Monolithic heater for thermionic cathodes

Abstract

A monolithic graphite heater for heating a thermionic cathode includes first and second conductive arms, each of the first and second conductive arms having an electrode mount at a proximal end, a thermal apex at a distal end, and a transition region between the electrode mount and the thermal apex, a cathode mount electrically and mechanically coupling each of the thermal apexes to form an area of ​​maximum Joule heating at or near the cathode mount and reducing Joule heating along each of the transition regions, and a press-fit opening formed in the cathode mount, the press-fit opening sized to receive at least a portion of the thermionic cathode to facilitate thermionic emission generated therefrom in response to operational thermoelectric power generation provided by the area of ​​maximum Joule heating.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 266,717, filed January 12, 2022, the entire disclosure of which is incorporated herein by reference. [Technical field]

[0002] The present disclosure relates generally to thermionic emitters. In particular, the present application relates to a heater formed of a common material that is shaped to localize heat to a thermionic cathode. [Background technology]

[0003] Applied Physics Technologies, Inc. (APTech) of McMinnville, Oregon, specializes in the development of thermionic and field emission cathodes. For example, APTech offers CeBix® cathodes (cerium hexaboride), LaB6 cathodes (lanthanum hexaboride), HfC cathodes (hafnium carbide), CFE, and ESE sources. The company's cathodes are used in a variety of applications, including microscopy, microanalysis, additive manufacturing, and other industries that employ electron sources in their products and workflows.

[0004] Thermionic cathode sources are heated to a temperature where the high energy tail of the Fermi-Dirac density of states exceeds the work function of the material. A good thermionic emitter therefore has a low work function, good material stability at high temperatures, and good vacuum compatibility. The operating temperature of thermionic cathode sources is about 1,800 Kelvin or higher, and typically thermionic cathode sources operate in high vacuum (about 1E-6 mbar) to ultra-high vacuum (about 1E-9 mbar).

[0005] The heater structures that bring the thermionic cathode sources up to operating temperature are typically made from refractory metals such as tungsten, rhenium, tantalum, molybdenum, or combinations thereof. All physical quantities are temperature dependent, so creep, thermal expansion, evaporation, drift, and other physical quantities are all involved in the design of the heater structure.

[0006] One type of conventional heater for a thermionic cathode source is called a Mini Vogel Mount (MVM) and is available from APTech. MVMs are used as heaters for thermionic emitters employing the thermionic cathodes described above. Figure 5 of U.S. Patent No. 7,544,523 shows a typical MVM. In this MVM, two posts are rigidly fixed to a thick ceramic base and bent toward the center in an inverted V-shape. The posts are typically made of a molybdenum-rhenium alloy or other material that maintains a high modulus of elasticity even at high temperatures. During assembly, the posts are slightly expanded to accommodate the thermionic cathode and pyrolytic graphite block. When the posts are then released, the thermionic cathode and pyrolytic graphite block are held in place by the clamping force of the posts. However, a common failure mode of MVMs is stress fracture of the thermionic cathode source during operation or relaxation of the multiple MoRe posts.

[0007] Other examples of heaters include filaments machined into standard shapes, and traditional wire or ribbon filaments. Long, thin filaments that are refractory metal wire or ribbon have temperature and mechanical stability issues. For example, the filament (wire) can physically change or move over time due to recrystallization, spot weld variations, and thermal stress relaxation. Summary of the Invention

[0008] The disclosed monolithic heater is designed in shape and structure such that the hottest part, which acts as a filament, is in an optimal position for the thermionic cathode source. Selectable limited areas of the structure are made hotter by adjusting the shape of the filament. The filament is constructed from a single material. The monolithic device is mechanically stable with respect to mechanical stress due to the lower coefficient of thermal expansion of graphite compared to metals at 2,000 Kelvin. It is believed that the increased mechanical stability results in less electron beam movement. The disclosed embodiments reduce the risk of filament burnout, improve heater stability and operating life, reduce radiation losses, allow higher operating temperatures for a given (standard) power supply, and have the ability to heat large objects.

