Silicon carbide

Silicon carbide (ceramics but also occurs in nature as the extremely rare mineral moissanite.

Production

Due to the rarity of natural moissanite, silicon carbide is typically man-made. Most often it is used as an carbon in an Acheson graphite electric resistance furnace at a high temperature, between 1600 and 2500 °C.

The material formed in the Acheson furnace varies in purity, according to its distance from the iron, which decrease conductivity.

Purer silicon carbide can be made by the more expensive process of physical vapor transport method commonly known as modified Lely method.

Purer silicon carbide can also be prepared by the thermal decomposition of a polymer, poly(methylsilyne), under an inert atmosphere at low temperatures. Relative to the CVD process, the pyrolysis method is advantageous because the polymer can be formed into various shapes prior to thermalization into the ceramic.

Discovery

  The material was manufactured by Edward Goodrich Acheson around 1893, and he not only developed the electric batch Mohs scale).

In nature

Naturally occurring moissanite is found only in minute quantities in certain types of meteorite and in 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.

Analysis of SiC grains found in the Murchison carbonaceous chondrite meteorite has revealed anomalous isotopic ratios of carbon and silicon, indicating an origin from outside the solar system.^[2] In fact 99% of these SiC grains originate around carbon-rich Asymptotic Giant Branch stars. SiC is commonly found around these stars as deduced from their infrared spectra.

Properties

Silicon carbide exists in at least 70 crystalline forms. Alpha silicon carbide (α-SiC) is the most commonly encountered sphalerite), is formed at temperatures below 2000 °C and is shown in the structure at the top of the page. Until recently, the beta form has had relatively few commercial uses, although there is now increasing interest in its use as a support for heterogeneous catalysts, owing to its higher surface area compared to the alpha form.

Silicon carbide has a specific gravity of 3.2, and its high sublimation temperature (approximately 2700 °C) makes it useful for bearings and furnace parts. Silicon carbide does not melt at any known pressure. It is also highly inert chemically. There is currently much interest in its use as a phase transitions that would cause discontinuities in thermal expansion.

Pure SiC is clear. The brown to black color of industrial product results from silicon dioxide that forms on the surface.

Uses

Semiconductor

Pure α-SiC is an band gaps of 3.28 eV (4H) and 3.03 eV (6H) respectively.

Possibly the earliest electrical application of SiC was in lightning arresters in electric power systems. These devices must exhibit high resistance until the voltage across them reaches a certain threshold V_T, at which point their resistance must drop to a lower level and maintain this until the applied voltage drops below V_T.

It was recognized early on that SiC had such a voltage-dependent resistance, and so columns of SiC pellets were connected between high-voltage power lines and the earth. When a lightning strike to the line raises the line voltage sufficiently, the SiC column will conduct, allowing the worst of the stroke to pass harmlessly to the earth instead of along the power line. Unfortunately, such SiC columns proved to conduct significantly at normal power-line operating voltages and thus had to be placed in series with a ionized and rendered conductive when lightning raises the voltage of the power line conductor, thus effectively connecting the SiC column between the power conductor and the earth, where it then operates as before.

The trouble here is that spark gaps used in lightning arrestors are notoriously unreliable: they either fail to strike an arc when needed or fail to turn off afterwards: this last is due to material failure or contamination by dust or salt. In fact, the whole idea of using the SiC column was to eliminate the need for the spark gap in a lightning arrester.

But with some intensive engineering, the gapped SiC lightning arrester proved to be a reasonably good lightning-protection tool for many years. There were several brand names; GE and Westinghouse made them, among others. The gapped SiC arrester has been largely displaced by no-gap varistors that use columns of zinc oxide pellets.

Silicon carbide is used for blue silicon dioxide has hampered the development of SiC based power MOSFET and IGBTs. Extensive research is going on to solve the problem. Due to its high silicon dioxide on the surface of a silicon carbide wafer than it is with diamond.

Pure SiC is a poor electrical conductor. Addition of suitable dopants significantly enhances its conductivity. 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 Electroluminescence of silicon carbide was observed by Captain Henry Joseph Round in 1907 and by O. V. Losev in the Soviet Union in 1923.^[6]

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. The components were intended to replace superalloy turbine blades or nozzle vanes. However, none of these projects resulted in a production quantity, mainly because of its low impact resistance and its low fracture toughness.

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.

While rare on Earth, silicon carbide is remarkably common in space. It is a common form of stardust found around carbon-rich stars, and examples of this stardust have been found in pristine condition in primitive (unaltered meteorites). The silicon carbide found in space and in meteorites is almost exclusively the beta-polymorph.

Silicon carbide may be a major component of the mantles of as-yet hypothetical carbon planets.

Grit

Silicon carbide is a popular abrasive in modern lapidary due to the durablility and low cost of the material. It is also used in coarse to fine grit sandpapers and as a grip tape in skateboards.

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. These discs are used on some sports cars, including the Porsche Carrera GT.

Clutch

Porsche Carrera GT

Diesel particulate filter

Silicon carbide is used in a sintered form for diesel particulate filters.

Thin filament pyrometry

Silicon carbide fibers are used to measure gas temperatures in a diagnostic technique called thin filament pyrometry.

