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.Encyclopædia Britannica, eb.com 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, G. (1893) "Production of artificial crystalline carbonaceous material" 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.Dunwoody, Henry H.C. (1906) Wireless telegraph system (silicon carbide detector) 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 and . 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 .
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.
Pure silicon carbide can be made by the Lely process, in which SiC powder is sublimed into high-temperature species of silicon, carbon, silicon dicarbide (SiC2), and disilicon carbide (Si2C) in an argon gas ambient at 2,500 °C and redeposited into flake-like single crystals,Lely SiC Wafers. Nitride-crystals.com. Retrieved on 2013-05-04. sized up to 2 × 2 cm, at a slightly colder substrate. This process yields high-quality single crystals, mostly of 6H-SiC phase (because of high growth temperature).
A modified Lely process involving induction heating in graphite crucibles yields even larger single crystals of 4 inches (10 cm) in diameter, having a section 81 times larger compared to the conventional Lely process. Silicon carbide wafers are often grown using this method, known in the industry as physical vapor transport with a seed wafer made of SiC. SiC powder is heated until it sublimes, and then the SiC vapor cools and deposits below the seed crystal.
Cubic SiC is usually grown by the more expensive process of chemical vapor deposition (CVD) of silane, hydrogen, and nitrogen. Homoepitaxial and heteroepitaxial SiC layers can be grown employing both gas and liquid phase approaches.
To form complexly shaped SiC, preceramic polymers can be used as precursors which form the ceramic product through pyrolysis at temperatures in the range 1,000–1,100 °C. Precursor materials to obtain silicon carbide in such a manner include polycarbosilanes, poly(methylsilyne) and polysilazanes.Europe Makes Ceramics Preceramics Silicon carbide materials obtained through the pyrolysis of preceramic polymers are known as polymer derived ceramics or PDCs. Pyrolysis of preceramic polymers is most often conducted under an inert atmosphere at relatively 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.
SiC can also be made into wafers by cutting a single crystal either using a diamond wire saw or by using a laser. SiC is a useful semiconductor used in power electronics.
Structure and properties
+ Structure of major SiC polytypes.(β)3C-SiC 4H-SiC(α)6H-SiC
Silicon carbide exists in about 250 crystalline forms. Through inert atmospheric pyrolysis of preceramic polymers, silicon carbide in a glassy amorphous form is also produced. The polymorphism of SiC is characterized by a large family of similar crystalline structures called polytypes. They are variations of the same chemical compound that are identical in two dimensions and differ in the third. Thus, they can be viewed as layers stacked in a certain sequence.
Alpha silicon carbide (α-SiC) is the most commonly encountered polymorph, and is formed at temperatures greater than 1,700 °C and has a hexagonal crystal structure (similar to Wurtzite). The beta modification (β-SiC), with a zinc blende crystal structure (similar to diamond), is formed at temperatures below 1,700 °C. 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.
+ Properties of major SiC polytypesPolytype3C (β)4H6H (α)Crystal structure Zinc blende (cubic) Hexagonal HexagonalSpace group T2d-F3m C46v-P63mc C46v-P63mcPearson symbol cF8 hP8 hP12Lattice constants (Å) 4.3596 3.0730; 10.053 3.0810; 15.12Density (g/cm3) 3.21 3.21 3.21Bandgap (eV) 2.36 3.23 3.05Bulk modulus (GPa) 250 220 220Thermal conductivity (W⋅m−1⋅K−1)@300 K (see Silicon Carbide. Thermal properties. Ioffe Institute Semiconductors Database. for temp. dependence) 320 348 325Thermal expansion coefficient (10−6 K−1)@300 K (see for temp. dependence) -- 2.28 (⊥ c); 2.49 (∥ c) 2.25
Pure SiC is colorless. The brown to black color of the industrial product results from iron impurities. The rainbow-like luster of the crystals is due to the thin-film interference of a passivation layer of silicon dioxide that forms on the surface.
The high sublimation temperature of SiC (approximately 2,700 °C) makes it useful for bearings and furnace parts. Silicon carbide does not melt but begins to sublimate near 2,700 °C like graphite, having an appreciable vapor pressure near that temperature. It is also highly inert chemically, partly due to the formation of a thin passivated layer of SiO2. 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. SiC has a very low coefficient of thermal expansion of about 2.3 × 10−6 K−1 near 300 K (for 4H and 6H SiC) and experiences no phase transitions in the temperature range 5 K to 340 K that would cause discontinuities in the thermal expansion coefficient.
Electrical conductivity Silicon carbide is a semiconductor, which can be doped n-type by nitrogen or phosphorus and p-type by beryllium, boron, aluminium, or gallium. Metallic conductivity has been achieved by heavy doping with boron, aluminium, or nitrogen.
Superconductivity has been detected in 3C-SiC:Al, 3C-SiC:B and 6H-SiC:B at similar temperatures ~1.5 K. A crucial difference is however observed for the magnetic field behavior between aluminium and boron doping: 3C-SiC:Al is type-II. In contrast, 3C-SiC:B is type-I, as is 6H-SiC:B. Thus, the superconducting properties seem to depend more on dopant (B vs. Al) than on polytype (3C- vs 6H-). In an attempt to explain this dependence, it was noted that B substitutes at C sites in SiC, but Al substitutes at Si sites. Therefore, Al and B "see" different environments, in both polytypes.
Uses Abrasive and cutting tools
In manufacturing, it is used for its hardness in abrasive machining processes such as grinding, honing, water-jet cutting and sandblasting. SiC provides a much sharper and harder alternative for sand blasting as compared to aluminium oxide. Particles of silicon carbide are laminated to paper to create sandpapers and the grip tape on skateboards.Fuster, Marco A. (1997). "Skateboard grip tape", .
In the arts, silicon carbide is a popular abrasive in modern lapidary due to the durability and low cost of the material.