JPH054947B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field) The present invention relates to a novel method for producing a silicon carbide sintered body having an excellent microstructure. (Background Art) Ceramic sintered bodies obtained by sintering fine powder of silicon carbide are expected to find expanded use as high-temperature structural materials. Notable advantages of silicon carbide sintered bodies include high strength at high temperatures, excellent corrosion resistance, high thermal conductivity, and low coefficient of thermal expansion. Therefore, silicon carbide sintered bodies having these advantages can be used as gas turbine parts and engine parts for automobiles.
Research and development is actively underway for applications such as high-temperature heat exchangers, bearings, and burners for combustion furnaces. A pressureless sintering method is suitable for industrially producing ceramic sintered bodies with complex shapes. However, since silicon carbide powder is a typical material that is difficult to sinter, special measures are required to enable pressureless sintering. Conventionally, several methods including patents have been proposed for sintering silicon carbide, and the representative methods include:
The following 2 uses boron and carbon as sintering aids
These include two US patents and corresponding Japanese patent applications. Namely: (1) U.S. Patent No. 4004934, (Japanese Patent Publication No. 57-32035) (2) U.S. Patent No. 4321954, (Japanese Patent Publication No. 59-34147) On the other hand, 0.3～
After adding and mixing a boron compound in an amount equivalent to 3.0% by weight of elemental boron and a carbon source in an amount equivalent to 0.1 to 1.0% by weight of elemental carbon, this mixture is shaped and heated under an inert atmosphere without any additives. 1900～ at pressure
This method involves heating to a high temperature of 2100°C to obtain a silicon carbide sintered body. Method (2) uses a boron compound in an amount equivalent to 0.15 to 3.0 weight % of elemental boron and an amount equivalent to 0.5 to 5.0 weight % of elemental carbon to silicon carbide powder containing 5 weight % or more of α crystals. A carbon compound is added and heated to a high temperature to form a silicon carbide powder and a sintered body without substantially changing the crystal form, that is,
This is a method for obtaining a silicon carbide sintered body by making the crystalline form of silicon carbide in silicon carbide powder, which is a starting material, have essentially the same proportion as the crystalline form in the final sintered body. The silicon carbide sintered body obtained by these methods is
It is a dense sintered body with more than 95% of the theoretical density of silicon carbide. However, the silicon carbide sintered bodies obtained by these methods have the problem of low mechanical strength and too large variations in strength for applications that require large mechanical loads such as gas turbines for automobiles. This has become an obstacle to industrial practical application. It is generally known that there is a strong relationship between the mechanical properties of ceramic sintered bodies and the microstructure of sintered bodies.
In order to obtain a sintered body with high strength, it is said that it is preferable that its microstructure satisfies the following three conditions (a) to (c). Here, the microstructure of a sintered body refers to a three-dimensional structure of particles and defects (voids) that constitute the sintered body. (a) The defects contained in the sintered body are few and small. (b) The particle size of the single particles constituting the sintered body is small. (c) There are no specifically grown coarse particles exceeding 50μ in the sintered body. Furthermore, the following (d), which has recently become an established theory,
This is to satisfy the following conditions. (d) There is anisotropy in the shape of the particles that make up the sintered body. Here, the generation of coarse particles in (c) above has conventionally been considered to be caused by phase transition during sintering, and efforts have been made to understand sintering conditions that do not cause phase transition. For example, in the above-mentioned Japanese Patent Publication No. 57-32035, it states that the phase transition from β crystal to α crystal causes grains to grow coarsely.
