Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound of silicon and carbon. A wide bandgap semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as powder and crystals since 1893 for use as an abrasive. Grains of silicon carbide can be bonded together by sintering to form very hard ceramics that are widely used in applications requiring high endurance, such as disc brakes, clutches and ballistic plates in bulletproof vests. Large single crystals of silicon carbide can be grown by the Lely method and they can be cut into synthetic moissanite gemstones. Electronic applications of silicon carbide such as light-emitting diodes (LEDs) and detectors in early radios were first demonstrated around 1907. Silicon carbide is used in semiconductor electronic devices that operate at high temperatures, high voltages or both.
Natural occurrence
Naturally occurring moissanite is found in only minute quantities in certain types of meteorite, corundum deposits and kimberlite. Virtually all the silicon carbide sold in the world, including moissanite jewelry, is synthetic. Natural moissanite was first found in 1893 as a small component of the Canyon Diablo meteorite in Arizona by Ferdinand Henri Moissan, after whom the material was named in 1905. Moissan's discovery of naturally occurring SiC was initially disputed because his sample may have been contaminated by silicon carbide saw blades that were already on the market at that time. While rare on Earth, silicon carbide is remarkably common in space. It is a common form of star dust found around carbon-rich stars, and examples of this star dust have been found in pristine condition in primitive (unaltered) meteorites. The silicon carbide found in space and meteorites is almost exclusively the beta-polymorph. Analysis of SiC grains found in the Murchison meteorite, a carbonaceous chondrite meteorite, has revealed anomalous isotopic ratios of carbon and silicon, indicating that these grains originated outside the Solar System.
History
Early experiments Non-systematic, less-recognized, and often unverified syntheses of silicon carbide include:
César-Mansuète Despretz's passing an electric current through a carbon rod embedded in sand (1849) Robert Sydney Marsden's dissolution of silica in molten silver in a graphite crucible (1881) Paul Schuetzenberger's heating of a mixture of silicon and silica in a graphite crucible (1881) Albert Colson's heating of silicon under a stream of ethylene (1882).
Wide-scale production
Wide-scale production is credited to Edward Goodrich Acheson in 1891. Acheson was attempting to prepare artificial diamonds when he heated a mixture of clay (aluminium silicate) and powdered coke (carbon) in an iron bowl. He called the blue crystals that formed carborundum, believing it to be a new compound of carbon and aluminium, similar to corundum. Henri Moissan also synthesized SiC by several routes, including dissolution of carbon in molten silicon, melting a mixture of calcium carbide and silica, and reducing silica with carbon in an electric furnace. Acheson patented the method for making silicon carbide powder on February 28, 1893. Acheson also developed the electric batch furnace by which SiC is still made today and formed the Carborundum Company to manufacture bulk SiC, initially for use as an abrasive. In 1900 the company settled with the Electric Smelting and Aluminum Company when a judge's decision gave "priority broadly" to its founders "for reducing ores and other substances by the incandescent method". The first use of SiC was as an abrasive. This was followed by electronic applications. In the beginning of the 20th century, silicon carbide was used as a detector in the first radios. In 1907 Henry Joseph Round produced the first LED by applying a voltage to a SiC crystal and observing yellow, green and orange emission at the cathode. The effect was later rediscovered by O.V. Losev in the Soviet Union, in 1923.
Production
Because natural moissanite is extremely scarce, most silicon carbide is synthetic. Silicon carbide is used as an abrasive, as well as a semiconductor and diamond simulant of gem quality. The simplest process to manufacture silicon carbide is to combine silica sand and carbon in an Acheson graphite electric resistance furnace at a high temperature, between 1,600 °C (2,910 °F) and 2,500 °C (4,530 °F). Fine SiO2 particles in plant material (e.g. rice husks) can be converted to SiC by heating in the excess carbon from the organic material. The silica fume, which is a byproduct of producing elemental silicon and ferrosilicon alloys, can also be converted to SiC by heating with graphite at 1,500 °C (2,730 °F). The material formed in the Acheson furnace varies in purity, according to its distance from the graphite resistor heat source. Colorless, pale yellow, and green crystals have the highest purity and are found closest to the resistor. The color changes to blue and black at greater distance from the resistor, and these darker crystals are less pure. Nitrogen and aluminium are common impurities, and they affect the electrical conductivity of SiC.
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