Heating element for a furnace for firing and / or sintering workpieces

Abstract

The invention relates to a heating element (7) for a furnace, which heating element comprises a sapphire glass tube (8) for receiving a heating coil (17) inside the sapphire glass tube (8). The sapphire glass tube (8) is gas-tightly connected to a borosilicate tube (14) and also to a quartz glass tube (15). The electrical connections at the ends lead through in a gas-tightly compressed manner. According to the invention, the tube element is made of sapphire (8) and is gas-tightly joined by transition glasses and glass solder to compensate for the different coefficients of thermal expansion. The joined elements (14, 15) are located outside the firing chamber (2) so that they ensure operational reliability in the case of a thermal effect of up to 500°C. Furthermore, for firing / sintering the ceramic element (4) with shortwave infrared radiation in the range of from 0.8 µm to 2.5 µm, the heating element (7) has stable optical, electrical and mechanical properties and thus high-quality, permanently uniform firing and sintering results are ensured with significantly shortened firing-sintering times. No changes to the surface of the heating elements (7) occur which are caused by chemical influences or evaporations from the fired-sintered elements. No contamination of the sintered objects and the firing chamber occurs which is caused by the heating element (7). The heating element (7) can be used at an operating temperature up to 1900°C and thus ensures use over long uptimes.

Description

[0001] The invention relates to a heating element for a furnace for firing and / or sintering workpieces, in particular workpieces made of dental ceramic materials.

[0002] Application areas of the invention include the sintering or firing of workpieces used in a wide variety of industries, such as gears and other components, particularly those used in the automotive industry. Another key application of the invention is in the dental sector. There, zirconium oxide and other dental ceramic materials based on dental alloys, especially zirconium and / or other ceramic materials, are sintered and fired to create metal-free dental prostheses.

[0003] For dental furnaces used for firing or sintering veneering ceramics with or without glazes, or for glaze firing, electric resistance heating elements are known, in which a heating wire, preferably made of Kanthal and drawn into a quartz tube, is used. However, these known heating elements can only be used up to a maximum of 1200 °C. Further known dental furnaces are described in WO 2018 / 011061 A1 and WO 2020 / 088943 A1.

[0004] A temperature of 1650 °C is required for the sintering of zirconium oxide (SiO2), which has been increasingly used in dental restorations for several years.

[0005] Three different heating systems are known for dental furnaces on the market for firing and / or sintering metal-free dental prostheses such as zirconium oxide ceramics or similar materials, namely: 1. Heating systems using molybdenum disilicide (MoSi2 heating systems), 2. Heating systems using silicon carbide (SiC) and 3. Heating systems using induction Regarding 1. Molybdenum disilicide:

[0006] Molybdenum disilicide is a dense metal-ceramic material consisting of molybdenum disilicide and an oxide component, predominantly a glass phase. This glass component, or protective layer, changes during the heating phases, leading to flaking and thus contamination of the sintered object (workpiece) and / or the firing chamber. The flaking is clearly visible as small glass fragments and glass dust.

[0007] After several firing cycles, this requires a so-called cleaning firing lasting more than four hours at a temperature above 1400 °C, without loading the furnace, which entails a considerable expenditure of time and energy.

[0008] The cleaning firing must be carried out without sintered objects until a uniform protective layer is again visible on the molybdenum disilicide heating elements.

[0009] The dissolution of the oxide layer on the heating element leads to the formation of MoO3 (molybdenum(VI) oxide), also known as "pest oxidation," which in turn causes an undesirable green-yellow discoloration of the restorations (workpieces). To protect sintered objects from contamination / discoloration, the use of a sintering tray covering the workpiece is necessary.

[0010] Furthermore, the molybdenum disilicide heating elements are very fragile even after a short operating time and are subjected to stress up to their maximum operating range in the temperature range up to 1650°C, meaning that failures after short operating times are the norm. Regarding point 2, silicon carbide:

[0011] Silicon carbide heating elements exhibit unfavorable temperature-resistance characteristics, necessitating a very complex thyristor-controlled regulation system. Furthermore, it is essential to ensure that only elements with the same electrical resistance or similar state of aging are connected together. This means that individual defective heating elements within a system cannot be replaced; in such cases, the entire heating system must be replaced.

