Heating element for 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, comprising a sapphire glass tube (8) for receiving a heating coil (17) within the sapphire glass tube (8). The sapphire glass tube (8) is hermetically connected to the borosilicate tube (14) and to the quartz glass tube (15). The end of the electrical connection is threaded in a gas-tight compression manner. According to the invention, the tube elements are made of sapphire (8) and are connected in a gas-tight manner by means of transition glass and glass solder in order to compensate for different coefficients of thermal expansion. The connecting elements (14, 15) are located outside the firing chamber (2) in order to ensure reliable operation even under thermal effects of up to 500 DEG C. Furthermore, in order to sinter / sinter the ceramic element (4) with short-wave infrared radiation in the wavelength range of 0.8 [mu] m to 2.5 [mu] m, the heating element (7) has stable optical, electrical and mechanical properties, so that a high-quality, permanently homogeneous firing and sintering result is ensured, and the firing and sintering time is significantly shortened. The surface of the heating element (7) does not change due to chemical effects or evaporation from firing the sintered element. Consequently, the sintered object and the firing chamber are not contaminated by the heating element (7). The heating element (7) can be used at an operating temperature of up to 1900 DEG C, thereby ensuring normal operation for a long period of time.

Description

Technical Field

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

[0002] Fields of application of the invention include the sintering or firing of workpieces that can be used in various industrial fields, for example gears and other components, especially in the automotive industry. Another focus of use of the invention is in the dental field. In this field, zirconium oxide and other dental ceramic compounds based on dental alloys are sintered and fired to metal-free dentures, in particular made of zircon and / or other ceramic materials. Background Art

[0003] In dental furnaces for firing and / or sintering veneer ceramics with or without enamel for glaze firing, resistance heating is known, wherein heating wires preferably made of Kanthal and incorporated in quartz tubes are used. However, such known heating elements can only be used at temperatures of up to 1200°C.

[0004] Other known dental furnaces are described in WO 2018 / 011061 A1 and WO 2020 / 088943 A1.

[0005] Zirconia (SiO 2 ) requires a temperature of 1650°C.

[0006] For commercially available dental furnaces for firing and / or sintering metal-free dental prostheses (e.g. zirconium oxide ceramics or similar materials), three different heating systems are known, namely

[0007] 1. Molybdenum disilicide heating system (MoSi 2 Heating system),

[0008] 2. A heating system using silicon carbide (SiC), and

[0009] 3. Use induction heating system

[0010] Item 1: Molybdenum disilicide:

[0011] Molybdenum disilicide is a dense cermet material consisting of molybdenum disilicide and an oxide portion (mainly a glass phase). The glass portion or the protective layer changes during the heating phase, leading to spalling and thus contamination of the object to be sintered (workpiece) and / or the firing chamber. These spallings are clearly visible in the form of small glass fragments and glass dust.

[0012] After some firing cycles, this requires a so-called clean firing at temperatures of more than 1400° C. for more than four hours without charge in the furnace, which implies considerable expenditure of time and energy.

[0013] The clean firing must be carried out several times without the object to be sintered until a uniform protective layer is seen again at the molybdenum disilicide heating element.

[0014] The dissolution of the oxide layer on the heating element leads to the formation of MoO 3 (Molybdenum(VI) oxide), also known as "harmful oxidation", instead leads to an undesirable discoloration of the restoration (workpiece) to a yellow-green color.

[0015] In order to protect the object to be sintered from contamination / discoloration, a sintering shell is used which covers the workpiece.

[0016] Furthermore, MoSi2 heating elements become very sensitive to breakage after a short period of use and at the same time they are subjected to stresses in the maximum use range in a temperature range of up to 1650°C and therefore often fail after a short period of use.

[0017] Item 2: Silicon Carbide:

[0018] The temperature resistance of silicon carbide heating rods is poor and requires very complex thyristor-controlled closed-loop control. In addition, care must be taken that only those rods with the same resistance and the same state of aging are connected. This results in a single defective heating rod in the system that cannot be replaced. In this case, the entire heating system must be replaced.

