A heating element for a furnace for firing or sintering a workpiece, and a furnace having at least one such heating element

Abstract

The present invention relates to a heating element (7) for a furnace, the heating element comprising a sapphire glass tube (8) for receiving a heating coil (17) inside the sapphire glass tube (8). The sapphire glass tube (8) is hermetically connected to a borosilicate tube (14) and a fused silica tube (15). The end electrical connections pass in a hermetically compressed manner. According to the invention, the tube elements are made of sapphire (8) and are hermetically joined by a transition glass and a glass solder to compensate for different coefficients of thermal expansion. The joining elements (14, 15) are arranged outside the firing chamber (2) so as to guarantee operational reliability in the case of thermal influences up to 500 °C. Furthermore, in order to sinter / fire the ceramic element (4) using short-wave infrared radiation in the range from 0.8 μm to 2.5 μm, the heating element (7) has stable optical, electrical and mechanical properties, and thus, with a significantly shortened firing-sintering time, high-quality, permanently uniform firing and sintering results are guaranteed. No changes are caused on the surface of the heating element (7) by chemical influences or evaporation from the sintered elements. No contamination of the sintered body and the firing chamber is caused by the heating element (7). The heating element (7) can be used at operating temperatures up to 1900 °C, thus ensuring long-term use.

Description

Technical Field

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

[0002] The fields of application of the present invention include, in particular, sintering or firing of workpieces that can be used in a variety of industrial fields such as gears and other elements used in the automotive industry. Another focus of the use of the present invention is found in the dental field. In this field, other dental ceramic compounds based on zirconium oxide and dental alloys are sintered and fired from zircon and / or other ceramic materials as metal-free dentures.

Background Art

[0003] In dental furnaces used for firing and / or sintering veneer ceramics, with or without glaze paint for glaze firing, electric resistance heating using a heating wire preferably made of Kanthal and incorporated in a quartz tube is known. However, such known heating elements can only be used up to a maximum of 1200°C.

[0004] Other known dental furnaces are described in International Publication No. WO 2018 / 011061 and International Publication No. WO 2020 / 088943.

[0005] In recent years, 1650°C has been required for sintering zirconium oxide (SiO2) as used in dental restorations.

[0006] For commercially available dental furnaces for firing and / or sintering metal-free dentures such as zirconium oxide ceramics or similar materials, three different heating systems are known. That is, 1. Heating system by molybdenum disilicide (MoSi2 heating system) 2. Heating system using silicon carbide (SiC), and 3. Induction heating system

[0007] Item 1. Molybdenum disilicide: Molybdenum disilicide is a high-density metal ceramic material composed of molybdenum disilicide and oxide pieces that are mainly in a glassy phase. This glassy part or this protective layer changes and peels off during the heating stage, resulting in contamination of the object (workpiece) to be sintered and / or the firing compartment. The peeling is clearly seen in the form of small glass fragments and glass dust.

[0008] After several firing cycles, a so-called cleaning firing is required, which is carried out at a temperature exceeding 1400 °C for more than 4 hours without putting materials into the furnace, consuming a considerable amount of time and energy.

[0009] The cleaning firing needs to be carried out several times without the object to be sintered until a uniform protective layer is seen again in the molybdenum disilicide heating element.

[0010] The dissolution of oxides in the heating element leads to the formation of MoO3 (molybdenum(VI) oxide), also called "pest oxidation", resulting in an undesirable discoloration of the repair object (workpiece) to greenish-yellow.

[0011] To protect the sintering object from contamination / discoloration, it is necessary to use a sintering shell that covers the workpiece.

[0012] Also, since the molybdenum disilicide heating element is stressed up to its maximum use range in the temperature range up to 1650 °C, it becomes very sensitive to breakage when the usage time is short, and usually malfunctions occur when the usage time is short.

[0013] Item 2. Silicon carbide: Since silicon carbide heating rods have unfavorable temperature resistance performance, very complex thyristor-controlled closed-loop control is required. Furthermore, it must be noted that only rods with the same electrical resistance and the same aging degradation state are connected. This results in the fact that a single defective heating rod within the system cannot be replaced. In such a case, the entire heating system must be replaced.

[0014] Silicon carbide heating rods are expensive and very sensitive to breakage. When replacing them, a high spare cost is incurred compared to other heating systems. As mentioned above, if one silicon carbide heating element fails, the connected heating elements must have the same electrical resistance, so the entire system must be replaced. Otherwise, other heating elements will fail within a very short period. Replacing all heating elements incurs even higher costs.

[0015] Item 3. Induction: It is known to operate a dental furnace by induction for sintering workpieces. Induction furnaces have induction coils. Induction coils are also called "inductors". They are usually water-cooled, which is disadvantageous when used in dental laboratories or dental clinics. The inductor through which current flows generates a time-varying magnetic field, and as a result, the workpiece is controllably heated by eddy currents.

