Furnace for sintering ceramics and process for sintering ceramics

Abstract: In a furnace for sintering non-oxidic ceramics, particularly Si₃N₄, which comprises a space for accommod­ ating a ceramic shaped body (12), carbon heaters (8) arranged around the ceramic shaped body in said space and heat insulating layers (13) of carbon fiber mat that cover the inner walls of the furnace, sheetings (18) of laminated graphite leaves having an ash content of not more than 0.3% by weight are provided between said heat insulating layers (13) and said ceramic shaped body (12). The carbon heaters are of high purity graphite having a carbon content of at least 99.9980%, a silicon content of not more than 5 ppm and an iron content of not more than 9 ppm by weight. These measures protect the shaped body from harmful influence of carbon fiber dusts liberated from the insulating layers during sintering.

Description: The present invention relates to furnaces for sintering ceramics, particularly non-oxide ceramics. This invention further relates to processes for sintering using such furnaces, wherein shaped bodies molded from a mixture of non-oxide ceramic powdery materials and sintering aids are heated at a high temperature under an inert gas atmosphere in the furnace. Nitride ceramic materials such as silicon nitride, Si₃N₄, boron nitride, BN, or the like are refractory materials and generally include 5 ∼ 10% of metal oxides (MeO), such as MgO, Aℓ₂O₃ or the like, or a mixture of the metal oxides with metal nitrides, as sintering aids to promote the sintering. Further, for example, Si₃N₄ green bodies before sintering generally have about 40 vol% voids. Now, the mechanism of strength development of the silicon nitride during sintering is accounted as formation of a kind of FRC (Fiber Reinforced Ceramics) wherein β-type silicon nitride needle crystals are dispersed as a reinforcement in glassy phases of metal oxides added as the sintering aids, whereby excellent strength characteristics are developed. Additionally, if an example is given for Si₃N₄, shaped bodies thereof are generally fired at a high temperature under an inert atmosphere, particularly, at a temperature of 1,700°C∼1,900°C under nitrogen gas atmosphere. A typical furnace to maintain such a high temperature stably with an inert atmosphere comprises a space for accommodating the ceramic shaped bodies, carbon heaters arranged around the ceramic shaped body in said space and heat insulating layers of carbon fiber mat that cover the inner walls of the furnace, which is further provided with a vacuum port and an inert gas feed opening. The above carbon fiber mat has an extremely large volume porosity, usually 70∼95 vol.% interstices, resulting in a bulk density averaging about 0.2 g/cm³, ensure its excellent heat insulating properties. Alternatively, particularly when the furnace is relatively of a small size, there may be a carbon cylinder to define the space for the shaped body and the graphitic carbon heaters are further arranged on inner side of the carbon fiber mat. JP-A-58-93804 describes a furnace having a carbon heater and a carbon fibre mat around the heated space, and between the mat and the heated space a gas-tight carbon sheet. During firing of the Si₃N₄ in a furnace as mentioned above, the carbon fiber mat having a bulk density of about 0.2 g/cm³ comes into contact with O₂ and H₂O remaining in the furnace or a trace of oxygen, oxides or oxynitrides generated from the metal oxide containing Si₃N₄ shaped bodies at high temperatures, so that carbon fibers in surface layers of the mat undergo an oxidation reaction. Therefore, the carbon fibers disintegrate, though as small bits. As a result, not only do the heat insulating properties of the mat gradually deteriorate whereby the life of the furnace is shortened but also characteristics of the sintered body are markedly impaired by the disintegrated carbon fiber dusts that fly and suspend in the furnace and eventually adhere to the high porous Si₃N₄ shaped bodies during or before sintering, and also by gases such as CO, CO₂ or the like formed by oxidation of the carbon dusts that diffuse and contact with the Si₃N₄. Namely, when the carbon fiber dusts adhere onto the high porous Si₃N₄ shaped bodies before or during sintering as mentioned above, the shaped bodies can draw these carbon fiber dusts inside thereof as the shaped bodies contract during sintering. The drawn-in carbon dusts react with sintering aids, metal oxides, to form CO or CO₂ which comes out to diffuse in the furnace atmosphere and simultaneously the metal oxides are reduced into low melting metals which vaporize. Thus, the metal oxides that