JPH0152325B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a silicon carbide powder suitable for producing a high-density and high-strength silicon carbide sintered body, and a method for producing the same.In particular, the present invention relates to a silicon carbide powder for sintering containing nitrogen and a method for producing the same. Regarding. Silicon carbide has extremely excellent chemical and physical properties, and its high-density sintered body is particularly useful for high-temperature structural materials used under harsh conditions such as gas turbine parts and high-temperature heat exchangers. This material is suitable for such uses. Conventionally, pressure sintering methods and reaction sintering methods are widely known as methods for sintering silicon carbide. However, according to the former pressure sintering method, it is difficult to manufacture sintered bodies with complicated shapes, and the productivity does not increase.
On the other hand, the latter reactive sintering method has the disadvantage that it is difficult to obtain a sintered body with high strength, and furthermore, it contains a large amount of free silicon, making it difficult to use in a high temperature range. In addition, the pressureless sintering method, which is commonly used to produce oxide ceramics, in which the formed body is molded at room temperature and sintered under no pressure, is not used for sintering silicon carbide. Although it was thought to be difficult until now, a pressureless sintering method has recently been reported in which a mixed powder consisting of silicon carbide, boron-containing additives, and carbonaceous additives is compacted and sintered in an inert atmosphere. has been done. Incidentally, silicon carbide is known to have many crystal forms, and has conventionally been roughly classified into two types: hexagonal α type and cubic β type. Of these, the former α
The mold has 4H and 6H, which are stable even at high temperatures such as 2000℃ or higher.
Two types of silicon carbide are known: silicon carbide that is stable at high temperatures such as molds, and 2H type silicon carbide that is unstable at temperatures below 1500°C. Conventionally, as a starting material for the pressureless sintering method of silicon carbide, for example, according to Japanese Patent Application Laid-Open No. 53-84013, silicon carbide, which is most commonly produced in a furnace of the Acheson process, is crushed and subjected to particle size classification. High-temperature-stable α-type silicon carbide powder obtained by In order to obtain a dense sintered body, it is necessary to increase the sintering temperature, and the sintered body obtained using high-temperature stable α-type silicon carbide as a starting material has a pseudospherical shape with no anisotropy. Since the structure consists of relatively coarse crystal grains, it has been difficult to obtain a high-strength sintered body. On the other hand, according to JP-A-50-160200, β-type silicon carbide powder with submicron particle size is produced from silicon halide and hydrocarbon by plasma jet reaction, and JP-A-54-67599 According to the publication, a method for producing high purity β-type silicon carbide powder obtained by thermally decomposing an organic silicon polymer compound is disclosed. Furthermore, the inventors of the present invention
In Japanese Patent No. 40527, we proposed a method for producing silicon carbide mainly consisting of β-type crystals using silica and carbon as starting materials. However, if silicon carbide powder consisting of β-type crystals described in the above-mentioned publications is used as a starting material for pressureless sintering, a gas phase will be generated due to thermal decomposition of SiC during sintering, and sintering will occur through this gas phase. As the sintering progresses, crystal grains made of α-type crystals grow rapidly and coarse plate-like crystals are formed, which has the drawback of hindering the densification of the sintered body. As mentioned above, it has been difficult to produce a pressureless sintered body of high density and high strength silicon carbide using conventionally known silicon carbide powder. An object of the present invention is to provide a silicon carbide powder that is extremely suitable for producing a pressureless sintered body and a method for producing the same. According to the present invention, it contains 0.015-0.20% by weight of nitrogen and 0.01-1.0% by weight of oxygen, with the balance being SiC.
