Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Reexamination Certificate
2000-05-26
2002-05-14
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
C501S090000, C264S643000, C264S676000
Reexamination Certificate
active
06387834
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sintered silicon carbide body such as a boat for heat-treating semiconductors or a liner tube, and to a method for efficiently producing the sintered silicon carbide body.
2. Description of the Related Art:
Conventionally, there has been the following problem in the production of a sintered silicon carbide body: when metallic silicon is, in a non-oxidizing atmosphere, impregnated and filled into a molded body containing silicon carbide and carbon so as to form an impregnated body and then the impregnated body is cooled, the impregnated body is broken, because the excess metallic silicon which has not reacted with the carbon volumetrically expands during the cooling and hardening.
Especially in a case of an elongated body, this problem occurs frequently, because the impregnated body hardens in an instant at the same time as the furnace is cooled. It has therefore been difficult to decrease the percentage of defective bodies even if the cooling rate is adjusted.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for producing a sintered silicon carbide body which solves the above-described conventional problem so as to produce a sintered silicon carbide body which does not crack or break, and to provide a sintered silicon carbide body which is produced by the method for producing and has excellent strength and the like.
In the method for producing a sintered silicon carbide body of the present invention for attaining the above object, metallic silicon is, in a vacuum atmosphere or in a non-oxidizing atmosphere, impregnated into a molded body containing silicon carbide and carbon so as to form an impregnated body, and the impregnated body is cooled in a state of being provided with a temperature distribution of 0.1-1.5° C./cm.
In the method for producing a sintered silicon carbide body, first, metallic silicon is impregnated into the molded body. At this time, the molten metallic silicon permeates into the molded body due to capillary action and the like. The permeated metallic silicon reacts with the carbon in the molded body, and silicon carbide is thereby generated. Next, the impregnated body is cooled in a state of being provided with a uniform temperature distribution. Accordingly, unreacted metallic silicon having expanded volume can be gradually formed from the lower temperature side toward the higher temperature side. As a result, a sintered silicon carbide body without cracks or fractures can be obtained.
The sintered silicon carbide body of the present invention for attaining the above object is produced by the method for producing a sintered silicon carbide body of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A method for producing a sintered silicon carbide body of the present invention includes an impregnated body forming step and a cooling step, and includes other steps as needed. The sintered silicon carbide body of the present invention is produced by the method for producing a sintered silicon carbide body of the present invention.
(Impregnated body forming step)
The impregnated body forming step is a step in which metallic silicon is, in a vacuum atmosphere or in a non-oxidizing atmosphere, impregnated into a molded body containing silicon carbide and carbon so as to form an impregnated body.
Molded Body
The molded body is obtained by mixing silicon carbide powder with an organic substance containing one or more types of carbon sources or with carbon powder so as to form mixed powder in a slurry-state, and then by molding the mixed powder. The obtained molded body may be referred to as a green body. This body is a molded body of non-sintered silicon carbide and carbon, which is obtained by removing the solvent from the mixed powder in a slurry-state, and has many pores therein.
The silicon carbide powder can be produced by the latter method using, as a raw material, a silicon carbide powder such as a powder of &agr;-type silicon carbide, &bgr;-type silicon carbide, amorphous silicon carbide, or a silicon carbide which is a mixture of these types.
In order to obtain a sintered silicon carbide body with high purity, it is preferable to use high purity silicon carbide powder as a raw material. The grade of the powder of &bgr;-type silicon carbide is not particularly limited, and, for example, a commercially available &bgr;-type silicon carbide powder can be used.
From the standpoint of obtaining a high density, it is preferable that the silicon carbide powder has a small particle diameter. Specifically, the particle diameter is preferably 0.01-10 &mgr;m, and more preferably 0.05-5 &mgr;m.
When the particle diameter is less than 0.01 &mgr;m, handling in the processes of measuring, mixing and the like may become difficult. On the other hand, when the particle diameter exceeds 10 &mgr;m, the specific surface area of the powder becomes small, i.e., the contact surface area between the particles becomes small, and it may become difficult to obtain a high density.
The high purity silicon carbide powder can be obtained, for example, by dissolving a silicon source including one or more types of silicon compound, a carbon source including one or more types of organic compound which generate carbon by heating, and a polymerization/crosslinking catalyst, in a solvent, and by drying the obtained solution, and then by burning the obtained powder in a nonoxidizing atmosphere.
A combination of a liquid silicon source and a solid silicon source can be used as the silicon source. The silicon source must include at least one type of liquid silicon source.
Examples of the liquid silicon source include (mono-, di-, tri-, tetra-) alkoxysilanes, polymers of tetraalkoxysilane, and the like.
Among the alkoxysilanes, tetraalkoxysilane is preferable, specifically, methoxysilane, ethoxysilane, propoxysilane, butoxysilane and the like are preferable. In terms of handling, ethoxysilane is especially preferable.
Examples of polymers of tetraalkoxysilane include polymers with a low polymerization degree of about 2-15 (oligomers), liquid polymers of silicic acid with a high polymerization degree, and the like. If desired, these polymers may be used in combination with silicon oxide.
Examples of the silicon oxide include, in addition to SiO, silica sol (such as a solution including extremely fine colloidal silica, which includes an OH group and an alkoxyl group therein), silicon dioxide (such as silica gel, fine silica, and quartz powder) and the like.
A single type or a combination of two or more types of the above silicon sources may be used.
Among the silicon sources, in terms of better uniformity and handling, an oligomer of tetraethoxysilane, a mixture of an oligomer of tetraethoxysilane and fine powder of silica, and the like are preferable. Further, silicon sources with high purity are preferable, and specifically, the initial impurities content thereof is preferably 20 ppm or less and more preferably 5 ppm or less.
In addition to a liquid carbon source, a combination of a liquid carbon source and a solid carbon source can be used as the carbon source, and an organic compound which has a high residual carbon ratio and is polymerized or crosslinked by the action of the catalyst or by heating is preferable. Specific examples of the organic compound preferably include monomers, prepolymers and the like of resins such as phenol resin, furan resin, polyimide, polyurethane and polyvinyl alcohol, as well as liquid carbon sources such as cellulose, sucrose, pitch and tar. Among these organic compounds, phenol resin of the resol type is especially preferable. A single type or a combination of two or more types of these carbon sources may be used. Further, the purity thereof can be suitably controlled and selected depending upon the purpose. When a silicon carbide powder with particularly high purity is required, it is preferable that an organic compound which does not include 5 ppm or more of any metal is used.
The polymerization/crosslinking catalyst can be
Odaka Fumio
Takahashi Yoshitomo
Ushita Kazuhiro
Bridgestone Corporation
Group Karl
Oliff & Berridg,e PLC
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