JP2008150263A - Method for producing silicon carbide powder - Google Patents

Abstract

A method for producing silicon carbide powder suitable as a raw material for producing a high-density, high-strength silicon carbide sintered body is provided.
A silica sol containing silica particles as core particles is produced, and then mixed by changing the quantitative ratio of silicon alkoxide, alcohol and aqueous ammonia solution, and set at temperature and pH to be hydrolyzed (A) (B). A bimodal silica sol is prepared by hydrolysis under different two-stage conditions, and then polymerized by adding phenols, formaldehyde and aqueous ammonia solution, and the core is coated with a phenol resin around the silica particles as the core A silicon carbide powder characterized in that a SiC precursor having a shell structure is prepared, heat-treated at 800 to 1000 ° C. in an oxygen-free atmosphere, and then silicified by heat treatment at 1400 to 2200 ° C. in an inert atmosphere. Manufacturing method.
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Description

  The present invention relates to a method for producing silicon carbide powder suitable for producing a silicon carbide sintered body having high density and excellent strength characteristics.

  For silicon carbide powder, a method of pulverizing and classifying bulk silicon carbide produced by the Atchison method has been known for a long time. However, since silicon carbide is an extremely hard substance, it is difficult to pulverize it into fine and spherical powder particles, and impurities are easily mixed in the pulverization and classification processes, and it is difficult to obtain a high-purity product.

  Therefore, a technique for producing fine silicon carbide fine powder of submicron grade by a gas phase process has been developed. For example, Patent Document 1 discloses an average particle diameter of silicon carbide powder obtained by thermally decomposing halogenated silane. Is an easily sinterable β-type silicon carbide powder in which the average ratio of the maximum particle size and the minimum particle size of each particle is 1.1 to 1.4.

  Further, Patent Document 2 discloses a spherical ultrafine particle β-type polycrystalline SiC that is an aggregate of β-type SiC having a crystallite of 50 angstroms or less and an average particle diameter of 0.01 to 1 μm. In addition, it is described that it is produced by vapor-phase pyrolysis of an organosilicon compound having at least one SiH bond in the molecule and containing no SiX (X is a halogen atom, etc.) bond at 750 ° C. or higher. ing.

Patent Document 3 discloses a spherical monodispersed β-SiC in which a mixture of silicon alkoxide and an alkoxysilane having at least one hydrocarbon group is hydrolyzed into spherical monodispersed gel particles, which are then fired to form β-SiC. A method for producing fine particles is disclosed.
JP 59-102809 Japanese Patent Laid-Open No. 60-096517 Japanese Patent Laid-Open No. 03-199115

  On the other hand, the silicon carbide sintered body is manufactured by a method in which a mixed powder obtained by mixing a silicon carbide powder with a sintering aid or the like is filled in a mold and subjected to hot-pressure sintering, but the raw material powder is uniformly mixed. There is a difficulty that it is difficult.

  Therefore, a method is widely used in which a silicon carbide powder, a sintering aid and the like are dispersed in a dispersion medium to prepare a slurry, and a compact is produced by a casting molding method and subjected to hot-pressure sintering. In this case, increasing the fluidity of the raw material slurry is important in obtaining a dense molded body, and the viscosity of the slurry is influenced by the particle size characteristics of the silicon carbide powder, and generally using silicon carbide powders having different particle sizes. The raw material powder adjusted in particle size is applied.

  In addition, in order to obtain a sintered body having a high density and excellent strength, it is effective to increase the density of the molded body. In order to increase the density of the molded body, the raw material silicon carbide powder whose particle size is adjusted is used. It will be preferred to use.

  However, it is not easy to prepare silicon carbide powder having a desired particle size by means such as pulverization because silicon carbide is an extremely hard substance, resulting in a decrease in product yield and an increase in cost. There's a problem.

