Chemistry of inorganic compounds – Silicon or compound thereof – Binary compound
Reexamination Certificate
1997-08-22
2001-06-26
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Binary compound
C423S346000, C501S088000
Reexamination Certificate
active
06251353
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a production method of silicon carbide particles, more specifically to a production method of silicon carbide particles not generating a sulfur compound derived from a hardening polymerization catalyst.
2. Description of the Related Art
As a production method of silicon carbide, conventionally the Atison method for obtaining silicon carbide particles using quartz sand and petroleum coke as the materials is known. It is known that according to the method, since many impurities are present in the above-mentioned materials, the silicon carbide particles obtained are not sufficiently pure, when using in sintered body material, many disadvantages in the various characteristics appear due to impurities in the sintered bodies. Thus they are not appropriate for the production of high purity sintered bodies.
As a method of producing silicon carbide particles of high purity, a method of using a high purity liquid type silicon source and a liquid type carbon source as the materials is disclosed in Japanese Patent Application Publication (JP-B) No. 1-42886.
In this method, toluene sulfonic acid is conventionally selected as a strong non-metallic acid in consideration of the strength of the pKa. However, it has a disadvantage in that equipment and a process are required for treating gases of sulfur compounds such as SO and SO
2
generated and continuously discharged outside the furnace in the carbonizing and baking processes.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a production method of high quality silicon carbide particles without generating sulfur compounds in the carbonizing and baking processes in the above-mentioned production method of silicon carbide of high purity.
The present inventors earnestly studied the mechanism of generation of the sulfur compound and found a method of homogeneously generating the hydrolysis and polymerization reaction without using a sulfur-containing catalyst represented by toluene sulfonic acid used in the production method of silicon carbide.
That is, a production method of silicon carbide of the present invention comprises a step of mixing at least one kind of silicon compound, liquid at room temperature, an organic compound having a functional group, which generates carbon upon heating and is liquid at room temperature, and a polymerization or crosslinking catalyst, which can homogeneously dissolve with the organic compound to obtain a mixture, a step of homogeneously solidifying the mixture to obtain a solid matter, and a step of heating and baking the solid matter in a non-oxidizing atmosphere, wherein the catalyst is a compound consisting of carbon atoms, hydrogen atoms and oxygen atoms, and has a carboxyl group.
More concretely, the catalyst is a compound having a carboxyl group, and is preferably at least one type selected from the group consisting of maleic acid and a derivative thereof.
It is preferable that the solid matter is previously heated and carbonized in a non-oxidizing atmosphere before heating and baking in the production method. Furthermore, it is preferable that the mixture further comprises a surfactant.
According to the method of the present invention, silicon carbide of high purity can be produced without generating a sulfur compound in the carbonizing and baking processes. Furthermore, the method of the present invention is preferable also from the viewpoint of environmental conservation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now the present invention will be described in more detail.
Silicon carbide particles of the present invention can be produced by heating and baking in a non-oxidizing atmosphere a mixture obtained by homogeneously mixing a liquid silicon compound, a liquid organic compound having oxygen in its molecular structure, which generates carbon upon heating (hereinafter optionally referred to as “carbon source”), and a polymerization or crosslinking catalyst, which homogeneously dissolves at least with the organic compound, and can be obtained preferably by a production method comprising a solidifying step of solidifying the mixture to obtain a solid matter, and a baking step of baking the solid matter in a non-oxidizing atmosphere. It is more preferable to further comprise a step of heating and carbonizing the obtained solid matter in a non-oxidizing atmosphere between the solidifying step and the baking step.
As a silicon source used in the production method of silicon carbide particles, at least one selected from the group consisting of tetraalkoxy silane of high purity, a polymer thereof, and silicon oxide is used. “Silicon oxide” in the present invention includes silicon dioxide and silicon monoxide. Concrete examples of a silicon source include alkoxy silanes represented by tetraethoxy silane, low molecular weight polymers (oligomers) thereof, silicic acids of a high polymerization degree, and silicon oxide compounds such as silica sol and fine particle silica. Examples of alkoxy silanes include methoxy silane, ethoxy silane, propoxy silane, and butoxy silane. Among these examples, ethoxy silane is preferably used in view of easy handling. “Oligomer” herein refers to a polymer of 2 to 15 polymerization degree.
Among these silicon sources, an oligomer of tetraethoxy silane and a mixture of an oligomer of tetraethoxy silane and fine particle silica are preferable in view of good homogeneity and handling. It is preferable to use a material of high purity as the silicon source according to the application. In this case, the initial impurity content is preferably 20 ppm or less, more preferably 5 ppm or less.
Concrete examples of materials used as an organic compound for generating carbon by heating, to be mixed with the above-mentioned silicon source include various saccharides such as monosaccharides including glucose, oligosaccharides including cane sugar, and polysaccharides including cellulose and starch; and coal tar pitch having a high actual carbon ratio; phenol resin; furan resin; epoxy resin; and phenoxy resin. Among these examples, those which are liquid at an ordinary temperature, those capable of dissolving with a solvent, those which soften upon heating, such as thermoplastic or heat-fusable ones or become liquid upon heating are preferably used for homogeneous mixing with a silicon source. In particular, compounds having a high actual carbon ratio, polymerized or crosslinked by a catalyst or heating, consisting of carbon atoms, hydrogen atoms and oxygen atoms are preferable. Concrete preferable examples thereof include phenol resin, polyvinyl alcohol, polyvinyl acetate.
The ratio of carbon and silicon (hereinafter abbreviated as “C/Si ratio”) in the production of silicon carbide particles, which are material particles of the present invention can be defined by elemental analysis of a carbide intermediate obtained by carbonizing the mixture. Free carbon in generated silicon carbide should be 0% when the C/Si ratio is 3.0 in stoichiometry. However, actually free carbon generates at a low C/Si ratio due to the vaporization of SiO gas generated at the same time. It is important to previously determine the proportion so that the free carbon amount in the generated silicon carbide particles is not inappropriate. In general, free carbon can be maintained at a C/Si ratio of 2.0 to 2.5 in the case of baking at 1600° C. or higher at about 1 atmospheric pressure, and thus the range can be preferably used. With a C/Si ratio of more than 2.5, although free carbon significantly increases, since the free carbon has an effect of restraining the grain growth, it may be optionally selected according to the purpose of the grain formation. However, in the case of baking under low or high atmospheric pressure, the C/Si ratio for obtaining pure silicon carbide fluctuates, so in this case the C/Si ratio is not always limited to the above-mentioned range.
In the method of the present invention where a mixture is obtained by thoroughly mixing the above-mentioned silicon compound, which
Hashimoto Masao
Kajiwara Meisetsu
Wada Hiroaki
Bridgestone Corporation
Langel Wayne A.
Nguyen Cam N.
Oliff & Berridg,e PLC
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