Forms of Boron Nitride

2 Dense Forms of Boron Nitride Cubic p-Boron Nitride 

Thermodynamic characteristics of p-BN have been calculated from the enthalpy of single crystals determined by drop calorimetry from 298 to 930 K [4] data are compiled in Table 4/2. 

The Debye temperature of p-BN has been determined to be 1680 70 K [6]. A method of computation of enthalpy increment of crystalline inorganic compounds at 298.15 K has been used to calculate enthalpies and entropies of over 200 compounds, including boron nitride, from literature data for details, see [5]. 

Thermodynamic properties of superhard materials produced from the dense modifications of boron nitride, especially of p-BN [7 to 9], have also been studied. The data for Cpfor Kiborit correspond closely to those of p-BN single crystals as given in Table 4/2. Also the Debye temperature of Kiborit [8] (1680 70 K) corresponds to the value for p-BN [6] see [9] for data up to 1400 K. 

The microstructure of p-BN during the a-BN p-BN phase transition [13 to 15] and the formation of multilayer polytypes of p-BN (deformation-induced polytypism) [16] have been studied by electron microscopy. Besides the phase transformation, structure compaction, plastic deformation, and structural relaxation stages have been detected. The chemical composition of the surface of p-BN, as studied by X-ray photoelectron spectroscopy (XPS), has a higher content of oxygen than the original a-BN annealing of p-BN at 800°C results in the depletion of N and the enrichment of oxygen and carbon in the surface layer [17]. 

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Wentorf R FI 1957 Cubio form of boron nitride J. Chem. Phys. 26 956... 

The cubic 2inc blende form of boron nitride is usually prepared from the hexagonal or rhombohedral form at high (4—6 GPa (40—60 kbar)) pressures and temperatures (1400—1700°C). The reaction is accelerated by lithium or alkaline-earth nitrides or amides, which are the best catalysts, and form intermediate Hquid compounds with BN, which are molten under synthesis conditions (11,16). Many other substances can aid the transformation. At higher pressures (6—13 GPa) the cubic or wurt2itic forms are obtained without catalysts (17). 

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 various forms of boron nitride are all relatively inert to chemical attack under normal conditions. For example, hexagonal boron nitride does not react with oxygen, chlorine, or steam up to 700 °C. Reaction with steam begins at 900 °C. Rapid reactions are observed with hot alkali or fused alkali carbonate, but reactions are slow with most acids and alcohols, and boron nitride is not wetted by most molten metals and glasses. The cubic forms have similar reactivity but rates are slower because of their dense structures. 

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. 

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... 

F. R. Corrigan and F. P. Bundy, Direct transition among allotropic forms of boron nitride at high pressures and temperatures, J. Chem. Phys. 1975, 63, 3812-3820. 

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. 

Dense Forms of Boron Nitride (P-BN, cubic y-BN, hexagonal)... 

Leonidov VYa, Timofeev IV, Lazarev VB, Bozhko AB (1988) Enthalpy of formation of the wurtzite form of boron nitride. Russ J Inorg Chem 33 906-908... 

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. 

Borazon. Trade-name (General Electric Co., USA) cubic form of boron NITRIDE (q.v.). 

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). 

Corrigan, F.R. and Bundy, F.P. (1975) Direct transitions among the allotropic forms of boron nitride at high pressure and temperature./. Chem. Phys., 6 (9), 3812-3820. 

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