List of therms» Carborundum

Silicon carbide is a ceramic compound of silicon and carbon. In lithography carborundum is used to grain the stone.
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.
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.

Anna Trojanowska