WO2009020378A2

WO2009020378A2 - Microwave sintering furnace and method for sintering artificial tooth using the same

Info

Publication number: WO2009020378A2
Application number: PCT/KR2008/004669
Authority: WO
Inventors: Byung Kwan Kim; Do Hyun Kim; Eun A Park
Original assignee: Byung Kwan Kim; Do Hyun Kim; Eun A Park
Priority date: 2007-08-09
Filing date: 2008-08-11
Publication date: 2009-02-12
Also published as: WO2009020378A3; EP2182881A4; EP2182881A2

Abstract

A method of sintering artificial teeth using a microwave sintering furnace, the method comprising the steps of: (1) providing first and second insulation material frames made of a ceramic material, at least one of which has formed therein a workpiece-receiving space for inserting a workpiece! (2) placing a first heating element in the workpiece-receiving space and placing artificial tooth cores around the first heating element; (3) combining the first and second insulation material frames with each other to form an assembly of insulation material frames; (4) placing the assembly of the insulation material frames in a microwave oven and operating the microwave oven for less than 4 hours to heat the artificial tooth cores.

Description

[DESCRIPTION] [Invention Title]

MICROWAVE SINTERING FURNACE AND METHOD FOR SINTERING ARTIFICIAL TOOTH USING THE SAME [Technical Field]

<i> The present invention relates to a microwave sintering furnace for sintering ceramic or ceramic composite materials and a method of sintering using the same. More particularly, the present invention relates to a low- power-consuming, inexpensive and stable microwave sintering furnace, which dose not cause a flame reaction, overcomes temperature variation, can significantly reduce the sintering time of a product to be sintered and can be used in dental clinics and the like, and the present invention relates to a method of sintering artificial teeth, made of a ceramic or ceramic composite material, using the microwave sintering furnace in a sample manner in dental clinics and the like. [Background Art]

<2> Generally, in microwave heating, particles of a material are sintered using high-frequency electric power obtained by an oscillatory circuit, when the particles oscillate by high-frequency microwaves generated in a vacuum tube called "a magnetron". However, in the sintering process, a flame reaction generally occurs due to an abrupt increase in temperature in order to solidify a product to be sintered, leading to severe temperature variations, and due to problems associated with said temperature variation, the sintering process is generally carried out for at least 8 hours. For this reason, efforts have been continued to reduce such temperature variations and sintering time in the microwave sintering furnace.

<3> Generally, microwave sintering refers to a heat treatment method in which particles are bound to each other to form a solid structure by mass transfer occurring at the atomic level. Specifically, when a material is heat-treated at a temperature lower than the melting point thereof, the material particles are solidified through the binding therebetween. When the particles are heated, mass transfer occurs in the direction of reducing surface energy, that is, the direction of reducing surface area, and thus the particles bind to each other. Accordingly, as industries employing the microwave sintering furnace have developed, the importance thereof has been considerably recognized, and the applications thereof in various fields also have become more diverse.

<4> However, in prior microwave sintering furnaces, runaway reactions (flame reactions) often occur due to the internal environment of the sintering furnaces, thus causing cracks and tilting in the product. For this reason, a thermocouple is damaged and, in addition to, because a rise and drop in temperature caused by the magnetron very rapidly occur, the use-value of the product is not maximized. Moreover, non-uniform temperature variations occur due to microwaves which are focused on only one site, and a significant amount of time is required for sintering, because the sintering process is carried out at a slow rate while reducing such temperature variation. To perform the sintering process while reducing the temperature variation, a time of at least 8 hours is generally required. As methods for reducing the temperature variation and sintering time, microwave heat sources in 4-8 directions were used in the prior art, but such microwave heat sources in various directions have problems in that sintering furnaces manufactured from such microwave heat sources are expensive.

<5> Generally, microwave sintering furnaces have a significantly short sintering time and a low electric power consumption compared to those of electric furnaces or gas furnaces, thus providing advantages in terms of cost or time, and achieve compact sintering. Despite such advantages, the microwave sintering furnaces have not yet been widely used in various fields, various problems as described above are not yet solved. Namely, due to the above-described problems, including runaway reaction (flame reaction) phenomena, temperature variations and heating time, the mechanical and electrical properties of the sintering furnaces are deteriorated, thus reducing the reliability of the sintering furnaces. In order to solve such problems, there has been a need to develop a heating element (referring to a container with which microwaves is brought into contact to generate heat) suitable for use in the sintering furnace and to provide the developed heating element to the sintering furnace in an inexpensive and stable manner, and efforts to develop sintering furnaces suitable for the heating element have been continued.

<6> Despite such efforts, the above-described shortcomings were not completely solved, and thus the desire of consumers was not satisfied.

<7> Namely, in the prior sintering furnaces, when the internal temperature of the sintering furnaces is not slowly elevated to 1500 °C , a temperature for solidifying the product, but is rapidly elevated, non-uniform heating occurs, thus causing a flame reaction, leading to cracks or tilting. For this reason, the completed product may not be uniform, or incomplete sintering may occur. In order to eliminate such flame reactions, cracks and tilting, the temperature must generally be elevated slowly to 1500 ⁰C over about 8 hours. However, in this case, an excessively large amount of time is required for heat treatment, leading to an unnecessary increase in the heat- treatment cost and equipment.

<8> Meanwhile, ceramic or ceramic composite materials which are used to sinter artificial teeth are permeable to electromagnetic waves, electromagnetic waves are absorbed into the materials, and the absorbed energy is converted into thermal energy, such that the materials can be sintered densely.

<9> Generally, sintering refers to a heat treatment method in which particles are bound to each other to form a solid structure by mass transfer occurring at the atomic level, and the use of sintering in various industrial fields is also currently increasing. Accordingly, methods for developing materials having excellent characteristics and properties, and methods for sintering materials into solid structures having higher strength have been continually studied. It is known to use high-frequency microwaves for good sintering. However, sintering furnaces for generating such high-frequency microwaves are disadvantageous in that they are significantly expensive and also large in size.

<io> Due to the limitation of places for the use of the sintering furnaces together with such reasons, the sintering furnaces have not been frequently used. Accordingly, there has been an urgent need to develop a method which is inexpensive and, at the same time, can perform a sintering process in an easy manner regardless of places. Namely, because the sintering furnaces cannot be generally used due to high equipment and manufacture costs, artificial teeth cannot be manufactured in general dental clinics and are order-made, thus increasing the manufacture cost, leading to an increase in expenses of consumers.

