Compounds covalent carbides

Carbon forms ionic carbides with the metals of Groups 1 and 2, covalent carbides with nonmetals, and interstitial carbides with d-block metals. Silicon compounds are more reactive than carbon compounds. They can act as Lewis acids. 

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. 

Covalent carbides, which have giant-molecular structures, as in silicon carbide (SiC) and boron carbide (B4C3). These are hard high-melting solids. Other covalent compounds of carbon (CO2, CS2, CH4, etc.) have covalent molecules. 

The difference in electronegativity between carbon and the other element forming a carbide is an important factor in determining the nature of the compound. As shown in Table 2.1, that difference in the interstitial carbides is large (Box A) while it is much less pronounced in the covalent carbides (Box B). 

In the previous chapter, the structure and composition of the two covalent carbides, i.e., silicon carbide and boron carbide, were reviewed. This chapter is an assessment of the properties and a summary of the fabrication processes and applications of these two compounds. 

In this section and the next three, the properties and characteristics of the covalent carbides are reviewed and compared whenever appropriate with those of the parent elements and of the refi tory compounds of titanium. For comparison with other carbides, nitrides, or borides, see the appropriate tables in Chs. 4-6. Reported property values often vary considerably and the values given here are a general average. 

Table 8.3 Density and Melting Point of Covalent Carbides and Other Refractory Compounds.

Both covalent carbides have high melting points which are slightly lower than the titanium compounds but higher than silicon and boron. Under most conditions, the thermal decomposition of SiC may occur well below its intrinsic melting poind l and decomposition can become significant at approximately 1700°C (see Sec. 3.7 and Fig. 7.8 of Ch. 7). The density of SiC is closer to that of diamond than it is to graphite, which can be expected since SiC has the structure of diamond. 

In a semiconductor material, the forbidden-energy gap is such that electrons in usable quantities are able to jump across it from the filled valence band to the empty eonduction band.1 1 The three elements that form the covalent carbides, i.e., boron, silicon, and carbon (in the form of doped diamond) are semiconductors and one would expect to find semiconductor properties in their compounds. 

Unlike the interstitial nitrides, the covalent nitrides are not metallic compounds. The differences in electronegativity and atomic size between the nitrogen and the other element are small and their electronic bonding is essentially covalent. In this respect, they are similar to the covalent carbides. They include the nitrides of Group mb (B, Al, Ga, In, Tl) and those of silicon and phosphorus. Of these, only three are considered refractory boron nitride, silicon nitride, and aluminum nitride. These are reviewed in Chs. 12 and 13. 

Covalent Carbides Compounds composed of carbon and low-electronegativity nonmetals or metalloids are covalent carbides. The most important covalent carbide is silicon carbide (SiC), a very hard material. Over 500,000 tons of silicon carbide are produced annually, mostly for use as an abrasive material in the cutting and polishing of metals. In a process analogous to the formation of calcium carbide, silicon carbide forms by the reaction of silicon oxide with coke at high temperatures. 

Important types of inorganic carbon compounds include carbides and carbonates. Carbon can form carbides with metalhc, covalent, or ionic properties. 

Carbides, which are binary compounds containing anionic carbon, occur as covalent and as salt-like compounds. The salt-like carbides are water-reactive and, upon hydrolysis, yield flammable hydrocarbons. Typical hydrolysis reactions include ... 

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

Boron carbide is a non-metallic covalent material with the theoretical stoichiometric formula, B4C. Stoichiometry, however, is rarely achieved and the compound is usually boron rich. It has a rhombohedral structure with a low density and a high melting point. It is extremely hard and has excellent nuclear properties. Its characteristics are summarized in Table 9.2. 

The four rather distinct forms of chemical bonding between atoms are metallic, ionic, covalent, and dispersive (Van der Waals). All of them are sub-topics of quantum electrodynamics. That is, they are all mediated by electronic and electromagnetic forces. There are also mixed cases, as in carbides and other compounds, where both metallic and covalent bonding occur. 

As we shall see later, borides (as well as oxides, nitrides, carbides, etc.) react with water to produce a hydrogen compound of the nonmetal. Thus, the reaction of magnesium boride with water might be expected to produce BH3, borane, but instead the product is B2ff6, diborane (m.p. -165.5 °C, b.p. -92.5 °C). This interesting covalent hydride has the structure... 

Compounds containing carbon in a negative oxidation state are properly called carbides, and many such compounds are known. In a manner analogous to the behavior of hydrogen and boron, carbon forms three types of binary compounds, which are usually called ionic, covalent, and interstitial... 

Small or negligible A rj carbides with more or less well-defined covalent bonding (compounds with some non-metals). 

---