and unavoidable impurities, and at least 80% by weight of the SiC is SiC consisting of β-type crystals,
The above object can be achieved by providing a sintering silicon carbide powder having a specific surface area of 5 to 50 m ² /g and a method for producing the same. Next, the present invention will be explained in detail. Conventionally, it has been thought that high-purity silicon carbide powder containing as few impurities as possible other than sintering aids such as carbon-containing additives and boron-containing additives is advantageous as a raw material for producing sintered bodies. However, it was not known that silicon carbide powder containing a relatively large amount of nitrogen is suitable as a raw material for producing a sintered body. When producing silicon carbide powder mainly composed of β-type crystals, the inventor charged nitrogen gas into a reaction furnace to prepare nitrogen-containing silicon carbide powder, and sintered the silicon carbide powder. When used as a raw material for manufacturing a sintered body, it was found that the nitrogen-containing silicon carbide powder, which had not previously been considered to be good as a raw material for manufacturing a sintered body, had excellent sintering properties. The sintered body manufactured using this method has extremely superior properties compared to the sintered body manufactured using conventional silicon carbide powder mainly composed of β-type crystals with low nitrogen content. I discovered something new. i.e. 0.015-0.20 wt% nitrogen and 0.01
Contains ~1.0% by weight of oxygen, with the remainder consisting of SiC and unavoidable impurities, and at least 80% of the SiC
By using silicon carbide powder for sintering that is SiC consisting of β-type crystals in weight% and has a specific surface area of 5 to 50 m ² /g, the thermal decomposition reaction of silicon carbide during sintering can be suppressed. It is possible to optimize the phase transformation of β-type crystals to α-type crystals, and prevent the formation of coarse plate-like crystals due to phase transformation, thereby obtaining a high-strength sintered body with a high density and uniform microstructure. be able to. According to the present invention, the silicon carbide powder is 0.015-0.20
It is necessary to contain % by weight of nitrogen and 0.01-1.0% by weight of oxygen, with the remainder consisting of SiC and unavoidable impurities. The reason for limiting the nitrogen content to within the range of 0.015 to 0.20% by weight is that if it is less than 0.15% by weight, the effect of optimizing the phase transformation of β-type crystals to α-type crystals will not be sufficiently exerted, Coarse plate-shaped crystals of α-type crystals are formed in the initial stage of sintering, and as a result, it is difficult to obtain a sintered body with a high density and uniform microstructure because it interferes with firing shrinkage. When the amount is more than 1% by weight, the β-type crystals in SiC are significantly stabilized, and the diffusion of SiC during sintering is suppressed, making it difficult for sintering shrinkage to occur during sintering, resulting in a high-density sintered body. This is because it becomes difficult to
Even better results are obtained in the range of 0.10% by weight. Next, the reason for limiting the oxygen content to within the range of 0.01 to 1.0% by weight will be described. Oxygen contained in the silicon carbide powder reacts with carbon added as a sintering aid during sintering, and is removed by a mechanism as shown in the following equation. SiO ₂ +C→SiO+CO……(1) SiO+2C→SiC+CO……(2) Therefore, if the oxygen is present in an amount greater than 1.0% by weight, a large amount of carbonaceous additive must be used. Moreover, a large amount of CO gas is generated, which not only makes sintering difficult, such as the necessity of degassing during sintering, but also makes it difficult to obtain a high-density sintered body. On the other hand, silicon carbide powder with an oxygen content of less than 0.01% by weight can be obtained, for example, by treatment with a mixed acid of hydrofluoric acid and nitric acid, but the highly purified silicon carbide powder obtained in this way is extremely active. However, if it is dried in an air atmosphere, it will easily oxidize even at room temperature, so in order to maintain a low oxygen content, the atmosphere after acid treatment must be kept non-oxidizing, which is not practical. Therefore, it is relatively easy to set the amount of oxygen contained in the silicon carbide powder within the range of 0.03 to 0.4% by weight, and it is suitable as a raw material for sintering. SiC constituting the silicon carbide powder is at least
