Heating element for a furnace for firing or sintering workpieces, and furnace having at least one such heating element

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] Heating element for a furnace for firing and / or sintering workpieces and furnace with at least one such heating element

[0002] The invention relates to a heating element for a furnace for firing and / or sintering workpieces, in particular workpieces made of dental ceramic materials, and to a furnace for firing and / or sintering workpieces, in particular workpieces made of dental ceramic materials.

[0003] Areas of application of the invention include the sintering and firing of workpieces used in a wide variety of industries, such as gears and other elements particularly useful in the automotive industry. Another focus of the invention is in the dental industry. There, zirconium oxide and other dental ceramic materials based on dental alloys, particularly zirconium and / or other ceramic materials, are sintered and fired as metal-free dental prostheses.

[0004] For dental furnaces used for firing or sintering veneering ceramics with or without gloss finishes, or for glaze firing, electric resistance heaters are known. These heaters utilize a heating wire, preferably made of Kanthai, threaded into a quartz tube. However, these heating elements can only be used up to a maximum of 1200 °C.

[0005] Other known dental furnaces are described in WO 2018 / 011061 A1 and WO 2020 / 088943 A1.

[0006] For sintering zirconium oxide (SiO2), which has been increasingly used in dental restorations in recent years, a temperature of 1650 °C is required.

[0007] There are three different heating systems available on the market for the firing and / or sintering of metal-free dental prostheses such as zirconium oxide ceramics or similar materials: 1. Heating systems with molybdenum disilicide (MoSi2 heating systems),

[0008] 2. Heating systems with silicon carbide (SiC) and

[0009] 3. Heating systems using induction to 1. Molybdenum disilicide:

[0010] 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 and leads to flaking and thus to contamination of the sintered object (workpiece) and / or the combustion chamber. The flaking is clearly visible in the form of small glass splinters and glass dust.

[0011] This requires, after several firing cycles, a so-called cleaning firing lasting more than four hours at a temperature above 1400 °C, without loading the kiln, which entails a considerable expenditure of time and energy.

[0012] The cleaning firing must be carried out without sintered objects until a uniform protective layer is visible on the molybdenum disilicide heating elements.

[0013] 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 leads to 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 required.

[0014] Molybdenum disilicide heating elements are also very fragile after a short period of operation and are subjected to temperatures up to 1650°C, reaching their maximum operating range, which means failures after short periods of operation are common. 2. Silicon carbide:

[0015] Silicon carbide heating elements exhibit unfavorable temperature-resistance behavior, necessitating very complex thyristor-controlled regulation. Furthermore, care must be taken to ensure that only those elements with the same electrical resistance or the same state of aging are connected together. This means that individual defective heating elements in a system cannot be replaced. In such a case, the entire heating system must be replaced.

[0016] Silicon carbide heating elements are expensive and highly fragile. Replacing them results in high spare parts costs compared to other heating systems. If a silicon carbide heating element fails, the entire system must be replaced, as already mentioned, because heating elements with the same electrical resistance must be connected together. Otherwise, other heating elements will fail within a very short time. Replacing all heating elements, in turn, is associated with even higher costs. 3. Induction:

[0017] It is common practice to operate dental furnaces using induction to sinter workpieces. An induction furnace has an induction coil. These induction coils are also called inductors. They are usually water-cooled, which makes them unsuitable for use in dental laboratories or dental practices. The current-carrying inductor generates an alternating magnetic field, which, through eddy currents, leads to a controllable heating of the workpieces.

[0018] Since non-conductive materials are used in the dental field, a susceptor must also be used. This is an element that has the ability to absorb electromagnetic energy, convert it into heat, and transfer it to the workpiece by convection. 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, especially by the size of the workpieces.

[0019] The combustion chamber size of the known induction furnaces is max. 3 crowns, i.e. a length of 38 mm and a height of 20 mm must not be exceeded.

[0020] Due to the high levels of interference, extensive safety regulations must be observed. For example, induction furnaces may not be used in patient environments.

[0021] Only limited materials are permitted for the heat treatment of workpieces by induction.