[0009] In one aspect, a monolithic graphite heater for heating a thermionic cathode includes first and second conductive arms. Each of the first and second conductive arms has an electrode mount at a proximal end, a thermal apex at a distal end, and a transition region between the electrode mount and the thermal apex. The monolithic graphite heater also electrically and mechanically couples each of the thermal apexes to form a maximum Joule heating area at or near the cathode mount and reduce Joule heating along each of the transition regions. The monolithic graphite heater has a press-fit opening formed in the cathode mount. The press-fit opening is sized to receive at least a portion of the thermionic cathode to facilitate thermionic emission generated therefrom in response to operational thermoelectric power generation provided by the maximum Joule heating area.

[0010] Further features and advantages will become apparent from the following detailed description of the embodiments, which proceeds with reference to the accompanying drawings. [Brief description of the drawings]

[0011] To easily identify the description of a particular element or act, the most significant digit(s) of a reference number refers to the number of the figure in which that element is first introduced.

[0012] [Figure 1] FIG. 1 is a diagram of a monolithic graphite heater for heating a cylindrical thermionic cathode attached to a heater cathode mount according to one embodiment. [Diagram 2] FIG. 2 is an isometric view of the monolithic graphite heater shown in FIG. 1 prior to drilling the cathode mounts and mating the electrodes to their respective electrode bases. [Diagram 3] FIG. 2 is an isometric view of the thermionic emitter showing the monolithic graphite heater of FIG. 1 in an assembled state. [Figure 4] 2 is a ParaView visualization of a multiphysical software simulation showing the Joule thermal gradient model for the monolithic graphite heater shown in FIG. 1. [Diagram 5] FIG. 2 is another simulation image showing a surface temperature gradient model of the side view of the monolithic graphite heater shown in FIG. 1. [Figure 6] 2 is an image capture of a thermal imaging software application showing the results of a thermal test on a side view of the cathode mount of the monolithic graphite heater shown in FIG. 1. [Figure 7] FIG. 7 is an image capture of the thermal imaging software application showing the results of a thermal test on a top view of the cathode mount of the monolithic graphite heater shown in FIG. [Figure 8] 13 is another ParaView visualization image showing the Joule heat gradient model of another monolithic graphite heater. [Figure 9] FIG. 9 is another simulation image showing the surface temperature gradient model of the side view of the monolithic graphite heater shown in FIG. [Figure 10] FIG. 13 is an isometric view of another variation of a monolithic graphite heater lacking localized heating at the cathode mount. [Figure 11] 1 is a first part of a sequence of isometric views illustrating a mount for a thermionic cathode source according to another embodiment. [Figure 12]11 is a second part of a sequence of isometric views illustrating a mount for a thermionic cathode source according to another embodiment. [Figure 13] 1 is a flowchart of a method for manufacturing a thermionic emitter according to an embodiment. [Figure 14] It is a monolithic graphite heater with a cylindrical cathode mount. [Figure 15] It is a monolithic graphite heater having a stepped cylindrical cathode mount with an embedded thermionic cathode. [Figure 16] 13 is a ParaView visualization of a multiphysical software simulation showing a Joule heating gradient model of a monolithic graphite heater according to another embodiment. [Figure 17] 13 is a ParaView visualization of a multiphysical software simulation showing a Joule heating gradient model of a monolithic graphite heater according to another embodiment. [Figure 18] 13 is a ParaView visualization of a multiphysical software simulation showing a Joule heating gradient model of a monolithic graphite heater according to another embodiment. [Figure 19] 13 is a ParaView visualization of a multiphysical software simulation showing a Joule heating gradient model of a monolithic graphite heater according to another embodiment. [Figure 20] 13 is a ParaView visualization of a multiphysical software simulation showing a Joule heating gradient model of a monolithic graphite heater according to another embodiment. [Figure 21] 13 is a ParaView visualization of a multiphysical software simulation showing a Joule heating gradient model of a monolithic graphite heater according to another embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] 1 is a diagram illustrating a monolithic graphite heater 100 for heating a thermionic cathode 102. The monolithic graphite heater 100 includes a first conductive arm 104 and a second conductive arm 106.

[0014] The first conductive arm 104 includes a first electrode mount 108 at a proximal end 110, a first thermal apex 112 at a distal end 114, and a first transition region 116 between the first electrode mount 108 and the first thermal apex 112. The first electrode 118 is disposed in a first opening 120 in the first electrode mount 108. The grommet 142 is a circular ring pressed against the first electrode 118.