  Therefore, the present inventor has developed a slurry manufacturing technique (Japanese Patent Application No. 2006-091098) in which silica particles having a bimodal particle size distribution previously developed and proposed by the present applicant, and the surroundings of the silica particles. High-density, high-strength silicon carbide is researched based on silicon carbide powder manufacturing technology (Japanese Patent Application No. 2006-012432), which is obtained by firing, carbonizing, and then silicifying core-shell SiC precursors coated with phenolic resin. Silicon carbide powder suitable as a raw material for producing a sintered body was successfully produced.

  That is, an object of the present invention is to provide a method for producing silicon carbide powder suitable as a raw material for producing a high-density, high-strength silicon carbide sintered body.

In order to achieve the above object, the method for producing silicon carbide powder according to the present invention comprises adding an aqueous ammonia solution to a mixed solution of silicon alkoxide and alcohol, stirring and hydrolyzing to prepare a silica sol. After adjusting the temperature and pH at the time to produce a silica sol having an average particle diameter of 400 nm or less of silica particles serving as core particles,
(A) The silica sol is added and mixed with varying amounts of silicon alkoxide, alcohol and aqueous ammonia solution, and is hydrolyzed by setting the temperature at 35 ° C. to less than the boiling point of the alcohol and the pH at 8 to 13. To increase the size of silica core particles,
(B) Next, silicon alkoxide, alcohol and aqueous ammonia solution are added and mixed at different ratios, hydrolyzed by setting the temperature at 35 ° C. to less than the boiling point of the alcohol, and the pH at 8 to 13, and further silica nuclei. While increasing the size of the particles, small particles are formed in the large particles.
Hydrolysis under different conditions of at least (A) and (B) to prepare a silica sol having a bimodal particle size distribution of silica particles, and then polymerize by adding phenols, formaldehyde and an aqueous ammonia solution, An SiC precursor having a core / shell structure in which silica particles are used as cores and coated with phenol resin is prepared, heat-treated at 800 to 1000 ° C. in an oxygen-free atmosphere and fired, and then 1400 to 2200 ° C. in an inert atmosphere. It is structurally characterized by silicidizing by heat treatment.

  In the above configuration, the average particle diameter Dl of the silica large particles of the silica sol having a bimodal particle size distribution is preferably 500 to 2000 nm, and the average particle diameter Ds of the silica small particles is preferably 10 to 500 nm. The ratio Dl / Ds between the average particle diameter Dl of the silica and the average particle diameter Ds of the small silica particles is preferably 2 to 100.

  According to the present invention, a silica sol in which silica particles having at least a bimodal particle size distribution are dispersed is prepared, and an SiC precursor having a core / shell structure in which the silica particles of the silica sol are coated with a phenol resin around the core is prepared. Then, after firing, carbonizing, and silicifying, a silicon carbide powder in which silica particles having a large particle size and a small particle size are mixed can be manufactured. A high-strength silicon carbide sintered body can be produced.

  The basic process of the method for producing the silicon carbide powder of the present invention is to add a solution of ammonia to a mixed solution of silicon alkoxide and alcohol, stir to hydrolyze the silicon alkoxide to produce a silica sol in which silica particles are dispersed, An SiC precursor having a core / shell structure in which silica particles in silica sol are used as cores and the periphery thereof is coated with a phenol resin is prepared, and carbonized and silicified to be converted into silicon carbide powder.

  In this basic process, first, silicon alkoxide is dissolved in alcohol, an aqueous ammonia solution is added to the mixed solution, and the temperature and pH are adjusted to hydrolyze the silicon alkoxide, whereby silica particles having a core particle size of 400 nm or less. Produces a silica sol in which is dispersed. In this case, it is preferable to produce silica particles having a uniform particle diameter. For example, silicon alkoxide is dissolved in an alcohol having a volume ratio of 3 to 8, an aqueous ammonia solution is added, the temperature is 20 to 70 ° C., and the pH is 8. The silica core particles having an average particle diameter of 400 nm or less are generated by adjusting to ˜13 and hydrolyzing.