[0016] Since the materials used in the dental field are non-conductive, a susceptor, that is, an element having the property of absorbing electromagnetic energy, converting it into heat, and transferring it to the workpiece by convection, must be additionally used.

[0017] Since the inductor must be well adapted to the properties of the material being processed to achieve the desired thermal behavior, the possible use of different non-conductive materials in dental technology is limited, especially with respect to the size of the workpiece.

[0018] The size of the firing compartment in a known induction furnace must not exceed the maximum of about three crowns, i.e., a length of 38 mm and a height of 20 mm.

[0019] Due to high interference radiation, extensive safety regulations should be complied with. For example, induction furnaces must not be used in the patient's environment.

[0020] Only a limited set of materials is permitted for the heat treatment of workpieces by induction. Summary of the Invention Problems to be Solved by the Invention

[0021] An object of the present invention is to provide a heating element for a sintering and / or firing furnace characterized by high effectiveness and high temperature resistance. Means for Solving the Problems

[0022] This object is achieved by the present invention, which a sapphire glass tube, and a heating coil made of tungsten and / or molybdenum provided in the sapphire glass tube and having connection lead wires led to the outside, and provides a heating element for a furnace for firing and / or sintering a workpiece, particularly a workpiece made of a dental ceramic material, wherein the end of the sapphire glass tube is hermetically closed by a closure made of particularly quartz glass, and the connection lead wires of the heating coil are led to the outside through the closure, particularly a closure made of quartz glass.

[0023]

[0024]

[0024] At both of its ends, the sapphire glass is hermetically sealed, in particular by fused silica or another heat-resistant material, and the connecting leads of the heating coils pass hermetically through the two closures. In the following, for the sake of brevity, the material of these closures will be referred to as "fused silica". At the end opposite the sapphire glass tube, a piece of fused silica tube "tightly squeezed" is used. The material of this piece of fused silica tube is also different from the synthetic sapphire material and can be processed in a softened state, i.e., it is synthetic and allows for hermetic sealing by compression (squeezing) when the connecting lead passes through it.

[0025] The sapphire glass tube and the piece of fused silica tube are joined together. Preferably, for a better transition between materials with different temperature coefficients, sapphire and fused silica, a borosilicate intermediate piece is arranged between the two. This borosilicate tubular piece is also joined to, i.e., hermetically connected to, the sapphire glass and fused silica closures. The corresponding joining techniques are known in the prior art. Thereby, for example, it can be joined by a glass material such as glass solder. By gradually adapting to different coefficients of thermal expansion, the resulting change in length caused by the heat generated thereby can be compensated. Thereby, thermal or mechanical stresses and cracks in and between the individual glass materials, sapphire, fused silica, and borosilicate, are avoided, and a vacuum tightness or leak rate of 10 - 8 mbar*l / s is guaranteed.

[0026] In a state where a heating element is incorporated in the furnace, it is convenient if the borosilicate intermediate tubular piece or, in the case where there is no borosilicate intermediate tubular piece, the fused silica closure is provided outside the firing chamber of the furnace. Furthermore, it is possible to cool such an intermediate tubular piece and the fused silica closure, as a result of which the generation of stress is significantly reduced.

[0027] The zone of the heating element provided outside the firing chamber is vacuum tight within a temperature range up to 500 °C, and the leak rate is only 10 - 8 mbar*l / s.

[0028] The melting points of tungsten and molybdenum used in the heating coil and the supply line are 2000°C to 2500°C. Therefore, in this regard, these materials are suitable for use as the heating coil of the heating element according to the present invention. The heating coil is made of tungsten, and each of the ends of the coil is electrically connected via a molybdenum intermediate piece to a common conductive material, preferably a conductor made of a material with enhanced heat resistance.

[0029] According to an advantageous embodiment of the present invention, the heating coil can emit 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 can be made transparent to the electromagnetic radiation in the range of 0.17 μm to 6 μm.

[0030] As described above, with regard to the adaptation of different temperature coefficients, it is advantageous for intermediate tubular pieces made of borosilicate to be provided hermetically between the quartz glass closure and the ends of the sapphire glass tube, respectively.

[0031] In another advantageous embodiment of the present invention, it is provided that the sapphire glass tube and, if any, the intermediate tubular piece of borosilicate are filled with a rare gas or other inert gas.

[0032] A further advantage of the heating element according to the present invention is that when operated with an appropriate current, the heating element emits electromagnetic radiation with a rapid response (the time from the "low temperature" state of the heating element to reaching its radiation maximum is preferably several seconds, particularly less than 10 seconds, or less than 5 seconds, or less than 3 seconds), and furthermore, no contamination occurs on the workpiece, and it is also found that the workpiece is not contaminated even by vapors from the coating on the workpiece, such as the glaze composition, or substances generated during sintering and / or firing.

