion of Insulation from the Vacuum Chamber") defines many combinations, often while avoiding repetitive and redundant descriptions of the details of various aspects and elements. For all of the following combinations, the detailed features are believed to be described in a manner to which the present application pertains and which will enable one skilled in the art having this disclosure to successfully implement them. FIG. 26A shows a ceramic retort furnace 26000, as in FIG. 14, having an inlet tube 26002 for injecting process gas from a process gas source 26001 and a sweep gas supply tube 26003 for injecting a Peclé sweep gas, supplied by a sweep source 26004, into the outer Peclé seal of the main retort seal. (In some cases, the sweep gas and the process gas are the same and flow from the same source.) The furnace chamber can be evacuated through extreme temperature pumping tube 26005, which terminates in a heated, high-temperature sealed tube adapter 26006 and adapts the vacuum pumping tube to metal exhaust tube 26007, which operates at progressively lower temperatures as it extends and can be sealed to an optional three-way valve 26008. During debinding, the valve directs the exhaust flow to a binder trap 26009, and a vacuum pump 26010 vacuum pumps the exhaust while maintaining a vacuum pressure within a predetermined pressure range. After debinding and during sintering, the three-way valve 26008 can be activated to direct process flow gases to the pump 26010 via bypass tube 26011. The retort functions as a vacuum chamber and is surrounded by furnace insulation and a furnace heater (not shown in FIG. 26A but should be understood from other figures, e.g., FIGS. 14-15). Note that the various components herein can be configured and operated together according to the detailed description above. For example, during debinding, the high temperature tube adapters are heated by internal heater cartridges (not shown) to maintain all of the tubes at elevated temperatures, e.g., above 300°C, often near the debinding temperature. These cartridge heaters can be turned off for a time interval after debinding is complete as the tops of the tubes approach the sintering temperature of the parts being sintered in the chamber.

[0169] FIG. 26B shows a double-ended tube furnace 26020 like that of FIG. 20A with metal end plates 26021 and 26021′ according to FIGS. 24A-26C, using a double seal 26022 supplied with sweep gas by a sweep gas source 26023. The joints between the tubes and each end plate function as sealed tube adapters and can be implemented in a variety of ways, for example, as disclosed earlier in this specification with reference to FIGS. 24A-C. One end plate 26021 includes a high-temperature heated vacuum pumping tube 26024 welded or machined thereto, which transitions to a vacuum exhaust tube 26025 that supports a temperature gradient from one end to the other, operating at progressively lower temperatures as it extends through a binder trap 26027 toward a pump 26026. The exhaust tube may be maintained at a temperature sufficient to avoid clogging, but need not be maintained at a temperature sufficient to prevent binder contamination therein. As previously mentioned, high-purity process gas can flow through the pumping tube 26024 to provide a pneumatic seal that prevents binder vapors and other gaseous contaminants from backflowing into the tube furnace. Note that in this description, the vacuum pumping tube is referred to interchangeably as the exhaust tube. During debinding, the entire furnace tube, end plates, and pumping tube are maintained at a sufficiently high temperature, well above 250°C, to prevent binder contamination throughout the critical vacuum jacket consisting of the inlet tube, the processing section of the furnace tube 26028, the furnace tube extensions 26029 and 26029' terminating in the end plates 26021 and 26021', the inside of the end plates, and the high-temperature metal pumping tube. The main section of the tube can be insulated by furnace insulation 26031 and furnace heater 26032, while the extensions and tube are surrounded by separate high-temperature insulation 26033 and 26033' and separate high-temperature heater 26034. The pumping and exhaust tubes can be fabricated or welded together with appropriate flanges 26035 to accommodate any changes in diameter and / or shape.The inlet tube 26030 can be supplied with process gas from a process gas source 26036, which also serves as a sweep gas for the pumping tube. In some cases, the sweep gas for the end plate and the process gas for the inlet tube can be the same and flow from the same source. The end plate 26021' can be hermetically sealed and / or fabricated with a metal inlet tube 26030 that receives process gas flow from the process gas source 26036. In some cases, the process gas and sweep gas can be the same and flow from the same source. The exhaust tube can terminate in an optional three-way valve 26038, which can direct the outflow of gas from the furnace through a binder trap 26039 during debinding and redirect the flow through a bypass tube 26040 after debinding.

[0170] Figure 26C shows a single-ended tube furnace with one end plate containing the inlet tube and pumping tube. All features are numbered consistently with those in 26A.

[0171] Figure 26D shows a vertically oriented single-ended tube furnace configured similarly to Figure 26, but using the same basic double seal design as the systems of Figures 14-17. With this system, the tube is held down by gravity, and the double seal does not require a clamping and holding mechanism as in the vacuum retort of Figure 14.

[0172] Figure 26C shows a vertically oriented tube furnace with a metal base plate constructed similarly to Figures 14-17, but this is metal rather than ceramic. The single-ended tube furnace can be constructed similarly to Figure 14. The vertical orientation eliminates the need for clamping or holding mechanisms. All part numbers are consistent with previous figures.