<π> A conventional sintering method uses a phenomenon in which particles of any material bind to each other, when the material is heat-treated at a temperature lower than the melting point thereof. In this case, the binding force and strength of the particles differ depending on the sintering temperature and the kind of material. An importance for improving the sintering method has been significantly recognized in various fields, and the applications of the sintering method have become more diverse.

<i2> However, in the prior sintering method, as described in Korean Patent Application No. 2007-79938, when the internal temperature of the sintering furnace is not slowly elevated to 1500 °C , a temperature for solidifying a product to be sintered, but is rapidly elevated, runaway phenomena occur during sintering, thus causing cracks and tilting in the sintered product, leading to damage to a thermocouple. In addition, because the rise and drop in temperature caused by the magnetron very rapidly occur, the sintering effect is not maximized, and due to microwaves which are concentrated on only one site, non-uniform temperature variations occur.

<13> Moreover, a time of at least 8 hours is required to achieve sintering while reducing such a temperature variation, and thus labor cost is increased, leading to an increase in the sintering cost.

<14> As described above, because the sintering furnace capable of sintering teeth must be used to sinter artificial teeth, the artificial teeth have been professionally produced in processing factories provided with sintering equipment. However, to cast a mold in a dental clinic and sent the cast mold to a processing factory to make an artificial tooth, various processes must be carried out, and thus a time of at least one week is generally required to manufacture the artificial tooth.

<15> Accordingly, it has been needed to implant artificial teeth into consumers, who require the artificial teeth, in a more rapid manner. The reason why it takes at least one week to order and manufacture the artificial tooth as described above is thought to be because molds is cast in several dental clinics and sent to a processing factory, and artificial teeth are manufactured and sintered through various processes according to the orders sent from the dental clinics.

<16> However, it is considered that, if many dental clinics can cast and process molds therein, traffic expenses, labor cost, production cost and time will be significantly reduced, thus making the present invention. [Disclosure] [Technical Problem]

<17> Accordingly, the present invention has been made to solve the above- described problems occurring in the prior art, and it is an object of the present invention to provide a sintering furnace, which does not cause a flame reaction until it reaches a significantly high temperature, and can reduce temperature variations while reducing the sintering time.

<18> Another object of the present invention is to provide a microwave oven- type magnetron system, which can be manufactured at a low cost by combining some sintering elements to a domestic microwave oven which is commercially widely available, and to provide a method for sintering artificial teeth, which allows artificial teeth to be sintered using the microwave sintering furnace in dental clinics in an easy and quick manner without using separate electrical devices.

<19> Still another object of the present invention is to provide a method for sintering artificial teeth, in which the sintering temperature can be controlled within a specific time by controlling at least one variable selected from among the thickness of an insulation material, the size of a workpiece-receiving space, the surface area and weight of a heating element, and the distance between the insulation material and the heating element. [Technical Solution]

<20> To achieve the above objects, according to one aspect of the present invention, there is provided a method for sintering artificial teeth, which comprises filling an insulation material in a space other than a workpiece- receiving space in a domestic microwave oven, and placing a heating element in the workpiece-receiving space, such that artificial teeth can be easily sintered. Namely, the present invention has an advantage in that, because a dental mold can be cast and sintered in a short time using a magnetron system, the production cost and production time can be reduced.

<2i> In other words, the present invention provides a method of easily sintering artificial teeth, which comprises filling an insulation material in a magnetron system so as to be apart from the inner wall of themagnetron system, placing a heating element, having a diameter of about 9 nm and a height of about 5 mm, in the central space of the magnetron system that is a workpiece-receiving space, placing a crucible in the first heating element, and placing a product in the crucible. The method is characterized in that heat can be blocked by spraying thermal insulation powder is sprayed so as to prevent heat from being transferred to the insulation material, and the internal temperature of the sintering furnace can be elevated to the sintering temperature by controlling the sizes of the insulation material, the first heating element and the crucible and the workpiece-receiving space.

<22> According to second aspect of the present invention, there is provided a microwave sintering furnace, which is primarily heated by a heating element, manufactured to have a porosity of less than 27% by adding at least one selected from mullite, alumina and zirconia to silicon carbide that is a main component, and in which, as shown in FIGS. 5, 6 and the like, a groove is formed in the first heating element to enlarge the cross-sectional area of the first heating element so as to increase the radiation of heat from the heating element, and no flame reaction, cracking or tilting phenomena occur even when the temperature is rapidly elevated, that is, the temperature is rapidly elevated to 1500-1600 "C within 2 hours during sintering, because microwaves pass through the grooves of the heating element to come into contact with other surfaces.

<23> Namely, in the prior art, microwaves are concentrated on only one portion during sintering, and thus a time of at least 8 hours is required to heat the product to the sintering temperature, whereas, in the microwave sintering furnace of the present invention, microwaves pass through the heating element (a kind of heater for generating heat) and a plurality of grooves so as to emit the highest possible heat, microwaves passed through the grooves come in direct contact with the surface of the heating element to make it possible to rapidly increase the temperature throughout the inside of the sintering furnace, thus making it possible to sinter the product rapidly.

<24> Also, the applicant has found through a few tens of experiments that, when the central space of the sintering furnace (in which the first heating element is to be placed) other than a refractory material made of alumina, silicon dioxide or zirconia so as to have low thermal conductivity has a size of less than 20 cm (W) x 20 cm (L) x 20 cm (H), preferably less than 12 cm (W) x 12 cm (L) x 6 cm (H), and most preferably less than 10 cm (W) x 10 cm (L) x 6 cm (H), the rise in temperature to 1600 ^°C was achieved within the shortest time.

<25> According to a third aspect of the present invention, there is provided a microwave oven-type microwave sintering furnace, wherein a first insulation material frame has a plate-shaped structure having a workpiece-receiving space formed in the upper surface thereof, and a second insulation material frame is made of a plate-shaped member which can be combined closely with the first insulation material frame.

<26> Also, a second heating element may be disposed in a first heating element, and artificial tooth cores may be placed in the second heating element. The second heating element may be made of a thermally conductive ceramic material. The second heating element may have a ball shape, a plate shape, a column shape or a number of irregular granular shapes, and may also have a combination of two or more of these shapes.

<27> Herein, the first heating element is preferably placed apart from the first or second insulation material frame, such that it can be prevented from being brought into contact with the first or second insulation frame to damage the first or second insulation material frame. In one embodiment, insulation material powder is sprayed onto the portion of the first or second insulation material frame with which the first heating element can be brought into direct contact, and then the first and second insulation material frames are combined with each other.