[0019] Silicon carbide heating elements are expensive and highly sensitive to breakage. Replacing them results in higher spare parts costs compared to other heating systems. As mentioned above, when one silicon carbide heating element fails, the entire system must be replaced because the connected heating elements must have the same resistance. Otherwise, other heating elements will fail in a very short time. Replacing all heating elements will result in higher costs.

[0020] Item 3: Induction:

[0021] It is known to operate dental furnaces by induction to sinter workpieces. Induction furnaces have induction coils. Induction coils are also called "inductors". They are usually water-cooled, which is a disadvantage if used in a dental laboratory or dental clinic. The inductor, through which current flows, generates a time-varying magnetic field, which results in controllable heating of the workpiece by eddy currents.

[0022] Since the materials used in the dental field are not electrically conductive, it is necessary to additionally use susceptors, i.e. elements that have the property of absorbing electromagnetic energy, converting it into heat and transferring it to the workpiece by convection.

[0023] Since the inductor must be well matched to the properties of the material to be treated in order to achieve the desired thermal behavior, the possible use of different non-conductive materials in dental technology is limited, especially with regard to the size of the workpiece.

[0024] The dimensions of the firing chamber in known induction furnaces are approximately a maximum of 3 kronor, ie the length must not exceed 38 mm and the height must not exceed 20 mm.

[0025] Due to the high interference emissions, extensive safety regulations need to be observed. For example, induction furnaces may not be used in patient environments.

[0026] Heat treatment of workpieces by induction is permitted only for a limited set of materials. Summary of the invention

[0027] The object of the present invention is to provide a heating element for a sintering furnace and / or a firing furnace which is distinguished by high efficiency and high temperature resistance.

[0028] This object is achieved by the present invention, which provides a heating element for a furnace for firing and / or sintering workpieces, in particular workpieces made of dental ceramic material, which heating element is equipped with:

[0029] Sapphire glass tube, and

[0030] A heating coil made of tungsten and / or molybdenum, arranged in a sapphire glass tube and having connecting leads leading out to the outside,

[0031] wherein the ends of the sapphire glass tube are closed in a gas-tight manner by means of a closure piece, which is made in particular of quartz glass, and

[0032] In this case, the connecting leads of the heating coil are led to the outside through a sealing member, in particular a sealing member made of quartz glass.

[0033] Similarly, the invention relates to a heating element, the heating coil of which is made of tungsten and the connecting leads of which contain molybdenum and are arranged in a sapphire glass tube. The material of the glass tube is synthetic sapphire, i.e. aluminum oxide with a purity of 100%.

[0034] The two ends of the sapphire glass are sealed in a gas-tight manner, in particular by quartz glass or another heat-resistant or heat-insulating material, wherein the connecting leads of the heating coil are passed through the two sealing parts in a gas-tight manner. In the following, for the sake of simplicity, the material used for these sealing parts is referred to as "quartz glass". A quartz glass tube is used, the end of which is remote from the sapphire glass tube and is "tightly squeezed". The material of this quartz glass tube is also synthetic and, unlike synthetic sapphire, can be processed in the softened state, i.e. a gas-tight closure is achieved by compression (squeezing) when the connecting leads are passed through.

[0035] The sapphire glass tube and the quartz glass tube are connected together. Preferably, in order to make the transition between the materials sapphire and quartz with different temperature coefficients better, a borosilicate intermediate piece is arranged between the two. The borosilicate tubular piece is also connected to the sapphire glass and quartz glass sealing pieces, that is, connected in an airtight manner. The corresponding connection technology is known in the prior art. Thus, for example, the connection can be achieved by glass solder or similar glass materials. By gradually adapting to the different thermal expansion coefficients, the resulting thermally induced length changes can be compensated. Thermal stress or mechanical stress and cracks within and between the individual glass materials sapphire, quartz and borosilicate are thereby avoided, and a 10 - 8 Vacuum tightness or leakage rate in mbar*l / s (millibar*liter / second).

[0036] In the state where the heating element is built into the furnace, it is convenient to arrange the quartz glass closure outside the firing chamber of the furnace if the intermediate tubular part of borosilicate or if the latter is not present. In addition, such intermediate tubular part and quartz glass closure can be cooled, thereby greatly reducing the occurrence of stresses.