[0173] As mentioned above, furnace insulation in conventional furnaces is considered by applicants to be one of the most detrimental sources of contamination during sintering. One reason for this is that the furnace insulation's primary function is to maintain a high-temperature zone separated from an outer, low-temperature zone. As a result, during debinding, the low-temperature zone (within the outer insulation and surrounding vacuum chamber) tends to collect binder through condensation, which tends to be re-emitted during sintering. To eliminate the presence of a low-temperature zone during debinding, several embodiments have been described that use an additional, separate "external" heater outside the furnace insulation. While these systems and methods tend not to achieve the phenomenal levels of purity associated with the insulation-free solutions described herein, they have nevertheless proven highly effective in tube management of binder contamination, demonstrating impressive results. There are at least two reasons for the relatively low purity. First, door seals tend to be sealed with large elastomeric O-rings or gaskets (not shown), which can prevent the chamber adjacent to the door from heating much above 250°C. As mentioned above, most common binders are expected to condense and contaminate surfaces at a moderately high temperature of 250°C. However, wall temperatures during debinding, such as 30°C, will result in much less buildup than condensation on the walls. While it is possible to distribute the external insulation and chamber heater to temperatures significantly higher than this, as long as measures are taken to protect the elastomeric seal, this fundamental problem still remains, and some binders may condense on the chamber walls. Furthermore, compared to embodiments without insulation, the exposed surface area of ​​the furnace insulation within the chamber tends to be orders of magnitude greater. At 250°C, this surface area is generally greater than that of a furnace without insulation, but much less than without the external heater. The full benefits of this approach cannot be realized without the use of high-temperature pumping tubing, typically deployed as a Peclet seal separating the inlet from the outlet.Without this step, the pumping tube would be prone to contamination and clogging, and without the pecre sealing, the contaminated section of the tube would remain in gaseous communication with the vacuum chamber, potentially allowing some of that contamination to diffuse into the vacuum chamber.

[0174] FIG. 22B shows a system operating according to this approach, combining an outer heater 298 with a high-temperature vacuum pumping tube 73. As described above with reference to FIG. 22, there are three sets of dashed lines indicating three possible positions for the outer heater. Applicants recognize that high-temperature airtight door seals can be implemented in the manner described herein such that the insulation and chamber can be heated during and after debinding to temperatures well beyond the 250-300°C limit common to elastomeric seals. With such a configuration, superior results are expected in terms of sintering atmosphere purity.

[0175] Having described various embodiments of the sealed tube adapter, various aspects will now be summarized with some additional explanation.

[0176] We begin by clarifying the term "tight" as it applies in the context of these discussions. Those skilled in the art of vacuum technology will be familiar with helium leak testers capable of detecting helium leaks of 10E-9 Torr Liters per second (TL / S) or less. In fact, state-of-the-art equipment is now available for detecting helium leaks that reach sensitivity levels of 10E-12 TL / S, an extremely minute leak by any standard. In practice, when helium leaks become large enough to saturate the leak detector, they are often referred to as gross leaks. For the purposes of these discussions, we emphasize that properly installed and operating elastomeric gaskets do not constitute a true hermetic seal. In this regard, we maintain standards similar to those employed in state-of-the-art semiconductor manufacturing equipment, which does not use elastomeric gaskets but instead uses all-metal copper gaskets. As will be appreciated by those skilled in the art to which this disclosure pertains, typical elastomeric O-rings and gaskets can exhibit significant diffusion when tested using a helium leak tester, ultimately achieving a steady-state signal of 10E-8 or greater after 10 minutes or more of continuous exposure to helium gas, depending on the elastomer and seal geometry. Systems and methods are described herein that achieve orders of magnitude better hermeticity, even at high temperatures of 200-700°C in some embodiments and even at extremely high temperatures exceeding 800°C in other embodiments. For all embodiments described herein, we routinely interface state-of-the-art helium leak testers to our systems and consistently achieve hermeticity that is more than two orders of magnitude better than elastomers. Generally, we routinely achieve hermeticity of 10E-9 TL / S or better when evaluated using a helium leak tester.

[0177] If the gasket is highly porous and has a significant leak, the leak rate can be detected and measured using a standard technique known to vacuum engineers, in which the vacuum valve and pump are isolated and the operator monitors the rate of chamber pressure rise over a period of seconds or minutes. This technique, sometimes called a "leak-up test," is useful for detecting and measuring leaks large enough to saturate sensitive equipment like helium leak testers. We often use leak-up tests to measure the leak rate from certain grafoil gaskets.

[0178] With respect to the term "adapter," it is emphasized that the adapter arrangements described herein are configured for operation at high and even extreme temperatures limited only by the gasket, flange, and tubing materials used. Those skilled in the art of vacuum technology will understand that commercially available adapters, particularly ceramic-to-metal adapters, are generally not widely available for temperatures much above 400 degrees.

[0179] Tube-sealing tube adapters, including ceramic-to-ceramic and ceramic-to-metal tube adapters, are described that operate at high and / or extreme temperatures limited only by the tube, gasket, and flange materials. In particular, adapters are described that employ an inner non-sealing gasket surrounding a Peclet seal supplied by a high-purity sweep gas, and are positioned such that during operation, the inner seal is surrounded by this high-purity sweep gas rather than by ambient air.

[0180] In one embodiment, this configuration ensures that the gas leaking around the inner seal into the tube contains only (i) the sweep gas and (ii) any trace gas that may have diffused out of the tube. With respect to the sweep gas, operators and / or system designers can provide the sweep gas at the required purity and operate with confidence that the sweep gas is not contaminated by outside air to parts per billion or greater.

[0181] It should be noted that the embodiments described herein are generally operable at high and / or extreme temperatures because these configurations tend to tolerate relative movement within the assembly that can result from mismatches in coefficients of thermal expansion (CTE). For example, various embodiments of high-temperature ceramic-to-metal adapters described herein generally tolerate different rates and amounts of thermal expansion between the ceramic tube and the metal flange. These configurations generally can provide a gas-tight connection up to and including the maximum operating temperatures of the materials used for the tube, gasket, and flange. In particular, the adapters tend to be operable as long as they do not experience melting, warping, or other unpredictable or mal-behaving deformation due to heat exceeding the expected degree of thermal expansion. We are not aware of any commercially available ceramic-to-metal tube adapters that can tolerate CTE mismatches (between the ceramic and the metal) and provide gas-tight performance over such temperature ranges.

[0182] In yet another aspect, it should be understood that an adapter between a given tube and a given flange can also be configured to fit from a first tube to a second tube, with the second tube configured to be in gas communication with the flange. Indeed, in most embodiments described herein, an airtight fit between one tube and another is intended, and an airtight fit between the first tube and a flange (where the flange is connected to the second tube) facilitates this goal. The term flange is used very broadly throughout this disclosure and may be considered or described by other terms, such as "adapter body" or "adapter arrangement." In this regard, the term flange is not to be considered limiting with respect to shape or geometry, so long as the adapter arrangement or body includes the features and functionality described herein.