<28> The first heating element is preferably made of a ceramic composite material, and particularly preferably a material based on silicon carbide (SiC). The second heating element may be made of a thermally conductive ceramic material. The first and second heating elements may have a ball shape, a plate shape, a column shape or a number of irregular granular shapes and may also be made of a combination of materials having such shapes.

<29> In the first, second and third aspects of the present invention, the insulation material filled in the sintering furnace is deposited in a block shape to act to refract radiant heat during sintering and, in addition, minimize the leakage of heat to the outside. In addition, as shown in the figures, the space for receiving the heating element is provided in the internal central space of the sintering furnace, and this portion is rotated by a rotating device rotating with a motor, such that heat in the portion is uniformly distributed, while electromagnetic waves are applied to the portion to prevent tilting. Moreover, a product is placed in the crucible in the heating element and is allowed to rotate and heated with dispersion of heat, thus solidifying the product.

<30> Namely, in the present invention, the insulation material was made of a material capable of resisting a temperature of 1500-1600 °C , and the heating element was fabricated by adding at least one selected from among alumina, zirconina and mullite to silicon carbide that is a main component, and was designed such that it not brought into direct contact with refractory materials in order to prevent cracks from occurring due to thermal shock between the heating element and the insulation material.

<3i> Furthermore, for uniform temperature distribution, a rotatable microwave oven was used, microwaves having a power of at least 700W were used, the insulation material was made of an insulation material having good electromagnetic permeability, and a ceramic material was placed in the heating element to induce a secondary rise in temperature and maintain the temperature, such that the rise in temperature to 1500-1600 °C was achieved within the shortest time.

<32> For uniform temperature correction, the temperature in the sintering furnace can be periodically measured by a temperature checking ring, a separate temperature sensor can be introduced to measure temperature versus time, and a current temperature can be measured using a pyrometer, thus controlling the performance of sintering elements as a function of time. Also, separate electric devices can be provided to control temperature.

<33> The first method of sintering artificial tooth cores using the above- described microwave oven-type microwave sintering furnace of the present invention comprises the steps of:

<34> (1) placing an insulation material in the magnetron system to form a workpiece-receiving space separated from the inner wall of the magnetron system by a specific distance;

<35> (2) placing in the workpiece-receiving space a first heating element, at least one side of which is open, so as to be spaced from the insulation material;

<36> (3) placing a crucible in the heating element;

<37> (4) placing artificial teeth in the crucible; and

<38> (5) operating the magnetron system so as to reach a temperature of 1500-1600 °C, such that the artificial teeth are heated indirectly through the first heating element and the crucible,

<39> (6) wherein the heating time in step (5) is controlled by controlling one or more variables selected from the group consisting of the thickness of the insulation material, the volume of the workpiece-receiving space, the surface area and weight of the heating element, and the distance (D) between the insulation material and the first heating element.

<40> The second method of sintering artificial tooth cores using the above- described microwave sintering furnace of the present invention comprises the steps of:

<4i> (1) providing first and second insulation material frames made of a ceramic material, at least one of which has formed therein a workpiece- receiving space for inserting a workpiece;

<42> (2) placing a first heating element in the workpiece-receiving space and placing artificial tooth cores around the first heating element",

<43> (3) combining the first and second insulation material frames with each other to form an assembly of insulation material frames; and

<44> (4) placing the combined insulation material frames in a microwave oven and operate the microwave oven for less than 4 hours to heat the artificial tooth cores.

<45> While the methods for sintering artificial tooth cores using the above- described microwave oven-type microwave sintering furnace of the present invention have been described with respect to the case of placing and heating the product in the crucible, it is to be understood that the same effect can also be obtained if the product is placed directly and heated in the heating element without using the crucible. [Advantageous Effects)

<46> In the prior sintering furnaces, when the internal temperature of the sintering furnaces is not slowly elevated to 1500 °C , a temperature for solidifying the product, but is rapidly elevated, non-uniform heating occurs, thus causing a flame reaction, leading to cracks or tilting. For this reason, the completed product may not be uniform, or incomplete sintering may occur. In order to eliminate such flame reaction, cracking and tilting phenomena, the temperature must generally be elevated slowly to 1500 °C over about 8 hours. However, according to the present invention, only the microwave sintering elements are simply placed in a microwave oven in a manner deviating from the concept of conventional sintering furnaces and are simply placed to elevate the temperature within 2hours of the microwave oven, and there is an advantage in that sintering can be performed more than 4 times over 8 hours. Namely, in the present invention, even when the temperature of the microwave oven is rapidly elevated to 1500-1600 °C within 2 hours, no flame reaction, cracking or tilting phenomenon occurs. [Description of Drawings]

<47> FIG. 1 shows a microwave sintering furnace according to one embodiment of the present invention.

<48> FIG. 2 shows a microwave sintering furnace according to another embodiment of the present invention.

<49> FIG. 3 shows a microwave sintering furnace according to still another embodiment of the present invention.

<50> FIG. 4 shows the electrical construction of the microwave sintering furnace of the present invention.

<5i> FIG. 5 is a detailed diagram illustrating the heating operation of a heating element in the sintering furnace of the present invention.

<52> FIG. 6 shows the detailed shape of the heating element in the sintering furnace of the present invention.

<53> FIG. 7 is a cross-sectional view of the microwave sintering furnace of the present invention on the assumption that an artificial tooth is sintered in the sintering furnace.

<54> FIG. 8 shows a detailed structure in which a heating element is placed and heated in a sintering furnace according to the present invention.

<55> FIG. 9 shows that an insulation material is formed in various manners in order to form a receiving spacer in a sintering furnace according to the present invention. <56> FIGS. 10 to 12 show the formation of first and second insulation materials in a sintering furnace according to the present invention. <57> FIG. 13 is a flowchart showing a method of sintering artificial teeth according to one embodiment of the present invention. <58> FIG. 14 is a flowchart showing a method of sintering artificial teeth according to another embodiment of the present invention. <59> FIG. 15 is a graphic diagram showing a time versus temperature relationship when an artificial tooth was introduced in a sintering furnace according to the sintering method of the present invention. <60> FIGS. 16 to 18 show the relationship between microwaves and temperature in a microwave oven-type microwave sintering furnace according to the present invention. <6i> FIG. 19 shows the relationship between temperature and time in a microwave oven-type microwave sintering furnace according to the present invention. <62> FIG. 20 shows artificial teeth manufactured according to an embodiment of the present invention.

