HIGH-TEMPERATURE BEHAVIOR OF HEXAGONAL BORON-NITRIDE

Three boron nitride processes have been studied-synthesis of turbostra tic powder, sintering of hexagonal ceramics and transformation into di amond-like phases. It is shown that the crystallization of boron nitri de proceeds in three distinctive steps. The first step is the recombin ation of boron and nitrogen forming semi-organic oligomers; this is fo llowed by a double coalescence on the basis of this mechanism, three f undamental sub-polytypes of layered boron nitride (turbostratic, mesog raphitic and graphitic) have been defined. Compared with most other in organic materials, the chemical properties and the character of the in teraction with the environment are strongly dependent on the crystallo graphic direction. The crystal structure of boron nitride is thus a re sult of a competition between the combination of boron nitride monolay ers and their chemical interaction with the environment.