[0015] Similarly, the second conductive arm 106 includes a second electrode mount 122 at the proximal end 110, a second thermal apex 124 at the distal end 114, and a second transition region 126 between the second electrode mount 122 and the second thermal apex 124. A second electrode 128 is disposed in a second opening 130 in the second electrode mount 122. The grommet 144 is a circular ring pressed against the second electrode 128.

[0016] The cathode mount 132 spans between the first thermal vertex 112 and the second thermal vertex 124, electrically and mechanically coupling each of the thermal vertices to form a maximum Joule heat area 134 adjacent the cathode mount 132 and decreasing along each of the first transition region 116 and second transition region 126. The maximum Joule heat area 134 is thus the area of ​​highest resistance for a constant current.

[0017] The cathode mount 132 is formed with a press-fit opening 136. The press-fit opening 136 is sized to receive at least a portion (e.g., a bottom cylindrical section) of the thermionic cathode 102 to facilitate thermionic emission generated therefrom in response to the operating thermoelectric power generation provided by the maximum Joule heat area 134. The monolithic graphite heater 100 operates like a filament in that current flows upward on one side and downward on the other side. Further details regarding heating are shown and described below with reference to Figures 4-7.

[0018] The thermionic cathode 102 is press-fit into the press-fit opening 136 of the monolithic graphite heater 100, eliminating the need for welding. This allows more design space for the shape of the thermionic cathode 102, improving emission characteristics. Additionally, because the monolithic graphite heater 100 is made from a single piece of machined graphite, it can accommodate hard materials such as borides or carbides without breaking when pressed in.

[0019] In some embodiments, the monolithic graphite heater 100 is machined from one piece of material. The monolithic graphite heater 100 is machined with a mechanical isolation 138 between the first conductive arm 104 and the second conductive arm 106 so that the monolithic graphite heater 100 is stable during machining and attachment of the thermionic cathode 102. FIG. 1 shows that at least a portion of the mechanical isolation 138 is located within the gap 140.

[0020] 2 shows the monolithic graphite heater 100 before drilling the press-fit openings 136 and before bonding the first electrode 118 and the second electrode 128. In some embodiments, the monolithic graphite heater 100 can be formed using precision machining, molding, sintering, electrical discharge machining (EDM), or can be 3D printed. Additionally, the cathode mount 132 can have a variety of shapes (e.g., rectangular, cylindrical, or other shapes for mounting a variety of thermionic cathode sources). Examples of various shapes are shown in FIGS. 14 and 15.

[0021] 2 also shows details of each electrode mount and transition region. In some embodiments, each transition region includes a rectangular base 202, a rectangular top 204 that is smaller than the rectangular base 202, a pair of opposing truncated triangular sides 206, and a uniform thickness 208 between the pair of opposing truncated triangular sides 206.

[0022] In some embodiments, each electrode mount is chair-shaped. For example, the first electrode mount 108 includes a seat 210 for receiving the first electrode 118 (FIG. 1) and a back 212 from which the first transition region 116 extends.

[0023] FIG. 3 shows a thermionic emitter 300 including a machined monolithic graphite heater 100 mounted on a ceramic heater base 302, a thermionic cathode 102 pressed into a cathode mount 132, and first and second electrodes 118 and 128.

[0024] The ceramic heater base 302 is brazed to the first and second electrodes 118, 128 on one side and faces the proximal end 110 on the other side. The first and second electrodes 118, 128 are secured with grommets 142, 144, so no welding is required. This also provides greater mechanical stability. Because the integrity of the spot weld depends on many parameters, this attachment method relies on machining tolerances, which are more controllable. It is more difficult to precisely control the location of a spot weld to within 0.001 inches than it is to maintain a machining tolerance of 0.001 inches.

[0025] 3 shows the thermionic emitter 300 in a vertical layout, one skilled in the art would recognize that horizontal layouts or other orientations are also possible. Thermionic emitter 300 can also be retrofitted (i.e., replacing multiple MVMs) into existing customer sources.

