Refractory covalent nitrides

The location in the Periodic Table and the atomic number of the four elements forming the refractory covalent nitrides is as follows ... 

The previous chapter was a review of the structure and composition of the three refractory covalent nitrides boron nitride, aluminum nitride, and silicon nitride. This chapter is an assessment of the properties and a suimnary of the fabrication processes and applications of these three materials. 

The refractory covalent nitrides have remarkable properties and are industrial materials of major importance, produced on a large scale in the form of powders, monolithic shapes, and coatings. 

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. 

This chapter is a review of the general characteristics of the refixictory nitrides and their classification. Like the refiactory carbides (see Ch. 2), the refractory nitrides can be divided into two major types the interstitial nitrides reviewed in Chs. 10 and 11 and the covalent nitrides, reviewed in Chs. 12 and 13. 

In the five categories listed above, only some of the interstitial and covalent nitrides qualify as refractory, i.e., the nitrides of the elements of Groups IV and V and the covalent nitrides of boron, aluminum, and silicon. These elements are shown in bold type in Table 9.1. Unlike the carbides of Group VI elements, the Group VI nitrides are not refractory and consequently are not considered in any depth in this book. 

The third factor governing the structure of nitrides is the nature of the bond between the nitrogen atom and the other element forming the compound. As mentioned in Ch. 2, the bond is the force of attraction that holds together the atoms of a molecule.l The bonds in refractory carbides can be ionic (saltlike nitrides), covalent (covalent nitrides), or a combination of metallic, covalent, and ionic (interstitial nitrides) (for a discussion of electronic bonding, see Ch. 2, Sec. 5.0). 

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. 

As mentioned in Ch. 9, the refractory nitrides consist of two structurally different types generally known as interstitial and covalent nitrides. This chapter provides a general review of the structural characteristics and composition of the interstitial nitrides and follows the outline of Ch. 3, Interstitial Carbides Structure and Composition. Some of these interstitial nitrides, titanium nitride in particular, are major industrial materials. 

Table 13.1 Density and Melting Point of Covalent Nitrides and Other Refractory Compounds...

Diamond-like nitrides in which N bonds covalently to elements of groups IIIB and IVB they are chemically stable, refractory, and nonconducting. 

As shown in subsequent chapters, most elements form carbides and nitrides and these can be divided into several types with different physico-chemical structures and characteristics. Of these, however, only the interstitial and covalent materials meet the refractory qualification. This includes the carbides and nitrides of the nine transition elements of Groups rv, V, and VI and the 4th, 5th, and 6th Periods, the carbides and nitrides of boron and silicon, and aluminum nitride. 

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... 

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. 

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