The reason why it is necessary to use SiC with 80% by weight of β-type crystals will be described below. Usually, the crystals mixed in silicon carbide, which mainly consists of β-type crystals, are 2H-type crystals, which are more stable at lower temperatures than β-type crystals, or α-type crystals, such as 4H and 6H types, which are more stable at higher temperatures than β-type crystals. The 2H type silicon carbide is extremely unstable in the temperature range where normal sintering reactions occur, and tends to cause abnormal grain growth during sintering.On the other hand, the 4H and 6H types, which are stable in the high temperature range, are If it is contained, the phase transformation from β-type crystal to α-type crystal is promoted during sintering, and coarse plate-like crystals of α-type silicon carbide are generated as the α-type crystal changes, which hinders sintering shrinkage and increases the density. It is difficult to obtain a high-strength sintered body having a large and uniform microstructure. Therefore, in order to obtain a high-strength sintered body, which is the object of the present invention, it is necessary that at least 80% by weight of the SiC constituting the silicon carbide powder be SiC consisting of β-type crystals, and especially 90% by weight of SiC. At least % by weight consists of β-type crystals
More preferably, it is SiC. According to the present invention, the main constituents of the inevitable impurities contained in the silicon carbide powder of the present invention are:
01-3.0% by weight free carbon, 0.5% by weight or less aluminum, 0.01-0.5% by weight iron, 0.01-0.5% by weight alkali metals and alkaline earth metals. The reason why it is preferable to keep the content of the unavoidable impurities within the above range is because free carbon has the effect of functioning as a carbonaceous additive, but if it is contained in an amount larger than the above range, it will significantly inhibit sintering. death,
In addition, the inclusion phase in the sintered body increases, which significantly reduces the physical properties, especially the strength, of the sintered body.Aluminum has the effect of generating plate crystals in the sintered body, so the amount of aluminum in the sintered body is larger than the above range. This is because if the iron, alkali metal, or alkaline earth metal is contained in an amount greater than the above range, it will be difficult to obtain a high-density sintered body, and if the iron, alkali metal, or alkaline earth metal is contained in an amount greater than the above range, the physical properties of the sintered body will be significantly deteriorated. According to the present invention, in order to obtain a high-density sintered body, it is important to uniformly generate sintering initiation bonding points, that is, necks, which occur at the mutual contact areas of silicon carbide powder particles when sintering starts. The specific surface area of the silicon carbide powder must be within the range of 5 to 50 m ² /g. The reason why the specific surface area is limited to a range of 5 to 50 m ² /g is that the specific surface area is 5 m ² /g.
If it is smaller than g, there will be fewer areas where necks are formed at the start of sintering, and shrinkage during sintering will be uneven, making it difficult to obtain a sintered body with the high density and strength that is the objective of the present invention. On the other hand,
Silicon carbide powder with a specific surface area larger than 50 m ² /g is considered to be excellent in forming a sintered body because it has a large number of areas where there are necks, but it is difficult to obtain such silicon carbide powder, and even if it is not available, Even if it were, it would be extremely expensive and impractical;
More suitable results are obtained within the range of m ² /g. Next, a method for producing silicon carbide powder of the present invention will be explained. The silicon carbide of the present invention can also be produced by heating a mixture of carbon powder with metallic silicon, or by causing a gas phase reaction between silicon halide and a mixed gas such as a hydrocarbon; The silicon carbide thus produced is very expensive. Contains 0.015 to 0.20% by weight of nitrogen of the present invention, and
Silicon carbide, which is SiC in which at least 80% by weight of SiC is composed of β-type crystals, is produced by charging a raw material made by mixing silica powder and carbon powder into granules and charging it into an indirect heating furnace. It can be produced by heating at a temperature of 1,700 to 2,150°C while maintaining a nitrogen partial pressure within a range of 5 to 200 mmHg. The reason why the nitrogen partial pressure in the furnace is limited to a range of 5 to 200 mmHg is that if the nitrogen partial pressure in the furnace is lower than 5 mmHg, silicon carbide with the nitrogen content targeted by the present invention cannot be produced. However, the crystal grains tend to become coarse during the SiC production reaction, making it difficult to produce fine silicon carbide powder.
If it is higher, the amount of nitrogen contained in the silicon carbide powder will increase significantly, and it will not only be difficult to keep the nitrogen content within the range targeted by the present invention, but also
This is because the SiC production reaction is significantly suppressed, making it difficult to efficiently produce silicon carbide. Furthermore, the reason why the reaction temperature is limited to a range of 1700 to 2150°C is that according to the present invention, as mentioned above, by maintaining the nitrogen partial pressure in the furnace relatively high, SiC
If the temperature is lower than 1700℃, the production reaction will be suppressed.