A method of preventing this using a nitrogen atmosphere is proposed on the right side of the page. Furthermore, on the left side of page 3 of Japanese Patent Publication No. 59-34147, it is stated that ``The crystalline form of silicon carbide in the starting material has essentially the same proportion as the crystalline form of the final non-pressure sintered body.'' clearly stated. Furthermore, the condition (d), that the particle shape is anisotropic, means that the fracture surface that occurs when the sintered body breaks increases; It is presumed that this is to increase the energy required for destruction. In other words, the sintered body is difficult to break. Here, the anisotropy of particle shape refers to the presence of three-dimensional anisotropy in the shape of particles, such as plate-like or rod-like shape. Needless to say, the sintered body as a whole must have no grain orientation in a specific direction. Conventional techniques have not produced a silicon carbide sintered body that satisfies all of the above conditions (a) to (d). For example, according to the experiments of the present inventors, the above (1)
In the method (method of Japanese Patent Publication No. 57-32035),
It is not possible to overcome the drawback that coarse particles are likely to be generated, and method (2) (method of Japanese Patent Publication No. 59-34147) does not have anisotropy in a single particle of the sintered body. However, there is a problem that a sintered body with high strength cannot be obtained. Furthermore, recently, the sintered bodies obtained by the pressureless sintering methods (1) and (2) have been subjected to HIP
A method has also been developed in which the sintered body is heated again to around 2000℃ in an inert gas atmosphere of several thousand atmospheres to reduce the internal defects contained in the sintered body and make it more compact (for example, the 3rd press process). Next Generation Industrial Infrastructure Technology Symposium - Fine Ceramics - Proceedings 13
~28 pages). This method is an effective method for reducing internal defects in a sintered body by heating the surface of the sintered body while pressurizing it with high-pressure gas. However, in a sintered body with many defects, the high-pressure gas permeates into the inside of the sintered body, and the densification effect is difficult to manifest. According to the experiments of the present inventors,
It has been found that a density of 3.05 g/cc or more is desirable before HIP treatment. Method (1) above has the disadvantage that coarse particles are generated before the sintered body density reaches 3.05 g/cc, and method (2) has the disadvantage that the particle shape is anisotropic even if HIP treatment is performed. Does not occur. (Basic idea) The present inventors aimed to obtain a silicon carbide sintered body with high mechanical strength and small variations in mechanical strength, and the microstructure of the particles constituting the sintered body has been used as a guideline for this purpose. As a result of intensive studies with the goal of obtaining a sintered body that satisfies all of the proposed conditions (a) to (d) above, we determined that the following raw material powders were used and the sintering conditions were specified. By doing this, we found that a microstructure that satisfies the conditions (a) to (d) above can be obtained with good reproducibility, and furthermore, it is true that a sintered body with a microstructure that satisfies the conditions (a) to (d) above is It was confirmed that the mechanical strength was high and the variation was small, and the present invention was completed. (Detailed Disclosure of the Invention) That is, according to the present invention, 0.05 to 0.3 parts by weight of boron and 0.5 to 3.0 parts by weight of carbon are added and mixed to 100 parts by weight of silicon carbide powder, and then the mixture is molded. In the method of obtaining a silicon carbide sintered body by heating and sintering at ~2050°C, the physical properties of the silicon carbide powder are as shown below (a):
It satisfies the conditions (b) and (c): (a) The average particle diameter ρ is 0.1 to 0.2μ. (b) The standard deviation of particle diameter is 0.8×ρ or less. (c) Crystal form: 80% by weight or more of β crystals and 20% by weight
Contains the following low-temperature alpha crystals. and sintering that satisfies the following conditions (d) and (e) by heating in an inert gas atmosphere to a temperature range such that the α-crystal content in the silicon carbide powder does not exceed 5% by weight. Obtain an intermediate: (d) Density is 3.05 g/cc or more. (e) The crystal form shall contain 95% by weight or more of β crystals and 5% by weight or less of α crystals. After that, it is heated to 1800-2000℃ in an inert gas atmosphere of 100 atmospheres or more, and the following (f), (g),
A novel method for producing a silicon carbide sintered body, which comprises obtaining a sintered body characterized by the characteristics (h). (f) The density of the sintered body must be 3.10 g/cc or more. (g) The average particle diameter of the microstructure is 3 to 15μ and the aspect ratio is 5 to 20. (H) The crystal form shall contain 50% by weight or more of β crystals and less than 50% by weight of high-temperature α crystals. is provided. The present invention will be explained in detail below. First, the raw material silicon carbide powder used in the present invention is required to satisfy the following three requirements. The first requirement is that the average particle diameter ρ of the powder is 0.1 to 0.2μ.
It is to be. The reason is that when ρ exceeds 0.2μ, the density of the silicon carbide sintered body obtained is 3.10g/
This is because it is difficult to exceed cc, and if ρ is less than 0.1 μ, the driving force for sintering is too large, or coarse particles are likely to be generated in the sintered body. It cannot be reached. The second requirement is that the standard deviation of the powder particle size is 0.8×
The particle size distribution must be narrow below ρ.
