Silicon carbide (SiC), also called carborundum, is an exceedingly hard, synthetically produced crystalline compound of silicon and carbon that has been used since the 19th century for sandpapers and grinding wheels, as well as in refractory linings and heating elements for industrial furnaces, in wear-resistant parts for pumps and rocket engines, and in semiconducting substrates for light-emitting diodes. It occurs in nature as the mineral moissanite, but is typically manufactured on a large scale for commercial use.
Carbide smelters manufacture silica carbide from quartz sand and coal or petroleum coke (or wood chips) by heating these materials in an electrical resistance-type furnace to a high temperature, which produces a chemical reaction in which the sand and carbon combine to form SiC. This method of manufacturing is still the most common in many industrial applications, though there are a number of more advanced methods for producing silicon carbide on a larger scale.
The simplest method of production was developed by Edward Goodrich Acheson in 1890, using an electric furnace with a graphite resistor heat source. A mix of silica sand and finely ground coal (often from an auto exhaust) was heated in the furnace to about 2500 deg C, or 450 deg F. After several days, a small amount of sulfuric acid was added to the mixture in order to help dissolve the carbon from the sand.
Acheson's process was subsequently improved by Henri Moissan in France, who synthesized silica carbide from a mixture of quartz and alumina, which he later described as being similar to the blue crystals found in the Canyon Diablo meteorite that had been discovered in Arizona in 1911. The new method, which was more expensive than Acheson's, resulted in more uniformly sized lump aggregates of silica carbide than the Acheson method.
Reaction-bonded SiC is formed by mixing a powdered form of SiC with a plasticizer, forming the mixture into the desired shape, and then infusing it with gaseous or molten silicon to react with the carbon in the plasticizer. The fired object is then cooled and re-fired to produce additional SiC. Reaction-bonded materials have a lower melting point than pure SiC, and have more ductility, making them suitable for applications such as nozzles in injectors.
Various types of ceramics are made from silicon carbide by sintering, a technique that binds the grains together to create very hard ceramics. They are used in automobile brakes, clutches and ceramic plates that protect bulletproof vests from wear and tear.
Some types of SiC can be bonded to other compounds, such as silicon nitride or silicon-oxynitride, to make materials with higher corrosion resistance. These nitride-bonded SiC materials are particularly useful for pump parts, acid spray nozzles, and in aluminum reduction cells.
Vapour-deposition synthesis, a process in which volatile compounds containing silicon and carbon are reacted at high temperatures with hydrogen, can also be used to make wear-resistant layers of silicon carbide. This type of synthesis is especially useful for manufacturing large single crystals of SiC.
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