[0183] In yet another aspect, it should be understood that "outside air" and "outside ambient air" should be considered any gas surrounding the perimeter of the Pecre Seal. In most of the above embodiments, this is ambient air consisting of the natural atmosphere above ground. However, it is not intended that the disclosure be so limited, and it is emphasized that the systems and methods described herein may be used to hermetically isolate against the intrusion of any outside gases in which the assembly is used.

[0184] With the above considerations in mind, a sealed tube adapter has been described in which each of the tube and adapter arrangements is constructed of the same or different materials, both of which are hermetically impermeable to the diffusion of gas therethrough. The hollow tubes can be double-ended or single-ended, and the hollow interior of the hollow tube can include at least one open tube end such that the open tube end supports a first gasket sealing surface thereon that surrounds the open end of the first hollow interior. The tube cross-section need not be circular, and the systems and methods herein are readily adaptable to tubes of various cross-sectional shapes.

[0185] Adapter configurations can include: (i) A flange made of a highly airtight material. The flange can include a gasket sealing surface that has a shape that matches and faces the gasket sealing surface of the tube so that the vacuum gasket is sandwiched at the gasket sealing surface. (ii) A gasket that functions as a vacuum seal, consisting of a non-hermetic gasket material sandwiched between a first gasket sealing surface and a second gasket sealing surface. The gasket need not be hermetic; it only needs to (a) provide a sufficient degree of gas barrier so as not to overload the selected vacuum pump, and (b) result in gas flow at least an order of magnitude less than that consumed by the surrounding Peclet seal. Consider the gasket, for example, as performing its primary function of permitting a vacuum within a reasonable range of the base pressure of a given vacuum pump, while the outer Peclet seal performs its primary function of gas-tight isolation from the outside atmosphere. Regarding the former, anyone skilled in the art of vacuum technology will understand that mechanical vacuum pumps, such as scroll pumps, rotary vane pumps, and piston pumps, typically exhibit a predetermined base pressure that can be achieved in the absence of leaks or other gases entering the pump. In our embodiment, leakage through the Grafoil gasket is generally small enough that any of these mechanical pumps can approach equilibrium at an inlet pressure within an order of magnitude of their designated base pressure. However, without the outer Peclet seal, the equilibrium pressure would be primarily composed of air from the outside and should be considered far from hermetically isolated therefrom. However, these same embodiments utilizing an outer Peclet seal can prevent the presence of outside air (within the hollow tube and / or its connected chambers and systems) from being detected, even using state-of-the-art laboratory equipment such as helium leak detectors and other vacuum-based mass spectrometers. (iii) a mechanical clamping arrangement configured to mechanically engage both the hollow tube and the flange and clamp the first gasket sealing surface and the second gasket sealing surface toward each other to create a compressive force on the open gasket; Various embodiments of clamping arrangements are disclosed. More particularly, the heretofore described hereof's hereby unrestricted tube adapter relies on a sealing arrangement that relies on the arrangement of a series of features that may include the following three elements: (i) A gasket and Peclet seal in which the gasket, functioning as a non-hermetic vacuum seal, exhibits at least some gasket leakage as diffusion of any gas surrounding the periphery of the gasket through the non-hermetic gasket and / or into the periphery of the non-hermetic gasket. (ii) A groove in one or both of the tube end and flange, positioned around the periphery of the open gasket, configured to receive a flow of oxygen-free sweep gas injected therein through an injection channel in the flange so that the sweep gas surrounds the periphery of the gasket. The injection channel can be hermetically sealed to the flange or can be tubing machined with the flange. In many of our embodiments, we often utilize standard off-the-shelf tubing and fittings from SWAGELOK, a brand of Solon, Ohio, for this purpose. (iii) An external Peclet seal surrounding the vacuum gasket and disposed in series between the vacuum gasket and the ambient air, from its inner periphery in gas communication with the groove to its outer periphery in gas communication with the ambient air, capable of gas-tightly separating the open gasket from the ambient air. The external Peclet seal is configured to receive a flow of oxygen-free sweep gas and provide diffusion sealing through the flow of oxygen-free sweep gas. It is reemphasized that the term ambient air refers to any atmosphere surrounding the outside of the assembly. For example, during testing with a helium leak checker, the ambient air may be 100% helium gas.

[0186] In general, it is recommended that Peclet seals be configured so that the majority of the oxygen-free sweep gas flow passes through the Peclet seal, with only a small amount of the oxygen-free sweep gas leaking through and / or around the gasket. In other words, the gasket seal must be tight enough that only a small portion of the Peclet flow leaks into the gasket, as opposed to through the Peclet seal. Once this criterion is established, a properly configured Peclet seal, designed according to the theoretical formulas presented in the above discussion, can result in an operating mode in which a non-hermetic vacuum seal provides sufficient sealing to support and allow a desired predetermined vacuum pressure within the hollow tube, while the Peclet seal provides sufficient sealing isolation from the ambient air at sufficient flow rates so that leakage through and / or around the non-hermetic gasket seal is not sealed from the ambient air.

[0187] Generally, each Peclet seal has an inlet in gas communication with the groove for receiving sweep gas therethrough and an outlet in gas communication with the ambient air. The sweep gas can flow along the Peclet flow path length L from the inlet to the outlet, and the sweep gas flow suppresses back-diffusion of ambient air to achieve separation. The above description includes sufficient details to enable those skilled in the art to achieve this result, provided the Peclet gap G is highly uniform throughout the gap. In our embodiment, we achieve sufficient uniformity and flatness of the Peclet sealing surface by relying on precision grinding, as opposed to machining. When the tube is ceramic, we typically polish the Peclet sealing surface in a simple post-processing step using a conventional precision grinder equipped with a diamond abrasive tool.

