Polymorphism silicon carbide

Silicon carbide exhibits a two-dimensional polymorphism called polytypism. All polytypes have a hexagonal frame of SiC bilayers. The hexagonal frame should be viewed as sheets of spheres of the same radius and the radii touching, as illustrated in Figure 1.5. The sheets are the same for all lattice planes. However, the relative position of the plane directly above or below are shifted somewhat to fit in the valleys of the adjacent sheet in a close-packed arrangement. Hence, there are two inequivalent positions for the adjacent sheets. 

An interesting example of a mineral featuring remarkable hardness anisotropy and considerable hardness scatter is silicon carbide, SiC, widely used in electronic, electrical engineering and machining industries. It can crystallize in two polymorphous varieties, namely cubic / -SiC and hexag-... 

There are various polymorphs of silicon carbide made by high temperature interaction some have wurtzite (ZnS) or diamond structures. It is exceedingly hard and inert it finds uses in polishing products, furnace linings, and semiconductor technology. 

Silicon carbide (carborundum, SiC) is of especial interest on account of its rich polymorphism, no fewer than six structures being known. As is to be expected, each carbon and silicon atom is tetra-hedrally co-ordinated by four atoms of the other kind, and two of the forms of carborundum have the zincblende and wurtzite structures. The close relationship between these two structures has already been discussed ( 4.13), and is emphasized by the many AX compounds (including ZnS itself) in which both are found. It is illustrated in fig. 8.03, where the cubic zincblende structure has been drawn with one of the cube diagonals vertical and parallel to the principal axis of the wurtzite structure. When viewed in this way it will be seen that both structures can be visualized as formed by the superposition of a series of puckered sheets of atoms, but that in zincblende successive sheets are identical (albeit translated) whereas in wurtzite they differ and are related by a rotation through 180° about the principal axis. In the two structures the sequence of sheets can therefore be symbolized as... 

Silicon carbide carborundum) has several polymorphs the 3-form adopts the wurtzite structure Figure 5.20). It is extremely hard, resists wear, withstands very high temperatures, has a high thermal conductivity and a low coefficient of thermal expansion, and has long been used as a refractory material and abrasive powder. Recent development of suitable CVD... 

Figure 21.10 Reprinted from Guinel, M. J-F. and Norton, M.G. (2006) Oxidation of silicon carbide and the formation of silica polymorphs, J. Mater. Res. 21, 2550. With permission from the Materials Research Society.

Silicon carbide exists in several modifications being polymorphic and polytypical and crystallizing in a diamond lattice, like silicon [13]. 

Polymorphism and polytypism. Silicon carbide has two polymorphs. At temperatures above 2000°C alpha silicon carbide (a-SiC), with a hexagonal crystal structure, is the more stable polymorph with iridescent and twinned crystals with a metallic luster. At temperatures lower than 2000°C, beta silicon carbide ((3-SiC) exhibits a face-centered cubic (fee) crystal structure. 

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