are to form a glassy phase matrix are lost particularly in the surface layers, leaving skeletons behind. In the skeletonized state, the Si₃N₄ sintered bodies have no excellent characteristics, such as a high strength, high thermal shock resistance, high abrasion resistance or the like, any longer. Further, the Si₃N₄ shaped bodies that contact with CO, CO₂, etc. formed in the furnace repeat the following reactions to lose metal oxides (MeO) rapidly: Si₃N₄ + MeO + CO → Si₃N₄ + CO₂ + Me↑ CO₂ → CO + O C + O → CO These reactions accelerate the abovementioned formation of the Si₃N₄ skeleton. In order to prevent such bad influences of the carbon fiber dusts generated from the insulating layer forming carbon fiber mats, an attempt was made wherein a carbon cylinder was arranged on the inner side of the insulating layer as mentioned above. However, it usually has a wall thickness of about 10 mm, so that if the cylinder having such a high heat capacity is put in the furnace body, an excessively large electric load is naturally applied to the heaters, increasing the consumption of the heaters. Moreover, the manufacture of such a big sized cylinder is cost- and time-consuming that it is economically disadvantageous. Additionally, carbon materials that are denser, on the one hand, are less in self-consumption so that the atmosphere in the furnace can be kept clean and, on the other hand, since such materials have so high a thermal expansion coefficient that they are low in thermal shock resistance and repeated thermal stress, so that a cylinder made thereof develops cracks through which carbon fiber dusts pass to fly, doing harm to surfaces of the Si₃N₄ sintered body, same as described above. In order to prevent the crack development, if a cylinder made of a carbon material having a low thermal expansion coefficient is used, the aforementioned disadvantages caused by the high porosity of the material itself still will not be eliminated. The present inventors, as a result of continuing assiduous efforts that went into the research of the abovementioned problems and the investigation of the causes, have found that materials of the carbon heaters have a close interrelation and mutually act with the quality of nitride ceramic sintered bodies. Namely, since conventional carbon heaters have been aimed principally at the manufacture at a lowest possible cost as far as their heat generation performance is satisfiable, the purity of the constituting material, i.e., graphite, has been given less consideration, so that those having a carbon content of about 99.9% containing impurities such as silicon, iron or the like of about several hundreds of ppm have generally been employed. However, when such a carbon heater is heated at high temperatures, attacks and perforations of the graphite are commenced initiating at the sites of impurities such as silicon, iron or the like contained in the graphite and the carbon disintegrates to fly and eventually adhere to the nitride shaped bodies before or during sintering. Thus, in the same way as above, the skeletonization of the surface layers of the sintered bodies takes place. Simultaneously, oxygen, oxides or oxynitrides generating from the shaped bodies adversely enter micropores formed in the heater graphite and react with carbon in the depths, to encroach and disintegrate the skeletons of the graphite, emitting carbon particles, whereby the pores are enlarged until formicary-like pores are formed on the heater members. Thus, the skeletonization due to emitting carbon of the surface layers of the sintered bodies is further promoted to accelerate the degradation of the heaters. Such a heater loses its phase balance as required for a heater material, rendering not only an accurate temperature control impossible but also surface electric current increase locally at porous portions, resulting in breakage in an extreme case. Additionally, other than the above-described phenomena, a problem of a bad influence of the suspending carbon particles upon a thermocouple that functions as important temperature control has been realized as new. Namely, in temperature measurement in a high temperature nitrogen gas atmosphere at 1,700°C∼2,000°C, a two-color pyrometer that has usually been applied to high temperatures can hardly expect an accuracy due to fluctuation, etc. induced by convections of gases in the furnace. Accordingly, in order to prevent nitriding by nitrogen gas of tungsten, generally employed is a W/Re thermocouple that is encapsulated in a molybdenum protective tube typically enveloping argon gas. However, the molybdenum tube is carbonized, when the suspending carbon particles adhere