This is based on the experimental findings of the present inventors that in order to prevent the generation of coarse particles, the particle size distribution of the fine structure of the sintered body is narrow and the particle size distribution of the raw material powder is narrow. be. As a method for measuring the particle size, it is preferable to employ a method of determining the particle size from an electron microscope image, as detailed in the section [Particle Size Measurement Method] below. The third requirement is that the crystal form of silicon carbide powder is 80
It contains at least 20% by weight of β crystals and 20% by weight or less of low-temperature α crystals (2H). Low-temperature α crystal (2H)
20% by weight or less means that it contains 20% by weight or less of crystal forms, most of which undergo a phase transition to β crystals during sintering. The reason for this is based on the experimental findings of the present inventors that anisotropy (indicated by aspect ratio) occurs in the particles of the sintered body only when this third requirement is satisfied. . Further, the content of high-temperature α crystals (4H, 6H, 15R) contained in the silicon carbide powder is preferably a trace amount of 1% by weight or less. Silicon carbide powder that satisfies the above three requirements is
Basically, the method proposed by the present inventors in Japanese Patent Application Laid-Open No. 59-83922 is used, that is, once a decomposable silicon compound and a decomposable carbon compound are combined into a silicon oxide and an elemental carbon through a chemical reaction. It can be easily obtained by producing an extremely finely mixed powder, compressing this powder to a bulk density of 0.15 g/cc or more, and then heating it to about 1600 to 1900°C. In addition, as other methods for producing silicon carbide powder containing β crystals as a main component, the following two methods are conventionally known as typical methods. That is, (a) After mechanically mixing silicon oxide and carbon or silicon and carbon, heating to 2000°C or less,
A method of finely pulverizing the silicon carbide produced. (b) Silicon compounds such as SiH ₄ and Si(CH ₃ ) ₄ and CH ₄ ,
A method in which silicon carbide is produced by introducing a carbon compound such as C ₂ H ₆ into a plasma atmosphere heated to over 1200°C and causing a gas phase reaction in the presence of hydrogen. However, according to the results of follow-up experiments conducted by the present inventors on the above two methods (a) and (b), the powder obtained by method (a) has particles pulverized to a size of 0.1 micron or less and particles of several microns.
It is a powder with a wide particle size distribution that coexists with particles of
In addition, the powder obtained by method (b) is a single particle.
Although the size is about 0.1μ, the particles are tightly aggregated, making it a powder with a large apparent particle size and a wide particle size distribution. Therefore, neither of the silicon carbide powders obtained by the above methods (a) and (b) is suitable as a raw material for the present invention. Now, a silicon carbide sintered body is obtained by molding silicon carbide powder into a desired shape and then sintering it, but in the present invention, elemental boron or a boron compound and elemental carbon are added to silicon carbide powder prior to molding. A small amount is added and mixed as a sintering aid to accelerate sintering to form a mixture. The amount added is 0.05 to 0.3 parts of the former and 0.5 to 3.0 parts of the latter in terms of elemental boron per 100 parts of silicon carbide powder. The silicon carbide powder and the sintering aid can be mixed using a ball mill or the like, but for that purpose it is desirable to mix the sintering aid with the silicon carbide powder as uniformly as possible. The compound is preferably in the form of a fine powder with a specific surface area of 3 m ² /g or more. Note that examples of the boron compound include B ₄ C, BN, BP, AlB ₂ and the like. In addition, as a method for adding elemental carbon, hydrocarbons with a relatively high carbon content, such as phenol-
Formaldehyde condensate, resorcinol-formaldehyde condensate, glycerin, tarpitz, furan resin, etc. are dissolved in water or a solvent such as toluene, acetone, ethanol, etc., and this is mixed with a small amount of elemental boron or boron compound powder to form silicon carbide. This can be carried out by adding it to a powder, mixing it in a wet state using a ball mill, etc., then drying off the water or solvent, and then heating it in an inert gas atmosphere at 500 to 600°C to carbonize the hydrocarbon. Note that as another method of adding elemental carbon, finely powdered elemental carbon itself may be used. In this case, carbon black, acetylene black, etc. with a specific surface area of 30 m ² /g or more is used as the elemental carbon, and in order to obtain a more uniform mixture, water or methanol,
A preferred method is to add an organic solvent such as ethanol and mix the mixture wet using a ball mill, vibration mill, or the like. That is, after adding and mixing water or an organic solvent to elemental boron or boron compound fine powder, elemental carbon fine powder, and silicon carbide fine powder,
This is a method of removing water or organic solvent by evaporation. The thus obtained fine silicon carbide powder containing a small amount of sintering aid is then molded into a desired shape and once heated with a gas such as argon, helium, nitrogen, etc. at about atmospheric pressure or below atmospheric pressure or under vacuum. By heating in an active gas atmosphere, the density is 3.05 g/cc or more and the crystal form is 95% by weight.