The SiC production reaction is extremely slow and it is difficult to efficiently produce SiC, and on the other hand, if the temperature is higher than 2150°C, most of the silicon carbide produced undergoes crystal transformation to silicon carbide consisting of α-type crystals, so it is difficult to produce SiC efficiently. At least 80% by weight of SiC consisting of β-type crystals suitable as a raw material
This is because it is difficult to manufacture the silicon carbide contained therein. According to the present invention, C/SiO ₂ of silica and carbon
When charging raw materials with a molar ratio within the range of 3.2 to 5.0 into a vertical indirect heating furnace having nitrogen gas charging means at the bottom and causing a SiC production reaction while lowering the interior of the indirect heating furnace by its own weight. , it is preferable to bring the nitrogen gas into countercurrent contact with the nitrogen gas charged from the charging means. The reaction formula of silicon carbide produced from silica and carbon is generally shown by the following formula (3). SiO ₂ +3C→SiC+2CO……(3) However, the actual main generation mechanism is that SiO gas generated by the reaction of formula (1) above reacts with carbon according to formula (2) above. It is known that silicon carbide is produced by this process. By the way, according to the present invention, the nitrogen partial pressure in the furnace is 5
Maintained within ~200mmHg. Nitrogen in the furnace suppresses the reaction of formula (2), and the SiO
It has the effect of increasing the amount of gas. The SiO
As the gas amount increases, the following equation (4) in the preheating zone
The amount of mixed precipitates of SiO ₂ , Si, SiC, C, etc. generated according to (5) and (6) increases, which impedes the smooth fall of the raw material, which is important in the continuous production of silicon carbide. This makes stable continuous operation over a long period of time difficult. 2SiO→SiO ₂ +Si……(4) SiO+CO→SiO ₂ +C……(5) 3SiO+CO→2SiO ₂ +SiC……(6) Therefore, according to the present invention, the amount of carbon in the raw material is increased. It is desirable to increase the number of locations where the above formula (2) occurs by increasing the C/SiO ₂ molar ratio from 3.2 to 5.0.
It is preferable to set it within the range of. The reason for this is that if the C/SiO ₂ molar ratio is lower than 3.2, it is difficult to maintain the smooth weight fall of the raw material over a long period of time, as mentioned above, and on the other hand, if it is higher than 5.0, carbon that does not contribute to the reaction is heated to high temperatures. This is because the thermal efficiency decreases due to heating, and the cost required for the carbon raw material increases, so it is not economical. In addition, for the purpose of reducing the influence of precipitates in the preheating zone, the internal horizontal cross-sectional area of the upper part of the cylinder forming the preheating zone, as shown in Fig. 1, is calculated from the internal horizontal cross-sectional area of the heating zone. It is also advantageous to use a large vertical indirect heating furnace. Furthermore, in order to solve the above drawbacks, the average particle size of silica is set within the range of 75 to 250 μm, and the relationship between the average particle sizes of silica and carbon is adjusted so as to satisfy the following formula (7), Y ≦8.5×10 ⁻² RX+3.1×10 ⁴ /T (7) It is advantageous to maintain the reaction rate between formulas (1) and (2) above within a suitable range. In the above formula, X is the average particle size of silica (μ
m), Y is the average particle diameter of carbon (μm), T is the reaction temperature (〓), and R is the C/C of silica and carbon in the starting materials.
_SiO2 molar ratio. In addition, in the present invention, when charging raw materials from the upper part of a vertical indirect heating furnace having nitrogen gas charging means at the bottom and causing the SiC production reaction while lowering the inside of the heating furnace by its own weight, the charging means It is preferable to bring the nitrogen gas into countercurrent contact with the introduced nitrogen gas. This is because the SiC production reaction consisting of silica and carbon is as shown in equation (3) above, and a large amount of CO gas is produced as a result of the reaction, so when nitrogen gas is charged from the top of the furnace, the nitrogen gas is This is because the nitrogen gas is discharged together with the CO gas, making it difficult to efficiently bring the nitrogen gas into contact with the charge, and also making it extremely difficult to control the nitrogen partial pressure in the heating furnace. According to the present invention, in order to produce silicon carbide containing 0.015 to 0.20% by weight of nitrogen and in which at least 80% by weight of SiC is β-type crystal, the nitrogen partial pressure in the furnace and the reaction temperature are set as follows. It is advantageous to set the value within a range that satisfies relational expression (8). 3.3×10 ^-4 T ² −1.35T＋1384≦PN ₂ ≦8.5×10 ^-4 T ² −3.46T＋3593 ………(8) In the above formula, PN ₂ is the nitrogen partial pressure in the furnace
Hg), T is the reaction temperature (〓). Next, an example of a vertical indirect heating furnace that can be used in the method of the present invention will be described with reference to the drawings. The indirect heating furnace that can be used in the method of the present invention has a raw material charging port 1 and a preheating zone 2 as shown in FIG.