thereto, to form MoC that is very brittle and different in thermal expansion coefficient from Mo, so that cracks develop after several firing operations. From the cracks, the enveloped argon gas leaks out and nitrogen gas enters instead, whereby the tungsten is nitrided causing a change of an electromotive force that eventually results in loss of its accurate function. The present invention aims to solve the abovementioned various problems. A principal object of the present invention is to provide high quality non-oxide ceramic sintered bodies, particularly Si₃N₄ sintered bodies, having a high strength and excellent abrasion resistance and thermal shock resistance. Another object is to obtain such high quality Si₃N₄ sintered bodies with industrial feasibility and economical advantages. A different object is to provide a furnace for sintering Si₃N₄ shaped bodies, with a relatively low cost, which has a prolonged life of the furnace body, being provided with insulating layers resistant to consumption. A further object is to prevent deterioration of carbon heaters in such a furnace to extend the life thereof. Still a further object is to make it possible to maintain an accurate temperature control for a long time during sintering. The furnace of the invention is set out in claim 1. The method according to the present invention for firing a non-oxide ceramic shaped body is set out in claim 6. The carbon heater used in the furnace of the present invention is preferably composed of a high purity graphite having a carbon content of at least 99,9980%, a silicon content of not more than 5 ppm and and an iron content of not more than 9 ppm, by weight. In the process of the present invention, a preferred inert atmosphere is a nitrogen gas atmosphere, most preferably under pressure. The graphite leaf used in the invention has an ash content of preferably not more than 0.2%, more preferably not more than 0.1%, by weight as well as preferably a bulk density of 1.75 g/cc or more. Additionally, the sheeting composed of laminated graphite leaves desirably has a thickness of about 0.2∼0.4 mm. To interpose such sheetings between the heat insulating layers and the shaped body, it is preferred for the sheetings to be attached onto the inner surface of the heat insulating layers or, when a carbon cylinder is provided inside, onto the inner surface of the cylinder. Further, the concept of the present invention is suitably applicable to the process as well as the apparatus for firing not only nitride ceramics but also other non-oxide ceramics such as carbide ceramics or the like. However, the non-oxide ceramic which the present invention can be most suitable applied to is silicon nitride. The high purity graphite to be used for the carbon heater preferred for the furnace of the present invention has a carbon content of preferably at least 99.9985%, more preferably at least 99.9995%, a silicon content of preferably not more than 4 ppm, more preferably not more than 2 ppm, and an iron content of preferably not more than 8 ppm, more preferably not more than 3 ppm, by weight. Additionally, the above high purity graphite has a bulk density of preferably at least 1.75 g/cm³, more preferably 1.76 g/cm³. The construction and features of the present invention will be further explained in more detail with reference to the preferred embodiments taken in connection with the accompanying drawings, wherein: Fig. 1 is a vertical cross-sectional view illustrating an embodiment of the furnace according to the present invention for sintering silicon nitride; and Fig. 2 is a schematic vertical cross-sectional view illustrating a different embodiment of the furnace according to the present invention. In Fig. 1, a furnace body is comprised of a vertical cylinder 1 having a cylindrical, prismatic or other outline, provided with an upper lid 2 that hermetically closes the top end of the cylinder and a lower lid 4 that is releasably fixed with clamps 3 on the bottom end of the cylinder. The cylinder, the upper lid and the lower lid are provided with a water jacket, respectively, which has a cooling water inlet 5 and a cooling water outlet 6. Graphitic carbon heaters 8 supported by a heater supporting member 7 are arranged around a shaped body accommodated in space A in the center of the furnace and connected with an electric source via a heater terminal 9. Further, on the lower lid 4, a table 10 is supported with rods 11, on which a shaped body 12 is loaded. Each inner wall surface of the cylinder, upper lid and lower lid is covered and thermally shielded by an insulating layer 13 composed of a carbon fiber mat. An exhaust conduit 14 is connected