Carborundum, SiC

Silicon, like carbon, is relatively inactive at ordinary temperatures. But, when heated, it reacts vigorously with the halogens (fluorine, chlorine, bromine, cmd iodine) to form halides and with certain metals to form silicides. It is unaffected by all acids except hydrofluoric. At red heat, silicon is attacked by water vapor or by oxygen, forming a surface layer of silicon dioxide. When silicon and carbon are combined at electric furnace temperatures of 2,000 to 2,600 °C (3,600 to 4700 °F), they form silicon carbide (Carborundum = SiC), which is an Importeint abrasive. When reacted with hydrogen, silicon forms a series of hydrides, the silanes. Silicon also forms a series of organic silicon compounds called silicones, when reacted with various organic compounds. 

Would you expect carborundum (SIC) to adopt a diamond structure or one of higher coordination Explain why. 

A very clear-cut example for the influence of the electronic factor in supported catalysts, again involving a thin-layer metal type, is represented in Fig. 2. Here the carriers are commercially available samples of doped carborundum (SiC) which by itself is catalytically entirely inactive. In the abscissa of Fig. 2 we have arranged these samples in the order of their conductivity as stated by their manufacturers. The concentration of positive holes increases towards the right and that of the quasi-free electrons towards the left. Grains of these supports approximately 1 mm in size were covered with a thin layer of silver by the usual mirror produc-... 

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. 

Silicon combines with carbon to form silicon carbide or carborundum, SiC, and forms are known that have the wurtzite and zinc blende structures. It is a very hard, tough material that is used as an abrasive and a refractory. The powdered material is crushed after mixing with clay and heated in molds to make grinding wheels. It is prepared by the reaction... 

Very great hardness would thus be expected in nitrides and carbides in which, starting from the assumption of ionic bonding, tri- and quadrivalent negative ions would be present. In fact the extremely hard substances, such as carborundum SiC as well as AIN, TiN, TiC and WC employed as Widia metal, belong to these groups. 

Diamond belongs, as a prototype, to this group of solid substances, also boron, silicon, germanium, carborundum SiC,... 

Carborundum, SiC crystallizes in a large number of forms. The cubic form (/3-SiC) has the zinc-blende structure (p. 102), and in addition there are numerous crystalline modifications all with structures closely related to the zinc-blende and wurtzite structures, which are collectively referred to as a-SiC. The atomic positions... 

Calculate the percent composition of each of the following compounds (a) menthol, CjgHjpOH (b) carborundum, SiC (c) aspirin, C9Hg04. 

Silicon carbide, or carborundum, SiC, is one of the hardest substances known and is used as an abrasive. It has the structure of diamond with half of the carbons replaced by silicon. It is prepared industrially by reduction of sand (Si02) with carbon in an electric furnace. 

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... 

The hardness of the diamond is due to its symmetrical structure. Each C atom has four others arranged tetrahedrally and perfectly symmetrically round it (Fig. 5). The diamond is thus one huge molecule with no weak spot. Carborundum, SiC, has a similar structure and is also extremely hard. It might be thought that close-packing would explain the hardness, but the diamond has a relatively open structure if close packed it would be possible for... 

Carborundum, SiC, is widely used as an abrasive in industrial grinding wheels. It is prepared by the reaction of sand, Si02, with the carbon in coke Si02 + 3 C — SiC + 2 CO. How many kilograms of carborundum can be prepared from 727 kg of coke that is 88.9% carbon ... 

The increased Si—C bond lengths in the environment of C(l) illustrate the strong tension at least in this part of the molecule. This increase in bond distance is the result of the moving away from each other of atoms Si(2), Si(4) and Si(6) from 307 pm, as in carborundum SiC, to 313 pm. That is necessary in order to accommodate the axial CH3 groups C(202), C(404) and C(601). It can easily be shown that a separation... 

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