Bonding boron nitrides

For tool manufacture, the hard boron nitride alloy composites have to be bonded to refractory metals or hard steel alloys. Interlayers used for this purpose should have an adapted expansion coefficient and form mixed phases with the alloy-bonded boron nitride and the metallic substrate. It also should withstand the high temperatures developed during metal working. Thus, TiC [88] or Co/Ni [89] are used in these interlayers. On the other hand, the alloy... 

Boron nitride is used in metal-bonded boron nitride grinding disks. It is also used in the form of loose grains. 

Boron nitride is chemically unreactive, and can be melted at 3000 K by heating under pressure. It is a covalent compound, but the lack of volatility is due to the formation of giant molecules as in graphite or diamond (p. 163). The bond B—N is isoelectronic with C—C. 

Materials made of siHcon nitride, siHcon oxynitride, or sialon-bonded siHcon carbide have high thermal shock and corrosion resistance and may be used for pump parts, acid spray nozzles, and in aluminum reduction ceUs (156—159). A very porous siHcon carbide foam has been considered for surface combustion burner plates and filter media. It can also be used as a substrate carrying materials such as boron nitride as planar diffusion source for semiconductor doping appHcations. 

C21-0047. Boron nitride (BN) is a planar covalent solid analogous to graphite. Write a portion of the Lewis structure and describe the bonding of boron nitride, which has alternating B and N atoms. 

Evolution did not use this element, only in certain plants is it important as a trace element. The element became well-known because of heat-resistant borosilicate glasses. Boranes are chemically interesting as B-H bonds react very specifically. Perborates are used in laundry detergents (Persil). The hardness of cubic boron nitride approaches that of diamond. Amorphous (brown) boron burns very quickly and gives off much heat and is therefore used in solid-propellant rockets and in igniters in airbags. 

Under high pressure and temperature, boron nitride can be converted to a cubic form. The cubic form of (BN) is known as borazon, and it has a structure similar to that of diamond. Its hardness is similar to that of diamond, and it is stable to higher temperatures. The extreme hardness results from the fact that the B-N bonds possess not only the covalent strength comparable to C-C bonds, but also some ionic stabilization due to the difference in electronegativity between B and N. 

There is mass spectroscopic evidence for clusters C59B, C68B2,. ..C54 B6 in the products of laser ablation of boron nitride/graphite composites (Guo et al. 1991). It is proposed that these may be versions of the fullerene cage in which heteroatoms have occupied carbon sites with little disturbance of the framework. This is a plausible hypothesis for low doping ratios, in view of the ability of carbon to substitute for boron in carboranes. Bond-energy considerations suggest that... 

However, since the elements boron and nitrogen are only two groups apart, we are justifiably reluctant in representing the bonds in the compound boron nitride as B3+N3. Finally it would seem to show very bad chemical intuition to represent the bonds in graphite or diamond as C+4C-4. 

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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