  Examples of the silicon alkoxide include alkyl silicate, (mono-, di-, tri-, tetra-) alkoxysilane, tetraalkoxysilane polymer, and the like. Examples of the alkyl silicate include methyl silicate, ethyl silicate, butyl silicate and the like, and ethyl silicate is preferable from the viewpoints of handleability and reactivity. As the alkoxysilane, tetraalkoxysilane is preferable, and as the polymer of tetraalkoxysilane, a low molecular weight polymer having a polymerization degree of about 2 to 15 is preferable.

  The alcohol that dissolves the silicon alkoxide is not particularly limited as long as it is water-soluble, and methanol, ethanol, and the like can be used. However, it is preferable that the heating temperature for producing the silica sol is not higher than the boiling point of the alcohol.

  (A) The silicon alkoxide, alcohol, and aqueous ammonia solution were added to and mixed with the silica sol that produced the silica core particles in this manner, and the silicon alkoxide was hydrolyzed by controlling the temperature and pH to obtain silica. Grows core particles. Specifically, silica particles are enlarged to about 900 nm by hydrolysis at a temperature of 35 ° C. to less than the boiling point of alcohol and a pH of 8 to 13.

Next, (B) the silicon alkoxide is hydrolyzed by changing and adding the silicon alkoxide, the alcohol and the aqueous ammonia solution again, mixing and controlling the temperature and pH.
Specifically, the temperature is set to 35 ° C. to less than the boiling point of the alcohol, and the pH is set to 8 to 13, whereby the silica particles are further grown to become large particles and the small particles are formed in the large particles. Let

  In this way, the silica sol in which silica particles having an average particle diameter of 400 nm or less serving as core particles are dispersed is added with varying amounts of silicon alkoxide, alcohol, and aqueous ammonia solution, and at least hydrolysis of the silicon alkoxide is performed (A). (B) By carrying out under two different conditions, a silica particle having a large particle size and a silica particle having a small particle size are generated, and a bimodal silica sol having two peaks in the particle size distribution of the silica particle is prepared. Is done.

  In this case, by changing the quantity ratio of silicon alkoxide, alcohol, and aqueous ammonia, and adjusting the temperature and pH during hydrolysis, the particle size characteristics of the bimodal silica sol are controlled, and the average particle size of the large silica particles By controlling Dl to 500 to 2000 nm, the average particle diameter Ds of silica small particles to 10 to 500 nm, and the ratio Dl / Ds to 2 to 100, a high-density, high-strength silicon carbide sintered body is produced. A silicon carbide powder having suitable particle size characteristics can be produced.

  After diluting by adding water to the bimodal silica sol having two peaks in the particle size distribution of the silica particles thus obtained, phenols, formaldehyde and an aqueous ammonia solution are added to the diluted solution, By polymerizing formaldehyde, a polymer of phenols and formaldehyde is formed around silica particles, and a core / shell structure SiC precursor in which the core (core) of silica particles is coated (shell) with a phenol resin. Make it.

  The diluent is preferably diluted by adding 0.5 to 2 times the volume of water to silica sol by volume. When the amount of water added is 0.5 times or less, the amount capable of dissolving phenols is remarkably small. On the other hand, when the amount is 2 times or more, the concentration of phenols and formaldehyde becomes dilute, and the polymerization reaction does not proceed smoothly. is there.

  The SiC precursor having a core / shell structure is preferably adjusted so that the ratio of the phenol resin to the silica particles is about 33 to 500 by volume with respect to the silica particles 100. For example, phenols It is preferable to polymerize by adding formaldehyde in a molar ratio of 0.5 to 3.

  This is because when the volume ratio of the phenol resin obtained by polymerization is 33 or less, the silica particles cannot be completely coated, and when the volume ratio of the phenol resin is 500 or more, free carbon when the resin is baked to carbonize the resin. Since the amount increases, it takes a lot of work to remove it, and productivity is lowered.

  The polymerization reaction is preferably performed under conditions of pH 9 to 13 within a temperature range of room temperature to 100 ° C. This is because the reaction rate of the polymerization reaction is slow at room temperature or lower, and the solvent volatilization occurs at 100 ° C. or higher and the reaction proceeds very quickly, which may cause reaction runaway. In addition, the reaction is very slow at a neutral pH of 6 to 8, and the polymer is gelled or spongy at a pH of 5 or less, and a pulverization process is required to obtain a fine powder. .

