1. Field of the Invention
The present invention concerns a process for producing silicon carbide whiskers at high yield, at a reduced cost, with less content of powdery silicon carbide and at high purity.
2. Description of the Prior Art
Since silicon carbide whiskers are excellent in the strength, modulus of elasticity, oxidation resistance, heat resistance and chemical stability as compared with other fibers for use in composite materials, they have attracted attention as composite reinforcing material for ceramics, metals and plastics and, particularly, as composite reinforcing material for metals because they have satisfactory wettability with various kind of metals.
The production process for such silicon carbide whiskers is generally classified into (A) a method of depositing from liquid silicon carbide at high temperature and high pressure, (B) a method of dissolving carbon into a molten liquid of metal silicon and crystallizing silicon carbide, (C) a method of crystallizing by sublimating silicon carbide at high temperature from silicon carbide powder and (D) a method of growing crystals of silicon carbide formed by the heat decomposing reaction of silicon compounds.
Among them, the methods (A) and (B), however, result in a great difficulty in the production in view of the production facility because of the use of extremely high temperature and high pressure or the use of molten metal liquid. Further, the method (C) also requires an extremely high operation temperature and can not be operated with ease, as well as the facilities are complicated and fractional collection of the resultant whiskers is difficult. Accordingly, all of the methods (A) to (C) have fetal defects as the industrial production process.
Accordingly, the method (D) as above in which silicon carbide is deposited in the form of whiskers while forming silicon carbide by various heat decomposing reactions is predominant at present as the process for producing silicon carbide and various methods have further been proposed therefor. That is, as the method (D), there have been known (1) a method of conducting solid phase reduction for silicon dioxide with carbon or metal silicon and carbon, (2) a method of reacting an organic silicon compound or a mixture of a silicon compound and a carbonaceous compound in a gaseous form at high temperature and (3) a method of gas phase growing utilizing the reaction between a fluorine-containing silicate and carbon.
Among them, the method (2) only produces extremely small amount of silicon carbide whiskers per unit volume of the reaction chamber since the reaction is conducted in a gas phase with a large volume of the chamber, requires to maintain the inside of the large capacity reaction chamber at a high temperature upon production in an industrial scale since the reaction is conducted at high temperature, resulting in high cost, as well as involves a problem that corrosive reaction by-products are formed. Furthermore, in the method (3), a fluorine-containing silicate is melted and reducing reaction with carbon added thereto is taken place and evolved vapors are cooled to form silicon carbide whiskers. Also the method (3) involves the same problems as that for the method (2) above in view of the utilization of gas phase reaction. Moreover, since the method (3) is necessary to handle a great amount of molten salt, it brings about problems that the material for the production facility is limited severely, and these molten salts are incorporated as impurity to the produced silicon carbide whiskers. Therefore, the method (3) is not preferable as the industrial production process for the silicon carbide whiskers.
While on the other hand, in the method of utilizing the solid phase reduction (1) above, most easily available silicon dioxide is used as the starting material and it can be classified into two types depending on the reaction scheme.
.circle.1 A process for producing silicon carbide whiskers by using silicon dioxide and carbon as the starting material by the following reaction: EQU SiO.sub.2 +C.fwdarw.SiO+CO EQU SiO+2C.fwdarw.SiC+CO
That is, fundamentally by the same method as the Acheson's method generally known as the industrial production process for silicon carbide.
.circle.2 A production process by using silicon dioxide, metal silicon and carbon as the starting material by the reaction of: EQU SiO.sub.2 +Si.fwdarw.2SiO EQU SiO+2C.fwdarw.SiC+CO.sub.2
Among them, it is essential in the process .circle.2 to at first prepare silicon monoxide by the reaction of silicon dioxide and metal silicon. Accordingly, it is necessary, for example, as disclosed in Japanese Patent Publication No. Sho 47-18531 to preferentially conduct the reaction of forming silicon monoxide at first by silicon dioxide and metal silicon while avoiding the contact between silicon dioxide and carbon so that no unreacted metal silicon remains, for example, by a method of disposing a carbon or graphite substrate at a place apart from the portion of forming silicon monoxide. Accordingly, the process .circle.2 inevitably increases the capacity of the production facility and, also brings about defect of resulting in the reduction of the yield.
While on the other hand, the process .circle.1 is free from the defect in the process .circle.2 described above and it is advantageous as the industrial production process since it can be conducted by merely mixing silicon dioxide and carbon uniformly and heating the mixture to a predetermined temperature.
In connection with the process .circle.1 , various studies have been made also to the starting silicon source material for producing silicon carbide whiskers by the process .circle.1 and there have been known, for example, (a) a method of using silicon component present in hulls of gramineous plants, (b) a method of using silica sand as the starting material, (c) a method of using silas or glass dust as the starting material, (d) a method of using active silica with high specific surface area such as silica sol and silica gel as the starting material, etc.