A reaction vessel 6 has a heating zone 3, a cooling zone 4, and a sealable product discharge port 5, which are connected in the vertical direction, and a cylinder 7 forming the heating zone is made of graphite. It is equipped with means 8 and 9 for indirectly electrically heating the charge in the heating zone, and a nitrogen gas charging port 10 is provided in the cooling zone at the bottom of the reaction vessel. The reaction vessel 6 is installed in the center of the indirect heating furnace, and the indirect heating means 8 and 9 are composed of a graphite heating element 8 and a graphite reflector tube 9 provided close to the outside of the heating element. A heat insulating layer 11 made of carbon or graphite fine powder is provided on the outside of the reflective cylinder. In the space surrounded by the cylinder forming the heating zone and the graphite reflector cylinder, for example, argon, helium, nitrogen, carbon monoxide,
Hydrogen and other non-oxidizing gases are sealed to prevent the graphite heating element from being consumed by oxidation due to air intrusion. Next, an example of a method for manufacturing a sintered body using the silicon carbide powder of the present invention will be described. A sintered body using the silicon carbide powder of the present invention has a boron-containing additive of 0.1 to 3.0 parts by weight in terms of boron content and fixed carbon content in terms of boron content of 100 parts by weight of the silicon carbide powder of the present invention. 0.1 to 3.0 parts by weight of a carbonaceous additive are added thereto and mixed homogeneously, and the homogeneously mixed mixture is formed into a formed body of any desired shape. It can be produced by sintering at 2050 to 2250°C in a gas atmosphere consisting of at least one selected from xenon and hydrogen. It is known that when producing a sintered body using β-type silicon carbide powder with a low nitrogen content, it is possible to create a sintered body containing nitrogen by setting the atmosphere to a nitrogen gas atmosphere. However, according to such a method, the nitrogen concentration distribution dissolved in the solid solution in the sintered body is non-uniform, so the sintered body lacks uniformity and its uses are limited. Examples of the present invention will be described below. Example 1 100 parts by weight of silica powder (SiO ₂ = 99.7% by weight) with an average particle size of 153 μm, 76 parts by weight of petroleum coke powder (C = 98.7% by weight) with an average particle size of 29 μm, and silica powder with an average particle size of 43 μm 7 parts by weight of pitchch powder (C=50.4% by weight) was blended and mixed for 10 minutes using a vertical screw mixer. While spraying a 0.5% CMC aqueous solution onto the above-mentioned blended raw materials, the mixture is molded using a pan-type granulator, sized using a sieve and a burr grizzly, and then placed in a band-type ventilation dryer and dried with hot air at 150°C for 90 minutes. A molding raw material having an average particle diameter of 10.5 mm, a bulk specific gravity of 0.60 g/cm ³ and a C/SiO ₂ molar ratio of 4.0 was obtained. This forming raw material is charged from the upper part of an indirect heating furnace having the structure shown in Fig. 1 and the specifications shown in Table 1, and the charged raw material is continuously lowered under its own weight to form a graphite cylinder. The temperature of the outer wall of the sample was controlled to 2150°C in a heating zone, and the SiC production reaction was carried out for about 1 hour by indirect electrical heating in the horizontal direction within the heating zone, and then lowered under its own weight into a cooling zone and discharged. The reaction product was continuously discharged from the outlet.

【table】

[Table] The nitrogen partial pressure in the furnace was controlled by charging nitrogen gas from a nitrogen gas charging port provided in the cooling zone, and was maintained within the range of 100 to 130 mgHg. When the obtained reaction product was purified to remove free carbon and _SiO2 , it contained 0.06% by weight of nitrogen,
A silicon carbide powder was obtained in which 88% by weight of the total SiC was composed of β-type crystals. The silicon carbide powder was further classified to a particle size of 7 μm.
The above coarse particles were removed. The characteristics of the silicon carbide powder subjected to particle size classification are shown in Table 2. The silicon carbide powder 98.7 and commercially available 200 mesh boron carbide particles were crushed and classified to have a specific surface area of 24.3.
