Structure of Boron Nitride

There is, however an important difference, as the smaller boron atoms lay over and above the larger nitrogen atoms, so that there are sublayers composed of either boron or nitrogen atoms. This introduces polarity and reduces the covalency of the B-N bonds with an associated reduction in electrical conductivity, as opposed to the well-conducting graphite with an extended network of tr-bonds. The reduced electron delocaUzation in h-BN results in a large band gap consequently, h-BN is white, in contrast to the black graphite. 

The thermal conductivity of h-BN is highly anisotropic, showing in single crystals very high values of about 400 Wm K in the basal plane, but low values of about 2 W m K in the direction perpendicular to the stacked 6-ring planes. 

---

FIGURE 13.9 The layered structure of boron nitride. Compare this structure to that of graphite shown in Figure 13.11. 

FIGURE 14.31 The structure of boron nitride, BN, resembles that of graphite, consisting of flat planes of hexagons. In boron nitride, however, the hexagons consist of alternating B and N atoms (in place of C atoms) and are stacked differendy. (Compare with Fig. 14.34.)... 

The crystal structure of boron nitride resembles that of graphite. The boron and nitrogen atoms form plane regular hexagonal nets which are arranged parallel to one another at a distance of 3.33 A. An essential difference between graphite and boron nitride is that in the latter there are no free electrons. Pure boron nitride is white and does not conduct electricity. 

It is stable up to 2000 K and melts under pressure at 2500 K. The crystal structure of aluminium nitride resembles that of boron nitride and diamond, but unlike both of these it is rapidly and exothermically hydrolysed by cold water ... 

C21-0047. Boron nitride (BN) is a planar covalent solid analogous to graphite. Write a portion of the Lewis structure and describe the bonding of boron nitride, which has alternating B and N atoms. 

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

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. 

The decabome cage structure has also been used with some comonomers, specifically diamines, to give relatively high molecular weight polymers. Fibers formed from these chains can be pyrolyzed to give products that consist largely of boron nitride, BN. 

The traditional method for the preparation of boron nitride is by the fusion of urea with boric acid in an atmosphere of ammonia at 750 °C.54 The product from these reactions is hexagonal boron nitride with a layer structure like that of graphite. Unlike graphite, it is colorless and is not an electronic conductor. Conversion of the hexagonal form to a cubic modification requires heating at 1,800 °C at 85,000 atmospheres pressure. 

Ronning C., Banks A. D., McCarson B. L. et al.. Structure and electronic properties of boron nitride thin films containing silicon, J. Appl. Phys., 84 (1998) pp. 5046-5051. 

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. 

---