<63>

<64> *Description of important reference numerals used in the figures*

<65> 20: first insulation material ;

<66> 40: second insulation material ;

<67> 50: heating element;

<68> 60: second heating element;

<69> 13: crucible;

<70> 90: artificial tooth;

<7i> 100: sintering furnace(magnetron system);

<72> 27: grooves;

<73> 4: door;

<74> 51 : investment mater i al ;

<75> 22 : workpiece-receiving space ; <76> 26 : motor ;

<77> 110 '• rotat ing device',

<78> 120: support ;

<79> 150: cool ing fan;

<80> 170 : refractory mater ial ;

<8i> 160: di splay uni t ;

<82> 220 : magnetron;

<83> 190 : transformer ;

<84> 210 : thermocouple ;

<85> 180: control ler ;

<86> 200: 180SSR

<87> 300: heating element portion; and

<88> 240: shock absorption plate. [Mode for Invention]

<89> Hereinafter, a microwave oven-type microwave sintering system according to the present invention and a method of sintering artificial teeth using the same will be described in further detail with reference to embodiments and the accompanying drawings.

<90> Embodiment 1-1

<9i> FIG. 1 shows a method of sintering artificial teeth according to one embodiment of the present invention. Namely, FIG. 1 shows a case in which the insulation material and heating element of the present invention are inserted into a magnetron system. As shown therein, in the internal space of a magnetron system 100, an insulation material 10 in a block form is formed so as to form a circular cross-sectional receiving space for receiving artificial teeth. In the central space of the insulation material 10, the artificial tooth-receiving space having a plate-like structure is formed, and several insulation materials are filled in the microwave oven 100. Herein, the insulation materials are made of a ceramic material, particularly alumina, and are deposited in the form of blocks so as to block heat transfer. In the center of the insulation materials, that is, in the center of the magnetron system, a heating element 50 is provided in the receiving space.

<92> The first heating element 50 is made of a thermally conductive ceramic material, for example, silicon carbide or a material obtained by adding alumina or zirconia to silicon carbide that is a main component. When the process of providing the first heating element 50, it is important to spray insulation material onto the lower surface 22 of the artificial tooth- receiving space so as to prevent heat from being transferred to the insulation material 10. For this purpose, the first heating element 50 is preferably apart from the insulation material 50 by distance D (about 2-3 mm), that is, is not brought into contact with the insulation material 10.

<93> In the above-described state, a crucible 13 is placed in the inside of the first heating element 50, and an artificial tooth 90 to be sintered is placed in the crucible. Herein, the crucible may have a ball shape, a plate shape, a column shape or a number of irregular granular shapes and may also be made of a combination of materials having such shapes. In the crucible 13, the artificial tooth 90 is sintered.

<94> Meanwhile, an electric power is applied to the magnetron system to sinter the artificial tooth, and the sintering time is preferably about 1 hour as described above.

<95> FIG. 9(a), 9(b) and 9(c) show that an insulation material is formed in various shapes in order to form a receiving space in a magnetron system according to one embodiment of the present invention. Specifically, FIG. 9a) shows that a receiving space is formed in an upper insulation material 40, and the first heating element 50 and the crucible 13 are placed therein. FIG. 9(b) shows that the first heating element 50 and the crucible 13 are placed in a receiving space formed in a right first insulation material 20, and FIG. 9(c) shows that a receiving space is formed in a left first insulation material 40. In addition, according to the shapes of the insulation material, various receiving spaces may be formed.

<96> Embodiment 1-2 <97> A sintering furnace according to a second embodiment of the present invention preferably comprises- inlet/out let cooling fans which operate by external electric power; a controller for controlling electrical devices; a thermocouple for measuring the temperature of the sintering furnace; a magnetron for generating microwaves; a transformer for supplying high voltage to the magnetron; a microwave shock-absorbing plate for absorbing microwave shock; SSR (contact-free relay) having resin molded therein; and a rotating device for rotating the heating element or the crucible.

<98> Also, the inside of the microwave sintering furnace preferably comprises: a refractory material deposited in the form of blocks for refracting radiant heat to minimize the leakage of heat to the outside; and either a heating element, which resists thermal shock, shows strong resistance to oxidation, does not react with a crucible and consists of, in addition to silicon carbide as a main component, mullite, alumina, zirconia, etc., or a crucible together with the heating element.

<99> The sintering time in the microwave sintering furnace is preferably less than 2 hours.

<ioo> The first heating element is preferably fabricated to have porosity in order to transfer microwaves, absorb shock and make processing smooth, and it preferably comprises a plurality of grooves 27 in order to disperse the concentration of microwaves and enlarge the cross-sectional area of the heating element to maximize the radiation of heat.

<ioi> The center of the sintering furnace preferably comprises: a container (first heating element) for primary heating from the outside toward the inside of the sintering furnace; an investment material for primary heating and shock absorption; a container (crucible) for secondary heating; and an investment material for secondary heating and shock absorption.

<iO2> Herein, the investment material consists of alumina or the like, is interposed between the heating element and the crucible to suppress oxidative reactions and absorb thermal shock caused by high temperature, and serves absorb the shock of the crucible, the heating element and the refractory material. In addition, it functions to suppress the oxidative reaction of the first heating element and, at the same time, suppress the reaction of the first heating element with the refractory material.

<iO3> The second embodiment of the present invention will now be described with reference to FIG. 2.

<iO4> As shown in FIG. 2, the present invention adopts a structure in which input/output cooling fans 150 are provided at the left and right sides of the outer case of a microwave sintering furnace 100 so as to surround the outside of the case, thus performing forced air cooling. At the right side of the front of the outer case, a display unit 160 indicating the condition of the microwave sintering furnace or the internal temperature of the sintering furnace is provided.

<iO5> Meanwhile, at the lower end of the inside of the sintering furnace, a rotating device 110 which rotates by a motor 26 is provided, a support 120 at the upper end of the rotating device 110 is made of a circular flat plate, such that it can rotate while it supports the heating element 50 or the crucible. Also, the inside of the sintering furnace other than the space in which the support and the heating element can rotate by the rotating device 110 is filled with a block-shaped refractory material 170. Although not shown in the figures, a refractory block corresponding to the size of a door 4 is inserted during the operation of the sintering furnace, such that the support locates and rotates in the center of the sintering furnace.

