JPH0423397B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a carbon heater used for firing ceramics, and a powder compact made of a non-oxide ceramic raw material powder and a sintering aid in a firing furnace under an inert atmosphere at a high temperature. The present invention relates to a sintering furnace using the carbon heater for sintering by heating to . (Conventional technology) Nitride ceramic raw material, for example silicon nitride
Si ₃ N ₄ or boron nitride BN are materials that are difficult to sinter, and metal oxides (MeO) or metal oxides such as MgO and Al ₂ O ₃ are used as sintering aids to promote sintering. and a metal nitride.
It is common to add 10%, and a Si ₃ N ₄ molded body before sintering usually has about 40% by volume of pores. Here, the mechanism of the strength development of silicon nitride is
A type of fiber-reinforced ceramic, in which acicular final particles of β-type silicon nitride as a reinforcing agent are dispersed in a glass phase of a metal oxide added as a sintering aid, i.e.
It is said that excellent strength characteristics are developed by forming FRC (Fiber Reinforced Ceramics). Furthermore, taking Si ₃ N ₄ as an example, such compacts are generally produced in a high temperature inert atmosphere, especially in a nitrogen gas atmosphere.
It is fired at a temperature of 1700℃ to 1900℃. A typical firing furnace for stably maintaining such high temperatures in an inert atmosphere consists of a space for housing the compact, a carbon heater placed around the space, and a carbon fiber mat covering the inner wall surface. It has a built-in heat insulating layer. Carbon fiber mat has an extremely high porosity and an average bulk density of about 0.2 g/cc in order to provide good heat insulation. However, the carbon fibers in the surface layer of the pine come into contact with trace amounts of oxygen, oxides, or oxynitrides generated from the Si ₃ N ₄ compact containing metal oxides at high temperatures during high-temperature firing, resulting in oxidation.
It will peel off little by little. The flaked carbon fiber dust scatters and floats in the furnace, attaches to the Si ₃ N ₄ compact with high porosity before or during sintering, and can be taken into the compact when the compact shrinks due to sintering. It can happen. The carbon reacts with the metal oxide, which is a sintering aid, and becomes CO or CO ₂ and escapes into the furnace.At the same time, the metal oxide is reduced and becomes a low-melting-point metal, which evaporates, forming a glass phase matrix. The metal oxide that would have formed is lost, especially in the surface layer, leaving behind a skeleton of Si ₃ N ₄ . In the skeleton state, the Si ₃ N ₄ sintered body no longer has properties such as high strength, high thermal shock resistance, and wear resistance. In addition, when CO, CO _2, etc. generated in the furnace come into contact with the Si ₃ N ₄ molded body, the following reactions occur repeatedly.
Metal oxides (MeO) are rapidly lost. Si ₃ N ₄ +MeO+CO→Si ₃ N ₄ +CO ₂ +Me↑ CO ₂ →CO+O C+O→CO This also promotes the formation of the Si ₃ N ₄ skeleton described above. This prevents the carbon fibers that form the heat insulating layer from unraveling and scattering as carbon fiber dust, which would have an adverse effect on the surface of the molded product, making it a high-quality product with high strength and excellent abrasion resistance and thermal shock resistance. In order to provide a Si ₃ N ₄ sintered body, the present inventor first developed a sheet made by laminating and molding graphite flakes with an ash content of 0.3% by weight or less, and a heat insulating layer made of carbon fiber mat and a molded body. We proposed a method for manufacturing high-quality silicon nitride sintered bodies, which involves intervening carbon fiber dust that separates from the heat insulating layer and blocks contact between the molded body and the molded body. (Problems to be Solved by the Invention) The above-mentioned conventional problems have been largely solved by the proposal of the present inventor, Hiro, but with the rapid development of fine ceramics and the expansion of applications, It was still far from sufficient to meet the demands for higher performance and higher quality. As a result of continued efforts to investigate the remaining problems and investigate the causes, the inventor found that there is a close correlation between the material of the carbon heater and the quality of the nitride ceramic sintered body. We found that they interact. In other words, the main focus of conventional carbon heaters was to manufacture them as inexpensively as possible while still satisfying their heat generation performance. The carbon content is about three nines, and the content of impurities such as silicon and iron is several hundred ppm.
It has been used to some extent. However, when such carbon heaters are heated to high temperatures, the graphite begins to erode and perforate starting from impurity sites such as silicon or iron, causing the carbon to collapse and deteriorate.
It scatters and adheres to the nitride molded body before or during sintering, resulting in skeletonization of the surface layer of the sintered body, as described above. At the same time, oxygen, oxides, or nitride oxides generated from the molded object enter the pores formed in the graphite of the heater, react with carbon deep inside, and cause the bones of the graphite to decay. , while releasing carbon particles, further expanding the pores,
Eventually, an ant nest-like roughness is formed on the heater member.
