JPS649242B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is a novel method for producing silicon carbide powder in which carbon is uniformly dispersed. In recent years, from the standpoint of energy and resource conservation,
Ceramics are attracting attention as high-temperature structural materials. Among them, silicon carbide is expected to be the most promising material along with silicon nitride. Usually, carbon-silicon sintered bodies are produced by hot pressing, pressureless sintering, chemical vapor deposition, or the like. Among these, the pressureless sintering method is considered to be the most industrially advantageous method because it is a relatively simple method commonly used for molding, and can produce complex and large-sized products. However, since silicon carbide is a compound with strong covalent bonding properties, it cannot be sintered alone even in normal pressure sintering, and it is necessary to add a sintering aid. Carbon is effective as this sintering aid, and boron, aluminum, etc. are also commonly used. Moreover, even if these sintering aids are used, it is difficult to uniformly disperse the sintering aids on the particle surface using normal methods, so the structure of the sintered body becomes uneven after sintering, resulting in poor strength. does not rise much. Boron, aluminum, etc. are easily diffused during sintering even if they are not uniformly dispersed when added, and as a result, the dispersibility during addition is not much of a problem, but carbon does not diffuse much during sintering. First, therefore, the dispersibility of carbon in particles before sintering becomes a problem. The present inventors have conducted extensive studies on how to uniformly disperse carbon in SiC particles, and applied the technique of surface coating with polymers (hereinafter referred to as microencapsulation) to uniformly coat carbon onto SiC particles. As a result, they succeeded in obtaining SiC powder containing carbon very uniformly, and by sintering this powder, they were able to obtain an excellent sintered body with high strength. By such means, the amount of carbon contained can be smaller than in the case of sintering in the conventional method, and a uniform structure can be formed, so that the bending strength is also improved. In the present invention, when boron or aluminum is further added as a sintering aid, SiC powder to which these sintering aids have been added is microencapsulated, or carbon-coated SiC
It is also possible to obtain a powder and add these sintering aids to it, and a good sintered body can be obtained. Next, carbon coating of SiC powder by microencapsulation in the present invention will be described in detail. SiC powder is dispersed in an aqueous medium together with a vinyl monomer capable of radical polymerization or radical copolymerization, and a polymerization initiator is added to perform a polymerization reaction.
Coat the polymer uniformly on the SiC powder surface. The vinyl monomer used in the polymerization reaction may be one that undergoes radical polymerization or radical copolymerization, such as acrylic acid, methacrylic acid, acrylate, methacrylic ester, acrylonitrile, N-methylolacrylamide, etc. ,
Examples include vinyl chloride, vinyl acetate, styrene, divinylbenzene, and vinylidene fluoride. As a polymerization initiator, sulfur dioxide, sulfite aqueous solution, potassium persulfate, azobiscyanovaleric acid, V
-50 [2,2'-azobis-(2-aminodinopropane)-dihydrochloride, manufactured by Wako Pure Chemical Industries], etc. are used. To suitably carry out this polymerization reaction, 1 to 100 parts by weight of an organic solvent or 1 to 50 parts by weight of a surfactant, and 1 to 100 parts by weight of SiC powder are added to 100 parts by weight of water, and 0.1 to 100 parts by weight of a vinyl monomer. The polymerization initiator is added to the suspension thus obtained, and the polymerization initiator is added and stirred thoroughly. It is sufficient that the amount of the polymerization initiator added is 0.01 to 20% by weight based on the vinyl monomer. Although the reaction temperature is sufficient at room temperature,
When it is desired to shorten the polymerization time, it is sufficient to heat the polymer to about 70°C, and a high polymerization rate can be obtained in a short time of about 1 to 5 hours. After the polymerization reaction is completed, the slurry-like SiC powder is filtered, washed with water, and the residue is dried. The polymer coating composition for SiC powder obtained by such a method can be coated with any amount of resin, and the reaction conditions can be easily controlled. The thus obtained polymer-coated SiC powder is heat-treated in air at 250°C for 4 to 40 hours, and then baked in a non-oxidizing atmosphere at 800 to 1300°C for 2 to 10 hours to remove the coating polymer. Complete carbonization results in SiC powder that is uniformly coated with carbon. SiC powder containing carbon uniformly can be obtained as described above, but if other sintering aids such as aluminum and boron are coexisting before the microencapsulation treatment of SiC powder as described above, SiC At the same time, these sintering aids are also uniformly coated with the polymer, and in the above firing process, a mixed powder in which SiC and these sintering aids are uniformly coated with carbon is obtained. Further, when aluminum and boron are added as other sintering aids to SiC powder uniformly coated with carbon, the mixture can be sufficiently uniformized by ordinary stirring and mixing. In this way, the present invention can uniformly disperse other sintering aids such as carbon, aluminum, and boron in the SiC powder, so it can be molded and molded based on these powders.
By sintering, it is possible to obtain a sintered body that is harder and more uniform than ever before. In order to mold and sinter, it is molded by conventional extrusion, dry pressing, injection molding, etc., and is molded in a vacuum or with CO2.
This can be carried out by heating to 1900°C or higher in a gas atmosphere or an inert gas atmosphere. Since the SiC powder according to the present invention is uniformly coated with carbon, which is a sintering aid that was difficult to contain uniformly in the past, a sintered body with sufficiently high strength can be obtained even by pressureless sintering. It is something. In the present invention, it is preferable to perform microencapsulation by setting the content ratio of the coating polymer so that the amount of carbon coated on the SiC powder is 5% or less. It should be noted that if the carbon content exceeds 5%, it is not preferable because it becomes free carbon when sintered and adversely affects sinterability and formability. Hereinafter, the present invention will be explained in more detail based on examples. Example 1 A 500 ml three-necked flask was immersed in a constant temperature bath at 60°C, and 250 ml of water and SiC powder (β-SiC, SiC = 99
% or more, specific surface area 13.2 m ² /g), and 5 g of acrylonitrile, and while stirring and mixing, 4% V-
50 [2,2′-azobis-(2-amidinopropane)
-dihydrochloride] 50 ml of an aqueous solution was added to carry out a polymerization reaction. At this time, boron was also added in an amount of 1% by weight based on the SiC powder. The reaction time was set to 3 hr. After the reaction, the product was filtered, thoroughly washed with water, and then vacuum dried at 80°C. The content of polyacrylonitrile adhered at this time was 4.5 to the SiC powder.
It was in weight%. Thereafter, heat treatment was performed in Ar at 1000°C for 3 hours to almost carbonize the attached acrylonitrile. The content at that time was 1.9% by weight. Mix 1% by weight of polyethylene glycol with this powder,
After shaping, sintering was performed at 2000°C in an Ar gas atmosphere to obtain a sintered body with a theoretical density of 93%. When the three-point bending strength of this product was measured at room temperature, a high value of 60 Kg/mm ² was obtained. Examples 2 to 6 The same operations as in Example 1 were performed, except that the raw materials used for microencapsulation, their preparation conditions, and the amount of boron added required for sintering were changed. Table 1 shows the conditions, density, and bending strength.