[0022] The object of the invention is to create a heating element for sintering and / or firing furnaces which is characterized by high efficiency and high temperatures.

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

[0024] The sapphire glass tube is sealed at both ends, primarily by quartz glass or another heat-resistant material, with the connecting lines of the heating coil running gas-tight through these two closures. For the sake of simplicity, we will refer to quartz glass as the material for these closures. A piece of quartz glass tube is used, which is "pinched off" at the end facing away from the sapphire glass tube. The material of this piece of quartz glass is also synthetic, which, unlike synthetic sapphire material, can be processed in its softened state. By compressing (squeezing), it enables a gas-tight seal with the connecting line running through it.

[0025] The sapphire glass tube and the quartz glass tube sections are joined together. Preferably, a borosilicate spacer is arranged between the two materials, each of which has different temperature coefficients. This borosilicate tube section is also joined to the sapphire glass and the quartz glass closure, thus forming a gas-tight connection. Appropriate 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 thermal expansion coefficients, the resulting thermally induced length changes can be compensated. This prevents thermal or mechanical stresses and cracks in and between the individual glass materials (sapphire, quartz, and borosilicate), and ensures a vacuum tightness or leak rate of 10-8 mbar * l / s.When the heating element is installed in a furnace, it is advisable to place the borosilicate intermediate tubes or, if these are not available, the quartz glass seals outside the furnace's combustion chamber. Additionally, these intermediate tubes and quartz glass seals can be cooled, thus greatly reducing the occurrence of stresses.

[0026] The areas of the heating element located outside the combustion chamber are vacuum-tight in a temperature range up to 500 °C and have a leakage rate of only 10' 8 mbar * l / s.

[0027] The melting points of the materials used for the heating coil and the supply lines, tungsten and molybdenum, are between 2000°C and 2500°C. These materials are therefore suitable for use as the heating coil of the heating element according to the invention. The heating coil is made of tungsten, with the coil being electrically connected at each end via a molybdenum spacer to an electrical conductor made of a conventional electrically conductive material, preferably with increased temperature resistance.

[0028] 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 pm to 5 pm or from 0.8 pm to 2.5 pm and that the sapphire glass tube is permeable to the electromagnetic radiation in the range from 0.17 pm to 6 pm.

[0029] As already mentioned above, it is advantageous for adjusting the different temperature coefficients if an intermediate tube piece made of borosilicate is arranged in a gas-tight manner between the quartz glass closures and the ends of the sapphire glass tube.

[0030] In a further advantageous embodiment of the invention, the sapphire glass tube and, if present, the intermediate tube pieces made of borosilicate are filled with a noble or other inert gas. Further advantages of the heating element according to the invention are that the heating element, when operated with the specified current, emits electromagnetic radiation quickly (the time from the "cold" state of the heating element until it reaches its radiation maximum is preferably a few seconds, in particular less than 10 seconds or less than 5 seconds or less than 3 seconds) and, furthermore, does not cause any contamination on the workpiece and, moreover, is not itself contaminated by substances in the workpiece or by substances caused by this during sintering and / or firing, such as vapors from coatings on the workpiece, such as glaze masses.

[0031] In one embodiment of the invention, the heating coil is made of tungsten, wherein the ends of the heating coil are connected to electrical conductors made of a material other than molybdenum via electrical intermediate conductors made of molybdenum and / or wherein the molybdenum intermediate conductors extend through the closures of the sapphire glass tube.

[0032] The closures at the ends of the sapphire glass tube are made of a heat-resistant material whose thermal expansion coefficient roughly corresponds to that of molybdenum. It is advantageous for the molybdenum intermediate conductors to extend through these closures. 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 closure material for the sapphire glass tube and molybdenum as the electrical intermediate conductor. The electrical intermediate conductor extends through the "pinched-off" part of the closure and should therefore have essentially the same thermal expansion coefficient as the closure material, preventing thermal stress and cracking.