[0033] In one embodiment of the present invention, the heating coil is made of tungsten, and the end of the heating coil is connected to a conductor made of a material other than molybdenum via an intermediate conductor made of molybdenum, and / or the molybdenum intermediate conductor extends through the closed portion of the sapphire glass tube.

[0034] The closed portion at the end of the sapphire glass tube is made of a heat-resistant material whose coefficient of thermal expansion substantially corresponds to that of molybdenum. It is convenient for the molybdenum intermediate conductor to extend through these closed portions. Here, quartz glass is particularly suitable. Other combinations of materials are also conceivable, and thus the present invention is not limited to quartz glass as the closing material of the sapphire glass tube and molybdenum as the intermediate conductor. The intermediate conductor extends through the "squeezed" portion of the closed portion, that is, it should have substantially the same coefficient of thermal expansion as the closed portion material so that thermal stress and cracks do not occur.

[0035] The heating element according to the present invention can be used in a furnace according to the present invention, in which the heating element is incorporated such that, preferably, a quartz glass closed portion is provided outside the firing chamber. Conveniently, the heating element is guided through an opening on the opposite side of the wall of the firing chamber via a screw connection or a similar fixing element. These passages in the wall of the firing chamber for the heating element are hermetically sealed, and as a result, a vacuum can be generated inside the firing chamber.

[0036] In a convenient embodiment of the present invention, at least one reflecting element can be provided in the firing chamber in order to direct the electromagnetic radiation emitted by the heating element towards the workpiece.

[0037] In the above-described embodiment of the present invention, it can further be provided that the heating element is partially surrounded by the reflecting element, and / or the reflecting element has a design with a semi-circular cross-section and preferably extends over the entire length of the associated heating element, and / or the chamber wall formed in the firing chamber has high heat insulation properties.

[0038] In a preferred embodiment of the present invention, the furnace has a receiving element provided in the firing chamber for receiving the workpiece, the receiving element having a radiation-absorbing material that particularly absorbs radiation and thus acts as a susceptor element that is a heat radiator and transfers thermal energy to the workpiece.

[0039] In a convenient embodiment of the present invention, the receiving element contains silicon carbide.

[0040] In another advantageous embodiment of the present invention, it is provided that temperature measuring means are provided in the firing chamber near the ceramic element to be fired.

[0041] In a convenient embodiment of the present invention, temperature measuring means for sensing the temperature within the zone of the workpiece are provided.

Advantages of the Invention

[0042] Using the heating element according to the present invention, it is possible to operate a sintering furnace or a firing furnace at an extremely high temperature of at least 1900 °C. In this case, the furnace can be used in permanent operation within this temperature range, and as a result, a long service life can be achieved. Thus, with a furnace equipped in this way, both general ceramic firing and sintering processes can be carried out at extremely high temperatures under atmospheric pressure or in a vacuum. In particular, in the "firing by sintering" process in the temperature range of about 1600 °C, high requirements are imposed on the heating element. Due to the use of sapphire glass, the heating element according to the present invention has the characteristics of reliability, long service life, rapidity, cleanliness, energy saving, and high heat dissipation in a short time. According to the present invention, this is achieved by using a sapphire glass tube and a heating coil composed of tungsten and molybdenum transitions.

[0043] The physical properties of synthetic sapphire glass satisfy all the necessary requirements imposed on resistance to aggressive acids and vapors, density (important for airtightness), hardness, and compressive strength.

[0044] The temperatures required for the technical and dental sintering processes of about 1600 °C are significantly lower than the melting point of about 2050 °C, which is important for sapphire glass. Therefore, the sapphire glass tube can withstand firing chamber temperatures up to 1900 °C as provided according to the present invention.

[0045] Another relevant property of sapphire is its transmittance, i.e., its transparency to the electromagnetic radiation of the heating coil within the range of 0.8 μm to 2.5 μm. Therefore, the firing and sintering processes can be carried out in an extremely short time. The radiation range within the wavelength range of less than 2 μm has the characteristic that the penetration depth into the workpiece is greater, and as a result, high-quality results can be achieved in an extremely short time. Thermal energy reaches the workpiece substantially only by radiation, and heat convection is irrelevant.

[0046] Another advantage of the present invention can be seen in the fact that the use of the heating element and the physical properties of the sapphire glass tube provide excellent transmittance in the short-wave infrared range of 0.8 μm to 2.5 μm.

[0047] It is further advantageous that the sapphire glass tube has excellent resistance to chemical and aggressive media and has excellent optical, mechanical, and thermal properties, so that no limitations or variations in the quality of the results of the firing and / or sintering processes are seen even with permanent stress.