[0012] Silicon carbide heating elements are expensive and highly fragile. Replacing them incurs high spare parts costs compared to other heating systems. As mentioned earlier, if one silicon carbide heating element fails, the entire system must be replaced, since heating elements with the same electrical resistance must be connected together. Otherwise, further heating elements will fail very quickly. Replacing all heating elements, in turn, is even more expensive. Regarding point 3, induction:

[0013] It is known that dental furnaces are operated using induction to sinter workpieces. An induction furnace contains an induction coil. These coils are also called inductors. They are usually water-cooled, which is a disadvantage for use in dental laboratories or dental practices. The current-carrying inductor generates an alternating magnetic field, which, via eddy currents, leads to a controlled heating of the workpieces.

[0014] Since non-conductive materials are used in dentistry, a susceptor must also be used, i.e., an element that has the property of absorbing electromagnetic energy, converting it into heat and transferring it to the workpiece by convection.

[0015] Since the inductors must be well matched to the properties of the materials being treated in order to achieve the desired thermal behavior, the possible applications for the various non-conductive materials in dental technology are limited, as is the size of the workpieces.

[0016] The combustion chamber size of known induction furnaces is a maximum of 3 crowns, meaning a length of 38 mm and a height of 20 mm must not be exceeded.

[0017] Due to the high levels of electromagnetic interference, extensive safety regulations must be observed. For example, induction ovens must not be used in patient environments.

[0018] Only a limited number of materials are permitted for the heat treatment of workpieces by induction.

[0019] DE 40 14 246 A1 describes a heating device with a heating element (22) arranged in a protective tube (12), characterized in that the protective tube (12) is made of a monocrystalline ceramic material and is hermetically sealed and provided with feedthroughs (28) for the electrical connection of the heating element (22).

[0020] US 4,885,454 A describes a device for heating samples to temperatures in the range of 2000 °C in a non-neutral atmosphere, the device comprising: a plurality of elongated, parallel sapphire tubes of substantially equal length and evenly spaced around a heating area generally located centrally along the lengths of the tubes; within each of the tubes, an elongated resistance heater, each heater having at each end an elongated, rod-shaped conductor, with a filament-like tungsten heating element between the rod-shaped conductors, the heating elements forming a means for heating the area; an insulating sheath around the area, the tubes and rod-shaped conductors extending through the sheath, the sheath having an opening to allow the insertion of a sample into the area; and a means forming a common antechamber at each end of the tubes.within the anterooms an electrical connection device for applying electric current to the heating elements; a conduit device for introducing a controlled flow of non-reactive gas into one of the anterooms and for venting gas from the other of the anterooms, in order to induce a protective gas flow through the pipes around the heating elements.

[0021] The object of the invention is to create a heating element for sintering and / or firing furnaces that is characterized by high efficiency and high temperatures.

[0022] The present invention is described in the independent claims. Preferred embodiments are described in the dependent claims.

[0023] To solve this problem, the invention provides a heating element for a furnace for firing and / or sintering workpieces, in particular workpieces made of dental ceramic materials, which is equipped with a sapphire glass tube and a heating coil made of tungsten or molybdenum, which is arranged in the sapphire glass tube and has connecting leads leading to the outside, wherein the ends of the sapphire glass tube are sealed gas-tight by means of, in particular, quartz glass closures and wherein the connecting leads of the heating coil are led to the outside through, in particular, quartz glass closures.

[0024] The invention relates, in essence, to a heating element whose heating coil is made of tungsten and whose connecting leads are made of molybdenum, and which is arranged in a sapphire glass tube. The material of this glass tube is synthetic sapphire, i.e., an aluminum oxide with a purity of 100%.

[0025] The sapphire glass tube is sealed gas-tight at both ends, primarily with quartz glass or another heat-resistant material. The heating coil's connecting leads pass through these two seals, creating a gas-tight seal. For simplicity, the material used for these seals will be referred to as quartz glass. A section of quartz glass tubing is used, with the end furthest from the sapphire glass tube crimped. This quartz glass tubing is also a synthetic material, but unlike the synthetic sapphire, it can be worked in its softened state. This allows for a gas-tight seal by crimping the connecting lead through the tubing.