[0033] The heating element according to the invention can be used in a furnace according to the invention, in whose combustion chamber 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 passed through opposite openings in the combustion chamber wall by means of screw connections or similar fixings. These passages in the combustion chamber wall for the heating element are sealed gas-tight so that a vacuum can be created within the combustion chamber.

[0034] In an advantageous embodiment of the invention, at least one reflection element can be arranged in the combustion chamber in order to direct the electromagnetic radiation emitted by the heating element towards the workpiece.

[0035] In the previously described embodiment of the invention, it can further be provided that the heating element is partially surrounded by the reflector element and / or that the reflector element is semicircular in cross section and preferably extends over the entire length of an associated heating element and / or that the chamber walls forming in the combustion chamber have a high degree of thermal insulation.

[0036] 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.

[0037] In an advantageous embodiment of the invention, the receiving element comprises silicon carbide.

[0038] In a further advantageous embodiment of the invention, a temperature measuring device is provided in the combustion chamber near the ceramic element to be fired.

[0039] In an advantageous embodiment of the invention, a temperature measuring device is provided which detects the temperature in the area of ​​the workpiece.

[0040] The heating element according to the invention makes it possible to operate sintering or firing furnaces at extremely high temperatures of at least 1900°C. The furnace can be used continuously in this temperature range, thus achieving long operating times. In furnaces equipped in this way, both general ceramic firing and sintering processes can take place at extremely high temperatures, either atmospherically or under vacuum. Particularly for sintering 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 properties of reliability, long operating life, speed, cleanliness, energy efficiency, and high heat radiation delivered in a short time. This is achieved according to the invention by using the sapphire glass tube and the heating coil made of tungsten and molybdenum junctions.

[0041] 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.

[0042] The temperatures of approximately 1600 °C required for technical and dental sintering processes are significantly below the critical melting point of sapphire glass of approximately 2050 °C. This means that the sapphire glass tube can withstand the combustion chamber temperatures of up to 1900 °C provided for by the invention.

[0043] Another relevant property of sapphire is its transmission, i.e., its permeability to electromagnetic radiation from the heating coil in the range of 0.8 pm to 2.5 pm, which allows the firing or sintering process to take place in a very short time. Radiation in the wavelength range below 2 pm has the property of greater penetration depth into the workpiece, allowing high-quality results to be achieved in a shorter time. The thermal energy reaches the workpiece almost exclusively through radiation; thermal convection plays no role.

[0044] A further advantage of the invention is that the use of the heating element and the physical properties of the sapphire glass tube provide excellent transmission in the short-wave infrared range from 0.8 pm to 2.5 pm.

[0045] Another advantage is that the sapphire glass tube is highly resistant to chemical and aggressive media, has excellent optical, mechanical and thermal properties and therefore no restrictions or changes in the quality of the firing and / or sintering process result occur under permanent loads.

[0046] Finally, another advantage is that the exposure of the workpiece to short-wave infrared radiation (0.8 pm to 2.5 pm) and the resulting high penetration depth of the radiation into the workpiece results in significantly shorter firing and / or sintering times. This, in turn, significantly reduces energy consumption.

[0047] The invention is explained in more detail below using an exemplary embodiment and with reference to the drawings. In detail:

[0048] Figure 1 is a simplified view of the essential components of a firing and / or sintering furnace in exploded view and perspective and

[0049] Figure 2 is a side view of the heating element according to an embodiment.

[0050] The invention is described below using two heating elements in a dental furnace. However, the invention is not limited to use in dental furnaces. More or fewer than two heating elements 7 can also be used in a furnace.

[0051] 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 acting as a reflector 13. Furthermore, the housing 1 is closed with a lid and sealing ring 15. In the illustrated embodiment, two heating elements 7 are arranged in the combustion chamber 2. The two heating elements 7 are each inserted into the combustion chamber 1 through a through-opening with vacuum-tight screw connections 12 provided in the housing 1 and secured. The screw connections 12 surround and secure the respective heating element in a vacuum-tight manner when installed.

[0052] The glass tube 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 pressed gas-tight with the connection of the heating coil 17 with the molybdenum connecting line 11, which is led to the outside.

