Covalent boron carbides

The covalent carbides These include boron carbide B4C and silicon carbide SiC the latter is made by heating a mixture of silica and coke in an electric furnace to about 2000 K ... 

Diamondlike Carbides. SiUcon and boron carbides form diamondlike carbides beryllium carbide, having a high degree of hardness, can also be iacluded. These materials have electrical resistivity ia the range of semiconductors (qv), and the bonding is largely covalent. Diamond itself may be considered a carbide of carbon because of its chemical stmeture, although its conductivity is low. 

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. 

SiC, like boron carbide, silicon carbide is nearly completely covalent and is e.g. used as grinding powder, in the manufacture of ball bearings, balls, outlet casings and jet nozzles, as well as in packing rings of pumps used to transport materials which are likely to cause wear. 

Boron carbide (B4C) is also an extremely hard, infusible, and inert substance, made by reduction of B203 with carbon in an electric furnace at 2500°C, and has a very unusual structure. The C atoms occur in linear chains of 3, and the boron atoms in icosahedral groups of 12 (as in crystalline boron itself). These two units are then packed together in a sodium chloride-like array. There are, of course, covalent bonds between C and B atoms as well as between B atoms in different icosahedra. A graphite-like boron carbide (BQ) has been made by interaction of benzene and BC13 at 800°C. 

In nonoxide ceramics, nitrogen (N) or carbon (C) takes the place of oxygen in combination with silicon or boron. Specific substances are boron nitride (BN), boron carbide (B4C), the silicon borides (SiB4 and SiBg), silicon nitride (SisN4), and silicon carbide (SiC). All of these compounds possess strong, short covalent bonds. They are hard and strong, but brittle. Table 22.5 lists the enthalpies of the chemical bonds in these compounds. 

Compare oxide ceramics such as alumina (AI2O3) and magnesia (MgO), which have significant ionic character with covalently bonded nonoxide ceramics such as silicon carbide (SiC) and boron carbide (B4C see Problems 19 and 20) with respect to thermodynamic stability at ordinary conditions. 

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 the two elements of the covalent carbides is small. The carbon atom is only slightly smaller than the other atom. The bonding is essentially covalent.1 1 Only two covalent carbides, silicon carbide and boron carbide, fully meet the refractory criteria. Other carbides such as beryllium carbide, Be2C, are only partially covalent and, while they have a high melting point, are generally not chemically stable and are not considered here. 

Covalence. Carbon bonding is covalent, that is, the atoms share a pair of electrons. Such covalent bonds are strong since the carbon atom is small and four of its six electrons (the four sp valence electrons) form bonds. This is the case for the two covalent carbides, silicon carbide and boron carbide (see Ch. 7). The bonding in interstitial carbides is not as straightforward and is a combination of covalent, metallic, and ionic bonding as reviewed in Sec. 6.0. 

As mentioned in Ch. 2, the refractory carbides include two structurally different types (a) the interstitial carbides of the transition metals of Group IV, V, and VI (reviewed in Ch. 3, 4, 5, and 6), and (b) two covalent carbides boron carbide and silicon carbide. The structural characteristics of these two carbides are reviewed in this chapter and their properties and general characteristics in the following chapter. 

The bonds between the carbon atoms and boron atoms as well as between the boron atoms themselves in the icosahedra are essentially covalent. But like silicon carbide (Sec. 3.3), the bonding of boron carbide is also partially ionic. 1 1 The difference between the atomic spacing of SiC and the sum of the covalent radii of carbon and silicon on one hand and the sum of the ionic radii (m the other hand show that the bonding, although mainly covalent, includes a certain d rce of ionicity. The calculated covalent bond energy E is 9.42 eV and die ionic bond energy Ep is 1.41 eV. 

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. 

Specific Heat. The specific heat (C ) of the covalent carbides as a function of temperature is shown in Fig. 8.1 On a weight basis (J/g K), the specific heat of silicon carbide and particularly boron carbide is higher than that of the other refractory carbides and nitrides listed in Table 8.2 Thermal Conductivity. The thermal conductivity or k (i.e., the time rate of transfer of heat by conduction) of covalent carbides, unlike that of the interstitial carbides, decreases with increasing temperature as shown in Fig. 8.2.P It is highly dependent on the method of formation which is reflected by the large spread in values. The thermal conductivity of silicon carbide... 

Unlike the transition-metal nitrides and unlike boron carbide and silicon carbide, the covalent nitrides are excellent electrical insulators. Their electrons are strtmgly and covalently bonded to the nucleus and are not available for metallic bonding (see Sec. 3.1 of Ch. 4). 

The five covalent refractory carbides and nitrides, silicon carbide, boron carbide, aluminum nitride, silicon nitride, and boron nitride are all produced on an industrial scale and all have a number of successfiil large-volume applications. Of these five, the latecomer silicon nitride, either in bulk/monolithic form or as a coating, may have the most promising future. 

The nitrides reviewed here are those which are commonly produced by CVD. They are similar in many respects to the carbides reviewed in Ch. 9. They are hard and wear-resistant and have high melting points and good chemical resistance. They include several of the refractory-metal (interstitial) nitrides and three covalent nitrides those of aluminum, boron, and silicon. Most are important industrial materials and have a number of major applications in cutting and grinding tools, wear surfaces, semiconductors, and others. Their development is proceeding at a rapid pace and CVD is a major factor in their growth. 

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... 

The term ceramics comes from the Greek keramikos, which means potter s clay or burnt stuff. While traditional ceramics were often based on natural clays, today s ceramics are largely synthetic materials. Depending on which ceramic and which definition is to be applied, ceramics have been described as inorganic ionic materials and as inorganic covalent (polymeric) materials. In fact, many ceramics contain both covalent and ionic bonds and can thus be considered to be or not to be (shades of Shakespeare) polymeric materials. Many of the new ceramics, such as the boron nitriles and the silicon carbides, are polymeric without containing any ionic bonds. 

As for hydrides, borides, and carbides, different types of nitrides are possible depending on the type of metallic element. The classifications of nitrides are similarly referred to as ionic (salt-like), covalent, and interstitial. However, it should be noted that there is a transition of bond types. Within the covalent classification, nitrides are known that have a diamond or graphite structure. Principally, these are the boron nitrides that were discussed in Chapter 8. 

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