with an evacuating device such as a vacuum pump (not shown), and an inert gas, e.g., nitrogen gas, supply conduit 15 is connected with a pressurized inert gas supply device. Additionally, the furnace body is usually equipped with a thermocouple 16 and a sight hole 17 for measuring, controlling and monitoring temperature conditions, etc., during operation. This furnace for sintering silicon nitride is provided with graphite sheetings 18 interposed between the shaped body 12 and the heat insulating layer 13, preferably covering uniformly all over the inner sufaces of the heat insulating layers, to intercept a free communication between the atmosphere surrounding the shaped body 12 and the atmosphere along the vicinity of the heat insulating layers 13. In the embodiment shown in Fig. 1, such sheetings are provided extending all over the inner surfaces of the heat insulating layers and, however, in the case where a carbon cap or cylinder enclosing the space A accommodating the graphite heaters 8 together with the shaped body is provided (not shown), it is preferred that the above sheetings are attached throughout the length and breadth of the inner wall of the cap or cylinder. However, the graphite sheetings according to the present invention are attached not necessarily extending all over the inner wall surfaces throughout the length and breadth thereof. It is apparent that only to attach to a portion where the heat insulating layer of carbon fiber mat is otherwise intensely worn, namely, a portion near the heaters, can exert an appreciable effect. The above graphite sheeting is composed of laminated high purity graphite leaves. The leaf is formed from graphite that has been subjected to a high purification treatment to reduce the ash content to not more than 0.3%, preferably not more than 0.2%, more preferably not more than 0.1%, by weight, in order to suppress impurities generating from the graphite itself at high temperatures in the minimal amount. Such a sheeting can enough withstand temperatures of at least about 2,500°C under nitrogen gas atmosphere. The amount of the ash this graphite sheeting contains has a close relation with the life of the graphite sheeting in the case of a repeated use at a temperature of about 2,000°C. When the ash content is 0.3% or less, preferably 0.1% or less, the life of the furnace materials is very much prolonged advantageously. It is preferred that the sheeting has a thickness of about 0.2 mm ∼ 0.4 mm. If too thin, it becomes so deficient in strength that a risk of breakage arises when it is attached or installed, while, if too thick, its machinability will undesirably decrease. To attach the above sheeting to the inner surfaces of the heat insulating layers, it may be fastened by sewing with carbon fiber threads or adhered with special carbon adhesives. However, for feasible and simplified working, it is most preferred to use a heat resistant carbon fastening material as proposed by the present inventors in Japanese Utility Model Registration Application No. 62-80,942, namely, a pin formed from graphite integrally into a whole body composed of a large diametric disc-like member having a flat lower contact surface and a small diametric rod-like fastener member extending vertically from the center of said contact surface. Fig. 2 is also a vertical cross-sectional view illustrating a modification of the embodiment shown in Fig. 1, wherein the same parts are designated by same numbers. The apparatus shown in Fig. 2 has a structure substantially same as that in Fig. 1, except that the upper lid 2 is releasably fixed to the cylinder 1 and the table 10 to be loaded with the shaped body 12 is suspended with rods 11 from the upper lid 2. The functions of the apparatus and process according to the present invention will be explained hereinafter with reference of the furnace shown in Fig. 1. At the outset, releasing the engagement of the clamp 3, the lower lid 4 is taken off together with the table 10 loaded thereon from the cylinder 1 and descended by means of a lift or the like. After an Si₃N₄ shaped body 12 containing metal oxide sintering aids that has been molded according to a conventional method is placed on the table 10, the lower lid is ascended again to put the above shaped body into the furnace and fixed to the cylinder with the clamp 3. Then, the vacuum pump is operated to evacuate air inside the furnace through the air exhaust conduit 14 and then an inert gas, preferably nitrogen gas, is fed in through the inert gas supply conduit 15 to replace the atmosphere inside the furnace by nitrogen gas. In this condition, a voltage is applied via the terminal 9 to the