Silicon carbide

Silicon carbide is a compound of silicon and carbon bonded together to form ceramics, but it 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 abrasive where it is often known by the trademark carborundum, and more recently as a semiconductor and diamond simulant of gem quality. The simplest manufacturing process is to combine silica sand and 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 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.

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 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 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 1900 the company settled with the Electric Smelting and Aluminum Company when a judges' decision gave "priority broadly" to its founders "for reducing ores and other substances by the incandescent method". It is said that Acheson was trying to dissolve carbon in molten corundum (alumina) and discovered the presence of hard, blue-black crystals which he believed to be a compound of carbon and corundum: hence carborundum. Or, he named the material "carborundum" by analogy to corundum, which is another very hard substance (9 on the Mohs scale).

In nature
Naturally occurring moissanite is found only in minute quantities in certain types of meteorite and in corundum deposits and kimberlite. Virtually all of the silicon carbide sold in the world, including moissanite jewels, is synthetic. Natural moissanite was first found in 1893 as a small component of the Canyon Diablo meteorite in Arizona by Dr. 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. 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 polymorph; it is formed at temperatures greater than 2000 °C and has a hexagonal crystal structure (similar to Wurtzite). The beta modification (β-SiC), with a face-centered cubic crystal structure (similar to diamond and zincblende or 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 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. In addition, it has strong coupling to microwave radiation, which together with its high sublimation point, permits practical use in heating and casting metals. SiC also has a very low coefficient of thermal expansion and experiences no phase transitions that would cause discontinuities in thermal expansion.

Pure SiC is colorless. The brown to black color of industrial product results from iron impurities. The rainbow-like lustre of the crystals is caused by a passivation layer of silicon dioxide that forms on the surface.

Semiconductor
Pure α-SiC is an intrinsic semiconductor with 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 VT, at which point their resistance must drop to a lower level and maintain this until the applied voltage drops below VT.

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 spark gap. This spark gap is 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 LEDs, ultrafast, high-voltage Schottky diodes, MOSFETs and high temperature thyristors for high power switching. A famous paper by Jayan Baliga shows enormous potential of SiC as a power device material.

However, some problems with the interface of SiC with silicon dioxide has hampered the development of SiC based power MOSFET and IGBTs. Extensive research is underway to solve this problem. Another problem is that SiC itself breaks down at high electric fields due to the formation of extended stacking faults, but this problem may have been resolved relatively recently, as described in Nature.

Due to its high thermal conductivity, SiC is also used as substrate for other semiconductor materials such as gallium nitride. Due to its wide band gap, SiC-based parts are capable of operating at high temperature (over 350 °C), which together with good thermal conductivity of SiC reduces problems with cooling of power parts. They also possess increased tolerance to radiation damage, making it a material desired for defense and aerospace applications. Its main competitor is gallium nitride. 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 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 ultraviolet detector. Nikola Tesla, around the turn of the 20th century, performed a variety of experiments with carborundum. Electroluminescence of silicon carbide was observed by Captain Henry Joseph Round in 1907 and by O. V. Losev in the Soviet Union in 1923.

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 nickel 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 aluminium oxide and silicon carbide 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 Gemstone used in jewelry, silicon carbide is called Moissanite after the jewel's discoverer Dr. Henri Moissan. Moissanite is similar to diamond in several important respects: it is transparent and hard (9, although a patent states 8.5-9.0, on the Mohs scale compared to 10 for diamond), with a refractive index between 2.65 and 2.69 (compared to 2.42 for diamond). Moissanite is somewhat harder than common cubic zirconia. Unlike diamond, Moissanite is strongly birefringent. This quality is desirable in some optical applications, but not in gemstones. For this reason, Moissanite jewels are cut along the optic axis of the crystal to minimize birefringent effects. It is lighter (density 3.22 vs. 3.56), and much more resistant to heat. This results in a stone of higher lustre, sharper facets and good resilience. Loose moissanite stones may be placed directly into ring moulds; unlike diamond, which burns at 800 °C, moissanite remains undamaged by temperatures up to twice the 900 °C melting point of 18k gold.

Moissanite has become popular as a diamond substitute, and may be misidentified as diamond, since its thermal conductivity is much closer to that of diamond than any other diamond substitutes. It can be distinguished from diamond by 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. 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 ladle deoxidizer, a source of silicon and carbon in the ladle, an electric furnace slag deoxidizer, and as a synthetic slag additive. According to Miller and Company, it costs less than ferrosilicon and 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. The blocks are typically made using an automated concrete block making machine, and utilize water and limestone cement as a binder.

Armour
Like some other hard ceramics (namely alumina and boron carbide), silicon carbide is used in composite armour (eg. Chobham armour), and in ceramic plates in bulletproof vests. For example, Dragon skin, which is produced by pinnacle armor. It is used on the front line as a light weight flexible, rifle round resistant armour.

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 by Arthur C. Clarke (and the related series of books and films) 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. Carborundum is a trademark of Saint-Gobain Abrasives.