Silicon carbide, crystal structure

If silicon atoms are substituted for half the carbon atoms in this structure, the resulting structure is that of silicon carbide (carborundum). Both diamond and silicon carbide are extremely hard, and this accounts for their extensive use as abrasives. In fact, diamond is the hardest substance known. To scratch or break diamond or silicon carbide crystals, covalent bonds must be broken. These two materials are also nonconductors of electricity and do not melt or sublime except at very high temperatures. SiC sublimes at 2700 °C, and diamond melts above 3500 °C. 

On silicon carbide, it is easier to see and measure step heights than in crystals like beryl, because SiC has polytypes, first discovered by the German crystallog-rapher Baumhauer (1912). The crystal structure is built up of a succession of close-packed layers of identical structure, but stacked on top of each other in alternative ways (Figure 3.24). The simplest kind of SiC simply repeats steps ABCABC, etc., and the step height corresponds to three layers only. Many other stacking sequences... 

Pure silicon carbide is colorless, but iron impurities normally impart an almost black color to the crystals. Carborundum is an excellent abrasive because it is very hard, with a diamondlike structure that fractures into pieces with sharp edges (Fig. 14.43). 

Silicon carbide occurs in two slightly different crystal structures a single cubic form, (3SiC, and a large number of hexagonal... 

Single-crystal silicon carbide, 22 525 manufacture and processing of, 22 532 Single-crystal silicon substrates, 23 39-40 Single-crystal structure determination information from, 26 426 macromolecule, 26 425-426 small-molecule, 26 423—425 Single-electron transistors (SETs), 22 169, 171-172... 

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. 

Crystal Structure. Silicon carbide may crystallize in the cubic, hexagonal, or rhombohedral structure. There is a broad temperature range where these structures may form. The hexagonal and rhombohedral structure designated as the a-form (noncubic) may crystallize in a large number of polytypes. 

Semiconducting Properties. Silicon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stability of its electronic structure, silicon carbide has been studied for uses at high (>500° C) temperature. The Hall mobility in silicon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-type crystals, activation energy for ionization of nitrogen impurity varies with polytype (50,51). 

As with the hydrides (Chap. 2), the carbides are divided into three classes—the covalent, the saltlike, and the metallic (or interstitial). The volatile covalent carbides (for example, CC14, (CN)2, CH4, and CS2) are discussed elsewhere of the nonvolatile covalent carbides, silicon carbide (carborundum, SiC), is by far the most important. Although there are three known crystal forms of this compound, we may, for simplification, imagine it as a diamond structure in which every alternate carbon atom is replaced by a silicon atom. Thus it is not surprising that this compound is almost as hard and chemically inert as is diamond itself. 

A completely amorphous structure was found by X-ray diffraction on fibers which were pyrolysed at temperatures up to 1300 °C. A crystallization starts around 1400 °C and nanocrystalline silicon carbide is formed with a crystallite size of about 2 nm. Compared to an uncured sample the crystallization is retarded. A significant crystallite growth occurs around 1500 °C connected with a decreasing of the fiber properties. The oxygen content of these SiC fibers is less than 1 wt. % found by neutron activation... 

Silicon carbide occurs in a large number of different crystal structures... 

Although the silicon atom has the same outer electronic structure as carbon its chemistry shows very little resemblance to that of carbon. It is true that elementary silicon has the same crystal structure as one of the forms of carbon (diamond) and that some of its simpler compounds have formulae like those of carbon compounds, but there is seldom much similarity in chemical or physical properties. Since it is more electro-positive than carbon it forms compounds with many metals which have typical alloy structures (see the silicides, p. 789) and some of these have the same structures as the corresponding borides. In fact, silicon in many ways resembles boron more closely than carbon, though the formulae of the compounds are usually quite different. Some of these resemblances are mentioned at the beginning of the next chapter. Silicides have few properties in common with carbides but many with borides, for example, the formation of extended networks of linked Si (B) atoms, though on the other hand few silicides are actually isostructural with borides because Si is appreciably larger than B and does not form some of the polyhedral complexes which are peculiar to boron and are one of the least understood features of boron chemistry. 

Each constituent atom of a covalent crystal is linked to its neighbours through directed covalent bonds. The crystal structure is determined by the spatial dispositions of these bonds. Because primary valence forces are involved, such solids are hard and have high melting points, e.g. diamond, silicon carbide, etc. Relatively few entirely covalent solids have been studied at elevated temperatures and it is, therefore, premature to comment on their decomposition characteristics. 

Microstructure of Chemical Vapour Deposition SiC Figure 6.8 shows the X-ray diffraction (XRD) patterns of CVD SiC deposited in a temperature range of 1000 to 1300°C. Detailed analyses of the X-ray results indicate that the deposits are pure silicon carbide mainly composed of //-SiC (cubic 3C crystal structure) with a small amount of er-SiC (hexagonal 4H crystal structure). It is clear from the figure that the diffraction angles of 35.6°, 41.3°, 60.1°, 72.1° and 75.5° correspond to //-SiC and the diffraction angle of 33.7° corresponds to er-SiC. As the deposition temperatures decrease, the deposits become poorly crystallised because the diffraction peaks become broader or its intensity shown in Y axis become lower. At the deposition temperature of 1000°C, the deposits are in a quasi-amorphous state. 

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