[0026] The one-dimensional, time-independent heat equation for a Joule-heated wire [Equation 1] describes the physics of heater structures that are highly nonlinear with temperature-dependent coefficients and non-negligible blackbody radiation at the operating temperature. This equation is analytically intractable even in the one-dimensional case for the simplest heater geometries, such as a filament (heated wire).

number

[0027] Thermal conductivity k(T), resistivity ρ(T), emissivity JPEG2025503706000003.jpg109 are all temperature dependent. Furthermore, since k(T) is in the derivative, nonlinearity occurs in the second derivative, T 4 Another nonlinearity due to the term also occurs.

[0028] Numerical methods have been employed to obtain quantitative results for a variety of heater geometries and materials. Multiphysical simulation software (e.g., Elmer FEM, available as open source software) and visualization tools (e.g., ParaView software developed by Kitware Inc., Sandia National Laboratories, Los Alamos, New Mexico) have been used to design the disclosed heater structures.

[0029] FIG. 4 shows the results of such a numerical method in the form of a Joule heating simulation view 400 of a monolithic graphite heater 100 showing the relative magnitude of Joule heating in a tapered structure. Due to the tapered shape, the Joule heating gradient 402 increases at each transition region, reaching a maximum Joule heating region 134 adjacent the cathode mount 132. Joule heating is greatest near each apex and decreases in the arms. The tapered arms conduct enough heat to the base, creating a "pinch point" near the cathode mount 132, without the arms cooling down enough to burn out. In some other embodiments, the maximum Joule heating region may be at the cathode mount itself.

[0030] The Joule heat power per unit volume is balanced between the radiation losses of the surface area and the conduction losses through the cross-sectional area so that the temperature of any point on the heating structure is as close as possible to the operating temperature of the thermionic cathode 102. When heated in a vacuum, energy is only lost through conduction and radiation. By optimizing the geometry using CAD and multiphysics solvers, the structure can be designed to balance the conduction and radiation losses, keeping the heater structure at a safe operating temperature and extending its useful life. To this end, the net power loss can be adjusted.

[0031] Additionally, the total power input is minimized whenever possible, staying within the achievable limits of a typical power source. Power minimization is desirable in some use cases, but not all. Operating at minimum power input is desirable given power supply limitations and heat losses in the emitter environment, but higher power inputs may be acceptable if mechanical stability and heater life are optimized.

[0032] FIG. 5 shows an example of thermal modeling of the temperature distribution 500 of the monolithic graphite heater 100. In this example, the thermionic cathode 102 is heated to approximately 1,800 Kelvin. At the operating temperature of 1,800 Kelvin, the hottest area is concentrated at the thermal apex. Also, because each arm gets larger as it extends toward the base, each arm conducts more power and the power generation of the multiple arms decreases. The peak power generation per unit volume is also greatest at the thermal apex.

[0033] FIG. 6 shows an example of a thermal image 600 test result of a monolithic graphite heater 100 with a 600 μm CeB6 crystal as the thermionic cathode 102. The data from the test is shown in the table below, and the thermal image 600 is centered at Tgright in [Table 1]. The cathode mount 132 (Tgleft, Tgright), where the crystal is mounted, is cooler than the edge of the crystal (Tcrystal). The arms of the structure cool very quickly, causing the temperature to drop below the capability of the device.

[0034] [Table 1]

[0035] FIG. 7 shows an example of additional test results of a thermal image 700 of a monolithic graphite heater 100 with a 600 μm CeB6 crystal as the thermionic cathode 102, where the thermal image 700 is centered on a Tcrystal. In this example, the crystal is set at 1,800 Kelvin (emissivity @ .77). The ROI2 temperature in thermal image 600 is similar to the ROI4 temperature in thermal image 700.

[0036] Figures 8 and 9 show examples of alternative designs that do not localize heat at the cathode mount. For example, Figure 8 shows that Joule heating is greatest at the arm. Because the thermal conductivity is not high enough, it cannot conduct enough heat to the base, and the arm burns out at the operating temperature of the thermionic emitter (see Figure 9). In other words, the power generation is too high at the arm compared to the cathode mount.

