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Silicon carbide, also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. Historically, natural SiC was first found in meteorite in 1893 by Dr. Ferdinand Henri Moissan (Moissan 773), thus natural SiC, an extremely rare mineral, came to be known as moissanite. Although rare on earth, SiC is very common in outer space, where it is found around carbon-rich stars in the form of stardust. The natural SiC found in meteorites and that found in space are the beta-polymorph suggesting the possibility of having the same origin. This is substantiated by analysis of SiC grains found in the Murchison carbonaceous chondrite meteorite that revealed anomalous isotopic ratios of carbon and silicon, indicating an origin from outside the solar system; indeed, 99% of these SiC grains originate around carbon-rich asymptotic giant branch stars (Alexander 715).
Due to the extreme rarity of natural SiC, virtually all the SiC sold in the world market is synthetic in origin. The simplest and the commonly used approach for synthesis of SiC is the Acheson process. This approach offers the possibility of large quantity synthesis of SiC of adequate quality that it suitable for electronic application and for use in other applications. When high purity SiC is desired, methods such as the Lely process, modified Lely process, chemical vapor deposition and thermal decomposition of poly(methylsilyne) are employed. The structure and properties of SiC make it suitable for many applications. This term paper investigates the processing, properties and applications of SiC....
Processing of silicon carbide Acheson process As mentioned above, natural SiC is an extremely rare mineral. Thus synthetic methods have been developed to meet the demand for SiC. Among these method, Acheson process is the widely used (Knippenberg 161). The Acheson process involves the high temperature (1600 – 2700 °C) heating of a mixture of silica, carbon, sawdust and common salt (e.g. 50% silica, 40% coke, 7% sawdust and 3% common salt) in an Acheson graphite electric resistance furnace (Knippenberg 161). The active ingredients in the mixture are the silica, in the form of silica sand and the coke. By making the mixture porous, the sawdust facilitates the removal of carbon (II) oxide formed during the reaction process. High pressures of gas may locally be built up to form voids and channels to more porous parts of the mixture to eventually find its way out. The common salt serves as a purifier of the mixture. The chlorine will react with impurities to form volatile chlorides which escape. The heating is accomplished by means of graphite resistor placed centrally in the furnace. The mixture is place around the resistor. The purity of the SiC, which depends on the distance from the resistor, is adequate for many applications. Nitrogen and aluminum are common impurity in the SiC obtained by the Acheson process. These impurities affect the electrical conductivity of silicon carbide. Lely process High purity SiC can be obtained by the Lely process, which involves sublimating SiC powder in argon atmosphere at 2500 °C followed by re-depositing into flake-like single crystals, at a slightly colder substrate (Lely 229). This process is similar to the Acheson process in the voids and channels formed by built-up gas
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