A sintered intermediate containing the above β crystals and 5% by weight or less of α crystals can be obtained. Here, the heating temperature is basically required to be maintained within a temperature range such that the content of α crystals in the silicon carbide fine powder does not exceed 5%, but usually this heating temperature is 1850°C. A temperature range of ~2000°C is generally good. However, more preferably, it should be selected appropriately depending on the physical properties of the silicon carbide powder, and according to the experimental findings of the present inventors, the heating temperature should be set at a density of 3.05.
It is preferable to set the temperature within the range of the minimum temperature required to reach g/cc (hereinafter referred to as Ts) to Ts + 50°C in order to prevent generation of coarse particles in the sintered intermediate. According to the experimental findings of the present inventors, there is a positive correlation between (standard deviation of particle size) /ρ and Ts of silicon carbide powder, as shown in Figure 12 of the attached drawings, and the particle size 3.05 for narrow distribution powders at lower temperatures
There is a tendency to reach densities of g/cc. Note that the α crystal in this sintered intermediate means the sum of both low-temperature type and high-temperature type. Next, in the present invention, the sintered intermediate having the predetermined characteristic values obtained as described above is heated in an atmosphere of an inert gas such as argon, helium, or nitrogen at a sufficiently high pressure. It can be a sintered body. Here, the pressure of the inert gas is desirably at least 100 atm or higher, more preferably 500 atm or higher.
There is no need to set the upper limit of pressure, but it is 10000
Setting the pressure above atmospheric pressure is less effective considering the increased equipment cost. As mentioned above, the heating temperature is 1800-2000℃
Although it is generally good if it is in the range of , it is more preferable to take the above Ts into consideration and set it in the range of Ts - 50 to Ts + 50°C. According to the method of the present invention as described in detail above,
A silicon carbide sintered body having the following three properties can be obtained. Namely: First, the density of the sintered body is 3.10 g/cc or more, and second, the average particle size of the microstructure is 3 to 15 μm.
It is large and has an aspect ratio of 5 to 20. As mentioned above, the microstructure refers to the three-dimensional structure of the particles and defects that make up the sintered body, but the measurement of the average particle diameter and aspect ratio is performed by smoothing the surface of the sintered body and The particle size is determined from an image magnified (usually 100 to 1000 times) with a microscope, and the particle diameter is the average value of the maximum length (L) and minimum length (D) of each particle in this image. The diameter is the arithmetic mean value of particle diameters, and the aspect ratio is the arithmetic mean value of L/D. It is usually sufficient to calculate these for 300 or more particles. Furthermore, thirdly, the crystal form of the sintered body of the present invention is 80
It is a sintered body containing % or more of β crystals and 20% or less of high-temperature α crystals, and more preferably a sintered body containing 5% or less of high-temperature α crystals. According to the experimental findings of the present inventors, the low-temperature α
20% or less of crystals (the rest is β crystals), more preferably 5
If silicon carbide powder containing ~20% is used as a raw material and sintered under the conditions described above, silicon carbide sintered with a composition of 20% or less of high-temperature α crystals (the rest is β crystals), more preferably 5% or less. body can be obtained with extremely high reproducibility. It goes without saying that this sintered body has the excellent microstructure that is the object of the present invention. (Operations and Effects of the Invention) The present invention is directed to the production of a silicon carbide sintered body having an excellent microstructure, which has been strongly desired as a technical concept, but for which no production method has been established in reality. The method was established by specifying the physical properties of the raw material silicon carbide powder and the sintering conditions. The results of measuring the mechanical strength of the sintered body obtained by the method of the present invention show that the strength is clearly higher than that of the conventional sintered body and the variation is small, as shown in the examples below. It was confirmed that the toughness value (K _IC ) was also high. The silicon carbide sintered body obtained by the method of the present invention has an excellent microstructure as described above, and the reason for this is presumed to be as follows. That is: (a) Since the raw material powder is easily sinterable, sintering is possible with the amount of boron added as a sintering aid of 0.05 to 0.3 parts by weight, which is much smaller than in the conventional technology. Therefore, generation of coarse particles in the sintered body is suppressed. (b) Since raw material powder that is easily sinterable is used, the sintering temperature can be lowered to 1,900 to 2,050°C, which is lower than before, and therefore, coarsening of the sintered body particles can be suppressed. (c) In the sintering process, anisotropy occurs in the sintered particles because the sintering process passes through a sintered intermediate containing 5% or less of α crystals. (Examples and Comparative Examples) Hereinafter, more specific embodiments of the present invention will be described with reference to Examples. In addition, in the following, % is weight %
represents. Silicon Carbide Powder Silicon carbide powder having the quality shown in Table 1 was used in the Examples and Comparative Examples. Powders 1 to 3 in Table 1 are prepared using the method previously proposed by the present inventors in JP-A-59-83922.