[0037] The heating element area located outside the firing chamber is vacuum-tight in the temperature range up to 500°C and has a leakage rate of only 10 -8 mbar*l / s.

[0038] The melting temperatures of the material tungsten and the material molybdenum used for the heating coil and the supply line are above 2000° C. to 2500° C. Therefore, in this respect, these materials are suitable for use as heating coils for the heating element according to the invention. The heating coils are made of tungsten, wherein the coils are electrically connected at their ends via molybdenum intermediate pieces, each of which has an electrical conductor made of a common electrically conductive material, preferably a material with increased heat resistance.

[0039] According to an advantageous embodiment of the invention, it can be provided that the heating coil emits electromagnetic radiation in the near infrared range of 0.8 to 5 μm or 0.8 to 2.5 μm and that the sapphire glass tube is transparent to said electromagnetic radiation in the range of 0.17 to 6 μm.

[0040] As already mentioned above, for the adaptation of different temperature coefficients it is advantageous if an intermediate tubular piece made of borosilicate is arranged in a gas-tight manner between the quartz glass closure and the end of the sapphire glass tube, respectively.

[0041] In a further advantageous embodiment of the invention, provision is made for the sapphire glass tube and the borosilicate intermediate tube, if present, to be filled with a rare gas or other inert gas.

[0042] A further advantage of the heating element according to the invention can be seen in the fact that, when operated with an appropriate current, the heating element emits electromagnetic radiation with a fast response (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) and without any contamination of the workpiece, moreover, not even by the substance of the workpiece or substances generated thereby during the sintering and / or firing process, for example vapors from a coating on the workpiece, such as a glaze composition.

[0043] In one embodiment of the invention, the heating coil is made of tungsten, wherein the ends of the heating coil are connected to an electrical conductor made of a material other than molybdenum via an intermediate electrical conductor made of molybdenum, and / or wherein the molybdenum intermediate conductor extends through the closure of the sapphire glass tube.

[0044] The closures at the ends of the sapphire glass tubes are made of a heat-resistant material, whose coefficient of thermal expansion corresponds approximately to that of molybdenum. Conveniently, the molybdenum intermediate conductors extend through these closures. Quartz glass is particularly suitable here. Other combinations of materials 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 intermediate conductor. The intermediate conductor extends through the "extruded" part of the closure, i.e., it should have essentially the same coefficient of thermal expansion as the closure material, so that thermal stresses and cracks do not occur.

[0045] The heating element according to the invention can be used in a furnace according to the invention, said heating element being incorporated in its firing chamber, preferably in such a way that a quartz glass closure is arranged outside the firing chamber. Conveniently, the heating element is guided through relative openings in the wall of the firing chamber by means of screw connections or similar fixing elements. These passages in the wall of the firing chamber of the heating element are sealed in a gastight manner, so that a vacuum can be generated inside the firing chamber.

[0046] In an expedient embodiment of the invention, at least one reflecting element may be arranged in the emission chamber in order to direct the electromagnetic radiation emitted by the heating element towards the workpiece.

[0047] In the above-mentioned embodiments of the present invention, it can also be arranged that: the heating element is partially surrounded by the reflector element, and / or the cross-section of the reflector element is of semicircular design and preferably extends to the entire length of the relevant heating element, and / or the chamber wall formed in the firing chamber has high thermal insulation.

[0048] In a preferred embodiment of the invention, the furnace has a receiving element arranged in the firing chamber for receiving the workpiece, wherein the receiving element has a radiation absorbing material which in particular absorbs radiation and thus acts as a susceptor element which acts as a heat radiator and transfers heat energy to the workpiece.

[0049] In an expedient embodiment of the invention, the receiving element comprises silicon carbide.

[0050] In a further advantageous embodiment of the invention, the temperature measuring device is arranged in the firing chamber close to the ceramic component to be fired.

[0051] In an expedient embodiment of the invention, a temperature measuring device is provided which senses the temperature in the region of the workpiece.