graphitic carbon heater 9, to raise the furnace internal temperature up to about 1,700°C∼1,900°C that is kept for about 1 hour to effect sintering. During the sintering, the furnace walls, since shielded with the heat insulating layers 13 and further covered by the water jackets, are kept at a safety temperature of at most several hundred degrees. During sintering at a high temperature, a trace of oxygen, oxides, oxynitrides or the like liberated from the sintering aids and/or silicon nitride is blocked by barriers of the graphite sheetings 18 and prevented from contact with high temperature oxidizable, carbon fiber insulating layers. Alternatively, fibrous dusts such as carbon fiber fine fibrils formed by breaking and disintegrating by virtue of the action of a trace of surface oxygen originally held by the heat insulating layer constituting carbon fibers or oxygen incidentally entering through the above barriers, are confined by the graphite sheetings 18 within the vicinity of the furnace walls, so that the flying and floating fibrous dusts never come out through the sheetings to the inner side to contact with the shaped bodies. Additionally, since the graphite sheeting itself is composed of a high purity graphite having an extremely reduced ash content, impurities such as oxygen or metal oxides generating from the sheeting are limited in an amount within a virtually harmless range, so that the shaped body accommodating space is kept under a very clean atmosphere. Thus, the wear of the sintering aids decreases markedly and virtually no skeletonization of the Si₃N₄ takes place. Consequently, a high quality Si₃N₄ sintered body wherein Si₃N₄ needle crystals are uniformly dispersed in glassy phases of sintering aids even at the surface layers can be obtained. Further, as to carbon heaters to be used in furnaces, conventional ones have generally been fabricated by the steps of: kneading a carbon material comprising pulverized coke, etc., admixed with pitch, etc., to form a paste; extruding or injection-molding the paste into a rod-like structure; and graphitizing by firing the rod-like structure with a desired shape. Such graphite materials, on the one hand, have been extensively used because they are manufacturable at the lowest cost and can achieve a required high temperature. However, on the other hand, they are appreciably high in ash content including silicon and iron and, moreover, low in density such as about 1.65 g/cm³, which have constituted main causes for the abovementioned problems. As graphite materials to be used for the carbon heaters preferred in the furnace of the present invention, suitable ones are fabricated by graphitization through firing according to a conventional method of a body material which has been molded not by anisotropic molding, for example, extrusion-molding, injection-molding, etc., but by isotropic molding by means of a die molding, more preferably a cold isotropic press (CIP) molding, followed by a high purification treatment wherein heating is conducted under an inert gas atmosphere introducing a halogen gas thereinto, to eliminate impurities. The graphite material obtained by the above-mentioned process has a carbon content of at least 99.9980%, preferably at least 99.9985%, more preferably at least 99.9995%, by weight, and in its impurities, a silicon content of not more than 5 ppm, preferably not more than 4 ppm, more preferably not more than 2 ppm, and an iron content of not more than 9 ppm, preferably not more than 8 ppm, more preferably not more than 3 ppm, by weight. If the carbon content is less than 99.9980% and the silicon content and iron content exceed 5 ppm and 9 ppm, respectively, improvements in surface strength and antioxidation property of the sintered body are not substantially recognized and elongation of the life of the heater as well as prevention of the deterioration of the thermocouple are not achievable. Additionally, the aforementioned isotropic molding process can provide a graphite material with a density of 1.75 g/cm³ or more, which is desirable for the carbon heater. If the density is too low, it is not preferred because opportunities for oxygen, oxides, etc. to enter between graphite molecules increase. The carbon heaters made of such a high purity graphite material are suitably applicable to a furnace for sintering non-oxide ceramics, such as not only nitride ceramics but also carbide ceramics or the like, and further can be advantageously employed in a furnace for growing Si single crystals, etc. By applying the above high purity graphite material to the carbon heaters, liberation and flying of the carbon particles caused by