Boron Nitride (redirected from White carbon)

boron nitride [′bȯ‚rän ′nī‚trīd]

(inorganic chemistry)

BN A binary compound of boron and nitrogen, especially a white, fluffy powder with high chemical and thermal stability and high electrical resistance.

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Boron Nitride 

BN, a compound of boron and nitrogen. It is produced from these elements at a temperature above 2000° C or during the heating of a mixture of B₂O₃) with reducing agents (carbon, magnesium) in an ammonia atmosphere; this reaction produces the ordinary form of BN—a white powder, resembling talc, which is similar to graphite in crystalline structure. At pressures above 6,200 meganewtons per sq m (62,000 kilograms-force per sq cm) and temperatures above 1350° C in the presence of catalysts (alkali and alkali-earth metals) the graphite-like hexagonal α-form is converted into the cubical diamond-like β-form (borazone) which is markedly different in its properties. The hardness of borazone (10 on the mineralogical scale), approximates the hardness of diamond. At the same time, borazone is much more stable at high temperatures.

Ordinary boron nitride (the graphite-like form) is chemically inert at room temperature and reacts only with fluorine (giving BF₃ and N₂) and with HF (forming NH₄BF₄); hot solutions of alkalis decompose it, yielding NH₃. The chemical stability of borazone is significantly higher than that of the α-form. Boron nitride is used in the production of high-refractory materials, semiconductors, dielectrics, and neutron absorbers; the α-form serves as a dry lubricant in bearings; borazones are used in the production of superhard abrasive materials.

REFERENCES

Samsonov, G. V., and K. I. Portnoi. Splavy na osnove tugoplavkikh soedinenii. Moscow, 1961.
Shmartsev, Iu. V., Iu. A. Valov, and A. S. Borshchevskii. Tugoplavkie almazopodobnye ppluprovodniki. Moscow, 1964.
Niedenzu, K., and D. Dawson. Khimiia borazotnykh soedinenii. Moscow, 1968. (Translated from English.)

V. L. VASILEVSKII