Beta-silicon carbide

Silicon carbide occurs in two sUghtly different crystal structures the cubic pSiC, and a large number of hexagonal rhombohcdral varieties known collectively as aSiC.1 11 1 The single cubic form, pSiC, is obtained vdien the carbide is synthesized below 2100 C. It is a fiice-centered cubic (fee) structure of the zincblende type shown in Fig. 7.1. Zincblende is a mineral of zinc sulfide also known as sphalerite. In this illustration, the zincblende structure is represented with the cube diagonals vertical and appears as series of identical (although translated) puckered sheets of atoms widi an AA layer sequence. Another view of the pSiC crystal is shown in Fig. 7.2 (the carbon atoms, all located in the 4/ sites, are omitted for clarity] The pSiC structure has no polytype (see Table 7.3 for crystal structure data). 

The layer sequence of the 111 plane is ABCABCABC which means that every third layer is identical (Fig. 7.3).1 1 This gives a Ramsdell notation of 3C SiC where the numeral 3 refers to the number of layers of carbon atoms and silicon atoms necessary to produce a unit cell and C indicates cubic symmetry. It is analogous to the diamond structure and is also the structure of cubic boron nitride (see Ch. 12). 

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Pyrolysis analogous to polymer carbon formation has also been applied to methylchlorodisilane. This is converted to beta silicon carbide fibers of high tenacity. 

Milewski JV, Gac FD, Petrovic JJ, Skaggs SR (1985) Growth of beta-silicon carbide whiskers by the VLS process. J Mater Sci 20 1160-1166... 

Krstic, V., Production of Fine High-Purity Beta Silicon Carbide Powder, J. Am. Ceram. Soc., 75(1) 170-174 (1992)... 

The promising electronic properties of beta-silicon carbide are compared to those of other semiconductor materials in Table 8.3 of Ch. 8. A major advantage of this material is its high-temperature potential (>1000"C) which far surpasses that of other semiconductors. Beta-SiC should also be more effective than silicon or gallium arsenide particularly in microwave and millimeter-wave devices and in high-voltage power devices. The development of SiC as a semiconductor is still in the laboratory state. 

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. 

Cambaz, Z.G. et al.. Formation of carbide-derived carbon on beta-silicon carbide whiskers. Journal of the American Ceramic Society, 2006. 89(2) 509-514. 

Silicon carbide exists in a cubic form, beta silicon carbide, and in several noncubic forms lumped under the term alpha silicon carbide. 

Nesquehonite Calcium fluosilicate Beta silicon carbide Sodium sulfide Sodium fluoride Magnesium carbonate chloride hydroxide hydrate Magnesium pyrophosphate Anorthoclase Alpha cristobcdite Sodium hydroxide Calcium aluminum oxide sulfate... 

SR Nutt, DJ Smith, HJ Kim, RE Davis. Interface structures in beta-silicon carbide thin films. Appl Phys Lett 50 203, 1987. 

Turan, S. and Knowles, K.M., (1997), Interphase boundaries between hexagonal boron nitride and beta silicon nitride in silicon nitride-silicon carbide particulate composites , J. Eur. Ceram. Soc., 17 (15/16), 1849-1854. 

The susceptor materials used in high-temperature processing include zirconia, boron nitride, graphite, carbon black, sodium-beta alumina, zinc oxide, and silicon carbide. While each of these susceptor materials has relatively high dielectric losses at room temperature, silicon carbide is also refractory with a relatively good resistance to oxidation at temperatures up to roughly 1500°C.t ° ... 

P.B. Pickar et al [Engineering Sciences Lab., Dover, NJ, Research on Optical Properties of Single Crystal of Beta Phase Silicon Carbide. Summitry Tech. Rept. no. OR8394, May 5, 1965-June 5, 1966. Contract no. DA-28-017-AMC-2002, A, Oct. (1966) 85p. AD-641 198 ]... 

Improved stiffness, strength and thermal properties are offered by beta-Ti alloys reinforced with silicon carbide fibres. A high probability exists that these composites will make their way into the design of the actively cooled airframe of the future US NASP X30 spaceplane. 

As silicon carbide exists predominantly in its beta form at temperatures below 2000 °C, this is referred to as the low-temperature modification. Cubic P-SiC is metastable and, in accordance with Ostwald s rule, is formed initially in SiC production from silicon dioxide and carbon. P-SiC can also be prepared at about 1450 °C from simple mixtures of silicon and carbon, or by the hydrogen reduction of organosilanes at temperatures below 2000 ° C. Above 2000 ° C, only the hexagonal and... 

Silicon carbide has self-heating and beta-emitting glow characteristics and as such is a standard material for heating elements (see Ch. 15). The anisotropy of the electrical conductivity of boron carbide is low, between 70 and 700 K.0 1... 

Silicon carbide is produced most often by the Acheson process, discovered in 1891 by Edward Goodrich Acheson. Later in 1891 he formed Carborundum Corporation to manufacture Sic abrasives. In this process current is passed through a mixture of carbon and sand causing the reaction SiOj -H 3C — SiC + 2CO. When carried out at 2600°C or higher the result is alpha the beta form can be achieved at 1500 to 1600°C. ... 

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