disintegration, poromerization, etc. of the graphite itself are decreased, whereby the internal atmosphere of the furnace can be kept very clean. A process of the invention for firing non-oxide ceramics by using such carbon heaters in the furnace will be explained. Nitride powder such as Si₃N₄, BN or the like admixed with metal oxidic sintering aids is molded by means of a cold isotropic press molding such as die molding, rubber pressing or the like, to form shaped bodies. The furnace is loaded with the thus fabricated shaped body, of which internal atmosphere is replaced by an inert gas, particularly nitrogen gas, and pressurized to increase the partial pressure of the gas, if required. Under such conditions, a voltage is applied to the carbon heaters to raise the internal temperature of the furnace to at least about 1,700°C and below the sublimating temperature of the nitride, usually up to about 1,800°C, which temperature is kept for 1 hour to effect sintering. In the present invention, the use of the carbon heaters composed of a high purity graphite material having an extremely high carbon content and very low impurity content noticeably decreases liberation and flying-out of carbon fine particles from the graphite during sintering at a high temperature. Accordingly, the formation of formicary-like pores in the graphite itself of the heaters is virtually prevented and so the internal atmosphere of the furnace that contacts with the shaped body is kept in a clean condition that contains extremely reduced carbon particles. Therefore, the wearing of the sintering aids due to drawing-in by shaped body of the carbon particles is prevented and the skeletonization of the nitrides also noticeably decreases, so that a high quality nitride sintered body wherein nitride needle crystals are dispersed uniformly in glassy phases of the sintering aids to the surface layers of the sintered body is obtained. Additionally, since generation of gases such as oxygen, oxides or the like from the shaped body during the sintering is suppressed a great deal, virtually no attack on the graphite is induced and even if extremely small quantities of these generating gases contact with surfaces of the dense graphite, they cannot enter into the depths, so that disintegration of the graphite skeleton decreases and the heaters can remain in a good condition for a long period of time. In sintering Si₃N₄, shaped bodies are generally encased in SiC crucibles, Si₃N₄ crucibles or carbon crucibles having SiC densely deposited surfaces and then fired. It is an effect of the crucibles to suppress an influence exerted by carbon fiber dusts existing in the furnace, liberated from insulating layers, or by gases such as CO, CO₂ or the like generated by decomposition of the heater material. Additionally, the crucibles fill the role of firing the sintered bodies with high efficiency in a geometrically piled up state. Needless to say, also in the case where such crucibles are employed, the present invention can afford the same effect. It is additionally noted that, when the crucibles are made of Si₃N₄, etc., the present invention exerts an effect in respect of extending the life of the crucibles by preventing their skeletonization. The present invention will be further explained by way of examples. In the following examples, "percent" and "part" are all by weight. The ash content in the graphite sheeting was determined in accordance with JIS R 7223, namely, a method wherein the sheeting specimen was put into a platinum crucible, and after igniting at 800°C in an oven, the remaining ash was weighed. Example 1 To 90% of powdery Si₃N₄, were added 1% of SrO, 4% of MgO and 5% of CeO₂ as sintering aids, and after mixing thoroughly, the mixture was molded using a mold press into a plate of 10 mm x 60 mm x 60 mm. A furnace as shown in Fig. 1 was loaded with the above plate, and the internal atmosphere was replaced by N₂ gas and then kept at 1,700°C for 1 hour to sinter the plate. Graphite sheetings were attached to all over inner surfaces of heat insulating layers composed of carbon fiber felt in the furnace. The graphite sheetings were fabricated by laminating graphite leaves and adhering to each others with graphitic adhesive V58a (Registerer Trade Mark-manufactured by SIGRI, West Germany), followed by firing in nitrogen gas at about 600°C. The graphite sheetings had a thickness of about 0.4 mm and an ash content of 0.1%. Example 2 An Si₃N₄ sintered body was obtained in the same manner as the above Example 1 except that the graphite sheetings were not attached on to the inner surfaces of the heat insulating layers but to all over inner wall surfaces of a graphite