In this way, the skeletonization of the surface layer of the sintered body due to the emitted carbon further increases and the deterioration of the heater is accelerated. Such a heater not only destroys the phase balance of the heater material and makes accurate temperature control impossible, but also causes a local increase in surface current in the porous portion, leading to disconnection if suspected. In addition to the above-mentioned development, the problem of the adverse effects of suspended carbon particles on thermocouples, which perform an important temperature control function, has been recognized. i.e. 1700~2000℃
When measuring temperature in a high-temperature nitrogen gas atmosphere, it is difficult to ensure accuracy using a two-color thermometer that is normally used for high temperatures due to fluctuations caused by convection of gas in the furnace. Therefore, it is common to use a W/Re thermocouple by incorporating it into a molybdenum protection tube typically filled with argon gas to prevent nitridation of tungsten by nitrogen gas. However, molybdenum protection tubes become carbonized when floating carbon particles adhere to them.
Because MoC is extremely brittle and has a coefficient of thermal expansion different from that of Mo, cracks appear after several firing operations, and the enclosed argon gas leaks out, allowing nitrogen gas to enter.
As a result, tungsten is nitrided, and the electromotive force of the W/Re thermocouple changes, causing it to lose its correct function. The present invention has been made to solve the above-mentioned various problems all at once, and its main purpose is to:
High-quality non-oxide ceramic sintered bodies with high strength and outstanding wear resistance and thermal shock resistance, especially
To provide a Si ₃ N ₄ sintered body. Another purpose is to prevent deterioration of the carbon heater and extend its service life. Yet another objective is to maintain accurate temperature control during firing for a long period of time. (Means for Solving the Problems) The carbon heater according to the present invention to achieve the above-mentioned object has a carbon content of 99.9980% (weight) or more, a silicon content of 5 ppm (weight) or less, and an iron content of
It is characterized by being made of high-purity graphite of 9 ppm (weight) or less, and the sintering furnace for sintering ceramics using this heater is made of non-oxide ceramic raw material powder and a sintering aid. When sintering the powder compact by heating it to a high temperature using a carbon heater in an inert atmosphere in a firing furnace, the carbon heater material has a carbon content of 99.9980% (weight) or more and a silicon content of 99.9980% (weight) or more.
This is a ceramics sintering furnace characterized by keeping the atmosphere inside the furnace clean by using high-purity graphite with an iron content of 5ppm (weight) or less and an iron content of 9ppm (weight) or less. The non-oxide ceramic to which the present invention can be most preferably applied is silicon nitride. The high-purity graphite applied to the present invention preferably has a carbon content of 99.9985% (weight) or more, more preferably 99.9995% (weight) or more, and preferably 4 ppm (weight) or less, more preferably 2 ppm ( It has a silicon content of not more than 8 ppm (by weight), and more preferably an iron content of not more than 3 ppm (by weight). The high purity graphite used in the present invention preferably has a bulk density of at least 1.75 g/cc, more preferably 1.76 g/cc. A suitable inert atmosphere in the present invention is a nitrogen gas atmosphere, most preferably applied under pressure. In the present invention, when performing sintering work in a high-temperature inert atmosphere surrounded by a heat insulating layer made of carbon fiber mat, a method separately proposed by the present inventor is used. The best results can be obtained if the method is used in combination with a method in which a sheet formed by lamination molding is interposed between the heat insulating layer and the molded body to isolate the heat insulating layer and the molded body. Graphite, which is the material for carbon heaters, is traditionally made by kneading carbon fibers made by adding pitch to crushed coke or other materials to form a paste.
