Silicon carbide crystalline forms

Properties For silicon carbide, crystalline form ranges from small to massive crystals in the hexagonal system, the crystals varying from transparent to opaque, with colors from pale green to deep blue or... 

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.,... 

A -C-Si-C- network is formed on the (partially crystalline) oxide surface. This coating can therefore be treated as a silicon carbide coating. 

Thin silicon (oxy)carbide coatings can be formed using the liquid phase CSC method. This involves the liquid phase modification of the silica substrate with APTS, followed by a thermal treatment under inert atmosphere. At temperatures of 1873 K the material is a mixture of graphite and partially crystalline silica, coated with a molecular layer of silicon carbide. 

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 polycarbosUanes (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) structme a-SiC, or the cubic diamond (zinc blende) structme /3-SiC. The structmes differ in the way that the layers of atoms are stacked, with Si being fom-coordinate in all cases. 

Salinger (44) reported the successful conversion of methyltrichloro-silanes to silicon carbide in a 50-kW RF plasma torch. The liquid methyltrichlorosilanes were fed to the tail flame of various plasmas and the solid products were recovered in an acid-resistant bag filter. Up to 85% recovery of theoretical solid product was reported, which was subsequently heated at 500°C to remove elemental carbon. Under the best condition (20-25% vol. hydrogen in argon plasma at 36 kW), up to 70%o conversion to j3-SiC was obtained with ca. 10% conversion to amorphous SiC. Salinger suggested that the good crystallinity of the )3-SiC so obtained meant that the reaction occurred in a gas temperature range in which )3-SiC was the stable crystalline form (i.e., < 2300°C). 

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. 

Silicon carbide has been manufactured commercially since 1891 and the current world market is about 500 000 tons. This material is dense and crystalline. It is only recently, however, that a porous form has been reported. These two forms can be regarded as the analogues of quartz (dense, crystalline silicon oxide) and silica gel (porous, amorphous silicon oxide). We were interested in the properties of the porous silicon carbide, and in particular its stability. It is not improbable that this be higher than that of silica in view of the four-fold coordination of carbon compared to the two—fold coordination of oxygen. Although data on the stabilities of dense forms are ayailable, the information is not necessarily relevant to the properties of porous forms. 

Silicon carbide (SiC) is the most widely used nonoxide ceramic. Its major application is in abrasives because of its hardness (surpassed only by diamond, cubic boron nitride, and boron carbide). Silicon carbide does not occur in nature and therefore must be synthesized. It occurs in two crystalline forms the cubic P phase, which is formed in the range 1400-1800°C, and the hexagonal a phase, formed at >2000°C. 

An X-Ray diffraction analysis was carried out to identify the crystalline phases constituting the material. The silicon carbide (SiC) is in its moissanite form, whereas the intergranular oxide phase is essentially composed of silica. 

Silicon deposited on glass or silicon carbide is widely used in manufacturing photovoltaic cells. It is critical to monitor both the proportion and distribution of amorphous and crystalline silicon in such materials. Raman microscopy is ideal for this application as the two forms are readily distinguishable and permit simple quantification using Beer s law (Deschaines, 2009). Raman spectroscopy using a Thermo Scientific DXR Raman microscope with a 532 nm laser permitted the authors to quantify the relative amounts of amorphous and crystalline silicon in thin layer deposits. Crystalline silicon has a sharp band at 521 cm , while amorphous silicon has a broadband centered at 480 cm , seen in Figure 4.84. 

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