To a mixture of 1.3 g of boron carbide powder prepared to m ² /g and 1.5 g of carbon black with a specific surface area of 123 m ² /g, 280 ml of water, 1.0 g of cellulose acetate, and 0.3 g of monoethanolamine were added, and the mixture was placed in a ball mill. After performing a dispersion treatment, freeze-drying was performed. An appropriate amount was taken from this dry mixture and temporarily formed into a disk shape using a metal mold at a pressure of 0.15 t/cm ² . Next, using an isostatic press machine
It was molded at a pressure of 2.0t/cm ² . The density of the resulting form was found to be 1.86 g/cm ³ . The formed body was placed in a Tammann type sintering furnace and sintered by holding it at a maximum temperature of 2100° C. for 30 minutes in an argon gas flow. The obtained sintered body had a density of 3.05 g/cm ³ and had a microstructure consisting of uniform crystal grains of about 3 to 7 μm, as shown in the scanning electron mirror photograph (750x magnification) in Figure 2. I found out that I have it. This sintered body is 3
It was processed into a rod shape of ×3 × 30mm, and finally polished with 3μm diamond abrasive grains.The three-point bending strength was measured under the conditions of a span of 20mm and a crosshead speed of 0.5mm/min, and the average was 64Kg/ ^mm2 . It had strength. Comparative Example 1 Silicon carbide was produced using the same manufacturing method as in Example 1, but without introducing nitrogen gas into the reactor. The nitrogen gas partial pressure in the reactor was 0 to 3 mmHg. Table 2 shows the physical properties of the obtained silicon carbide powder. Using this silicon carbide powder, a sintered body was obtained in the same manner as in Example 1. The density of the obtained sintered body was as low as 2.83 g/cm ³ , and the average three-point bending strength was 48 Kg/mm ² at room temperature. This sintered body had a structure containing coarse plate-like crystals, as shown in the scanning electron micrograph (750x magnification) in Figure 3. Example 2, Comparative Example 2 Silicon carbide was produced using the same manufacturing method as in Example 1, but by changing the partial pressure of nitrogen gas in the furnace and the reaction temperature. Silicon powder was obtained. Using this silicon carbide powder, a sintered body was obtained in the same manner as in Example 1. The physical properties of the obtained sintered body were measured by the same method as shown in Example 1, and the results are shown in Table 2.

【table】

[Table] As shown in Table 2, the sintered body obtained using the silicon carbide powder with a high nitrogen content of Comparative Example 2 had deteriorated sintering shrinkage of 2.90 g/ ^cm3 . It was low density. As described above, according to the present invention, it is possible to inexpensively and easily supply silicon carbide fine powder capable of producing a silicon carbide sintered body having an extremely fine and uniform crystal structure, and the effects contributing to industry are Extremely large.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view of a vertical continuous manufacturing apparatus used to manufacture silicon carbide in Examples and Comparative Examples of the present invention, and FIG. Micrograph (750x) FIG. 3 is a scanning electron micrograph (750x) of the sintered body described in Comparative Example 1. 1... Raw material charging port, 2... Pre-preparation zone, 3... Heating zone, 4... Cooling zone, 5... Product discharge port, 6...
Reaction container, 7...Cylinder forming a heating zone, 8...Graphite heating element, 9...Graphite reflector tube, 10...Nitrogen gas charging port, 11...Insulating layer, 12...Non-oxidizing gas Sealing port, 13... Guide electrode, 14... Flexible conductor, 15... Bus bar, 16... Temperature measuring pipe,
17... Outer shell, 18... Firebrick, 20... Exhaust duct, 19... Raw material hopper.

Claims (1)

[Claims] 1 Contains 0.015 to 0.20% by weight of nitrogen and 0.01 to 1.0% by weight of oxygen, and contains 0.01 to 3.0% by weight of impurities.
wt% free carbon, 0.5 wt% or less aluminum, 0.01-0.5 wt% iron and 0.01-0.5 wt%
of alkali metals and alkaline earth metals, and the remaining SiC is at least 80% by weight of β
It is made of type crystal and has a specific surface area of 5 to 50
m ² /g of silicon carbide powder for sintering. 2 Silica powder and carbon powder are mixed in such a range that the molar ratio C/SiO ₂ of silica to carbon is 3.2 to 5.0, and then the raw material is formed into granules, and the raw material is heated with nitrogen in an indirect heating furnace. By charging it into a gas atmosphere and heating it while maintaining the reaction temperature in the furnace within the range of 1700 to 2150°C and at the same time maintaining the nitrogen partial pressure within the range of 5 to 200 mmHg, 0.015 to 0.20 weight % nitrogen and 0.01-1.0 wt% oxygen, as impurities, 0.01-3.0 wt% free carbon, not more than 0.5 wt% aluminum,
The SiC contains 0.01 to 0.5% by weight of iron and 0.01 to 0.5% by weight of alkali metals and alkaline earth metals, and the remaining SiC is composed of at least 80% by weight of β-type crystals and has a specific surface area of 5~ ^50m2 /
A method for producing silicon carbide powder for sintering, which is g.