[0188] In some embodiments, the hollow tube includes a first Peclet sealing surface on an end face of the tube perpendicular to the tube, and the flange of the adapter arrangement includes a second Peclet sealing surface facing the first Peclet sealing surface and conforming to the shape of the first Peclet sealing surface, such that the two Peclet sealing surfaces define a gap size, G, between them. In other embodiments, the Peclet sealing surfaces can surround the tube (e.g., in tube Peclet gap seal 20021 of FIG. 20D ); in particular, the first Peclet sealing surface (of the tube) can be located on an end extending axially along the tube from the rim of the tube for path length L, surrounding and surrounded by that end, facing radially outward. This sealing surface can be surrounded and surrounded by an opposing second Peclet sealing surface also extending axially along flow path length L and facing radially inward, such that the two opposing Peclet sealing surfaces define a gap size, G, between them. In our applications, the gap size G is typically between 0.003" and 0.030", with the Pecle flow path ranging from 2 mm to several cm. For larger gaps such as 0.030", the longer length and larger flow can easily compensate for this increased gap size, according to the theoretical equations mentioned earlier.

[0189] The sealed tube adapter can be arranged to hermetically connect one tube to another, such that the hollow tube functions as a first hollow tube, and the second hollow tube is sealingly attached to and / or machined with the flange so that the hollow interior of the second hollow tube is in gas communication with the hollow interior of the first hollow tube. In such cases, the gasket can be configured as an open gasket having an inner periphery defining an opening that allows gas communication between the interior of the second hollow tube and the hollow interior of the first hollow tube. In these embodiments, the open gasket can be configured as a peripheral gasket that extends around the open rim of the first hollow tube.

[0190] It is recognized that the gas-tight tube adapters described herein can operate at a variety of temperatures, including below room temperature. However, these systems and methods are generally most advantageous at high temperatures (i.e., above 300°C), especially when connecting thermally mismatched materials, such as ceramic tubing, to metal tubing with significantly different CTEs. Vacuum technology experts will be familiar with gas-tight ceramic-to-metal adapters that use brazing techniques to achieve a gas-tight bond between two thermally mismatched materials. These designs require careful design and analysis to prevent stress fracture of the ceramic tubing due to temperature changes, and commercially available ceramic-to-metal adapters generally cannot operate much above approximately 300-400°C, depending on the specific design and application. Many of the embodiments described herein are routinely used to provide ceramic-to-metal seals that operate at much higher temperatures, and these assemblies typically fail only when temperatures exceed the maximum operating temperature of the metal flange and / or tubing. We have routinely employed these techniques with ceramic tubes constructed from refractory ceramics capable of withstanding temperatures above 800°C, including (i) porcelain, (ii) mullite, (iii) alumina, and (iv) SiC. These ceramic tubes can be extended into the high-temperature zone of a sintering furnace insulated with extreme-temperature insulation, and the adapter is positioned outside the extreme-temperature furnace insulation but is nevertheless operated throughout the cycle at high temperatures, e.g., in the range of 300°C to 700°C (e.g., to avoid binder condensation). In such embodiments, the gas-tight adapter flange can be constructed from a high-temperature metal capable of withstanding temperatures above 300°C, and in some cases up to 700°C. Generally, gasket materials are selected that can meet and / or exceed the maximum operating temperature of the metal. If a gasket with a lower maximum temperature is selected, the gasket material becomes the limiting factor in determining the maximum operating temperature of the entire sealed tube adapter.

[0191] To facilitate clamping, the tube includes one or more engagement features disposed around the outer surface of the tube, each engagement feature including a ledge surface that slopes away from the end of the tube, each ledge surface engageable by the clamping mechanism such that at least a portion of the clamping force is generated as a counterbalancing force that opposes the engagement force at the ledge surface.

[0192] In many of our embodiments, there may be only one engagement feature that continuously surrounds at least a majority of the outer diameter of the hollow tube and provides a corresponding ledge surface around the diameter. For example, 2319 is illustrated in Figures 23A-D. In some embodiments, the engagement feature may include a recess having a ledge surface therein, such that each ledge surface is recessed inwardly from the outer surface into the wall of the tube. For example, indented features 2321 are illustrated in Figures 23A-C.

[0193] A retainer arrangement is provided that includes: (i) A main retainer body can be provided that surrounds the hollow tube and supports a first set of through-bolt conduits arranged in a bolt pattern that also surrounds the hollow tube. An adapter arrangement can include a second set of through-bolt conduits that match the first set. A set of adapter bolts can be configured to pass through the matching pattern of bolt holes to connect the main retainer body and the adapter arrangement, and when the set of bolts is tightened, can provide a clamping force that compresses the gasket. (ii) The one or more latches each include an insert portion extending into an associated recess and a protruding portion extending radially beyond the immediately adjacent outer surface of the tube, and the retainer arrangement can be positioned to surround the protruding portion of the latch and can transfer at least a portion of the clamping force component from the main retainer body, through the latch, and onto the ledge surface of the recess.

[0194] In one embodiment, the retainer arrangement can include a retainer lock configurable so that the protruding portion of each latch directly contacts it when deployed in its final position. Such a retainer lock radially includes a latch mechanism such that the inserted portion of the latch is securely trapped within the associated recess. This approach has proven to be tolerant of dimensional tolerances and highly robust against axial forces and gasket compression. In our prototypes, excessive axial forces do not appear to disengage the trapped latch mechanism unless and until the clamping force exceeds the mechanical strength of at least one of the ledges, latches, retainer locks, and main retainer body. Even when the bolts are severely overtightened, nothing in the assembly fails until the bolt snaps or one of the elements fails in shear. Surprisingly, we have not yet encountered a failure mode similar to that of the SiC ceramic tube. As a general design rule, we tend to size bolts to shear failure before exceeding mechanical destructive force, thereby preventing component loss or damage due to overtightening.