[0052] With the heating element according to the invention, it is possible to operate a sintering furnace or a firing furnace at very high temperatures of at least 1900° C., wherein the furnace can be permanently operated in this temperature range, so that a long service life can be achieved. Therefore, in a furnace equipped in this way, general ceramic firing and sintering processes can be carried out at very high temperatures in atmospheric pressure or in a vacuum. Especially in the "firing and sintering" process in the temperature range of about 1600° C., high requirements are placed on the heating element. Due to the use of sapphire glass, the heating element of the invention meets the characteristics of reliability, long life, fast, clean, energy-saving, and providing high heat radiation in a short time. According to the invention, this is achieved by using a sapphire glass tube and a heating coil made of tungsten and molybdenum transition layers.

[0053] The physical properties of synthetic sapphire glass meet all the necessary requirements for resistance to corrosive acids and vapors, density (important for airtightness), hardness and compressive strength.

[0054] The temperatures required for technical and dental sintering processes are approximately 1600° C., which is far below the melting point of approximately 2050° C., which is critical for sapphire glass. Therefore, according to the present invention, the sapphire glass tube can withstand firing chamber temperatures of up to 1900° C.

[0055] Another relevant property of sapphire is its transmissivity, i.e. the transmittance of the heating coil to electromagnetic radiation in the range of 0.8 μm to 2.5 μm, which allows the firing and sintering processes to take place in an extremely short time. The radiation range with a wavelength below 2 μm has the property of a greater penetration depth into the workpiece, so that high-quality results can be obtained in an extremely short time. The thermal energy reaches the workpiece almost entirely by radiation; thermal convection is not relevant here.

[0056] Another advantage of the present invention can be seen in that the use of the heating element and the physical properties of the sapphire glass tube provide excellent transmission performance in the short-wave infrared range of 0.8 μm to 2.5 μm.

[0057] Another advantage is that sapphire glass tubes have excellent chemical and corrosion resistance and have excellent optical, mechanical and thermal properties, so that even in the presence of permanent stresses, the quality of the results of the firing and / or sintering process is not limited or varied.

[0058] Finally, an advantage that can also be pointed out is that the short-wave infrared radiation of 0.8 μm to 2.5 μm acting on the workpiece and the associated great penetration depth of the radiation into the workpiece allow significantly shorter firing and / or sintering times. This in turn significantly reduces energy consumption.

[0059] The present invention will be explained in more detail below by way of examples and with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 a simplified diagram showing the main components of a firing and / or sintering furnace in exploded view and perspective view; and

[0061] Figure 2 A side view of a heating element according to an example is shown. DETAILED DESCRIPTION

[0062] In the following, the invention is described by using two heating elements in a dental furnace. However, the invention is not limited to use in dental furnaces. Moreover, more or less than two heating elements 7 may be used in a furnace.

[0063] The dental furnace has a housing 1 with a firing chamber 2. The interior of the housing 1 is equipped with a high-temperature insulation 6 and is provided with further refractory insulation elements 13, 14 and an element serving as a reflector 13. Furthermore, the housing 1 is closed by a cover and a sealing ring 15.

[0064] In the embodiment shown, two heating elements 7 are arranged in the firing chamber 2. Each of the two heating elements 7 is pushed into the firing chamber 1 through a passage opening and fixed therein by means of a vacuum-tight screw connection 12 arranged in the housing 1. In the installed state, the screw connection 12 surrounds and fixes the respective heating element in a vacuum-tight manner.

[0065] The glass tube 9 made of synthetic borosilicate and the glass tube 10 made of synthetic quartz glass of the heating element 7 and their leads 11 for the operating voltage are located outside the firing chamber 1 so that the connection of the quartz tube 10, for example joined by glass solder, to the heating coil 17 and the molybdenum connecting leads 11 are compressed in a gas-tight manner and the molybdenum connecting leads 11 are led to the outside.

[0066] The design of the heating element is as follows Figure 2In the embodiment shown, the sapphire glass tube 8 is connected in a gas-tight manner at both ends to each borosilicate intermediate tubular member 9, which is further connected in a gas-tight manner to the extruded quartz glass tube 10. In the adapted glass tube 10 whose ends are connected in a gas-tight manner, the heating coil 17 extending through the borosilicate intermediate tubular member 9 is connected to the connecting lead 11 through the molybdenum intermediate member 18, and the connecting lead 11 is led to the outside, and the connecting lead 11 is compressed in a gas-tight manner by the extruded portion.