That is, it was obtained by simultaneously injecting SiCl ₄ as a decomposable silicon compound and C ₉ fraction as a decomposable carbon compound into a flame obtained by burning hydrogen with air.
A soot-like extremely fine mixture containing 33.6% SiO ₂ and 66.3% elemental carbon was compressed to a bulk density of 0.5 g/cc and then heated to 1700°C, 1800°C, and 1900°C, respectively.
This product was obtained by heating the carbon to produce silicon carbide according to the reaction formula shown in equation (1) below, and then burning off the excess carbon. SiO ₂ +3C → SiC + 2CO ... (1) Powder 4 is obtained by directly reacting SiH ₄ , CH ₄ and H ₂ in the gas phase, using a conventionally known method [for example, Journal of the Chemical Society of Japan, 2 , 188 -193 (1980)] at a reaction temperature of 1400°C. Powders 5 and 6 are commercially available products, and their microscopic images show that they are made by mixing silicon dioxide or silicon with elemental carbon, heating the resulting coarse particles of silicon carbide, and grinding them for a long time using a vibrating mill or the like. It is assumed that the

[Table] As shown by the values in Table 1, Powders 1 to 3 satisfy all physical property conditions (a), (b), and (c) as raw materials of the present invention. In comparison, powders 4 to 6 have an average particle size ρ
and its standard deviation are both larger than the specified values of the present invention. Powder 6 does not contain any β-crystals and only has high-temperature α-crystals. Like this powder 4-6
does not satisfy any of the physical property conditions as a raw material specified in the present invention. The method for measuring the average particle diameter ρ and the standard deviation of particle diameter shown in Table 1 is described in the [Particle diameter measurement method] described in the summary of analysis methods below. I turned and asked. Transmission electron microscope images of Powders 1, 4, and 5 are shown in FIGS. 1, 2, and 3, respectively, and a powder X-ray diffraction image of Powder 1 is shown in FIG. 4. Examples 1 to 3 To 100 g of each of powders 1 to 3 shown in Table-1,
B ₄ C powder with a specific surface area of 10.4 m ² /g as a boron source
0.25g and a specific surface area of 120.5 as a single carbon source
Add 2 g of carbon black (m ² /g) and 50 c.c. of ethanol and mill them in a resin ball mill.
Mixed for 20 hours. 50 g of the powder obtained by heating the resulting mixture to remove ethanol by evaporation was placed in a cylindrical container and uniaxially compressed under a load of 0.5 T/cm ² .
It was molded by rubber pressing using a hydrostatic pressure of 2T/cm ² . Table 2 shows the density of each powder compact obtained. Next, these powder compacts were heated at a temperature of 1900° C. for 30 minutes in a nitrogen atmosphere under a vacuum of 10 ⁻¹ to 1 Torr to obtain a silicon carbide sintered intermediate. Table 2 shows the density and crystal form of the obtained sintered intermediate. The Ts (minimum temperature required to reach a density of 3.05 g/cc) of each powder compact was the value shown in Table 2. Ts was determined by the method described in the effect of standard deviation of particle size described below. Next, each sintered intermediate was heated at 1880° C. for 30 minutes in a nitrogen atmosphere of 1500 atmospheres to obtain a silicon carbide sintered body. Table 2 shows the density, average grain size aspect ratio of the microstructure, and crystal shape of the obtained sintered body. 20 pieces each from each silicon carbide sintered body obtained.