[0195] As noted above, this method is very tolerant of geometric variations, including within the ceramic tube groove. This can be an important consideration because ceramic parts tend to have somewhat loose tolerances during sintering, and you want to avoid costly post-processing by grinding. In embodiments using a recessed groove, the latch is configured as a ring-shaped element that drops into the groove and bends sufficiently to conform to any irregularities in the groove and / or ledge with a small fraction of the final gasket compression force, while remaining rigid enough to withstand the compression force without mechanical failure. In some embodiments, the groove is semicircular with loose tolerances, and a latch is used that consists of a tightly wound extension spring made of high-temperature steel, with an outer diameter that matches and can be accommodated within the semicircular cross-section of the groove (e.g., reference number 2323 in Figures 23A-C).

[0196] Although recessed engagement features have been described, it is recognized that the engagement features can form protrusions that continuously surround the outer diameter of the hollow tube, providing a corresponding ledge surface around the diameter. This approach can reduce the number of parts and simplify the overall mechanism.

[0197] The protruding engagement features may form a flange extending radially outward from the tube, with the ledge forming the major surface of the flange opposite the gasket sealing surface. Various flange embodiments will be readily apparent to those skilled in the art of mechanical design. Figures 23E-H illustrate several such embodiments, including (i) a flat flange where the ledge is generally perpendicular to the tube, (ii) a flange having a tapered shape where the radial element points away from the hollow tube and the ledge slopes outward at an angle, and (iii) a flange where the flange is rounded, either convexly or concavely.

[0198] Various embodiments relying on projections and / or flanges can employ a retainer arrangement that surrounds the hollow tube adjacent the flange so that the retainer arrangement can contact the ledge, with the main retainer arrangement supporting a first set of through-bolt conduits in a bolt pattern surrounding the hollow tube. The adapter body can include a second set of through-bolt conduits that match the first set. A set of bolts can be provided and arranged to pass through a matching pattern of bolt holes to connect the retainer and adapter arrangement and provide a clamping force on the retainer and adapter body, and thus the gasket, when the set of bolts is tightened. In such embodiments, it may be desirable to include a partial fit to provide tolerance for thermal expansion as well as physical shock. For example, one end or the other of each bolt can engage the retainer arrangement and adapter arrangement, respectively, via a spring mechanism, such as a wave spring or Belleville washer (e.g., 2416), that provides sufficient axial fit to accommodate CTE mismatch along that direction (axially of each bolt) throughout the entire operating temperature range.

[0199] Whatever the type of engagement feature, there is no particular requirement that the feature continuously surround the entire circumference of the tube. As shown in Figure 23I, two or more engagement features (23I, 2050), each forming a protrusion and / or depression in accordance with the above description, can be spaced apart from one another so as to be distributed around the circumference of the tube such that each engagement feature provides an associated ledge surface.

[0200] We have described various tube furnaces, with an emphasis on high-temperature tube adapter operation to reduce contaminants due to debinding. It should be understood that there are numerous applications in which these systems and methods could be advantageous. Examples include, but are not limited to, (i) applications in which an ultra-high purity atmosphere within the tube furnace is desirable; (ii) other applications in which organic and / or other volatile contaminants are temporarily introduced as part of the process but must subsequently be removed; and (iii) applications in which it is desirable to reduce the temperature gradient from the extreme temperature portion of the tube furnace to one or more ends of the tube. Regarding the last item, tube furnaces often fail due to the aforementioned temperature gradients, especially after repeated cycles. Replacement tubes are often very expensive, and in these cases, adopting our technology can significantly reduce these additional costs and result in significant overall savings.

[0201] Although a system and method have been described for providing a tube adapter capable of exhibiting truly superior air purity, it should be understood that in many applications it may not be necessary to achieve the ultimate high sealing performance just described.

[0202] Just because these systems and methods can achieve the near-perfect operation just described does not mean that they must always do so, and applicant recognizes that designers, builders, and operators can, and undoubtedly will, devise and find ways to compromise the performance of these systems and techniques while remaining within the scope of these teachings.

[0203] Compromises that may limit atmospheric purity, whether intentional or not, include, but are not limited to, (i) feeding the Péclet seal at a low flow rate to conserve gas usage, (ii) shortening the flow path length L to reduce costs or due to manufacturing limitations, (iii) enlarging the Péclet gap to compensate for tolerance variations across the gap size, (iv) using tubing or adapter materials that are slightly porous and not perfectly hermetic, (v) operating with Péclet gap sizes that vary by more than a few percent across the gap, and (vi) operating with Péclet gap sizes that are too small (e.g., to allow for low gas flow rates) making the gap overly sensitive to tolerance variations across the gap. Indeed, the last compromise was discovered accidentally during our early laboratory tests and experiments.

[0204] We recognize that the term "non-hermetic" with respect to a particular gasket seal can be considered somewhat vague, at least because different applications can have entirely different orders of magnitude requirements for sealing integrity. Furthermore, we recognize that the difficulty of measuring and characterizing ultimate limits of performance can create challenges in monitoring and describing performance, both verbally and in practice. In much of our work, the ultimate authoritative attribute of performance is the result achieved in terms of material composition and quality that meets the requirements of a specific application. In fact, during the process of sintering metals, our requirements for sealing integrity can vary by several orders of magnitude. For example, when debinding and sintering titanium, our data indicate that it is both possible and necessary to routinely achieve oxygen purity values ​​better than one part per billion. On the other hand, when sintering more tolerant materials such as copper or stainless steel, purity levels of one part per million are often sufficient. Given this variability in requirements, the term hermetic, as applied to the tube seals described herein, should be interpreted to specify that the entire tube seal, using a non-hermetic gasket surrounded by a Peclet seal, provides isolation from ambient air by at least one order of magnitude compared to the isolation level that would be achieved with the gasket alone and no Peclet seal. This definition serves as a very clear meaning in description as well as practice, at least because it is extremely easy to test and measure the operation of the sealing arrangement in situ, with or without a Peclet seal. After assembling the tube adapter according to our teachings, POOSITA simply stops the Peclet flow of sweep gas and measures the leak rate and atmospheric purity through the gasket using well-known techniques. The operator can then gradually increase the sweep gas flow while continuously monitoring the atmospheric purity until contaminant levels become unmeasurable and / or reach levels acceptable for the particular application. We routinely perform this testing and routinely obtain orders of magnitude improvements in ambient air leak rates into hollow tubes before even state-of-the-art helium leak testers fail to measure them.