<iO6> The electrical devices in the microwave sintering furnace according to the second embodiment of the present invention will now be described.

<iO7> FIG. 4 shows the electrical construction of the microwave sintering furnace according to the present invention.

<i08> As shown therein, the sintering furnace 100 of the present invention is structured such that the cooling fans 150 operate by an external power source, and when the power source is in an 0N-state to heat the heating material, high voltage is supplied to the magnetron 220 through the transformer 190, and microwaves are generated in the magnetron 220 to heat the product in the heating element or the crucible. Also, the control 180 is constructed such that it controls all electrical devices in the sintering furnace, including a thermocouple 210 and SSR 200. All parts of the electrical device controller 180 which is one characteristic of the present invention are codified while deviating from existing complex designs in order to minimize the failure rate, such that they are simply replaced when they are broken down. As the safety device thereof, the controller has a structure in which the cooling fans 150 automatically operate by external electric power in a plug-On state regardless of the operation of the sintering furnace in order to prevent a risk caused by heat generation from the magnetron 220 and the transformer 190. Also, in order to prevent a risk from occurring in the body and other parts due to high voltage during the operation of the sintering furnace, the operation of microwaves is automatically stopped when the door 4 of the microwave sintering furnace is opened due to spurious operation. Also, when the thermocouple 210 is broken down, the power is off in order to block excessively high microwave powder, and an SSR manner (contact-free relay manner) is adopted to eliminate all risky factors which can occur. In addition, a circuit constituting the electrical devices is designed to have a simple structure which deviates from the complex structure of prior sintering furnaces.

<iO9> FIG. 5 is a detailed diagram illustrating the heating operation of the heating element portion 300 of FIG. 2.

<πo> As shown therein, when microwaves (indicated by arrows) are generated by the magnetron, the support 120 rotates by the rotating device 110 in the sintering furnace in the direct indicated at the left side. When the support 120 rotates, the first heating element 50 placed on the support also rotates, and thus the crucible 13 also rotates. Microwaves generated from the left side during the rotation run against the heating element 50 to heat the sintering furnace. However, microwaves passed through the grooves 27 partially run against the crucible 13 to secondarily heat the sintering furnace, and the remaining portion runs against the opposite surface of the heating element, such that microwaves can reach throughout the heating element, thus providing good heat transfer compared to prior heating e1ements .

<iu> Herein, the investment material between the first heating element 50 and the crucible 13 and the investment material in the crucible function to absorb heat shock caused by an oxidative reaction and high temperature as described below so as to reduce the shock of the crucible, the first heating element and the refractory material.

<ii2> Next, the shape of the heating element 50 will be described in detail with reference to FIG. 6.

<H3> The first heating element must be manufactured to have a porous structure for microwave transfer and shock absorption as described above. If a heating element 50 is used in prior sintering furnaces, only one portion of the heating element receives microwaves, heat transfer to other opposite surfaces is slower than that in the place with which the microwaves are brought into direct contact, and a time of about 8 hours is required to heat the product to a desired sintering temperature of 1500 °C by elevating the temperature slowly without the flame reaction, cracking or tilting phenomena resulting from the variation in temperature between the direct contact surface and the other surfaces. However, in the present invention, the grooves 27 are formed in the heating element 50 so as to maximize the radiation of heat from the heating element, such that the heating element has not only portions with which microwaves come in direct contact, but also portions in which microwaves pass through the grooves 27 and come in direct contact with other surfaces of the heating element, having a larger cross- sectional area. Thus, in the sintering furnace of the present invention, heat can be uniformly transferred, no runaway (flame) phenomena occur, and heat can be radiated from a large cross-sectional area to the highest possible extent, such that the internal temperature of the sintering furnace can be elevated to 1500 ⁰C within 2 hours. Herein, while the description has been made with respect to the circular heating element having the grooves 27, the heating element may have a square shape.

<H4> Also, in the case of placing the crucible 13 in the heating element, placing a product to be sintered in the crucible 13 and heating the product by microwaves while rotating the crucible by the rotating devices 110, there is an advantage in that secondary heat transfer occurs rapidly, such that the product can be heated in a more stable manner, thus solidifying the product more rapidly.

<ii5> FIG. 7 shows a cross-sectional view of a microwave sintering furnace with the assumption that the produce is sintered in the sintering furnace 100. As shown in FIG. 7, the sintering furnace 100 has a structure surrounded by an outer case and includes an outer space in the case, such that forced air cooling can be performed by input/output cooling fans 150 provided at the left and right sides of the case. In the lower portion of the sintering furnace 100, a rotating device 110 is provided in order to rotate the first heating element 50 or the crucible 13 by the motor 26, and on the rotating device 110, a circular flat plate 120 is provided, such that the rotatable first heating element 50 and the crucible 13 made of porous alumina or the like can be placed thereon. The support 120 passes through the central space of the refractory material 17 filled in the sintering furnace and locates in the central portion of the sintering furnace. On the support 120, the first heating element 50 manufactured to have a porous structure using mullite, alumina and zirconia in addition to a main component silicon carbide as described above is placed, and at the left side of the first heating element 50, a microwave shock absorbing plate 240 for absorbing microwave shock generated from the magnetron is placed substantially parallel with the side of the first heating element in the vertical direction. At the left side of the microwave shock absorbing plate 240, a thermocouple 210 is provided in a direction parallel with the support in order to reduce impact. As shown in FIG. 7, on the support 120, the first heating element 50 is placed, and in the first heating element 50, the crucible 13 is placed. In the crucible, a product to be sintered can be placed together with zirconia powder, alumina powder and the like and heated so as to be solidified.

<H6> Meanwhile, between the first heating element and the crucible, an investment material 51 made of alumina or zirconia is filled in order to prevent thermal shock during heating. Also, as described above, the refractory material in a block shape is filled in a space other than the space having a size of less than 10 cm (W) x 10 cm (L) x 6 cm (H) and serving to receive the support, the heating element and the crucible, which locate in the central portion of the sintering furnace. Herein, if the other space was larger than the above-specified space, it was observed in experiments that the elevation in temperature during heating was slow and difficult to rise to 1500 ⁰C. For this reason, the space was limited to the above-specified value.