[0037] The table below shows a comparison of the structures of FIGS. 8 and 9 (with elongated arms typical of wire filament designs) with the monolithic graphite heater 100. [Table 2]

[0038] As shown in Table 2, at 1,800 Kelvin, the cathode mount (Tcenter) is slightly hotter than the crystal and the arms are at least 300 Kelvin hotter. At this high temperature, the arms will fail. Also, at the same temperature, the monolithic graphite heater 100 has a lower overall power.

[0039] Figure 10 shows other defects related to the operation of undesirable localized heat regions. Early attempts were made to minimize the cross-sectional area of ​​the arms. This created two problems when manufacturing filaments with thin sections: (a) achievable yield and durability during packaging / shipping, and (b) structural integrity to withstand drilling of the apertures and pressing of the crystals. If a filament is made with such a thin section and has no internal or external support at the cut-off point, the arms will easily snap under the stress of attaching the cathode.

[0040] 11 and 12 show an assembly sequence 1100 for a thermionic emitter 300, showing the step-by-step process of drilling the apertures, pressing the crystal in the press-fit aperture 136, and cutting the structure. The cutting is necessary to ensure that there is one path for current to flow between the electrodes. Those skilled in the art will appreciate that a relatively large monolithic structure is more mechanically stable than a multi-piece structure assembled using welds, springs, screws, epoxies, or other materials.

[0041] FIG. 13 illustrates a method 1300 for manufacturing a thermionic emitter. At block 1302, the method 1300 for manufacturing a thermionic emitter entails forming a monolithic graphite heater 100 (FIG. 1). Forming the monolithic graphite heater 100 may entail machining from a solid block, molding material, sintering, forming using EDM, or 3D printing. At block 1304, the method 1300 for manufacturing a thermionic emitter entails bonding an electrode to each of the electrode mounts. At block 1306, the method 1300 for manufacturing a thermionic emitter entails removing material in the cathode mount (e.g., drilling) to define an opening in the cathode mount for receiving at least a portion of the thermionic cathode. In other embodiments, different machined features (e.g., slots, pockets, or protrusions) may be formed to receive different cathode shapes.

[0042] The shape of the device the heater will be in will determine the shape of the heater. For example, the embodiments described above are suitable as replacements for MVMs, so the embodiments may conform to the same type factors, power, and electronic specifications. Figures 16-21 show several variations of monolithic graphite heaters that localize heat, but with different shapes for different use cases. For example, Figure 16 shows an elongated version.

[0043] Figure 17 shows a reduced height version, where the thermal apex is at the midpoint along the structure moving from the first electrode to the second electrode. The version in Figure 17 differs from the versions shown in Figures 8 and 9 in the size of the crystal mount and the size of the crystal (thermal mass), which results in the thermal apex being towards the center.

[0044] 18 shows a higher temperature version 1800. In this version 1800, the sidewalls 1802, rather than being straight sidewalls, are concavely curved from the base 1804 to the thermal apex 1806, forming substantially concave triangular faces 1808. These faces 1808 have a smaller surface area compared to straight triangular faces, effectively increasing the maximum Joule heating.

[0045] FIG. 19 shows another variation in which the triangular arms are arranged so that the major faces do not face each other.

[0046] Figure 20 shows another variation with an opening in each arm, which reduces heat loss in the legs and improves efficiency without unduly compromising mechanical stability.

[0047] FIG. 21 shows another variation in which the openings extend from the base to the thermal apex, thereby causing each leg to diverge.

[0048] Those skilled in the art will appreciate that many changes may be made to the details of the above embodiments without departing from the underlying principles of the disclosure. For example, the disclosed heaters may be made from materials other than graphite, including other refractory metals, borides, or carbides. Furthermore, the localization of heat may be used to heat rods, metal wires, metal coils, or other devices other than thermal cathode sources. The scope of the invention should therefore be determined solely by the following claims and equivalents. [Explanation of symbols]

