Carbides, interstitial

In a metallic bond, the atoms are considered to be ionized, with the positive ions arranged in the lattice positions. The electrons are delocalized, that is, they are able to move essentially freely throughout the lattice. The bonding occurs by the electrostatic attraction between the electrons and the positive metal ions. Most metals can be considered as close-packed arrays of atoms held together by these delocalized electrons. The metallic bond contributes to the bonding of interstitial carbides and is described in more detail in Ch. 3. 

The characteristics of the four categories of carbides can be summarized as follows. 

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The interstitial carbides These are formed by the transition metals (e.g. titanium, iron) and have the general formula M, C. They are often non-stoichiometric—the carbon atoms can occupy some or all of the small spaces between the larger metal atoms, the arrangement of which remains essentially the same as in the pure metal (cf. the interstitial hydrides). 

Metallic Carbides. This class of compounds comprises the interstitial carbides of the transition metals of Groups 4—6 (see Industrial,... 

The crystal stmeture and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, < 0.59. 

The carbides of Cr, Mn, Fe, Co and Ni are much more reactive than the interstitial carbides of the earlier transition metals. They are rapidly hydrolysed by dilute acid and sometimes even by water to give H2 and a mixture of hydrocarbons. For example, M3C give H2 (75%), CH4 (15%)... 

Carbon is the only Group 14/IV element that forms both monatomic and polyatomic anions. There are three classes of carbides saline carbides (saltlike carbides), covalent carbides, and interstitial carbides. The heavier elements in Group 14/IV form polyatomic anions, such as Si44 and Sn52, in which the atoms form a tetrahedron and trigonal bipyramid, respectively. 

The interstitial carbides are compounds formed by the direct reaction of a d-block metal and carbon at temperatures above 2000°C. In these compounds, the C atoms occupy the gaps between the metal atoms, as do the H atoms in metallic hydrides (see Fig. 14.9). Here, however, the C atoms pin the metal atoms together into a rigid structure, resulting in very hard substances with melting points often well above 3000°C. Tungsten carbide, WC, is used for the cutting surfaces of drills, and iron carbide, FesC, is an important component of steel. 

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. 

Handbook of Chemical Vapor Deposition 1.1 Refractory-Metal (Interstitial) Carbides... 

These carbides, also known as interstitial carbides, are crystalline compounds of a host metal and carbon. The host-metal atoms are generally arranged in a close-packed structure and the carbon occupies specific interstitial sites in that structure. Such a structure sets size restrictions on the two elements in order for the carbon atom to fit into the available sites and the population of these sites (if all are occupied) determines the stoichiometry of the carbide. 

The interstitial carbides have the following general characteristics ... 

Chemical Resistance. The chemical resistance of chromium carbide is superior to that of other interstitial carbides. Oxidation in air starts at 1000°C and a dense and strong oxide layer is formed. It is insoluble in cold HCl but dissolves in hot oxidizing acids. 

Hafnium carbide (HfC) is an interstitial carbide which, with tantalum carbide, is the most refractory compound known. Its characteristics and properties are summarized in Table 9.4. 

Tantalum carbide (TaC) is arefractory interstitial carbide with a high melting point. It is structurally and chemically similar to niobium carbide. It has two phases Ta and the monocarbide TaC. Thelatteris the only phase of industrial importance and the only one described here. The characteristics and properties of TaC are summarized in Table 9.7. 

Zirconium carbide (ZrC) is a refractory interstitial carbide with a high melting point. It is produced by CVD mostly on an experimental basis although it has some nuclear applications. Like TiC, cubic ZrC has a variable composition and forms solid solutions with oxygen and nitrogen over a wide range of composition. Its characteristics and properties are summarized in Table 9.10. 

The interstitial nitrides have several important characteristics in common with the interstitial carbides. 

The 90 electron trigonal prismatic cluster, [Rh C(CO) ], is electron rich and seems to behave as though there is a pair of electrons pointing out of each triangular face. Each metal atom is surrounded by five groups (1 terminal CO, 3 bridging CO s and the interstitial carbide) in an essentially square pyramidal... 

As we go from left to right across the transition metals in the periodic table, the metal atoms become smaller, much as in the lanthanide contraction (Section 2.6). Furthermore, the atoms of elements of the first transition series are smaller than those of corresponding members of the second and third. Consequently, interstitial carbides are particularly important for metals toward the lower left of the series, as with TiC, ZrC, TaC, and the extremely hard tungsten carbide WC, which is used industrially as an abrasive or cutting material of almost diamond like hardness. The parallel with trends in chemisorption (Section 6.1) will be apparent. 

The crystal structure and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, rx / rM < 0.59. When this ratio is larger than 0.59, as in the Group 7—10 metals, the structure becomes more complex to compensate for the loss of metal—metal interactions. Although there are minor exceptions, the H gg rule provides a useful basis for predicting structure. 

Carbides. As might he expecled from its position in the periodic table, carbon forms binary compounds with the metals in which it exhibits a negative valence, and binary compounds with the non-metals in which it exhibits a positive valence. A convenient classification of the binary compounds of carbon is into ionic or salt-like carbides, intermediate carbides, interstitial carbides, and cuvalent binary carbon compounds. 

There are three classes of carbides saline carbides (or saltlike carbides), covalent carbides, and interstitial carbides. 

FIGURE 14.48 The structure of an interstitial carbide, in which the carbon atoms (represented by the black spheres) lie between metal atoms (the gray spheres), thereby producing a rigid structure. 

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