cylinder (a bulk density of 1.75 g/cm³, and a wall thickness of 5 mm) that was arranged so as to enclose the carbon heaters 8 and the shaped body 12 in the furnace. Example 3 An Si₃N₄ sintered body was obtained in the same manner as the above Example 1 except that the graphite sheetings had an ash content of 0.3%. Comparative Example 1 An Si₃N₄ sintered body was obtained in the same manner and with the same apparatus as the above Example 1 except that the graphite sheetings were not attached. Comparative Example 2 An Si₃N₄ sintered body was obtained in the same manner and with the same apparatus as the above Example 2 except that the graphite sheetings were not attached. Characteristics of the sintered bodies obtained in the above Examples and Comparative Examples are shown altogether in Table 1 below. As is clear from the above Table 1, it has been exemplified that the Si₃N₄ sintered body obtained according to the process of the invention is very low in percent weight loss and loss of sintering aids of the sintered body as compared with conventional articles. This means the fact that virtually no skeletons of Si₃N₄ are formed which is also proved by the result showing that the sintered body according to the invention is extremely high in flexural strength of the sintered surface as compared with conventional ones. Additionally, the sintered body produced according to the invention is extremely stable against a high temperature oxidation reaction, and the fluorescent flaw detection has demonstrated it has a dense and substantially void-free texture. Thus, its excellence in abrasion resistance and thermal shock resistance is understood. Further, when the graphite sheetings have an ash content of 0.3%, though the life of the furnace is relatively short, a good silicon nitride sintered body is obtainable. It has been found that the ash content of the graphite sheetings used according to the present invention exerts a significant function on the characteristics of the sintered body, and an effect of the combined use of a graphite cylinder is also excellent. The following examples and discussion illustrate the effect of the carbon heaters which are preferred for use in the present invention. The cause of the deterioration of carbon heaters has so far been accounted as an action of oxygen contained in the ambient gas. The following experiment was conducted to confirm the above. Nitrogen gas was selected as the ambient gas. Admixing a very small quantity of oxygen, two kinds of nitrogen gas having a purity of 99.999% and 99.90%, respectively, were prepared. Under respective nitrogen gas atmospheres, heating at about 1,800°C for 1 hour with a carbon heater was repeated 100 times and in both cases no significant difference in state of deterioration was observed between two carbon heaters used in the different atmospheres. Examples 4∼8 90% of Si₃N₄ powdery material, 1% of SrO₂, 4% of MgO and 5% of CeO₂ were mixed and molded with a die-pressing machine into a square plate Si₃N₄ molded specimen of 6 mm × 60 mm × 60 mm. A furnace having an inside diameter of 400 mm and a height of 1,000 mm was loaded with the above specimen and kept at 1,700°C under nitrogen gas partial pressure of 100 KPa (1 atm.) for 1 hour to effect firing. The above firing was conducted for each of carbon heaters composed of six kinds of graphite materials, respectively, shown in Table 2 below. Impurity elements were measured by atomic-absorption spectroscopy. The result of the investigation of characteristics of the sintered body fired using each heater is shown in Table 3 below. As is clear from the result shown in Table 3, the Si₃N₄ sintered bodies obtained in Examples 4 to 8 are extremely high in flexural strength of the sintered surface as compared with the conventional one. Additionally, these sintered bodies are extremely stable against a high temperature oxidation reaction. The fluorescent flaw detection has demonstrated they have a dense and substantially void-free texture. Thus, their excellent abrasion resistance and thermal shock resistance is understood. Particularly, the Si₃N₄ sintered body obtained by Comparative Example 1 wherein heater No. 1 was used had a color shade difference such that surfaces exhibited a white shade and interior portions several millimeters inside from the surface became dark grey, and the surface layers had been skeletonized. Examples 9∼13 Si₃N₄ sintered bodies were obtained in the same manner as the above Examples 4∼8 and Comparative Example 3, except that the nitrogen gas partial pressure was 1000 KPa (10 atm.) and the sintering temperature was 1,750°C. The results are shown in Table 4 below. As is