Generally, a rod-like structure is formed by extrusion or injection molding and then fired to form graphite into a desired shape. Such graphite members have been widely used because they are the cheapest to manufacture and have sufficient ability to achieve the required high temperatures, but they have a high ash content, including silicon and iron, and a density of about 1.65. g/cc, which was the main cause of the above-mentioned problems. The graphite member using the carbon heater as a raw material applied to the present invention is not produced by anisotropic molding such as extrusion molding or injection molding, but by mold pressing method, more preferably cold isostatic pressing (CIP) method. It is preferable that the isotropically molded base material is fired and graphitized according to a conventional method, and then subjected to a high purification treatment in which halogen gas is introduced into an inert atmosphere and heated to remove impurities. . obtained by the method described above, with a carbon content of at least
99.9980% (by weight), preferably at least
99.9985% (by weight), more preferably at least
99.9995% (by weight), silicon content among impurities
Iron content is 5ppm (weight) or less, preferably 4ppm (weight) or less, more preferably 2ppm (weight) or less, and the iron content is 9ppm (weight) or less, preferably 8ppm (weight) or less, and more preferably 3ppm (weight) or less. A graphite member is applied as a material for the carbon heater of the present invention. When the carbon content is less than 99.9980% (by weight), the silicon content is more than 5 ppm (by weight), and the iron content is more than 9 ppm (by weight), there is little improvement in the surface strength and oxidation resistance of the sintered body, and the heater Extending the life of the thermocouple and preventing deterioration of the thermocouple are not substantially achieved. Further, by the above-mentioned isotropic molding method, it is possible to form a graphite member having a density of 1.75 g/cc or more, and a material having such a density is desirable as the heater material of the present invention. If the density is too low, there is an increased chance that oxygen, oxides, etc. will enter between the molecules of graphite, which is undesirable. Carbon heaters made from such high-purity graphite can be used not only with nitride ceramics, but also with
It is suitable as a heater for a firing furnace for non-oxide ceramics such as carbides, and can also be advantageously applied to heaters for Si single crystal growth furnaces. If a heat insulating layer made of carbon fiber molded into a pine shape is attached to the inner wall of the firing furnace, a graphite sheet may be uniformly interposed between the molded body to be fired and the heat insulating layer. Therefore, it is most desirable to apply the present invention after blocking free flow between the atmosphere surrounding the molded body and the atmosphere along the vicinity of the heat insulating layer. The above-mentioned graphite sheet is made by laminating and molding high-purity graphite flakes, and in order to minimize impurities generated from the graphite sheet itself at high temperatures,
It is formed using highly purified graphite with an ash content of 0.3% by weight or less, preferably 0.2% by weight or less, more preferably 0.1% by weight or less. Such sheets are heated at temperatures of at least about 2500°C under a nitrogen gas atmosphere.
can withstand temperatures of The thickness of the sheet is preferably about 0.2 mm to 0.4 mm; if it is too thin, the strength will be insufficient and there is a risk of breakage during attachment or stretching, while if it is too thick, the workability will deteriorate, which is undesirable. By using the above-mentioned high-purity graphite as the carbon heater material, the release and scattering of carbon due to the collapse of graphite itself, perforation, etc. is significantly reduced, and the atmosphere inside the furnace can be kept extremely clean. (Function) Si ₃ N ₄ with metal oxide sintering aid added by conventional method
Alternatively, a compact formed by molding nitride powder such as BN or by cold isostatic pressing such as a rubber press is charged into a firing furnace, and the atmosphere inside the furnace is replaced with an inert gas, especially nitrogen gas. Then, increase the gas partial pressure further if necessary. In this state, voltage is applied to the carbon heater to raise the temperature inside the furnace to approximately 1700℃.
The temperature is raised to below the sublimation temperature of nitride, usually about 1800°C, and the temperature is maintained for about 1 hour for firing. In the present invention, by using high-purity graphite with an extremely high carbon content and very few impurities as the carbon heater material, the release and scattering of carbon particles from the graphite during high-temperature firing is significantly reduced. Ant nest-like perforation of the graphite itself is almost prevented, and the atmosphere in the furnace that comes into contact with the molded body is maintained in a clean state in which carbon particles are significantly reduced. As a result, the depletion of the sintering aid due to carbon incorporation into the molded body is prevented, and skeletonization of nitride is also significantly reduced. A nitride-based sintered body is obtained.
In addition, by greatly suppressing the generation of gases such as oxygen and oxides from the compact during firing, it virtually eliminates the possibility of inducing erosion of graphite, and the extremely small amount of these gases generated becomes dense. Even if it comes into contact with the surface of graphite, it cannot penetrate deep inside, so the collapse of the bones of graphite is reduced, and the heater can maintain a good condition for a long period of time. (Example) The above-mentioned present invention will be explained with reference to an example. All "percents" and "parts" in the examples are based on weight. Experimental example The causes of carbon heater deterioration are conventionally
It was thought that this was due to the action of oxygen contained in the atmospheric gas, so the following experiment was conducted to confirm this. Nitrogen gas was selected as the atmospheric gas, and a trace amount of oxygen was mixed to prepare two types of nitrogen gases with purity of 99.999% and 99.90%. Heating was repeated 100 times at about 1800°C for 1 hour using each nitrogen gas, but no significant difference was observed in the state of carbon deterioration in both cases. Examples 1 to 5 90% Si ₃ N ₄ raw material powder, 1% SrO ₂ , 4% MgO and 5% CeO ₂ were mixed and molded into 6mm×60
The sample was a Si ₃ N ₄ molded body formed into a square plate of mm x 60 mm. This product was placed in a firing furnace with an inner diameter of 400 mmφ and a height of 1000 mmH, and fired at a nitrogen gas partial pressure of 1 atm. and maintained at 1700° C. for 1 hour. At this time, six types of graphite shown in Table 1 were prepared as carbon heater graphites. Note that impurity elements were measured by atomic absorption spectrometry.