A test piece was cut out and its bending strength was measured according to the method of JIS R-1601. Similarly, K _IC was measured from each of the 20 test pieces by the Vickers indentation method. These measured values are shown in Table-2. As shown by the values in Table 2, any silicon carbide sintered body produced by using raw material powder with physical properties that meet the conditions of the present invention and sintered under the conditions specified by the present invention satisfies the purpose of the present invention. It was something that had the characteristics of The powder X-ray diffraction images of the sintered intermediate and sintered body obtained in Example 1 are shown in FIG. 5 and FIG. 6, respectively.

[Table] Comparative Examples 1 to 3 Using the powders 4, 5, and 6 shown in Table 1 that do not meet the conditions specified in the present invention, the same method as in the example,
A silicon carbide sintered intermediate was obtained under the same conditions. As shown in Table 3, the results of measuring the density of the obtained sintered intermediate are all within the target value of the present invention.
It was found that it was significantly below 3.05g/cc.

【table】

[Table] Comparative Examples 4 to 6 Using powders 4, 5, and 6 shown in Table-1 that do not meet the conditions specified in the present invention, sintering conditions, etc. were searched to increase the density of the sintered body, and the results were obtained in Table-4. A silicon carbide sintered body was obtained by changing the conditions to those shown in (same conditions as in the example except for the conditions listed in Table 4). As shown in Table 4, the density of the obtained sintered body can be increased by increasing the amount of B ₄ C added for powder 4 (comparative example 4), and for powders 5 and 6 (comparative example 4). Examples 5 and 6) It was found that the heating temperature could be increased by increasing the heating temperature. In addition, Comparative Example 1 using Powder 4
The density of the powder compact containing 0.25 parts of B ₄ C did not reach 3.05 g/cc even when the heating temperature was increased.

[Table] Comparative Examples 7 to 9 The sintered intermediates with a density of 3.05 g/cc or more obtained in Comparative Examples 4 to 6 were prepared in the same manner as in the examples in a nitrogen atmosphere of 1500 atmospheres as shown in Table 5. The heating temperature was maintained for 30 minutes to obtain a silicon carbide sintered body. Next, the mechanical properties of these sintered bodies were measured in exactly the same manner as in the examples, and the results shown in Table 5 were obtained. As shown by the values in Table 5, the microstructure of any of the sintered bodies does not exhibit the physical property values targeted by the present invention, and the mechanical properties are also inferior. Although the density increased and the requirement (d) was satisfied, Comparative Examples 7 and 8
In Comparative Example 9, the average particle diameter was higher than the specified value, and in Comparative Example 9, the aspect ratio was lower than the specified value.
It has been found that a sintered body that achieves the objectives of the invention cannot be obtained. Furthermore, in comparison with Examples 1 to 3,
The average bending strength is low, the standard deviation is large, and the K _IC is also low.

