Silicon carbide crystalline structure

Twenty-five years later, Burhard reported the preparation of permethylated. polysilane (2). These materials were, however, highly crystalline, insoluble white solids which evoked little scientific interest until recently when it was discovered that silane polymers could be used as thermal precursors to / -silicon carbide fibers (3-5). In this regard, Yajima and co-workers reported that poly (dimethyl) silane could be converted by the two-step process shown below to / -silicon carbide, a structural material of considerable industrial importance. 

The atomic and crystalline structure of the two non-metallic carbides, boron and silicon carbides, is less complex than that of the... 

When Acheson found the hexagonal crystals in the voids, he sent some to B.W. Frazier, a professor at Lehigh University. Professor Frazier found that although the crystals were all silicon carbide, they differed in their crystalline structure. He had discovered the polytypism of SiC [18]. Polytypism will be explained in Section 1.3.2. 

The structure of presolar silicon carbide grains can provide information about the conditions of formation. Crystalline silicon carbide is known to form about 100 different polytypes, including cubic, hexagonal, and rhombohedral structures. Presolar silicon carbide exists in only two of these, a cubic (fi-SiC) polytype and a hexagonal (a-SiC) polytype (Daulton et al.,... 

Gogotsi, Y., Weltz, S., Ersoy, D.A., and McNallan, M.J. Conversion of silicon carbide to crystalline diamond-structured carbon at ambient pressure. Nature 411, 2001 283-287. 

As mentioned, elemental silicon has the diamond structure. Silicon carbide, SiC, occurs in many crystalline forms, some based on the diamond structure and some on the wurtzite structure (see Figures 7-6 and 7-8(b)). It can be made from the elements at high temperature. Carborundum, one form of silicon carbide, is widely used as an abrasive, with a hardness nearly as great as diamond and a low chemical reactivity. SiC has now garnered interest as a high-temperature semiconductor. 

Roewer G, Herzog U, Trommer K, Muller E, Friihauf S (2002) Silicon Carbide - A Survey of Synthetic Approaches, Properties and Applications 101 59-136 Rosa A, Ricciardi G, Gritsenko O, Baerends EJ (2004) Excitation Energies of Metal Complexes with Time-dependent Density Functional Theory 112 49-116 Rosokha SV, Kochi JK (2007) X-ray Structures and Electronic Spectra of the it-Halogen Complexes between Halogen Donors and Acceptors with it-Receptors. 126 137-160 Rowan SJ, Mather PT (2008) Supramolecular Interactions in the Formation of Thermotropic Liquid Crystalline Polymers. 128 119-149... 

The preparation, manufacture, and reactions of SiC have been discussed in detail in Gmelin, as have the electrical, mechanical, and other properties of both crystalline and amorphous of SiC. Silicon carbide results from the pyrolysis of a wide range of materials containing both silicon and carbon but it is manufactured on a large scale by the reduction of quartz in the presence of an excess of carbon (in the form of anthracite or coke), (Scheme 60), and more recently by the pyrolysis of polysilanes or polycarbosilanes (for a review, see Reference 291). Although it has a simple empirical formula, silicon carbide exists in at least 70 different crystalline forms based on either the hexagonal wurtzite (ZnS) structure a-SiC, or the cubic diamond (zinc blende) structure /i-SiC. The structures differ in the way that the layers of atoms are stacked, with Si being four-coordinate in all cases. 

The atomic and crystalline structures of covalent carbides are less complex and generally better understood and characterized than those of interstitial carbides. Bonding is essentially covalent where the carbon atoms bond to the silicon or boron atoms by sharing a pair of electrons and, like all covalent bonds, these atoms form definite bond angles. The bonding is achieved by the hybridization of the valence electrons of the respective atoms. 

Nonetheless, owing to its temperature stability, dopabUity and widely tunable optical behavior, amorphous SiC doped with hydrogen (a-SiC H) has been investigated for its application in photovoltaic devices. One of several structural renderings of amorphous SiC is shown in Figure 11.9. In this case, the refractive index can be varied from 2.3 to 3.7, and the band gap from 2.4 to 2.0eV, simply by altering a few parameters of the plasma-enhanced CVD technique that is used to deposit an amorphous silicon carbide layer with low defect density onto, for example, a substrate of RSiC. Attention has also been focused on the surface passivation performance of layers deposited on crystalline siUcon. Indeed,... 

Silicoaluminophosphates (SAPOs), along with their crystalline aluminum phosphate counterparts (ALPOs), first discovered by Union Carbide workers in the early 1970s [41, 42], derive their acidity through the substitution of framework phosphorous by silicon thereby creating the charge imbalance which, when compensated for by protons, creates acidic centers. SAPOs in general have seen limited use in bond-breaking applications primarily due to weaker acidity, framework stability, or technoeconomic reasons. Of the rich variety of structures available,... 

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