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 whiskers. This material proved to be exceptionally strong. Development of this laboratory-produced composite to a commercial product took only three years. 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. in the U.S. and EKL in Berlin. 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. The terminals were not attached to the hot zone, but were held in place by weights, or springs. 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. The development of reaction-bonding techniques led to the introduction of jointed elements. Initially, these featured larger diameter cold ends, but by the 1940s, equal diameter elements were being produced. From the 1960s onwards, one-piece elements were produced, with cold ends created by filling the pore volume with a silicon alloy. Another one-piece technique is to cut a spiral slot in a homogeneous tube where the hot section is desired. Further developments have included the production of multi-leg elements, where two or more legs are joined to a common bridge, and the production of high density, reaction-bonded elements, which provide additional resistance to oxidation and chemical attack. 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.

Nuclear Fuel

Silicon carbide is often used as a layer of the TRISO coating for the nuclear fuel elements of high temperature gas cooled reactors or very high temperature reactors such as the Pebble Bed Reactor.

Jewelry

As a gold.

Moissanite has become popular as a diamond substitute, and may be misidentified as diamond, since its birefringence and a very slight green, yellow, or gray fluorescence under ultraviolet light.

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.^[9] It can also be used to raise tap temperatures and adjust the carbon content.

90% silicon carbide is used by the steel industry as a deoxidizer, a source of carbon combination, produces cleaner steel due to low level of trace elements, it has a low gas content, it does not lower the temperature of steel, and it has an abundant world wide supply. 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.^{cement as a binder.}

Armour

Like some other hard ceramics (namely boron carbide), silicon carbide is used in composite armour (eg. Chobham armour), and in ceramic plates in bulletproof vests.

Catalyst Support

The natural resistance to oxidation exhibited by silicon carbide, as well as the discovery of new ways to synthesize the higher surface area beta form, has led to significant interest in its use as a heterogeneous catalyst support. The beta cubic form has already been employed for several years as a catalyst support for the oxidation of C4 hydrocarbons, such as n-butane, to maleic anhydride.

In popular culture

• In Edgar Rice Burroughs' Barsoom series, "carborundum" is used as building material for city walls.
• In 2001: A Space Odyssey and the related series of books and movies (by Arthur C. 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 movie Snatch, a pawn shop employee (Sol) determines a diamond is actually Moissanite, much to the dismay of the thief (Bad Boy Lincoln) who stole the ring.
• The name of the material is part of the pun "Illegitimi non carborundum" (Dog Latin for "don't let the bastards grind you down"), which figures into a football fight song of Harvard University among other things.
• In the BBC television show Top Gear, host Jeremy Clarkson expresses excitement over the mere mention of silicon carbide used in the brakes and clutch of the Porsche Carrera GT.
• Silicon carbide is the material used for the National Design Awards trophy, awarded by the Cooper-Hewitt National, Design Museum designed by William Drenttel and Jessica Helfand and manufactured by Norton Electronics.

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.^[13] Carborundum is a trademark of Saint-Gobain Abrasives.

See also

• Diamond simulant.
• Illegitimi non carborundum, mock-Latin using the trademark Carborundum as if it were a Latin verb gerund.

References

1. ^ Mabery, Charles F. (1900). "Notes, On Carborundum". Journal of the American Chemical Society XXII (Part II): 706-707. Johnson Reprint Company, via Google Books scan of Harvard University copy. Retrieved on 2007-10-28.
2. ^ http://img.chem.ucl.ac.uk/www/kelly/history.htm
3. ^ http://www.mdatechnology.net/techsearch.asp?articleid=174[1]
4. ^ Comparison of 6H-SiC, 3C-SiC, and Si for power devices, Bhatnagar, M., Baliga, B.J., IEEE Transactions on Electron Devices, March 1993
5. ^ http://www.qinetiq.com/home/commercial/information_communication_and_electronics/Electronics/optronics/quantum_electronics.html
6. ^ http://www.indiana.edu/~hightech/fpd/papers/ELDs.html
7. ^ Moissanite Education
8. ^ Patent #5,762,896 Espacenet record
9. ^ http://www.millerandco.com/products/briquettes_steel/
10. ^ http://www.millerandco.com/products/briquettes_steel/specifications/briq90.htm
11. ^ Miller and Company
12. ^ http://www.millerandco.com/products/briquettes_steel/specifications/briq65.htm
13. ^ U.S. Patent 492,767  -- Production of artificial crystalline carbonaceous material

^  Most in the jewelry industry may not recognize the 1/4 fractional intervals on the Mohs scale (a relative scale), and it is technically not correct since the Mohs scale only contains whole and half numbers. But the issuers of the patent use it in showing exactly where certain minerals are in relation to each other. On the original Mohs scale diamond was listed as a 10 and sapphire is listed as a 9. On an absolute scale, a diamonds hardness is between 5700–10400 on the Knoop scale, while a sapphire's hardness is 2000. The Knoop hardness of moissanite is 3000. This puts the Mohs hardness of moissanite around 8.5–9.25—as stated here, 9 1/4 may not be recognized, but 8.5–9.25 is the number used in the patents.

• A Brief History of Silicon Carbide Dr J F Kelly, University of London
• Material Safety Data Sheet for Silicon Carbide
• Mindat.org
• discovery of Moissanite by Moissan Moissan studied Meteorites. George Frederick Kunz describes this discovery in USGS annual report.