[0205] To the extent that any tube adapter system taught herein utilizes a given non-sealing gasket, the overall combination of that gasket seal surrounded by a Peclet Seal should be considered to function in accordance with these teachings for any operating mode involving ambient air leakage through the non-sealing gasket, where the Peclet Seal substantially reduces the amount of ambient air by at least one order of magnitude compared to sustained operation without Peclet Flow in steady-state operation. According to our teachings, leakage of a non-sealing gasket exhibiting a given level of ambient air intrusion by leakage and / or diffusion around and / or through the gasket can be substantially reduced by surrounding the gasket with a Peclet Seal using the techniques and methods described herein, thereby substantially improving ambient air isolation by one or more orders of magnitude compared to a steady-state test mode in which the Peclet Flow is deactivated.

[0206] While the disclosed subject matter has been described and illustrated with respect to embodiments thereof, it should be understood by those skilled in the art that features of the disclosed embodiments may be combined and rearranged within the scope of the invention to produce additional embodiments, and that various other changes, omissions, and additions may be made therein and thereto without departing from the spirit and scope of the invention.

Claims

1. a hollow tube constructed of a tubing material that is hermetically impermeable to the diffusion of gases, the hollow tube having at least one open tube end such that the hollow interior of the hollow tube terminates in the open tube end supporting a first gasket sealing surface thereon surrounding the open end of the hollow interior; (i) a flange made of a hermetically impermeable flange material including a second gasket sealing surface having a shape matching and opposing the first gasket sealing surface, such that a vacuum gasket can be sandwiched between the gasket sealing surfaces; (ii) a gasket that functions as a vacuum seal, comprising a non-hermetic gasket material sandwiched between the first and second gasket sealing surfaces; (iii) a clamping arrangement configured to mechanically engage both the hollow tube and the flange and clamp the first and second gasket sealing surfaces toward each other to create a compressive force on the open gasket; and an adapter arrangement comprising: The hollow interior of the tube comprises: (i) a gasket that functions as a non-hermetic vacuum seal that exhibits at least some gasket leakage as gas surrounding the periphery of the gasket diffuses through the non-hermetic gasket and / or into the periphery of the non-hermetic gasket; (ii) a groove in one or both of the tube end and the flange disposed around the outer periphery of the gasket and configured to receive a flow of oxygen-free sweep gas injected therein through an injection channel in the flange such that the sweep gas surrounds the outer periphery of the gasket; (iii) an external Peclet seal surrounding the vacuum gasket and disposed in-line between the vacuum gasket and the ambient atmosphere, the external Peclet seal configured to receive the flow of the oxygen-free sweep gas to provide diffusion sealing by gas flow of the oxygen-free sweep gas from an inner periphery of the Peclet seal in gas communication with the groove to an outer periphery of the Peclet seal in gas communication with the ambient atmosphere, such that the gasket can be substantially isolated from exposure to the ambient atmosphere; and a hermetic sealing arrangement including: A sealed tube adapter wherein a majority of the flow of oxygen-free sweep gas flows through the Peclet seal, and a trace amount of the oxygen-free sweep gas flows through and / or around the gasket as the leak, and wherein the non-hermetic vacuum seal provides a sufficient seal to support and enable vacuum pressure within the hollow tube but cannot provide sufficient airtightness against outside air without additional sealing around the perimeter, and the Peclet seal, acting as an additional seal around the perimeter, provides sufficient isolation from outside air so that the leak through and / or around the non-hermetic gasket seal is substantially free of outside air.

2. 2. The sealed tube adapter of claim 1, wherein the Peclet seal has an inlet in gaseous communication with the groove for receiving the sweep gas from the groove, an outlet in gaseous communication with the ambient air, and a Peclet flow length L from the inlet to the outlet, along which the flow of the sweep gas has a flow velocity sufficient to suppress back-diffusion of the ambient air and achieve the gas-tight separation.

3. 3. The closed tube adapter of claim 2, wherein the hollow tube includes a first Peclet sealing surface, the adapter includes a second Peclet sealing surface conforming to and matching the shape of the first Peclet sealing surface, and the two Peclet sealing surfaces face each other in opposing relationship to define a Peclet gap G as the gap size therebetween.

4. 3. The closed tube adapter of claim 2, wherein for an end extending axially along the tube from the rim of the tube for the flow length L, the first Peclet sealing surface surrounds and encircles the end and faces radially outward and is surrounded and encircled by an opposing second Peclet sealing surface extending axially for the flow length L and facing radially inward, the two opposing Peclet sealing surfaces defining the Peclet gap G as a radial gap therebetween having a gap size of G.

5. 2. The sealed tube adapter of claim 1, wherein the hollow tube functions as a first hollow tube, and a second hollow tube is gas-tightly attached to and / or machined together with the flange, the hollow interior of the second hollow tube being positioned in gas communication with the hollow interior of the first hollow tube.

6. 6. The closed tube adapter of claim 5, wherein the gasket is an open gasket having an inner periphery defining an opening that allows gas communication between the interior of the second hollow tube and the hollow interior of the first hollow tube.

7. 7. The closed tube adapter of claim 6, wherein the open gasket is a peripheral gasket extending around the open rim of the first hollow tube.

8. 3. The closed tube adapter of claim 2, wherein the rim of the hollow tube includes the first Pecle sealing surface that is flat and extends around the outer periphery of the groove, and the flange supports the second Pecle sealing surface that surrounds the outer periphery of the groove and extends radially along the flow path length, in opposed spaced apart relationship with the first Pecle sealing surface to define the Pecle gap G therebetween having the flow path length L.

