Silicon carbide
Encyclopedia : S : SI : SIL : Silicon carbide
| Silicon carbide | |
|---|---|
| |
| General | |
| Other names | |
| Molecular formula | SiC |
| Molar mass | 40.097 g/mol |
| Appearance | black-green odorless powder |
| CAS number | [409-21-2] |
| Properties | |
| Density and phase | 3.22 g/cm3, solid |
| Solubility in water | insoluble |
| Melting point | 2700°C |
| Boiling point | ? |
| Index of refraction | 2.65-2.69 |
| Mohs hardness | 9.25 |
| Hazards | |
| EU classification | not listed |
| NFPA 704 |   
|
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) [Chemical infoboxInfobox disclaimer and references] | |
Silicon carbide () 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 moissanite gemstones. The simplest manufacturing process is to combine silica sand and carbon at a high temperature, between 1600 °C and 2500 °C.The material formed in the Acheson furnace varies in purity, according to its distance from the graphite resistor that is the heat source. Clear, pale yellow and green crystals have the highest purity, and are found closest to the resistor. The colour changes to blue and black at greater distance from the resistor, and these darker crystals are less pure, and usually doped with aluminium or iron, which increases electrical conductivity.
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 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. 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 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. Natural moissanite was first found in 1905 as a small component of a meteorite in Arizona by Dr. Ferdinand Henri Moissan, after whom the material is named in the gem market. 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. Alpha SiC has the typical hexagonal crystal structure. Beta modification (β-SiC), with a face-centered cubic crystal structure, is formed at temperatures of below 2000 °C, but has relatively few commercial uses. 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. 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 and that, together with its high melting point permits practical use in heating and casting metals. SiC also has very low thermal expansion coefficient and no phase transitions that would cause discontinuities in thermal expansion.Pure SiC is clear. The brown to black color of industrial product is caused by iron impurities. The rainbowish lustre of the crystals is caused by the passivation layer of silicon dioxide that forms on its surface.
As a gemstone, silicon carbide, called Moissanite (named for the gemstones' discoverer Dr. Henri Moissan), is similar to diamond in several important ways: it is transparent and extremely hard (9.25 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). SiC has a hexagonal crystal structure.
Uses
Semiconductor
Pure α-SiC is an intrinsic semiconductor with a band gap of 3.26 eV (4H), respectively 3.00 eV (6H) eV.Silicon carbide is used for blue LEDs, ultrafast Schottky diodes, MESFETs and high temperature thyristors for high power switching. Due to its high thermal conductivity, SiC is also used as substrate for other semiconductor materials such as gallium nitride[link]. It 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. Lossev in the Soviet Union in 1923 [link]. 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.
Pure SiC is a bad 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.
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 they also make manufacturing and figuring 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. It is also used in "super fine" grit sandpapers.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.Diesel particulate filter
Silicon carbide is used in a sintered form for Diesel Particulate Filters.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. 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 aluminium 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 USA 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. 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.Gemstone
In 1998, C3, Inc. (Charles and Colvard) [Nasdaq: CTHR], a subsidiary of Cree Research, Inc., introduced gem-quality synthetic silicon carbide onto the market under the name "moissanite," marketing it as a lower-cost alternative to diamond. For example, a 1 carat (200 mg) moissanite gem sells for about $600 (2005 USD), while a diamond of similar size and color typically runs for upwards of $4500. Synthetic moissonite is almost as hard as diamond, with a slightly higher index of refraction and greater dispersion; these qualities make SiC a decent and durable diamond simulant. Moissanite's greater dispersion and index of refraction gives it more fire and brilliance than diamond.While some properties of moissanite are closer to those of diamond than to cubic zirconia, another synthetic 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. Jewellers were at first fooled by moissanite's thermal conductivity, which is close to that of diamond, rendering older thermal testers useless; what worked with cubic zirconia did not work with moissanite.
Moissanite is harder than cubic zirconia (9 1/4 vs. 8 1/2), lighter (SG 3.33 vs. 5.6), and much more resistant to heat. This results in a stone of higher lustre, sharper facets and good resilience: loose moissanites may be placed directly into ring moulds, as the synthetic stones remain undamaged by temperatures up to twice the 900 °C melting point of 18k gold.
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.[link] It can also be used to raise tap temperatures and adjust the carbon content.
90% silicon carbide is used 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.[link] 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.[link] The blocks are typically made using an automated concrete block making machine, and utilize water and limestone cement as a binder.
In popular culture
- In Edgar Rice Burroughs' Barsoom series, "carborundum" is used as building material for city walls.
- In [[2001: A Space Odyssey (novel)|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 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.
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.- [U.S. Patent 492767] -- Production of artificial crystalline carbonaceous material
See also
- Diamond simulant.
- Illegitimi non carborundum, mock-Latin using the trademark Carborundum as if it were a Latin verb gerund.
External links
- Charles & Colvard, Co. [Moissanite FAQ]
- [Moissanite Jewelry FAQ]
- [Mineral galleries - moissanite info.]
- [NASA Glenn High Temperature Integrated Electronics and Sensors Team]
- [Case Western Reserve University PECVD SiC Research Team]
- [Case Western Reserve University MINOLAB]
- [Semisouth, SiC Device and Materials Specialists]
- [University of Arkansas Silicon Carbide Device Modeling Team]
- [Device characteristics of sublimation grown 4H-SiC layers] Rafal R. Ciechonski, Universiy of Linkoping
- [Material Safety Data Sheet] for Silicon Carbide
- [link] SiC Whisker-Reinforced Ceramic Composites
- [Mindat.org]
- [Webmineral - moissanite info.]
- [History and Status of Silicon Carbide Research]
- [Arkansas Power Electronics International, Silicon Carbide Power Electronics]
- For a full list of external links to MSDSs, spectroscopic data, commercial chemicals suppliers etc. for this compound, see [Chemical sources].
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