[Table] Example 7 During microencapsulation, without adding boron,
Carbon-coated SiC powder with a carbon content of 2.0% by weight was obtained under the same conditions as in Example 1 except that only the SiC powder was microencapsulated. Boron was added to this powder to give a concentration of 1% by weight, and mixed and stirred to obtain a mixed powder. This mixed powder was mixed with 1% by weight of ethylene glycol and molded, followed by pressureless sintering at 2000°C in an Ar gas atmosphere to obtain a sintered body with a theoretical density of 92%. Moreover, the bending strength was 59Kg/mm ² . Comparative Examples 1, 2 100 g of silicon carbide powder (β-SiC, SiC = 99% or more, specific surface 13.2 m ² /g) 1 g of carbon, 1 g of boron
(Comparative Example 1), carbon 0.5g, boron 0.5
(Comparative Example 2) were wet-mixed in hexane in a plastic ball mill. After that, it is dried, mixed with phenol formaldehyde resin as a binder, and molded.
After degreasing, pressureless sintering was performed at 2000℃. The ratio of the obtained sintered body to the theoretical density and the bending strength of Comparative Example 1 were 95%, 50Kg/mm 2 , and 50Kg/mm ² , respectively.
The weight of Comparative Example 2 was 87%, 43 Kg/mm ² .

Claims (1)

[Claims] 1 A radical polymerizable or radical copolymerizable vinyl monomer and SiC powder, or boron,
A mixed powder containing a sintering aid such as aluminum is dispersed in an aqueous medium and polymerized. The powder is coated with a polymer and heated in air at a temperature below 250°C.
A method for producing carbon-containing SiC powder, which comprises carbonizing a coating polymer by firing at ~1300°C in a non-oxidizing atmosphere.