<ii7> Meanwhile, the refractory material 170 in the sintering furnace is deposited in the form of blocks to refract radiant heat so as to minimize the leakage of heat to the outside. For this purpose, it must be made of alumina, carbon dioxide, zirconia, etc. The heating element can resist thermal shock, does not undergo a reaction with the crucible caused by heating, must have strong resistance to oxidation and, at the same time, sufficiently resists sintering performed using a heat sources having a temperature of 1500 °C . For this purpose, the heating element is made of, in addition to silicon carbide as a main component, mullite, alumina, zirconia, etc.

<ii8> Next, the above-described structure in which the crucible 13 is placed and heated in the first heating element 50 will be descried with reference to FIG. 8.

<ii9> As shown in FIG. 8, the first heating element 50 includes a container (first heating element) for primary heating from the outside to the inside of the sintering surface, and a container (crucible, 13) for secondary heating. Between the container for primary heating and the container for secondary heating, the investment material 51 for absorbing the shock of the first heating, and in the container for secondary heating, the investment material 51 for absorbing the shock of the secondary heating is filled. In the crucible 13, a product is placed and solidified by microwave heating. As described above, it is very important to fabricate the heating element to have a porous structure for microwave transfer and shock absorption, in order to uniformly spread microwaves to facilitate heat transfer.

<12O> Embodiment 2-1

<i2i> A method of sintering ceramic materials such as zirconia, particularly artificial teeth, using the above-described microwave oven-type microwave sintering furnace of the present invention, will now be described in detail.

<i22> FIG. 13 shows an overall flowchart for embodying the present invention.

<i23> As shown in FIG. 13, an insulation material made of a ceramic material is formed such that it can be placed in a microwave oven (Sl). Then, the formed insulation material is filled in the space of the microwave oven other than a workpiece-receiving space (S2). Then, a first heating element obtained by adding alumina or zirconia to a ceramic composite material that is a main component is placed in the workpiece-receiving space (S3). Then, a crucible made of a thermally conductive ceramic material is placed in the first heating element (S4). Then, insulation material powder is sprayed, after which an artificial tooth is placed in the crucible (S5). Then, electric power is applied to the magnetron system, and after the internal temperature of the microwave oven reaches a specific temperature within a specific time, the product is sintered (S6).

<i24> Herein, the specific temperature can be controlled by controlling at least one variable selected from among the thickness of the insulation material, the workpiece-receiving space, the surface area and weight of the first heating element, and the distance (D) (about 2-3 mm) between the insulation material and the first heating element.

<125> Also, the variable is controlled by measuring the sintering temperature after performing sintering about 50 times.

<126> As described above, the sintering method of the present invention is performed. <127> FIG. 15 is a graphic diagram showing the relationship between time and temperature, when the artificial tooth was introduced in the sintering furnace according to the sintering method of the present invention. As can be seen from the graphs of FIG. 15, the internal temperature of the microwave oven rapidly rises to 1400 °C within 40-50 minutes, but once the temperature rises to 1400 ^°C , it becomes saturated, such that the temperature becomes substantially constant.

<128> Namely, as shown in FIG. 15, it could be observed through a few tens of experiments that the temperature was rapidly changed to 136 °C at 5 minutes, 304 ⁰C at 10 minutes, 482 ⁰C at 15 minutes, 1105 "C at 35 minutes, 1328 ⁰C at 45 and 1388 ⁰C at 50 minutes, but after 50 minutes, the temperature became saturated, such that it was not substantially changed with the passage of time. Accordingly, an artificial tooth-sintering time of a few hours in the prior art can be significantly reduced to one hour. Thus, according to this sintering method, an artificial tooth can be easily sintered in dental clinics, etc.

<i29> Embodiment 2-2

<13O> An artificial tooth can be sintered using another embodiment of the present invention, in addition to the above-described method.

<i3i> Another method for sintering artificial teeth comprises the steps of: (a) providing first and second insulation materials 20 and 40, at least one of which has formed therein a workpiece-receiving space for inserting a workpiece; (b) placing a first heating element 50 in the workpiece-receiving space, and placing artificial tooth cores 90 around the first heating element 50; (c) combining the first and second insulation materials with each other; and then (d) placing the combined insulation materials in a magnetron system and operating the magnetron system for a specific time, thus heating and sintering the artificial tooth cores 90.

<132> In a preferred embodiment, the first and second insulation material frames are made of a ceramic material, preferably a single ceramic material or composite ceramic material selected from among alumina, zirconia and mullite, and particularly preferably alumina. Also, the first heating element 50 is preferably made of a composite ceramic material, and particularly preferably, a silicon carbide (SiC)-based material.

<i33> The operating time of the magnetron system is preferably less than 4 hours, more preferably less than 2 hours, and particularly preferably, in the range from 20 minutes to 1 hour and 30 minutes.

<i34> The first heating element 50 may have a ring shape, a plate shape, a column shape, a number of irregular granular shapes, or a combination of two or more thereof.

<135> A second heating element 60 may additionally be placed around the first heating element 50, and in this case, the artificial tooth cores 90 are placed in the second heating element 60. The second heating element 60 is preferably made of a thermally conductive ceramic material. The second heating element 60 may have a ring shape, a plate shape, a column shape, a number of irregular granular shapes, or a combination of two or more thereof.

<i36> The first heating element 50 is preferably placed apart from the first or second insulation material 20 or 40, such that the first or second insulation material 20 or 40 can be prevented from being damaged due to the direct contact of the first heating element with the first or second insulation material 20 or 40. In one embodiment, insulation material powder is sprayed onto the portion of the first or second insulation material 20 or 40 with which the first heating element 50 can be brought into direct contact, and then the first and second insulation materials 20 and 40 are combined with each other.

<i37> Embodiment 2-3 ci38> In another embodiment, the first insulation material 20 has a plate- like structure having the workpiece-receiving space 22 formed in the upper surface thereof, and the second insulation material 40 is made of a plate- like member which can be combined closely with the first insulation material 20. In this embodiment, the second heating element 60 may be placed around the first heating element 50, and the artificial tooth core may be placed in the second heating element 60. The second heating element may be made of a thermally conductive ceramic material. The second heating element may have a ball shape, a plate shape, a column shape, a number of irregular granular shapes, or a combination of two or more thereof.

<139> In the above-described embodiment in which the first and second insulation materials 20 and 40 are made of the plate-shaped member, the first heating element 50 may have a ring shape which is circular, square or polygonal in cross-section. In this case, the insulation material powder is sprayed onto the lower surface of the workpiece-receiving space 22 of the first insulation material 20, and then the first heating element 50 is placed such that it is spaced from the inner wall of the workpiece-receiving space 22 of the first insulation material 20 by a specific distance. Then, the second heating element 60 is placed in the first heating element 50, and the artificial tooth cores 90 are placed in the second heating element, and then sintered using the microwave oven-type microwave sintering furnace 100.