[Particle size measurement]

In the present invention, the particle diameter is measured using a microscope image, which is the only method for directly observing particles, and a transmission electron microscope is used as the microscope. As is well known, when powder particles are supported on a sample support of an electron microscope, agglomeration and segregation of particles tend to occur, making it quite difficult to prepare a sample that represents the entire powder. In order to prevent this problem, studies have been carried out in the past (for example, "Powder Particle Size Measurement Method" edited by the Powder Engineering Research Group (published by Yokendo, 1962))
Various methods are described in Chapter 4). As a result of intensive study of these various methods, the present inventors sprayed a slurry in which powder particles were dispersed onto a sheet mesh using collodion as a support film using a nebulizer, and the powder particles were supported on the collodion film. It was confirmed that the sample gave the best results. In the present invention, the particle size of the powder is determined from an electron microscope image of a sample obtained by this spraying method. The slurry preferably uses isobutanol as a dispersion medium, and the powder slurry concentration is preferably in the range of 0.3 to 0.7%. When dispersing the powder, it is preferable to use a powerful ultrasonic dispersion device such as an ultrasonic cell disrupter and allow it to act for several minutes. The particle size of the powder is determined from the microscopic image of the sample obtained in this way, and here the particle size is defined as the equivalent circle diameter. The equivalent circle diameter is a value expressed by measuring the area of a particle image, assuming a circle with the same area as the image, and expressing the diameter of the circle. Here, in the present invention, in order to eliminate the possibility of arbitrariness as to whether a certain image is regarded as a single particle or a plurality of particles, for example, when an image is partially connected, such as a gourd-shaped image, All shall be considered as a single particle. Note that as the amount of powder supported on the support film increases, the probability that multiple particles will apparently connect and appear as a single particle increases, so the particle diameter in the present invention is defined as the area of the particle image in the entire image. It shall be determined from a sample prepared such that it is in the range of 5 to 15%. Note that the average particle diameter is the arithmetic mean value of equivalent circle diameters. [Crystal form determination method] Low-temperature α contained in silicon carbide powder and sintered body
Crystal 2H, β crystal 3C, high temperature α crystal 4H, 6H, 15
Quantification of R is determined from a pattern obtained by powder X-ray diffraction using CuKα radiation as a light source and a monochromator on the receiving side. The method of calculating each component from each peak height of a figure is described in Ceramics Association Journal, 87 , 576-582.
(1979)). In addition,
When measuring a sintered body using powder X-ray diffraction, the sintered body is ground into 100 meshes or less.

[Brief explanation of the drawing]

FIGS. 1 to 3 are photographs showing the particle state of powders 1, 4, and 6 used in Examples and Comparative Examples of the present invention, respectively, taken with a transmission electron microscope. The magnification is 8000x in both cases. FIG. 4 shows the powder X-ray diffraction pattern of Powder 1. 5 and 6 show powder X-ray diffraction patterns of a sintered intermediate and a sintered body, respectively, obtained using powder 1 of Example 1 as a raw material. 7 to 9 are photographs taken with an optical microscope showing the grain structure of the cross section of the sintered bodies obtained in Example 1, Comparative Example 8, and Comparative Example 9, respectively. The magnification is 500x in both cases. FIGS. 10 and 11 are photographs taken with an optical microscope showing the grain structures of the cross sections of the sintered intermediates obtained in Example 1 and Comparative Example 1, respectively. The magnification is 150x in both cases. FIG. 12 is a graph showing the relationship between (standard deviation of particle diameter/ρ) and Ts.
FIG. 13 is a graph showing the relationship between the α crystal content of the sintered intermediate and the aspect ratio of the microstructure of the crystal. FIG. 14 is a graph showing the relationship between the density of the sintered intermediate and the density of the sintered body.

Claims (1)

[Claims] 1. After adding and mixing 0.05 to 0.3 parts by weight of boron and 0.5 to 3.0 parts by weight of carbon to 100 parts by weight of silicon carbide powder, the mixture is molded and heated and sintered at 1900 to 2050°C. In the method for obtaining a silicon carbide sintered body, the physical properties of the silicon carbide powder are as shown below (a):
It satisfies the conditions (b) and (c): (a) The average particle diameter ρ is 0.1 to 0.2μ. (b) The standard deviation of particle diameter is 0.8×ρ or less. (c) Crystal form: 80% by weight or more of β crystals and 20% by weight
Contains the following low-temperature alpha crystals. And once in an inert gas atmosphere 1850~2000
C. to obtain a sintered intermediate that satisfies the following conditions (d) and (e): (d) Density is 3.05 g/cc or more. (e) The crystal form shall contain 95% by weight or more of β crystals and 5% by weight or less of α crystals. After that, it is heated to 1800-2000℃ in an inert gas atmosphere of 100 atmospheres or more, and the following (f), (g),
A novel method for producing a silicon carbide sintered body, which comprises obtaining a sintered body characterized by the characteristics (h). (f) The density of the sintered body must be 3.10 g/cc or more. (g) The average particle diameter of the microstructure is 3 to 15μ and the aspect ratio is 5 to 20. (H) The crystal form shall contain 50% by weight or more of β crystals and less than 50% by weight of high-temperature α crystals. 2 There are 5 to 5 low-temperature α crystals in the raw material silicon carbide powder.
20% by weight, and the resulting silicon carbide sintered body contains 5% by weight or less of high-temperature α crystals.