[0067] Use Burning Station 3 ( Figure 1 ) and a temperature sensor 5, the dental ceramic (workpiece 4) to be fired and / or sintered, which has been placed on the firing table 3, can be introduced into the firing chamber 2. The temperature of the firing chamber 2 is closed-loop controlled by the temperature sensor 5 and a corresponding electronic controller (not shown).

Claims

1. A heating element for a furnace for firing and / or sintering workpieces, in particular workpieces made of dental ceramic material, comprising: a sapphire glass tube (8), and a heating coil (17) made of tungsten and / or molybdenum, the heating coil (17) being arranged inside the sapphire glass tube (8) and having a connecting lead (11) leading to the outside, The end of the sapphire glass tube (8) is sealed in an airtight manner by a sealing piece made of a heat-resistant material, in particular by a quartz glass sealing piece (10), and The connecting lead wire (11) of the heating coil (17) is led to the outside through the sapphire glass tube sealing member (10).

2. The heating element according to claim 1, characterized in that The heating coil (17) emits electromagnetic radiation in the near infrared range of 0.8 μm to 5 μm, or 0.8 μm to 2.5 μm, and the sapphire glass tube (8) is transparent to electromagnetic radiation in the range of 0.17 μm to 6 μm.

3. The heating element according to claim 1 or 2, characterized in that The intermediate tubular member (9) is arranged in an airtight manner between the sapphire glass tube sealing member (10) and the end of the sapphire glass tube (8). The intermediate tubular member (9) is made of a heat-resistant material, and the thermal expansion coefficient of the heat-resistant material is between the thermal expansion coefficient of the sapphire glass and the thermal expansion coefficient of the sealing material of the sapphire glass tube (8). The intermediate tubular member (9) is particularly made of borosilicate.

4. The heating element according to any one of claims 1 to 3, characterized in that The sapphire glass tube (8) and the intermediate tubular piece (9) of borosilicate (if any) are filled with rare gas or other inert gas.

5. The heating element according to any one of claims 1 to 4, characterized in that The heating element (7) responds quickly to emit electromagnetic radiation when operated under an appropriate current and does not cause contamination to the workpiece (4). In addition, the heating element (7) is not even contaminated by the substance of the workpiece (4) or substances generated therefrom during sintering and / or firing, such as vapors from a coating on the workpiece, such as a glaze composition.

6. A heating element according to any one of claims 1 to 5, characterised in that The heating coil (17) is made of tungsten, and the ends of the heating coil (17) are connected to an electrical conductor made of a material other than molybdenum via an intermediate electrical conductor made of molybdenum, and / or the molybdenum intermediate conductor (11) extends through the closure of the sapphire glass tube (8).

7. A furnace for firing and / or sintering workpieces, in particular workpieces made of dental ceramic material, comprising: a firing chamber (1), and At least one heating element (7) according to any one of claims 1 to 5, said heating element (7) being arranged in said firing chamber (1).

8. The furnace according to claim 7, characterized in that At least one reflective element (13) is arranged in the firing chamber (1) in order to direct the electromagnetic radiation emitted by the heating element (7) towards the workpiece (4).

9. The furnace according to claim 7 or 8, characterized in that: The heating element (7) is partially surrounded by the reflector element (5).

10. The furnace according to any one of claims 7 to 9, characterized in that The reflector element (13) is of semicircular design in cross section and preferably extends over the entire length of the associated heating element (7).

11. The furnace according to any one of claims 7 to 10, characterized in that The chamber walls (6, 13, 14) formed in the firing chamber (1) have high thermal insulation properties.

12. The furnace according to any one of claims 7 to 11, characterized in that A receiving element (3) for receiving the workpiece (4) is arranged in the firing chamber (1), wherein the receiving element (6) has a radiation absorbing material which in particular absorbs radiation and thus acts as a susceptor element which, as a heat radiator, transfers heat energy to the workpiece (4).

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

14. A furnace according to any one of claims 7 to 13, characterized in that A temperature measuring device (5) is provided in the firing chamber (2) close to the ceramic element (4) to be fired.

15. The furnace according to any one of claims 7 to 14, characterized in that The temperature measuring device (5) senses the temperature in the region of the workpiece (4).

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