[0026] The sapphire glass tube and the quartz glass tube sections are joined together. Preferably, to improve the transfer between the materials sapphire and quartz, which have different thermal expansion coefficients, a borosilicate spacer is placed between each. This borosilicate tube section is also joined to the sapphire glass and the quartz glass closure, thus creating a gas-tight connection. Suitable joining techniques are known in the art. For example, the joining can be achieved using glass solder or similar glass materials. By gradually adapting to the different coefficients of thermal expansion, the resulting thermally induced changes in length can be compensated for. This prevents thermal and mechanical stresses and cracks in and between the individual glass materials sapphire, quartz, and borosilicate, and ensures a vacuum tightness or leakage rate of 10⁻⁸ mbar * l / s.

[0027] In theWhen installing the heating element in an oven, it is advantageous if the borosilicate glass intermediate tubes, or, if these are unavailable, the quartz glass closures, are located outside the oven's combustion chamber. Additionally, these intermediate tubes and quartz glass closures can be cooled, thus significantly reducing the occurrence of stresses.

[0028] The areas of the heating element located outside the combustion chamber are vacuum-tight in a temperature range up to 500 °C and exhibit a leakage rate of only 10 -8< mbar * l / s.

[0029] The melting point of the materials tungsten and molybdenum used for the heating coil and the connecting leads is above 2000 °C to 2500 °C. These materials are therefore suitable for use as the heating coil of the heating element according to the invention. The heating coil consists of tungsten, with each end of the coil being electrically connected via a molybdenum spacer to an electrical conductor made of an ordinary electrically conductive material, preferably with increased temperature resistance.

[0030] According to an advantageous embodiment of the invention, it can be provided that the heating coil emits electromagnetic radiation in the near-infrared range from 0.8 µm to 5 µm or from 0.8 µm to 2.5 µm and that the sapphire glass tube is transparent to electromagnetic radiation in the range from 0.17 µm to 6 µm.

[0031] As mentioned above, it is advantageous for adjusting the different temperature coefficients if a gas-tight intermediate tube made of borosilicate is arranged between the quartz glass closures and the ends of the sapphire glass tube.

[0032] In a further advantageous embodiment of the invention, it is provided that the sapphire glass tube and, if present, the intermediate tube sections made of borosilicate are filled with a noble or other inert gas.

[0033] Further advantages of the heating element according to the invention are that, when operated with the prescribed current, the heating element reacts quickly (the time from the "cold" state of the heating element to reaching its radiation maximum is preferably a few seconds, in particular less than 10 seconds or less than 5 seconds or less than 3 seconds) emits electromagnetic radiation and, furthermore, does not cause any contamination on the workpiece and, in addition, is not itself contaminated by substances of the workpiece or by substances caused by it during sintering and / or firing, such as vapors from coatings of the workpiece such as glazes.

[0034] In one embodiment of the invention, the heating coil consists of tungsten, wherein the ends of the heating coil are connected via electrical intermediate conductors made of molybdenum to electrical conductors made of a material other than molybdenum and / or wherein the molybdenum intermediate conductors extend through the closures of the sapphire glass tube.

[0035] The end caps of the sapphire glass tube are made of a heat-resistant material whose coefficient of thermal expansion is approximately that of molybdenum. This is because it is advantageous for the molybdenum intermediate conductors to extend through these caps. Quartz glass is particularly suitable for this purpose. Other material combinations are also conceivable, which is why the invention is not limited to quartz glass as the cap material for the sapphire glass tube and molybdenum as the electrical intermediate conductor. The electrical intermediate conductor extends through the "pinched-off" portion of the cap and should therefore have essentially the same coefficient of thermal expansion as the cap material, so that thermal stresses and cracks cannot occur.

[0036] The heating element according to the invention can be used in an oven according to the invention, in the combustion chamber of which the heating element is installed, preferably in such a way that the quartz glass closures are arranged outside the combustion chamber. The heating element is expediently guided through opposing openings in the combustion chamber wall by means of screws or similar fixings. These openings in the combustion chamber wall for the heating element are sealed gas-tight so that a vacuum can be created within the combustion chamber.

[0037] In a preferred embodiment of the invention, at least one reflector element can be arranged in the combustion chamber to direct the electromagnetic radiation emitted by the heating element towards the workpiece.