9. 3. The sealed tube adapter of claim 2, wherein a majority of the sweep gas surrounding the open gasket, flowing into and through the Peclet seal, passing through the gasket and / or around the gasket, and the sweep gas in the groove flowing through the Peclet seal into the hollow interior of the tube flows along a Peclet flow path at a velocity sufficient to inhibit diffusion of ambient air toward the interior volume, exposing the gasket to ambient air, resulting in at least one order of magnitude less air than if the sweep gas flow were continuously inactivated.

10. 10. The hermetic tube adapter seal of claim 9, wherein the sweep gas flows through the Peclet seal at a rate of between 0.01 to 0.1 standard liters per minute (SLM) and 10 SLM per linear centimeter of circumferential gasket circumference.

11. 11. The sealed tube adapter of claim 10, wherein the sweep gas flows through the Peclet seal at a rate between 0.1 and 1.0 SLM.

12. 10. The sealed tube adapter of claim 9, wherein the gasket material is one of: (i) sufficiently porous so that the majority of the leak flows through the gasket rather than around it, allowing the leak to be easily detected by monitoring the steady-state pressure rise in a vacuum closed system; or (ii) an elastomeric material that is sufficiently diffusive to oxygen so that diffusion therethrough can be detected by using a helium leak tester.

13. 3. The hermetic tube adapter seal of claim 2, wherein said sweep gas is a highly purified, oxygen-free inert gas.

14. 14. The seal tube adapter of claim 13, wherein the sweep gas is a pure laboratory grade inert gas having a purity of oxygen or other gaseous contaminants, including hydrocarbon gases, substantially better than 1 ppm.

15. 14. The sealed tube adapter of claim 13, wherein the sweep gas is predominantly high purity argon.

16. 3. The closure tube adapter of claim 2, wherein the Peclet seal comprises a Peclet gap having a Peclet gap length L of (i) at least 1 mm, (ii) at least 3 mm, (iii) at least 5 mm, or (iv) 10 mm or greater.

17. 17. The closure tube adapter of claim 16, wherein the Peclet gap is between or equal to 0.001 inches and 0.010 inches.

18. 17. The seal tube adapter of claim 16, wherein the Peclet gap is between or equal to 0.002 inches and about 0.005 inches.

19. 14. The sealed tube adapter of claim 13, wherein the Peclet seal provides isolation against diffusion of ambient air to a sufficient extent such that sweep gas in the sweep gas groove leaking into the sealed vacuum retort through the gasket leak is contaminated by ambient air at less than parts per million (ppm).

20. 20. The seal tube adapter of claim 19, wherein the amount of contaminants is less than 100 parts per billion (ppb).

21. 20. The seal tube adapter of claim 19, wherein the amount of the contaminants is less than 10 ppb.

22. 20. The seal tube adapter of claim 19, wherein the amount of the contaminants is less than 1 ppb.

23. 10. The sealed tube adapter of claim 1, wherein the hollow tubing has a first CTE and the flange material has a second CTE that is substantially different from the first CTE.

24. 24. The sealing tube adapter of claim 23, wherein the ceramic is a refractory ceramic capable of withstanding temperatures of 800 degrees Celsius or greater and is comprised of one of: (i) porcelain, (ii) mullite, (iii) alumina, and (iv) SiC.

25. 25. The sealed tube adapter of claim 24, wherein the flange is constructed of a high temperature metal that can withstand temperatures of 300 degrees or greater, and the gasket is a high temperature gasket material capable of operating at temperatures of 300 degrees or greater such that the entire sealed tube adapter is capable of operating at temperatures of 300 degrees or greater.

26. 26. The seal tube adapter of claim 25, wherein the metal can operably withstand temperatures of 400 degrees or greater without substantial and / or unpredictable deformation, softening, or melting, and the gasket is a high temperature gasket material operable at temperatures of 400 degrees or greater such that the seal tube adapter is operable at 400 degrees or greater.

27. 27. The sealing tube adapter of claim 26, wherein the metal is an ultra-high temperature and / or refractory metal capable of withstanding temperatures substantially in excess of 500 degrees, and the gasket is a high temperature gasket material capable of withstanding temperatures equal to or greater than the metal.

28. 28. The sealed tube adapter of claim 27, wherein the gasket is constructed of grafoil having sufficient porosity such that it is substantially non-hermetic in the absence of the peripheral Peclet seal, such that ambient oxygen leakage is not permitted.

29. 2. The closed tube adapter of claim 1, wherein the hollow tube includes one or more engagement features disposed about an outer surface of the tube, each feature including a ledge surface that slopes away from the end of the tube, each ledge surface engageable by the clamping mechanism, and at least one component of the clamping force can be generated as a reaction force that balances the engagement force on the ledge.

30. 30. The closed tube adapter of claim 29, wherein there is only one engagement feature that continuously surrounds at least a majority of the outer diameter of the hollow tube and provides a corresponding ledge surface around the diameter.

31. 30. The closed tube adapter of claim 29, wherein there are two or more spaced apart engagement features distributed around the circumference of the tube such that each engagement feature provides an associated ledge surface.

32. 30. The closure tube adapter of claim 29, wherein each engagement feature comprises a recess having a ledge surface therein such that the ledge surface is recessed from the outer surface inwardly into the wall of the tube.

33. an adapter arrangement including a main retainer body surrounding the hollow tube and supporting a first set of through-bolt conduits also surrounding the hollow tube and arranged in a bolt pattern, and a second set of through-bolt conduits matching the first set, wherein a set of adapter bolts are configured to pass through the matching pattern of bolt holes, thereby coupling the main retainer body and the adapter arrangement, and wherein tightening the set of bolts provides the clamping force; the one or more latches each include an insert portion extending into an associated recess and a protruding portion extending radially beyond an immediately adjacent outer surface of said tube; 33. The sealing tube adapter of claim 32, comprising a retainer arrangement disposed to surround a protruding portion of the latch and to transfer at least a portion of the clamping force from the main retainer body, through the latch, and onto the recessed ledge surface, which clamping force can be further transferred to the gasket causing compression thereof.