  Any phenol can be used as long as it generates a polymer by reaction with formaldehyde, such as phenol, 2-methylphenol, 3-methylphenol, 4-methylphenol, 1,3 dihydroxybenzene, 1 4, dihydroxybenzene and the like are exemplified, and preferably 1,3 dihydroxybenzene is used.

  The core-shell structured SiC precursor, in which the silica particles having a bimodal particle size distribution thus produced are used as cores (cores) and the periphery thereof is coated (shell) by filtration, is separated by filtration. After drying at 150 ° C., the phenol resin is baked and carbonized by heat treatment at 800 to 1000 ° C. in an oxygen-free atmosphere.

By this calcination and carbonization treatment, a calcination carbide of a SiC precursor having a C / SiO ₂ structure in which silica (SiO ₂ ) is used as a core and the periphery thereof is coated with a resin carbide is obtained. It should be noted that if the temperature during the calcining heat treatment is 800 ° C. or less, calcining carbonization is insufficient, while if the temperature is 1000 ° C. or more, a reduction reaction of silica with carbon occurs, which is not preferable.

Next, the calcined carbide having a C / SiO ₂ structure obtained by heat-treating this SiC precursor is heat-treated at a temperature of 1400 to 2200 ° C. in an inert atmosphere such as He, Ne, Ar, and N _{2 to} convert the silica into carbon. And then silicified by reaction of SiO ₂ + C → SiO + CO, SiO + 2C → SiC + CO. This is because the reaction proceeds slowly when the heat treatment temperature is 1400 ° C. or lower, and SiC decomposes when the heat treatment temperature is 2200 ° C. or higher.

  In addition, since free carbon exists in the silicified product, it is necessary to remove this, and the removal of free carbon is, for example, a method of burning and removing free carbon by heating at a temperature of 1000 ° C. or less in air. Simple and preferable. However, SiC is oxidized at a temperature of 1000 ° C. or higher. In this way, silicon carbide powder is produced.

  Examples of the present invention will be specifically described below in comparison with comparative examples.

Example 1
35 g of tetraethoxysilane is dissolved in 175 g of ethanol (5.8 times by volume), 70 g of 3% aqueous ammonia solution is added and stirred, and the temperature is adjusted to 25 ° C. and pH 12 to hydrolyze tetraethoxysilane. Thus, a silica sol having an average particle diameter of 370 nm of silica particles serving as core particles was produced.

  To 280 g of this silica sol, 350 g of (A) tetraethoxysilane, 1750 g of ethanol, and 700 g of 3% aqueous ammonia solution are added and stirred, and adjusted to a temperature of 25 ° C. and pH 12 to hydrolyze tetraethoxysilane to form silica core particles. The average particle size was increased to 920 nm to increase the size.

  Next, after separating 1/10 of the total amount of silica sol of (A), (B) 35 g of tetraethoxysilane, 175 g of ethanol, and 70 g of 3% ammonia aqueous solution were added and stirred to adjust the temperature to 40 ° C. and pH 12. Then, tetraethoxysilane is hydrolyzed to increase the size of silica particles, and the average particle size Dl grows to 1480 nm to increase the size of the particles. In addition, small particles are formed in the large particles, and the average particle size Ds is 280 nm. A silica sol having the following particle size distribution was prepared.

  The silica sol was diluted by adding ion exchange water at a volume ratio of 1: 1, and then 1,3 dihydroxybenzene and twice its amount (molar ratio) of formaldehyde were added to the diluted solution and stirred. The addition amount of 1,3 dihydroxybenzene was set so that the volume ratio of the phenol resin produced by polymerization of 1,3 dihydroxybenzene and formaldehyde was 300% of the silica particles in the silica sol.

  Next, an aqueous ammonia solution is added to adjust the pH to 12, the mixture is stirred and mixed at room temperature to polymerize 1,3 dihydroxybenzene and formaldehyde, and the core is coated with a phenol resin as a polymer around silica particles. -A suspension containing a SiC precursor having a shell structure was prepared. This suspension was filtered to separate and remove only the solid content, thereby producing a SiC precursor.