[0053] The design of the heating element is shown in Fig. 2. In the example shown, the sapphire glass tube 8 is joined at both ends in a gas-tight manner to a borosilicate intermediate tube piece 9, which in turn is further joined in a gas-tight manner to the pinched-off quartz glass tube 10. In the quartz glass tube 10, which is joined at the ends in a gas-tight manner, the heating coil 17, which extends through the borosilicate intermediate tube pieces 9, is connected via molybdenum intermediate pieces 18 to the outwardly leading connecting lines 11, which are pressed together in a gas-tight manner by means of the pinched-off part.

[0054] With the aid of a firing table 3 (Fig. 1 ) and a temperature sensor s, it is possible to introduce the dental ceramic to be fired and / or sintered (workpiece 4), 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

Patent claims Heating element for a furnace for firing and / or sintering workpieces, in particular workpieces made of dental ceramic materials, comprising a sapphire glass tube (8) and a heating coil (17) made of tungsten and / or molybdenum, which is arranged in the sapphire glass tube (8) and has connecting lines (11) leading to the outside, wherein the ends of the sapphire glass tube (8) are gas-tightly sealed by means of closures made of a heat- or heat-confirming material, in particular by means of quartz glass closures (10), and wherein the connecting lines (11) of the heating coil (17) are led to the outside through the sapphire glass tube closures (10). The heating element according to claim 1 is characterized in that the heating coil (17) emits electromagnetic radiation in the near-infrared range from 0.8 pm to 5 pm or from 0.8 pm to 2.5 pm and that the sapphire glass tube (8) is transparent to electromagnetic radiation in the range from 0.17 pm to 6 pm.Heating element according to claim 1 or 2, characterized in that an intermediate tube section (9) made of a heat-resistant material with a coefficient of thermal expansion that lies between that of sapphire glass and that of the sealing material of the sapphire glass tube (8), in particular made of borosilicate, is arranged gas-tight between the sapphire glass tube closures (10) and the ends of the sapphire glass tube (8). Heating element according to any one of claims 1 to 3, characterized in that the sapphire glass tube (8) and, if present, the intermediate tube sections (9) made of borosilicate are filled with a noble gas or other inert gas. Heating element according to any one of claims 1 to 4, characterized in that the heating element (7) emits electromagnetic radiation rapidly when operated with the specified current and furthermore does not cause any contamination of the workpiece (4) and is itself not contaminated by substances of the workpiece (4) or by substances produced during sintering and / or firing, such as vapors from coatings of the workpiece, such as glazes. Heating element according to any one of claims 1 to 5, characterized in that the heating coil (17) is made of tungsten and that the ends of the heating coil (17) are connected via molybdenum intermediate conductors to electrical 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).A furnace for firing and / or sintering workpieces, in particular workpieces made of dental ceramic materials, comprising a firing chamber (1) and at least one heating element (7) arranged in the firing chamber (1) according to any one of claims 1 to 5. A furnace according to claim 7, characterized in that at least one reflector element (13) is arranged in the firing chamber (1) to direct the electromagnetic radiation emitted by the heating element (7) towards the workpiece (4). A furnace according to claim 7 or 8, characterized in that the heating element (7) is partially surrounded by the reflector element (5). A furnace according to any one of claims 7 to 9, characterized in that the reflector element (13) has a semicircular cross-section and preferably extends over the entire length of an associated heating element (7).

11. Oven according to one of claims 7 to 10, characterized in that the chamber walls (6, 13, 14) forming in the combustion chamber (1) have high thermal insulation.

12. Oven according to one of claims 7 to 11, characterized by a receiving element (3) arranged in the combustion chamber (1) for receiving the workpiece (4), wherein the receiving element (6) 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 (4).

13. Oven according to claim 12, characterized in that the receiving element (6) comprises silicon carbide.

14. Oven according to one of claims 7 to 13, characterized in that a temperature measuring device (5) is provided in the combustion chamber (2) near the ceramic element (4) to be fired.

15. Oven according to one of claims 7 to 14, characterized by a temperature measuring device (5) that detects the temperature in the area of ​​the workpiece (4).

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