<14O> The insulation material is made of a material capable of resisting heat having a temperature of 1600 "C , and the first heating element is manufactured by adding alumina or zirconia to silicon carbide that is a main component. Also, in order to prevent cracks from occurring due to the thermal shock between the first heating element and the insulation material, the first heating element is designed such that it is not brought into direct contact with the refractory material. For uniform temperature distribution, a rotatable microwave oven is used, and microwaves having a powder of at least 700W are used. The heating space has a size of less than 12 cm (W) x 12 cm (L) x 6 cm (height), the insulation material is an insulation material having good electromagnetic permeability, and a ceramic material is placed in the heating material so as to elevate secondary temperature through the generation of heat therefrom and maintain the temperature. The applicant has found through a few tens of experiments that, when such constructions are applied, the internal temperature of the sintering furnace reaches 1600 ^°C within the shortest time. For uniform temperature correction, the temperature in the sintering furnace can be periodically measured by a temperature checking ring, a separate temperature sensor can be introduced to measure temperature versus time, and a current temperature can be measured using a pyrometer, thus controlling the performance of sintering elements as a function of time. Also, separate electric devices can be provided to control temperature.

<i4i> The microwave sintering elements can perform at least four sintering treatments over 8 hours, and high-frequency microwaves having a powder of at least 700W are preferably used in the sintering furnace. The insulation material is preferably an insulation material having good electromagnetic permeability. The weight of the heating element is preferably determined depending on the amount of electromagnetic energy, the degree of insulation and the heating space, the ceramic material is preferably placed in the second heating material to elevate and maintain the internal temperature of the sintering furnace, and the temperature rise time is preferably determined as a function of the temperature. In order to prevent tilting of the product, a rotatable microwave oven or a microwave oven structure having rotating devices mounted in the lower portion thereof is preferably used.

<i42> As a number of sintering treatments progress, the electromagnetic wave transmission time can be determined by periodically measuring the internal temperature of the sintering furnace using a temperature checking ring, a temperature sensor or a pyrometer. When it is required to continuously control the temperature, a variable control system can be provided in the sintering furnace.

<i43> FIG. 14 shows the above-described inventive method for sintering artificial teeth. As shown in FIG. 14, in the process of sintering the artificial tooth cores, the first and second insulation material frames 20 and 40, at least one of which has formed therein a workpiece-receiving space for inserting a workpiece, are provided (step 10).

<144> FIG. 10 shows one embodiment of the first and second insulation material frames according the present invention. In this embodiment, the first insulation material frame 20 has a plate-shaped structure having the workpiece-receiving space formed in the upper surface thereof, and the second insulation material frame 40 is made of a plate-shaped member which can combined closely with the first insulation material frame 20. In this embodiment, each of the first and second insulation materials 20 and 40 is made of an alumina material and has a rectangular shape having a one-side length of 20 cm and a height of 2.5 cm. Also, the workpiece-receiving space 22 consists of a circular cross-sectional groove having a depth of 1.5 cm.

<145> In the state in which the first and second insulation material frames 20 and 40 were provided as described above, the first heating element 50 is placed in the workpiece-receiving space 22 of the first insulation material frame 20 (step 12). In this embodiment, the first heating element 50 has a ring shape which is rectangular in vertical cross-section and circular in horizontal cross-section, and it is made of a thermally conductive material, for example, silicon carbide or a material based on silicon carbide (SiC). In the step of placing the first heating element (50), the insulation material powder is preferably sprayed onto the lower surface of the workpiece-receiving space 22, such that the heat of the first heating element 50 is not transferred to the first insulation material 20 during the sintering process. In addition, the outer diameter of the first heating element 50 is preferably smaller than the inner diameter of the workpiece- receiving space 22, such that the first insulation material frame 20 is not brought into direct contact with the first heating element 50.

<146> In the state in which the first heating element 50 was placed as described above, the second heating element 60 is filled in the first heating element 50, and the artificial tooth cores 90 are placed in the second heating element 60. Herein, the second heating material 60 is preferably made of a thermally conductive material. The second heating element may have a ball shape, a plate shape, a column shape or a number of irregular granular shapes and may also be made of a combination of materials having a combination of two or more of these shapes. <147> After the first heating element 50 is placed and the artificial tooth cores 90 are filled, the second insulation material frame 40 is placed closely on and combined with the first insulation material frame 20 (step 14).

<i48> Then, the combined insulation material frames are placed in a microwave oven, and the microwave oven is operated for a specific time, thus heating and sintering the artificial tooth coreCstep 16). The operating time of the magnetron system is less than 4 hours, preferably less than 2 hours, and particularly preferably in the range from 20 minutes to 1 hour and 30 minutes. Most preferably, the microwave oven is operated for 1 hour.

<149> FIGS. 11 and 12 show other embodiments of the first and second insulation material frames. As shown in FIG. 11, a lower first insulation material frame 20a placed in a lower layer is provided in the form of a plate, and a workpiece-receiving space can be provided at the lower side of a second insulation material frame 40a. Also, as shown in FIG. 12, first and second insulation material frames 20b and 40b may be fabricated such that they are combined with each other in a width-wise direction. Particularly, as shown in FIG. 12, a sleeve may also be provided at one side of first or second insulation material frames 20c or 40c in order to combine the insulation material frames closely with each other.

<i50> Embodiment 3-1

<i5i> Colored articles, TZ-3YB, TZ-3YB-E, TZ-3YSB-E, TZ-3YSB-C and TZ-3Y(Ai), were sintered in a microwave sintering furnace in the following manner according to the embodiments of the present invention. <152> 1. Samples to be analyzed

<153> -sintered zirconia materials (articles sintered by microwaves) <154> 1493 °C: TZ-3YB, TZ-3YB-E, TZ-3YSB-E, TZ-3YSB-C, and TZ-3Y (colored

A₁);

<155> 1520 ⁰C: TZ-3YB, TZ-3YB-E, TZ-3YSB-E, and TZ-3YSB-C <i56> -molding condition: CIP: 2000 bar(= 200 MPa) <157> -sintering condition: 1493 ⁰C x 1 hour, and 1520 °C x 1 hour (checked by temperature measuring ring).