[0048] Finally, it can also be noted as an advantage that by applying short-wave infrared rays of 0.8 μm to 2.5 μm to the workpiece and increasing the penetration depth of the radiation into the workpiece related thereto, an essentially shorter firing time and / or sintering time can be achieved. Thereby, the energy consumption is significantly reduced.

Brief Description of the Drawings

[0049] Hereinafter, the present invention will be described in more detail with reference to the drawings using examples. Specifically:

[0050]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0051] In the following, the present invention is described by using two heating elements in one dental furnace. However, the present invention is not limited to use in a dental furnace. Also, more than two or fewer heating elements 7 can be used in one furnace.

[0052] The dental furnace has a housing 1 having a firing chamber 2. The interior of the housing 1 is provided with a high-temperature heat insulating material 6, further refractory heat insulating materials 13, 14, and an element functioning as a reflector 13. Also, the housing 1 is closed by a lid and a seal ring 15.

[0053] In the illustrated embodiment, two heating elements 7 are provided 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 a vacuum-tight screw connection portion 12 provided in the housing 1. In the attached state, the screw connection portion 12 surrounds and fixes the corresponding heating element in a vacuum-tight manner.

[0054] The glass tube 9 made of synthetic borosilicate glass and the glass tube 10 made of synthetic quartz glass of the heating element 7 having lead wires 11 for operating voltage are outside the firing chamber 1. As a result, for example, the quartz tube 10 joined by glass solder is hermetically compressed by the connection of the connection portion of the heating coil 17 guided to the outside and the molybdenum connection lead wire 11.

[0055] The design of the heating element is shown in FIG. 2. In the illustrated embodiment, sapphire glass tubes 8 are hermetically joined at both ends to respective borosilicate intermediate tubular pieces 9, which are in turn hermetically joined to the said squeezed quartz glass tube 10. In the quartz glass tube 10 hermetically joined at the ends, a heating coil 17 extending through the borosilicate intermediate tubular piece 9 is connected via a molybdenum intermediate piece 18 to a connection lead 11 that leads to the outside and is hermetically compressed by the squeezing portion.

[0056] Using the firing stage 3 (FIG. 1) and the temperature sensor 5, dental ceramics (workpiece 4) to be fired and / or sintered placed on the firing stage 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 a workpiece, Sapphire glass tube and The device comprises a heating coil made of tungsten and / or molybdenum, which is provided inside the sapphire glass tube and has connecting leads that lead to the outside, A heating element in which the end of the sapphire glass tube is hermetically sealed by a closure made of a heat-resistant material, and the connecting lead of the heating coil is guided to the outside through the closure of the sapphire glass tube.

2. The heating element according to claim 1, characterized in that the heating coil 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 is transparent to the 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 an intermediate tubular piece made of a heat-resistant material having a coefficient of thermal expansion between the sapphire glass and the closure material of the sapphire glass tube is airtightly provided between the closure portion of the sapphire glass tube and the end of the sapphire glass tube.

4. The heating element according to claim 1 or 2, characterized in that the sapphire glass tube is filled with a noble gas or other inert gas.

5. The heating element according to claim 1 or 2, characterized in that when the heating element is operated with an appropriate current, it emits electromagnetic radiation in a rapid response, preventing contamination of the workpiece, and is not contaminated by the material of the workpiece or by the material generated by the workpiece during sintering and / or firing.

6. The heating element according to claim 1 or 2, characterized in that the heating coil is made of tungsten, the end of the heating coil is connected to a conductor made of a material other than molybdenum via an intermediate conductor made of molybdenum, and / or the intermediate conductor extends through the closed portion of the sapphire glass tube.

7. Firing chamber and A furnace for firing and / or sintering a workpiece, comprising at least one heating element according to claim 1, provided within the firing chamber.

8. The furnace according to claim 7, characterized in that at least one reflective element is provided in the firing chamber to direct electromagnetic radiation emitted by the heating element toward the workpiece.

9. The furnace according to claim 8, characterized in that the heating element is partially surrounded by the reflecting element.

10. The furnace according to claim 8 or 9, characterized in that the reflective element has a semicircular cross-section and extends over the entire length of the associated heating element.

11. The furnace according to claim 7 or 8, characterized in that the chamber wall formed in the firing chamber has high thermal insulation properties.

12. The furnace according to claim 7 or 8, comprising a receiving element provided in the firing chamber for receiving the workpiece, wherein the receiving element has a radiation-absorbing material that absorbs radiation and acts as a susceptor element which is a thermal radiator, and transmits thermal energy to the workpiece.

13. The furnace according to claim 12, characterized in that the receiving element contains silicon carbide.

14. The furnace according to claim 7 or 8, characterized in that a temperature measuring means is provided in the firing chamber near the workpiece to be fired.

15. The furnace according to claim 7 or 8, further comprising a temperature measuring means for sensing the temperature within the zone of the workpiece.

Images