SiC. A compound produced by fusing a mixture of carbon and silica; also known as carborundum. It has several crystalline modifications, all of which are high-melting and hard, and is used mainly as an abrasive.
Silicon carbide (SiC) is a ceramic compound of silicon and carbon.
The word moissanite is a trade name given to silicon carbide for use in the gem business.
Production
Most silicon carbide is man-made for use as an abrasive (when it is often known by the trademark carborundum), or more recently as a semiconductor and simulated moissanite jewels.
The material formed in the Acheson furnace varies in purity, according to its distance from the graphite resistor that is the heat source.
Purer silicon carbide can be made by the more expensive process of chemical vapor deposition (CVD). Commercial large single crystal silicon carbide is grown using a physical vapor transport commonly known as modified Lely method.
Purer silicon carbide may also be made by the thermal decomposition of a polymer, poly(methylsilyne), under an inert atmosphere at low temperatures. This has certain advantages over the CVD process in that the polymer may be readily formed into various shapes prior to thermolization into a silicon carbide ceramic.
Discovery
The material was discovered by Edward Goodrich Acheson around 1893, and he not only developed the electric batch furnace by which SiC is still made today, but also formed The Carborundum Company to manufacture it in bulk, initially for use as an abrasive.
In nature
Naturally occurring moissanite is extremely rare, as it is not formed naturally in any quantity within the Earth, and thus is found only in tiny quantities in certain types of meteorite and as microscopic traces in corundum deposits and kimberlite. Virtually all of the silicon carbide sold in the world, including moissanite gemstones, is synthetic. Moissan's discovery of naturally occurring SiC was disputed at first because his sample may have been contaminated by silicon carbide saw blades that were already on the market at that time.
Properties
Alpha silicon carbide (α-SiC) is most common, and is formed at temperatures greater than 2000 °C. Silicon carbide has a specific gravity of 3.2, and its high melting point (approximately 2700 °C) makes silicon carbide useful for bearings and furnace parts. There is currently much interest in its use as a semiconductor material in electronics, where its high thermal conductivity, high electric field breakdown strength and high maximum current density make it more promising than silicon for high-powered devices.
Pure SiC is clear. The rainbowish lustre of the crystals is caused by the passivation layer of silicon dioxide that forms on its surface.
As a jewel, silicon carbide, called Moissanite (named for the gemstones' discoverer Dr. Henri Moissan), is somewhat similar to diamond in several important ways: it is transparent and extremely hard (>9.0 on the Mohs scale, compared to 10 for diamond), with an index of refraction between 2.65 and 2.69 (compared to 2.42 for diamond).
Uses
Semiconductor
Pure α-SiC is an intrinsic semiconductor with band gaps of 3.28 eV (4H) and 3.03 eV (6H) respectively.
Silicon carbide is used for blue LEDs, ultrafast Schottky diodes, MESFETs and high temperature IGBTs and thyristors for high power switching. Although diamond has an even higher band gap, SiC-based devices are easier to manufacture due to the fact that it is more convenient to grow an insulating layer of silicon dioxide on the surface of a silicon carbide wafer than it is with diamond.
Pure SiC is a bad electrical conductor. Typically, such material has a negative temperature coefficient between room temperature and about 900 °C, and positive temperature coefficient at higher temperatures, making it suitable material for high temperature heating elements.
Silicon carbide is also used as an ultraviolet detector. Electroluminescence of silicon carbide was observed by Captain Henry Joseph Round in 1907 and by O.
Structural material
In the 1980s and 1990s, silicon carbide was studied on several research programs for high-temperature gas turbines in the United States, Japan, and Europe.
Astronomy
Silicon carbide's hardness and rigidity make it a desirable mirror material for astronomical work, although its properties also make manufacturing and designing such mirrors quite difficult.
Silicon carbide may be a major component of the mantles of as-yet hypothetical carbon planets.
Grit
Silicon carbide is a popular product in modern lapidary due to the durablility and low cost of the material.
Disc brake
Silicon-infiltrated carbon-carbon composite is used for high performance brake discs as it is able to withstand extreme temperatures. The silicon reacts with the graphite in the carbon-carbon composite to become silicon carbide.
Diesel particulate filter
Silicon carbide is used in a sintered form for Diesel Particulate Filters.
Ceramic membrane
Silicon carbide is used for producing ceramic membranes for industrial processes, yielding high fluxes due to the sintering process.
Cutting tools
In 1982 at the Oak Ridge National Laboratories, George Wei, Terry Tiegs, and Paul Becher discovered a composite of aluminum oxide and silicon carbide whiskers. In 1985, the first commercial cutting tools made from this alumina and silicon carbide whisker-reinforced composite were introduced by the Advanced Composite Materials Corporation (ACMC) and Greenleaf Corporation.
Heating element
References to silicon carbide heating elements exist from the early 20th century when they were produced by Acheson's Carborundum Co. Silicon carbide offered increased operating temperatures compared with metallic heaters, although the operating temperature was limited initially by the water-cooled terminals, which brought the electric current to the silicon carbide hot zone. Operating temperature and efficiency was later increased by the use of separate low resistance silicon carbide 'cold ends', usually of a larger diameter than the hot zone, but still held in place only by mechanical pressure. From the 1960s onwards, one-piece elements were produced, with cold ends created by filling the pore volume with a silicon alloy. Silicon carbide elements are used today in the melting of non-ferrous metals and glasses, heat treatment of metals, float glass production, production of ceramics and electronics components, etc.
As a Jewel
In 1998, Charles and Colvard [Nasdaq: CTHR] introduced jewel-quality synthetic silicon carbide onto the market under the name "moissanite," marketing it as a lower-cost alternative to diamond.
While some properties of moissanite are closer to those of diamond than to cubic zirconia, another diamond simulant, once its properties are known, moissanite is perhaps even easier to identify, as it is doubly refractive and has a slight green tint to it.
Synthetic moissanite is somewhat harder than common cubic zirconia, (9 1/4 vs.
Moissanite jewelry is sold at a variety of retail outlets.
Steel
Silicon carbide dissolved in a basic oxygen furnace used for making steel acts as a fuel and provides energy which increases the scrap to hot metal ratio.
90% silicon carbide is used by the steel industry as a ladle deoxidizer, a source of silicon and carbon in the ladle, an electric furnace slag deoxidizer, and as a synthetic slag additive. The silicon carbide used as a steel additive or fuel comes as a granular product in either bulk and bags.
50% and 65% silicon carbide are used in the steel industry for processing steel and iron scrap. Typically supplied as blocks and made from silicon carbide crucible scrap, it helps extend the hot metal supply and raises the tap temperature. Clarke and Stanley Kubrick, among others) the monoliths (or at least their exteriors) were made of silicon carbide In the Discworld novel Monstrous Regiment: Carborundum is the name of the Troll that enlists. In the BBC television show Top Gear, host Jeremy Clarkson climaxes over the mere mention of Silicon Carbide used in the brakes and clutch of the Porsche Carrera GT.
Patents and trademarks
Edward Goodrich Acheson (1856–1931) patented the method for making silicon carbide powder on February 28, 1893. On May 19, 1896, he was also issued a patent for an electrical furnace used to produce silicon carbide.
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