[Table] Table 2 shows the results of the sintered body characteristics investigation conducted using each heater.

[Table] As is clear from the results in Table 2, the Si ₃ N ₄ sintered body obtained by the present invention has extremely high bending strength on the fired surface compared to the conventional product. It is also extremely stable against high-temperature oxidation, and fluorescent flaw detection has demonstrated that it has a dense structure with few voids, indicating that it has excellent wear resistance and thermal shock resistance. In particular, the Si ₃ N ₄ sintered body obtained in Comparative Example 1 using heater number 0 exhibited a hue difference in that the surface was white and the inside several millimeters from the surface turned black-gray. It had become a skeleton. Furthermore, it has been found that the carbon and impurity contents of the graphite of the carbon heater applied to the present invention have a significant effect on the characteristic values of the sintered body. Examples 6 to 10 Si ₃ N ₄ sintered bodies were obtained in the same manner as in the Comparative Examples and Examples, except that the nitrogen gas partial pressure during firing was 10 atm and the firing temperature was 1750°C. The test results are shown in Table 3.

[Table] As is clear from comparing the results in Table 3 with Table 2, the strength of the sintered body is further improved by increasing the atmospheric nitrogen gas partial pressure and firing temperature. Examples 11 to 15 Using a carbon heater made of six types of graphite shown in Table 1 as heater materials, Si ₃ N ₄ was fired under the conditions of nitrogen gas partial pressure of 10 atm, firing temperature of 1800°C, and firing time of 1 hour. The durability of the heater and thermocouple was investigated. The results were as shown in Table 4.

[Table] From the results in Table 4, it is confirmed that the present invention has the effect of significantly extending the service life of the carbon heater itself and the functional maintenance period of the thermocouple compared to the conventional method. When sintering Si ₃ N ₄ , the compact is
SiC crucible, Si ₃ N ₄ crucible or surface
It is common to store it in a carbon crucible or the like in which SiC is densely deposited and then sinter it. This is because it has the effect of suppressing the effects of carbon fiber waste, which is an insulating material, that exists in the furnace, and the effect of suppressing the effects of gases such as CO and CO ₂ caused by the decomposition of the heater material. It plays the role of efficiently firing the sintered body by assembling the sintered body. It goes without saying that the present invention produces similar effects even when such a crucible is used. Furthermore, it should be noted that when the crucible is made of Si ₃ N ₄ or the like, it is effective in preventing skeletonization of the crucible and extending its life. (Effects of the Invention) As described above, according to the present invention, when non-oxide ceramics, particularly Si ₃ N ₄ ceramics, are fired, the atmosphere inside the firing furnace is less contaminated by carbon particles generated from the carbon heater, making it easier to clean. This prevents Si ₃ N ₄ skeletonization due to depletion of the sintering aid on the surface of the sintered body, resulting in high strength and wear resistance.
A homogeneous and high quality Si ₃ N ₄ sintered body with excellent thermal shock resistance can be obtained. These improvements in quality and performance are expected to further expand the range of applications for nitride ceramics. In addition, the present invention extends the service life of expensive heaters and W/Re thermocouples and maintains good functionality for a long period of time, resulting in cost savings due to reduced replacement frequency and quality quality due to stable manufacturing conditions. In addition to the effect of killing two birds with one stone by making it more uniform, the economic advantage of making continuous mass production possible is significant.

Claims (1)

[Claims] 1. Carbon content is 99.9980% (weight) or more, silicon content is 5 ppm (weight) or less, and iron content is 9 ppm (weight).
A carbon heater used for firing ceramics, characterized by being made of the following high-purity graphite. 2. The carbon heater according to claim 1, having a bulk density of at least 1.75 g/cc. 3. When firing a powder compact made of a non-oxide ceramic raw material powder and a sintering aid by heating it to a high temperature using a carbon heater in an inert atmosphere in a firing furnace, the carbon heater material is used as the carbon heater material. Carbon content 99.9980% (weight)
Above, silicon content is 5ppm (weight) or less, iron content is
A firing furnace for ceramics sintering that is characterized by maintaining a clean atmosphere inside the furnace by applying high-purity graphite of 9 ppm (weight) or less. 4. The firing furnace for sintering ceramics according to claim 3, wherein the non-oxide ceramic is silicon nitride.