Hexagonal a-Boron Nitride

3 Structural and Physical Properties 4.1.1.3.1 Hexagonal a-Boron Nitride 

Thermodynamic properties of a-BN (and of y-BN) have been determined for the range of 5 to 300 K [1]. The specific heats of the cited modifications have been measured by vacuum adiabatic calorimetry the temperature dependence of Cp for a-BN is shown in Fig. 4-2 [1]. 

The macrostructure of pyrolytic a-BN of density 2.19 g/cm has been studied by electron-positron annihilation. The annihilation photon angular distributions have been measured with and without superposition of a static magnetic field (B = 10 kG) at 85 K. The total concentration of point defects has been found to be about 10 cm [11, 12]. 

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Physical Adsorption on Hexagonal Graphitic Boron Nitride (a-BN)... 

Lithium Nitride. Lithium nitride [26134-62-3], Li N, is prepared from the strongly exothermic direct reaction of lithium and nitrogen. The reaction proceeds to completion even when the temperature is kept below the melting point of lithium metal. The lithium ion is extremely mobile in the hexagonal lattice resulting in one of the highest known soHd ionic conductivities. Lithium nitride in combination with other compounds is used as a catalyst for the conversion of hexagonal boron nitride to the cubic form. The properties of lithium nitride have been extensively reviewed (66). 

Hexagonal boron nitride is relatively stable in oxygen or chlorine up to 700°C, probably because of a protective surface layer of boric oxide. It is attacked by steam at 900°C, and rapidly by hot alkaU or fused alkaU carbonates. It is attacked slowly by many acids as well as alcohols (to form borate esters), acetone, and carbon tetrachloride. It is not wetted by most molten metals or many molten glasses. 

FIGURE 14.27 (a) The structure of hexagonal boron nitride, BN, resembles that of graphite, consisting of flat planes of hexagons of alternating B and N atoms (in place of C atoms but, as shown for two adjacent layers in part (b), the planes are stacked differently, with each B atom directly over an N atom and vice-versa (compare with Fig. 14.29). Note that (to make them distinguishable) the B atoms in the top layer are red and the N atoms blue. 

Boron nitride has two crystalline forms, hexagonal (h-BN) and cubic (c-BN), with much different properties. Hexagonal BN is the more important and has many industrial applications. Its structure is similar to that of graphite which it resembles in many ways. It has a very large anisotropy in the crystal with resulting anisotropic properties. 

Cubic boron nitride is obtained from hexagonal boron nitride at high pressure and temperature in the presence of lithium nitride as a catalyst. It is almost as hard as diamond and has superior chemical resistance and a much higher oxidation threshold.Efforts to... 

There is great interest in developing molecular precursors for boron-nitrogen polymers and boron nitride solid state materials, and one general procedure is described in this report. Combinations of B-trichloroborazene and hexamethyldisilazane lead to formation of a gel which, upon thermolysis, gives hexagonal boron nitride. The BN has been characterized by infrared spectroscopy, x-ray powder diffraction and transmission electron microscopy. 

Boron nitride may be obtained in three primary crystalline modifications (2) a, j3, and y. The most commonly encountered a form has a graphitic structure (hexagonal cell, a = 2.504 A, c = 6.661 A). For many years, this modification has been prepared from combinations of cheap boron and nitrogen containing reagents, e.g. B(0H)3 and (NH2)C0, B(0H)3, C and N2 or KBH4 and NH4C1 (3-5). More... 

The c-BN phase was first obtained in 1957 [525] by exposing hexagonal boron nitride phase (h-BN) to high pressures and low temperatures. A pressure of more than 11 GPa is necessary to induce the hexagonal to cubic transformation, and these experimental conditions prevent any practical application for industrial purposes. Subsequently, it has been found that the transition pressure can be reduced to approximately 5 GPa at very high temperature (1300-1800°C) by using catalysts such as alkali metals, alkali metal nitrides, and Fe-Al or Ag-Cd alloys [526-528]. In addition, water, urea, and boric acid have been successfully used for synthesis of cubic boron nitride from hexagonal phase at 5-6 GPa and temperature above 800-1000°C [529]. It has been... 

Cubic BC2N. Hetero-diamond B C—N compounds have recently received a great interest because of their possible applications as mechanical and optical devices. The similar properties and structures of carbon and boron nitrides (graphite and hexagonal BN, diamond, and cubic BN) suggested the possible synthesis of dense compounds with all the three elements. Such new materials are expected to combine the best properties of diamond (hardness) and of c-BN (thermal stability and chemical inertness). Several low-density hexagonal phases of B,C, and N have been synthesized [534] while with respect to the high-density phases, different authors report contradictory data [535-538], but the final products are probably solid mixtures of c-BN and dispersed diamonds [539]. 

Its structure resembles that of graphite, but the latter s flat planes of carbon hexagons are replaced in boron nitride by planes of hexagons of alternating B and N atoms (Fig. 14.31). Unlike graphite, it is white and does not conduct electricity. Under high pressure, boron nitride is converted to a very hard, diamondlike crystalline form called Borazon. In recent years, boron nitride nanotubes similar to those formed by carbon have been synthesized (Section 14.18), and they have been found to be semiconducting (see Box 14.2). 

PI5.9 Boron nitride (BN) is isoelectronic with carbon and the B, C, and N atoms are about the same size. The result is that BN forms crystal structures similar to those of carbon, in that it crystallizes in a hexagonal (graphite-like hBN) and a cubic (diamond like cBN) structure. The data summarized at the end of the problem are available for the two forms of BN.17... 

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