This SiC precursor was dried at 100 ° C., then heat-treated at 900 ° C. in an oxygen-free oxygen-barrier atmosphere, calcined and calcined, with silica (SiO ₂ ) as the core, and the surroundings coated with resin carbide. A calcined carbide of a SiC precursor having a / SiO ₂ structure was obtained.

Next, the calcined carbide is heat-treated in an Ar gas atmosphere at a temperature of 1700 ° C., and SiO ₂ is thermally reduced with C to be silicified, and then heated to 800 ° C. in air to burn and remove free carbon. A silicon carbide powder was produced.

Example 2
In Example 1, silicon carbide powder was produced in the same manner as in Example 1 except that (B) was hydrolyzed with an aqueous ammonia addition amount of 35 g, a temperature of 35 ° C., and a pH of 11.

Example 3
In Example 1, silicon carbide powder was produced in the same manner as in Example 1 except that hydrolysis of (B) was carried out by adding an aqueous ammonia solution amount of 140 g, a temperature of 70 ° C., and a pH of 13.

Comparative Example 1
In Example 1, silicon carbide powder was produced in the same manner as in Example 1 except that (A) and (B) were not hydrolyzed.

Comparative Example 2
In Example 1, silicon carbide powder was produced in the same manner as in Example 1 except that (B) was not hydrolyzed.

  For the silicon carbide powder thus produced, the particle size distribution was measured by a laser diffraction method to determine the average particle size. The obtained results are shown in Tables 1 and 2 in comparison with the production method.

  Next, the tap density of these silicon carbide powders was measured, and the powder filling properties were compared. The tap density was measured by the constant volume measurement method of JIS R1628-1997 “Method for measuring bulk density of fine ceramic powder”.

  From Tables 1-3, the SiC powder of the Example manufactured using the bimodal silica sol has high powder filling property, while the SiC powder of the comparative example produced from the silica sol having a single particle size distribution is higher than that of the Example. The powder filling property was inferior.

Claims (3)

When preparing a silica sol by adding an aqueous ammonia solution to a mixed solution of silicon alkoxide and alcohol and stirring and hydrolyzing, the average particle size of silica particles as core particles is adjusted by adjusting the temperature and pH during hydrolysis. After producing a silica sol of 400 nm or less,
(A) The silica sol is added and mixed with varying amounts of silicon alkoxide, alcohol and aqueous ammonia solution, and is hydrolyzed by setting the temperature at 35 ° C. to less than the boiling point of the alcohol and the pH at 8 to 13. To increase the size of silica core particles,
(B) Next, silicon alkoxide, alcohol and aqueous ammonia solution are added and mixed at different ratios, hydrolyzed by setting the temperature at 35 ° C. to less than the boiling point of the alcohol, and the pH at 8 to 13, and further silica nuclei. While increasing the size of the particles, small particles are formed in the large particles.
Hydrolysis under different conditions of at least (A) and (B) to prepare a silica sol having a bimodal particle size distribution of silica particles, and then polymerize by adding phenols, formaldehyde and an aqueous ammonia solution, An SiC precursor having a core / shell structure in which silica particles are used as cores and coated with phenol resin is prepared, heat-treated at 800 to 1000 ° C. in an oxygen-free atmosphere and fired, and then 1400 to 2200 ° C. in an inert atmosphere. A method for producing silicon carbide powder, wherein the silicon carbide powder is heat-treated and silicified.   The method for producing a silicon carbide powder according to claim 1, wherein the silica large particle of the silica sol having a bimodal particle size distribution has an average particle diameter Dl of 500 to 2000 nm, and the silica small particle has an average particle diameter Ds of 10 to 500 nm.   The method for producing silicon carbide powder according to claim 1, wherein the ratio Dl / Ds between the average particle diameter Dl of the large silica particles and the average particle diameter Ds of the small silica particles is 2 to 100.