However, the method (a) involves a drawback that hulls as the starting material have to be applied with pre-treatment of carbonizing or ashfying hulls and, in addition, a large-scaled hull processing facility is required since the silicon content of hulls is less than 20% by weight. Further, availability of hulls as the starting material is liable to suffer from the effects depending on the season and climate conditions and many impurities are incorporated into the silicon carbide whiskers produced. Further, the method (b) requires a step of previously pulverizing silica sands finely and the silica sands, being crystalline substance, show poor reactivity and results in lower yield as compared with amorphous silicic source starting material. The method (c) brings about problems that metal impurities other than silicon contained in the starting material may be incorporated into silicon carbide whiskers, or impurities are gasified and scattered in the heating step of the starting material, which are solidified by coagulation at the low temperature portion of the reaction chamber to hinder the continuous operation. The method (d) also involves problems that silica is expensive as the starting silicon source material, can provide no high reactivity as expected since the silica shrinks at high temperature and changes into deactivated silica with low specific surface area at the reaction temperature and, accordingly, provides no good yield. Therefore, it is difficult to provide silicon carbide whiskers with less impurity content at a high yield by using any one of starting silicon source materials (a) to (d).
Further, it has recently been disclosed a process for producing silicon carbide whiskers by using a hydrolyzable silicon compound as the starting silicon source material, admixing the hydrolyzable silicon compound and a carbon compound into a steam-containing hot gas, thereby forming a mixed aerosol of silicon oxide and carbon and producing silicon carbide whisker by using the mixed aerosol as the starting material (Japanese Patent Laid-Open No. Sho 60-16809 and Japanese Patent Publication No. Sho 61-2640). However, the process is disadvantageous in that the mixed aerosol formed by the process is extremely fine and of low bulk density to render the handling difficult, reduce the charging amount of the starting material per unit volume and worsen the productivity.
Furthermore, in the process .circle.1 above, various attempts have been made for other materials than the starting silicon source material and the following improvement has also been proposed for the method of using the starting silicon source materials (b) to (d) described above.
(i) A method of incorporating chlorine or hydrogen chloride, easily evaporizable hydrocarbon or halogenated hydrocarbon as a carbon source, etc. into an atmosphere for improving the reaction rate (Japanese Patent Publication No. Sho 52-28757 and U.S. Pat. No. 3933984).
(ii) A method of adding, as VLS catalyst, powder of metals such as Fe, Co, Ni, B, La, Mn, Al, Ti, etc. or the compounds thereof to a mixture of silicon dioxide and carbon and, further, using vapors of sodium chloride as the space-forming agent (U.S. Pat. No. 3622272, Japanese Patent Publication Nos. Sho 51-8760, 59-45637 and 60-44280 and Japanese Patent Laid-Open No. Sho 61-22000).
However, the method (i) involves a problems in using corrosive gases and the method (ii) involves problems in that it uses a great amount of the VLS catalyst and further uses sodium chloride, as the space-forming agent. Accordingly, the catalyst and sodium chloride are introduced as impurities to silicon carbide whiskers in the production step thereof to reduce the purity, thus bringing about the reduction in the wettability and adhesion between the resulting whiskers and the matrix, as well as reduction in the toughness of the whiskers.
Further, in any of the processes .circle.1 as described above, there is a high possibility that a great amount of powdery silicon carbide is by-produced together with the amied silicon carbide whiskers. Although the size and the content of the powdery silicon carbide vary depending on the whisker forming conditions, the powdery silicon carbide is by-produced while being broadly dispersed in the whiskers. When the whisker containing such by-products are composited with metals or plastics, the substantial whisker content composited is low and no expected compositing effect can be obtained. In addition, the strength of the composite material is rather reduced due to large grain size of granular or finely powderous product. Accordingly, the silicon carbide whiskers containing a great amount of powdery silicon carbide are often not suitable as composite reinforcing material for ceramics, metals and plastics. Furthermore, there has been a problem that the powdery products by-produced in the silicon carbide whiskers can not completely be separated even by the separation method utilizing the difference in the oleophilic and hydrophilic properties or by means of centrifugal precipitation.
In view of the foregoing situations, the present inventors have made earnest studies on the process for producing silicon carbide whiskers by improving the method (1) and, particularly, the process .circle.1 and have proposed a process of mixing a hydrolysis product of chlorosilane or chlorodisilane as the starting silicon source material with a carbonaceous material and reacting them at a high temperature from 1400.degree. to 1700.degree. C. (Japanese Patent Application Nos. Sho 60-26687 and Sho 61-53243). Although it is possible to produce silicon carbide whiskers at a yield as high as 80% or more, a further improvement is still required for suppressing the content of the by-produced powdery silicon carbide and reducing metal impurity.