34. the retainer arrangement includes a retainer lock configured such that the protruding portion of each latch directly contacts it when the retainer arrangement is positioned in a final position, the retainer lock radially containing the latch mechanism such that the inserted portion of the latch is fixedly trapped within its associated recess; an axial force will not disengage the trapped latch mechanism unless and until the clamping force exceeds the mechanical strength of at least one of the ledge, latch, retainer lock, and main retainer body; 34. The closure tube adapter of claim 33.

35. 35. The sealing tube adapter of claim 34, wherein the tube is SiC and the bolt is sized to fail in shear before the mechanical fracture force is exceeded.

36. 36. The closure tube adapter of claim 35, wherein the engagement feature is a recessed groove and the latch is a ring-shaped element that drops into the groove and bends sufficiently to conform to any irregularities in the ledge under a small fraction of the gasket compressive force, while being sufficiently rigid to withstand the compressive force without mechanical failure.

37. 37. The sealing adapter arrangement of claim 36, wherein the groove has a semicircular cross section and the latch is a tightly wound extension spring made of high temperature steel, the spring having an outer diameter that conforms to and can be received within the semicircular cross section of the groove.

38. 30. The closed tube adapter of claim 29, wherein there is only one engagement feature forming a protrusion that continuously surrounds the outer diameter of the hollow tube and provides a corresponding ledge surface around the periphery of said diameter.

39. 30. The sealed tube adapter of claim 29, wherein two or more engagement features, each forming a protrusion, are spaced apart from one another so as to be distributed around the circumference of the tube, each engagement feature presenting an associated ledge surface.

40. 39. The closure tube adapter of claim 38, wherein the projection forms a flange extending radially outward from the tube, and the ledge forms a major surface of the flange opposite the gasket sealing surface.

41. 41. The closed tube adapter of claim 40, wherein the flange is flat so that the ledge is generally perpendicular to the tube.

42. 41. The closed tube adapter of claim 40, wherein the flange has a tapered shape such that a radial element points away from the hollow tube and the ledge slopes outward at an angle.

43. 41. The closure tube adapter of claim 40, wherein the flange is either convex or concave rounded.

44. a retainer arrangement surrounding the hollow tube adjacent the flange, the retainer arrangement being capable of contacting the ledge, the main retainer arrangement supporting a first set of through-bolt conduits with a bolt pattern surrounding the hollow tube, the adapter body including a second set of through-bolt conduits matching the first set, and a set of bolts disposed through a matching pattern of bolt holes connecting the retainer and adapter arrangement and providing the force transmitted from the adapter arrangement to the ledge to function as the clamping force when the set of bolts is tightened; 41. The airtight tube adapter of claim 40.

45. 45. The sealing tube adapter of claim 44, wherein one or other end of each bolt engages the retainer arrangement and the adapter arrangement, respectively, via a spring mechanism having sufficient compatibility such that the bolt circle is sufficiently compliant to accommodate CTE mismatch along the axial direction of each bolt through a full range of operating temperatures.

46. 46. ​​The sealing adapter of claim 45, wherein the spring mechanism is one of: (i) a spring; (ii) a wave spring; (iii) a Belleville washer; or (iv) a flexure integral with or attached to the adapter arrangement.

47. a first hollow tube constructed from a first tube material that is hermetically impermeable to the diffusion of gases, the first hollow tube having at least one open tube end, the open tube end supporting a first gasket sealing surface thereon, the first hollow interior of the first hollow tube terminating in the open tube end, the first gasket sealing surface surrounding the open end of the first hollow interior; (i) a flange made of a hermetically impermeable flange material, the flange including a second gasket sealing surface having a shape that matches and faces the first gasket sealing surface, such that a vacuum gasket can be sandwiched between the gasket sealing surfaces; (ii) a second hollow tube constructed of a second tube material that is hermetically impermeable to the diffusion of air, the second hollow tube being hermetically sealed to and / or machined with the flange, the hollow interior of the second hollow tube being disposed in gas communication with the hollow interior of the first hollow tube; an adapter assembly including: an open gasket acting as a non-hermetic vacuum seal disposed about the open end of the first hollow interior, the open gasket being sandwiched between the first and second gasket sealing surfaces and having an inner periphery defining a gasket opening that provides the gas communication to the interior of the hollow tube; a clamping arrangement configured to mechanically engage both the first tube and the flange and clamp the first and second gasket sealing surfaces toward each other to create a compressive force on the open gasket; The hollow interiors of the tubes are in gas communication with each other and can be hermetically sealed from the external ambient air by employing a gas-tight sealing arrangement including: (i) the open gasket functions as a non-hermetic vacuum seal that exhibits at least some gasket leakage as gas diffusion through the non-hermetic gasket and / or into the surroundings of the non-hermetic gasket; (ii) a groove disposed on the outer periphery of the open gasket and configured to receive a flow of an oxygen-free sweep gas injected therein via an injection channel in the adapter assembly such that the sweep gas surrounds the open gasket; (iii) an external Peclet seal surrounding the vacuum gasket and disposed in series between the vacuum gasket and the ambient atmosphere, the external Peclet seal configured to receive the flow of oxygen-free sweep gas and provide a diffusion seal with radial gas flow of the oxygen-free sweep gas from the inner periphery of the Peclet seal to the outer periphery of the Peclet seal, such that the Peclet seal can hermetically separate the open gasket; wherein a majority of the flow of oxygen-free sweep gas flows through the Peclet seal, and a negligible amount of the oxygen-free sweep gas flows through and / or around the open gasket as the leak so that the non-hermetic vacuum seal provides sufficient hermeticity to support and enable vacuum pressure within the hollow tube, but would not provide sufficient airtightness to the outside air in the absence of the Peclet seal, and the Peclet seal provides sufficient hermetic separation from the outside air so that the leak through and / or around the non-hermetic gasket seal is substantially free of oxygen, hermetically sealing the gap.

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