[0049] 100 Monolithic Graphite Heater 102 Thermionic Cathode 104 First conductive arm 106 Second conductive arm 108 First electrode mount 110 Proximal end 112 The First Heat Peak 114 Distal end 116 First Transition Region 118 First Electrode 120 First opening 122 Second electrode mount 124 The Second Peak of Heat 126 Second Transition Region 128 Second Electrode 130 Second Opening 132 Cathode mount 134 Maximum Joule Heat Region 136 Pressfit opening 138 Mechanical separation section 140 Gap 142 Grommet 144 Grommet 202 Rectangular Base 204 Rectangular Top 206 Cut Triangular Face 208 Thickness 210 Seat 212 Back 300 Thermionic Emitter 302 Ceramic Heater Base 400 Joules Heat Simulation View 402 Joule heat gradient 500 Temperature distribution 600 Thermal Images 700 Thermal Images 1100 Assembly Sequence 1300 Method for manufacturing a thermionic emitter 1302 Block 1304 Block 1306 Block 1800 Version 1802 side wall 1804 Base 1806 Heat Peak 1808 faces

Claims

1. 1. A monolithic graphite heater for heating a thermionic cathode, comprising: first and second conductive arms, each of the first and second conductive arms having an electrode mount at a proximal end, a thermal apex at a distal end, and a transition region between the electrode mount and the thermal apex; a cathode mount electrically and mechanically coupling each of the thermal vertices to form an area of ​​maximum Joule heating at or near the cathode mount and to reduce Joule heating along each of the transition regions; a press-fit opening formed in the cathode mount sized to receive at least a portion of the thermionic cathode to facilitate thermionic emission generated therefrom in response to operational thermoelectric power generation provided by the maximum Joule heat area; A monolithic graphite heater comprising:

2. Each of the transition regions is a rectangular base, a rectangular top that is smaller than said rectangular base, a pair of opposing truncated triangular faces, and a uniform thickness between said pair of opposing truncated triangular faces; 2. The monolithic graphite heater of claim 1.

3. The maximum Joule heat area is configured to operate at a temperature range from about an operating temperature of the thermionic cathode to about 10 percent higher than the operating temperature; 2. The monolithic graphite heater of claim 1.

4. The temperature range is from about 1,800 Kelvin to about 1,980 Kelvin; 4. The monolithic graphite heater of claim 3.

5. the cathode mount includes a rectangular body; 2. The monolithic graphite heater of claim 1.

6. the cathode mount includes a cylindrical body; 2. The monolithic graphite heater of claim 1.

7. a first electrode disposed in a first opening of the electrode mount of the first conductive arm; a second electrode disposed in a second opening in the electrode mount of the second conductive arm; and 10. The monolithic graphite heater of claim 1 further comprising:

8. further comprising a ceramic base to which each of the electrode mounts is secured; 2. The monolithic graphite heater of claim 1.

9. The monolithic graphite heater of claim 1; the thermionic cathode mounted in the press-fit opening; A thermionic emitter comprising:

10. and at least a portion of the mechanical separator disposed in a gap between the first and second conductive arms. the mechanical isolation is configured to provide stability during assembly of the thermionic cathode mounted in the press-fit opening.

10. The thermionic emitter of claim 9.

11. 1. A method of manufacturing a thermionic emitter, comprising the steps of: forming a monolithic graphite heater; coupling an electrode to each of the electrode mounts; removing material within the cathode mount to define an opening therein for receiving at least a portion of the thermionic cathode; Including, The monolithic graphite heater comprises: a first and second conductive arms, each of the first and second conductive arms having an electrode mount at a proximal end, a thermal apex at a distal end, and a transition region between the electrode mount and the thermal apex; The monolithic graphite heater comprises: a cathode mount electrically and mechanically coupling each of the thermal vertices to form an area of ​​maximum Joule heating at or near the cathode mount and reducing Joule heating along each of the transition areas; method.

12. The forming step includes: machining said monolithic graphite heater from a solid block. The method of claim 11.

13. The forming step includes: heating a feedstock material to form the monolithic graphite heater; The method of claim 11.

14. Pressing the thermionic cathode into the opening; The method of claim 11 further comprising:

15. mounting the proximal end on a ceramic base; The method of claim 11 further comprising:

16. activating a flow of electrical current through the monolithic graphite heater to promote thermionic emission generated from the thermionic cathode in response to operating thermoelectric power generated in a thermal localized region; The method of claim 11 further comprising:

17. separating the first and second conductive arms by removing at least a portion of a separation portion connecting the first and second conductive arms; The method of claim 11 further comprising:

18. The step of removing the material comprises: drilling said opening; The method of claim 11.

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