clear from the comparison of the results in Table 4 with those in Table 3, the strength of the sintered bodies further improves when the ambient nitrogen gas partial pressure is increased and the sintering temperature is raised. Examples 14∼18 Using carbon heaters composed of 6 kinds of graphite materials, respectively, shown in Table 2, sintering of Si₃N₄ at a sintering temperature of 1,800°C under a nitrogen gas partial pressure of 1000 KPa (10 atm.) for 1 hour was repeated. The durabilities of the heaters and thermocouples were studied. The result was as shown in Table 5 below. It is understood from the above Table 5 that the appropriate choice of the carbon heater conspicuously extends the life of the carbon heater itself as well as the period of time to maintain the function of the thermocouple. By virtue of the improvement in quality and performance obtainable with the invention, the applicable fields of nitride ceramics are expected to be further extended and diversified. By the present invention, satisfactory improvements are readily achievable by attaching graphite sheetings on to the inner sufaces of the heat insulating layers, so that the sintering of Si₃N₄ shaped bodies is achieved with high industrial feasibility and economic advantage, and can be materialized with far small investment and running cost as well as without increasing power consumption, as compared with conventional processes and apparatuses such as that require, for example, an expensive, large sized graphite cylinder that forces the cost to increase, e.g., due to an increase of consuming rate of heaters caused by a thermal load increase. The present invention makes it possible to achieve excellent effects that have never been attained so far and accomplish further improvements of quality and characteristics, if such a graphite cylinder is employed in combination. Further, the present invention can also provide that prevention of contact of the carbon fiber dusts generated form heat insulating layers with the Si₃N₄ shaped body extends the lives of heat insulating layers and carbon heaters and also extends the life of the furnace body. Furthermore, since the present invention extends lives of expensive heaters and W/Re thermocouples and maintains good functions thereof for a long period of time, it has prominent economical advantages, rendering a continuous production possible in addition to its two-bird-one-stone effect, that is, savings of expenses by virtue of exchange frequency reduction and quality homogenization resulting from stabilization of manufacturing conditions.

Claims: A furnace for sintering non-oxide ceramics which comprises a space for accommodating a ceramic shaped body (12) to be sintered, at least one carbon heater (8) adjacent said space, a heat insulating layer or layers (13) of carbon fiber mat covering the inside of the furnace wall, and carbon sheeting or sheetings (18) between said heat insulating layer or layers (13) and said space for the ceramic shaped body, characterised in that said sheeting or sheetings (18) is or are composed of laminated graphite leaves having an ash content of not more than 0.3% by weight. A furnace as claimed in claim 1, wherein said graphite leaf has a bulk density of 1.75 g/cm³ or more. A furnace as claimed in claim 1 or claim 2 wherein said sheeting (18) has a thickness in the range 0.2 to 0.4 mm. A furnace as claimed in any one of claims 1 to 3, wherein carbon heater or heaters (8) are composed of a high purity graphite having a carbon content of at least 99.9980%, a silicon content of not more than 5 ppm by weight and an iron content of not more than 9 ppm by weight. A furnace as claimed in claim 4, wherein said graphite of said heater or heaters (8) has a bulk density of at least 1.75 g/cm³. A method of firing a shaped body (12) molded from a mixture of non-oxide ceramic powder and at least one sintering aid under a high temperature inert gas atmosphere surrounded by an insulating layer or layers (13) composed of a carbon fiber mat with carbon sheeting or sheetings (18) interposed between said insulating layer or layers (13) and said shaped body (12) characterized in that, to protect said shaped body (12) from the insulating layer or layers (13) said sheeting or sheetings (18) is or are composed of laminated graphite leaves having an ash content of not more than 0.3% by weight. A method as claimed in claim 6 wherein said non-oxide ceramic is silicon nitride.

Inventors: HANZAWA SHIGERU, KOBAYASHI KAZUO, HANZAWA, SHIGERU, KOBAYASHI, KAZUO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1988
Main classification: F27B17
IPC codes: F27D21, F27D1, F27B21, F27D23, F27B5