A form of boron nitride

The main source of boron is a complex compound of boron called borax. About half of the world supply of borax comes from a large deposit in California s Mojave Desert. Borax is used as a cleaning agent and as fireproof insulation. Another compound of boron, boric acid, is used as a disinfectant and as an eye wash. A form of boron nitride is the second hardest known material only diamond is harder. These materials are classified as superabrasives. They are used in grinding wheels, which shape manufactured parts and tools. 

Two forms of boron nitride are known. The ordinary form is a slippery while matenul. The second, formed artificially at high pressures, is the second hardest substance known. Both remain as solids at temperatures approaching 3000 °C. Suggest structures. 

Finally, an important form of boron nitride should be mentioned, pyrolytic boron nitride. It is manufactured by reacting ammonia and a boron halogenide at about 2000°C and depositing the BN vapor on a graphite substrate or mandrel. The characteristic feature of pyrolytic boron nitride is the high degree of crystal orientation with the hexagonal basal plane parallel to the mold surface and the c-direction perpendicular to the substrate. 

The hexagonal B3N3-motif in the layered form of boron nitride appears in a group of compounds called borazines. The parent compound (HBNH)3, 12.27, is isoelectronic and isostructural with benzene. It is prepared by reaction... 

Existence of a nitrogen-rich boron nitride denoted as h BN was suggested by Yoo et ai, who observed appearance of a low-density hexagonal compound after laser heating of boron in excess of nitrogen in a diamond cell [123], This compound was synthesized both in the stability fields of conventional hBN (2GPa, 1300 K) and of cubic BN (15 GPa, 1800 K). h BN is highly transparent and can be recovered as white polycrystals, which do not convert to other forms of boron nitride at ambient conditions [123]. Unfortunately, chemical composition of the compound obtained was not determined quantitatively. 

Heating the layered form of BN at rj2000 K and >50 kbar pressure in the presence of catalytic amounts of Li3N or MgsN2 converts it to a more dense polymorph, cubic-BN, with the zinc blende structure (see Section 6.11). Table 13.2 shows that the B—N bond distance in cubic-BN is similar to those in R3N-BR3 adducts and longer than in the layered form of boron nitride this further supports the existence of 7r-bonding within the layers of the latter. 

The cubic crystalline form of boron nitride has a diamond-like structure, each boron atom is bonded to four nitrogen atoms at a bond distance of 156 pm. The mean NB bond energy calculated from the energy of atomization is 321 kJ mol i.e. more than 2.5 times larger than the NB bond dissociation energy of amine borane. We conclude that the NB bond in amine borane is not only unusually long, but also unexpectedly weak. 

Boron forms a compound with carbon, B4C. This substance, boron carbide, is one of the hardest substances known and it has found extensive use as an abrasive and for the manufacture of small mortars and pestles for grinding very hard substances. The cubic form of boron nitride, BN, with a tetrahedral structure like that of diamond, has about the same hardness. 

Turbine Blades. See gas turbine. Turbostratic. A layered structure in which successive layers are oriented at random with respect to each other. One form of boron nitride has this structure. (US Pat 3,241,919 22/03/1966 E.I du Pont de Nemours Co). 

Reactions of boron ttihalides that are of commercial importance are those of BCl, and to a lesser extent BBr, with gases in chemical vapor deposition (CVD). CVD of boron by reduction, of boron nitride using NH, and of boron carbide using CH on transition metals and alloys are all technically important processes (34—38). The CVD process is normally supported by heating or by plasma formed by an arc or discharge (39,40). 

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