[0038] In the embodiment of the invention described above, it can further be provided that the heating element is partially surrounded by the reflector element and / or that the reflector element has a semicircular cross-section and preferably extends over the entire length of an associated heating element and / or that the chamber walls formed in the combustion chamber have high thermal insulation.

[0039] In a preferred embodiment of the invention, the furnace has a receiving element arranged in the combustion chamber for receiving the workpiece, wherein the receiving element has radiation-absorbing material which in particular absorbs radiation and thus acts as a susceptor element which, as a thermal radiator, transfers heat energy to the workpiece.

[0040] In a preferred embodiment of the invention, the receiving element comprises silicon carbide.

[0041] In a further advantageous embodiment of the invention, it is provided that a temperature measuring device is provided in the combustion chamber near the ceramic element to be fired.

[0042] In a preferred embodiment of the invention, a temperature measuring device is provided that detects the temperature in the area of ​​the workpiece.

[0043] With the heating element according to the invention, it is possible to operate sintering or firing furnaces at extremely high temperatures of at least 1900 °C, whereby the furnace can be used continuously in this temperature range, thus enabling long operating times. In furnaces equipped in this way, both general ceramic firing and sintering processes at extremely high temperatures can take place atmospherically or under vacuum. Particularly in "sinter firings" in the temperature range of approximately 1600 °C, high demands are placed on the heating element. Due to the use of sapphire glass, the heating element according to the invention fulfills the requirements of reliability, long service life, speed, cleanliness, energy efficiency, and high heat radiation provided in a short time. This is achieved according to the invention by the use of the sapphire glass tube and the heating coil made of tungsten and molybdenum junctions.

[0044] The physical properties of synthetic sapphire glass meet all necessary requirements for resistance to aggressive acids and vapors, density (important for gas tightness), hardness and compressive strength.

[0045] The temperatures of approximately 1600 °C required for technical and dental sintering processes are significantly below the critical melting point of sapphire glass, which is approximately 2050 °C. Therefore, the sapphire glass tube can withstand the combustion chamber temperatures of up to 1900 °C provided for in the invention.

[0046] Another relevant property of sapphire lies in its transmission, i.e., its permeability to electromagnetic radiation from the heating coil in the range of 0.8 µm to 2.5 µm, which allows the firing or sintering process to take place in a very short time. The radiation range in the wavelength range below 2 µm has the property of a greater penetration depth into the workpiece, enabling high-quality results to be achieved in less time. The heat energy reaches the workpiece almost exclusively through radiation; heat convection plays no role.

[0047] Another advantage of the invention is that the use of the heating element and the physical properties of the sapphire glass tube result in excellent transmission in the short-wave infrared range from 0.8 µm to 2.5 µm.

[0048] Furthermore, the sapphire glass tube is highly resistant to chemical and aggressive media, has excellent optical, mechanical and thermal properties, and therefore does not result in any limitations or changes in the quality of the firing and / or sintering process under continuous stress.

[0049] Finally, it should also be noted as an advantage that the application of short-wave infrared radiation (0.8 µm to 2.5 µm) to the workpiece, and the associated high penetration depth of the radiation into the workpiece, results in a significantly shorter firing and / or sintering time. This, in turn, significantly reduces energy consumption.

[0050] The invention is explained in more detail below using an exemplary embodiment and with reference to the drawing. In the Individuals demonstrate: Figure 1 shows a simplified exploded and perspective view of the essential components of a firing and / or sintering furnace, and Figure 2 shows a side view of the heating element according to an exemplary embodiment.

[0051] The invention is described below using the example of two heating elements in a dental oven. However, the invention is not limited to use in dental ovens. More or fewer than two heating elements 7 can also be used in an oven.

[0052] A dental furnace comprises a housing 1 with a combustion chamber 2. The interior of the housing 1 is equipped with high-temperature insulation 6 and further high-temperature-resistant insulating elements 13, 14, as well as an element functioning as a reflector 13. The housing 1 is also closed with a lid and sealing ring 15.

[0053] In the illustrated embodiment, two heating elements 7 are arranged in the combustion chamber 2. Each heating element 7 is inserted into the combustion chamber 1 and secured through a through-opening with vacuum-tight screw connections 12 provided in the housing 1. In the assembled state, the screw connections 12 surround and secure the respective heating element in a vacuum-tight manner.

