Hexagonal BN

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. 

The Ba.C) N, nanotubes and fullerene-like structures have been synthesized by various laboratories in recent years. The most popular method is the plasma arc technique. The first report on the synthesis of BN nanotubes was by Zettl and coworkers (55). Since BN is an insulator, a composite anode was prepared from a tungsten rod with an empty bore in the center, which was stuffed with a pressed hexagonal BN powder. For the cathode, a water-cooled Cu rod was used. The collected gray soot contained limited amount of multi wall BN nanotubes. It is possible that in this case, the tungsten serves also as a catalyst. By perfecting this method, macroscopic amounts of double-wall BN nanotubes of a uniform diameter (2 nm)... 

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]. 

Figure 4 B]2N12H12 model for solid hexagonal BN and molecular (BH2)3N at optimized geometries, with calculated N shieldings.

This combination of excellent properties of hexagonal BN (h-BN) opens a huge range of technical applications. 

Cubic BN is usually manufactured at about 5 GPa and 1500°C from a mixture of graphitic hexagonal BN and a catalyst solvent such as lithium or magnesium nitride. Many other catalyst solvent systems have been found and most of them involve a nitride-forming element. As pressure and temperature increase, the catalyst requirements relax as with carbon. 

Boron nitride (BN) can normally be prepared from the reaction of boric acid and urea or melamine. For example, the pyrolysis of MB can yield hexagonal BN. It is commonly referred to as white graphite because of its platy hexagonal structure similar to graphite. Under high pressure and at 1600°C, the hexagonal BN is converted to cubic BN, which has a diamond-like structure. 

Hexagonal BN is stable in inert or reducing atmosphere to about 2700°C and in oxidizing atmosphere to 850°C. It is an excellent thermal conductor and has been frequently quoted as a functional filler for fire-retardant encapsulants for E/E applications. 

Figure 7.27. Contact angle versus time for Al on sintered hexagonal BN in an He-3% H2 gas. Data from work reported in (Fujii et al. 1993) [24],...

Silicon has an affinity both for B (at 1500°C Si dissolves up to 17 at.% B (Massalski 1990)) and N (although Si3N4 is less stable than BN). Naidich (1981) achieved stationary contact angles of 95° and 110° for Si on cubic and hexagonal BN at 1500°C in a high vacuum in a few minutes. These values are much higher than those observed for Al, but significantly lower than the 140° or so observed for non-reactive metals. A detailed interpretation of these values is not possible in the absence of information on interfacial reactions which could occur in the Si/BN system. 

At high pressure and temperature (e.g., 2000°C and 60 kbar), hexagonal BN converts to cubic boron nitride, a material that has a tetrahedral structure analogous to diamond and is almost as hard it is employed as a high-temperature abrasive in situations where diamond forms carbides and hence cannot be used. 

The arrangement of carbon atoms in one plane of the graphite structure. Boron nitride can also form this structure (hexagonal BN) with atoms of B and N alternating as indicated by shaded and open circles. 

Rederive an expression for the covalent energy (sec Eq. 4-18) for the graphite structure, and evaluate the corresponding polarity for hexagonal BN, assuming that it has the graphite bond length (see Problem 3-1). 

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