<158> 2. Analysis items and methods <159> -Measurement of density of sintered materials: density measurement was performed using the Archimedes method.

<160> -Measurement of particle diameter of sintered materials'- the sintered materials were ground and polished, and the polished materials were thermally etched at a temperature which was lower than the sintering temperature by 50 °C. Then, the particle diameter of the materials was observed by SEM and calculated according to the planimetric method.

<161> 3. Analysis results <162> The density and particle diameter of the sintered materials are shown in Table 1 below.

<163> [Table 1]

<164> FIG. 20 shows artificial teeth sintered according to the embodiment of the present invention.

<165> For reference, FIGS. 16 to 19 show the microwave-temperature relationship and the temperature-time relationship in the microwave oven-type microwave sintering furnace according to the present invention. [Industrial Applicability]

<166> According to the present invention, a sintering time of 7-8 hours required to sinter artificial teeth in the prior art can be significantly reduced to 1-2 hours. Thus, according to the sintering method of the present invention, artificial teeth can be easily sintered in dental clinics and the like. In addition, the sintering of artificial teeth according to the sintering method of the present invention is easily achieved within a few hours in dental clinics and the like, and thus it reduces the expense of consumers, is time-consuming and also can lead to an increase in the income of dental clinics.

Claims

[CLAIMS] [Claim 1]

A method of sintering artificial tooth using a magnetron system, the method comprising the steps of:

(1) placing an insulation material in the magnetron system to form a workpiece-receiving space separated from the inner wall of the magnetron system by a specific distance;

(2) placing in the workpiece-receiving space a first heating element, at least one side of which is open, so as to be spaced from the insulation material ;

(3) placing a crucible in the heating element;

(4) placing artificial teeth in the crucible! and

(5) operating the magnetron system so as to reach a temperature of 1500-1600 ⁰C, such that the artificial teeth are heated indirectly through the first heating element and the crucible,

(6) wherein the heating time in step (5) is controlled by controlling one or more variables selected from the group consisting of the thickness of the insulation material, the volume of the workpiece-receiving space, the surface area and weight of the heating element, and the distance (D) between the insulation material and the first heating element.

[Claim 2]

The method of Claim 1, wherein the workpiece-receiving space has a size of less than 200 mm X 200 mm X 200 mm.

[Claim 3]

The method of Claim 1, wherein the insulation material is in the form of blocks or grooves.

[Claim 4]

The method of Claim 1, wherein the distance D is about 2-3 mm.

[Claim 5]

The method of Claim 1, wherein the heating element is made of a silicon carbide-based ceramic composite material having a space which is less than 9 mm in diameter and less than 5 mm in height.

[Claim 6]

A method of sintering artificial teeth using a magnetron system, the method comprising the steps of:

(a) providing first and second insulation material frames made of a ceramic material, at least one of which has formed therein a workpiece- receiving space for inserting a workpiece!

(b) placing a first heating element in the workpiece-receiving space and placing artificial tooth cores around the first heating element;

(c) combining the first and second insulation material frames with each other to form an assembly of insulation material frames;

(d) placing the assembly of the insulation material frames in a magnetron system and operating the magnetron system for less than 4 hours to heat the artificial tooth cores.

[Claim 7]

The method of Claim 6, wherein the first heating element is made of a silicon carbide-based ceramic composite material. [Claim 8]

The method of Claim 6, wherein step (b) further comprises a step of placing a second heating element around the first heating element and placing the artificial tooth cores in the second heating element. [Claim 9]

The method of Claim 8, wherein the second heating element is made of a thermally conductive ceramic material. [Claim 10]

The method of Claim 6, wherein insulation material powder is sprayed onto the lower surface of the workpiece-receiving space of the first insulation material frame, and then the first heating element is placed in the workpiece-receiving space of the first insulation material frame so as to be apart from the inner wall of the first insulation material frame. [Claim 11] The method of any one of Claims 6 to 10, wherein the first heating element is placed so as to be apart from the first and second insulation material frames. [Claim 12]

The method of Claim 11, wherein the first insulation material frame is made of a plate-shaped structure having the workpiece-receiving space formed in the upper surface thereof, and the second insulation material frame is made of a plate-shaped structure which can be combined closely with the first insulation material frame. [Claim 13]

The method of Claim 8, wherein the first heating element has a ring shape, and step (b) further comprises the steps of:

(bl) spraying the insulation material powder onto the lower surface of the workpiece-receiving space of the first insulation material frame;

(b2) placing the first heating element in the workpiece-receiving space of the first insulation material frame so as to be apart from the inner wall of the workpiece-receiving space;

(b3) placing the second heating element in the first heating element having a ring shape; and

(b4) placing the artificial tooth cores in the second heating element. [Claim 14]

A magnetron system for sintering artificial tooth cores, wherein the sintering furnace comprises a heating element comprising a plurality of grooves for microwave dispersion, and a refractory material deposited in a block form. [Claim 15]

A microwave sintering furnace, comprising: inlet/outlet cooling fans which operate by external electric power; a controller for controlling electrical devices', a thermocouple for measuring the temperature of the sintering furnace; a magnetron for generating microwaves; a transformer for supplying high voltage to the magnetron; a microwave shock-absorbing plate for absorbing microwave shock and the like; an SSR (contact-free relay) having resin molded therein', a rotating device for rotating a heating element or a crucible, wherein the sintering furnace comprises a heating element comprising a plurality of grooves for microwave dispersion, and a refractory material deposited in a block form. [Claim 16]

The microwave sintering furnace of Claim 14 or 15, wherein the heating element and the crucible can rotate in a space, which is provided in the sintering furnace, the space being filled with the refractory material and having a size of less than 12 cm (W) x 12 cm (L) x 6 cm (H). [Claim 17]

The microwave sintering furnace of Claim 15 or 16, wherein the heating element is manufactured to have a porous structure for microwave transfer and shock absorption. [Claim 18]

The microwave sintering furnace of Claim 15 or 16, wherein the central portion of the sintering furnace comprises: a container for primary heating from the outside toward the inside of the sintering furnace'» an alumina-filled investment material for primary heating and shock absorption; a container for secondary heating; and an alumina-filed investment material for second heating and shock absorption. [Claim 19]

The microwave sintering furnace of Claim 15 or 16, wherein the heating element is composed of silicon carbide as a main component and at least one selected from among mullite, alumina and zirconia. [Claim 20]

The microwave sintering furnace of Claim 15 or 16, wherein the crucible can be placed in the heating element.