[0054] The glass tubes 9 made of synthetic borosilicate and 10 made of synthetic quartz glass of the heating element 7 with its connections 11 for the operating voltage are located outside the combustion chamber 1, whereby the quartz tube 10, which is attached, for example, by glass solder, is gas-tightly pressed together with the connection of the heating coil 17 with the molybdenum connecting line 11, which is led to the outside.

[0055] The design of the heating element is in Fig. 2As shown, in the illustrated example, the sapphire glass tube 8 is gas-tightly joined at both ends to a borosilicate intermediate tube section 9, and this in turn is further gas-tightly joined to the crimped quartz glass tube 10. Within the quartz glass tube 10, which is gas-tightly attached at both ends, the heating coil 17, extending through the borosilicate intermediate tube sections 9, is connected via molybdenum intermediate tube sections 18 to the externally leading connecting lines 11, which are gas-tightly crimped by means of the crimped section.

[0056] Using a firing table 3 ( Fig. 1 ) and with a temperature sensor 5, it is possible to insert the dental ceramic (workpiece 4) to be fired and / or sintered, which is placed on the firing table 3, into the firing chamber 2. The temperature of the firing chamber 2 is regulated by means of the temperature sensor 5 and the corresponding electronic control (not shown).

Claims

1. A heating element for a furnace for firing and / or sintering workpieces, especially workpieces made of dental-ceramic materials, comprising a sapphire glass tube (8), and a heating coil (17) made of tungsten or molybdenum, that is provided in the sapphire glass tube (8) and has connection leads (11) routed to the outside, wherein the ends of the sapphire glass tube (8) are sealed in a gas-tight manner by means of closures made of a heat-resistant material, especially by means of quartz glass closures (10), and wherein the connecting leads (11) of the heating coil (17) extend through the closures (10) of the sapphire glass tube to the outside.

2. The heating element according to claim 1, characterized in that an intermediate tubular piece (9), which is made of a heat-resistant material having a coefficient of thermal expansion that is between that of sapphire glass and that of the closure material of the sapphire glass tube (8), especially made of borosilicate, is respectively arranged in a gas-tight manner between the closures (10) of the sapphire glass tube and the ends of the sapphire glass tube (8).

3. The heating element according to any of claims 1 to 2, characterized in that the sapphire glass tube (8) and, if any, the intermediate tubular pieces (9) of borosilicate are filled with a noble gas or another inert gas.

4. The heating element according to any of claims 1 to 2, characterized in that the heating coil (17) is made of tungsten, and that the ends of the heating coil (17) are electrically connected via intermediate conductors made of molybdenum to electric conductors made of a material other than molybdenum, and / or that the molybdenum intermediate conductors (11) extend through the closures of the sapphire glass tube (8).

5. A furnace for firing and / or sintering workpieces, especially workpieces made of dental-ceramic materials, comprising a firing chamber (1), and at least one heating element (7) according to any of claims 1 to 5, arranged within the firing chamber (1).

6. The furnace according to claim 5, characterized in that at least one reflector element (13) is provided in said firing chamber (1), in order to direct the electromagnetic radiation emitted by the heating element (7) towards the workpiece (4).

7. The furnace according to claim 5 or 6, characterized in that said heating element (7) is partially surrounded by said reflector element (13).

8. The furnace according to any of claims 5 to 7, characterized in that said reflector element (13) has a semicircular cross-section, and preferably extends over the entire length of a related heating element (7).

9. The furnace according to any of claims 5 to 8, characterized in that the chamber walls (6, 13, 14) to be formed in the firing chamber (1) have a high thermal insulation.

10. The furnace according to any of claims 5 to 9, characterized by a receiving element (3) for receiving said workpiece (4), provided in said firing chamber (1), wherein said receiving element (6) has a radiation-absorbing material, which absorbs radiation, in particular, and thus acts as a susceptor element, which, being e thermal radiator, transfers thermal energy to the workpiece (4).

11. The furnace according to claim 10, characterized in that said receiving element (6) comprises silicon carbide.

12. The furnace according to any of claims 5 to 11, characterized by a temperature measuring means (5) sensing the temperature in the zone of the workpiece (4).

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