process for the production of a form of bornitrid with hoeherer density than hexagonal bornitrid

摘要

PICT:1048973/C1/1 Boron nitride having a hardness equal to that of diamond is produced in the absence of a catalyst by preparing a specimen of hexagonal boron nitride having a graphite structure, subjecting the specimen for a period of at least one minute to simultaneous pressure and temperature conditions which lie in or above the region "Th" of the graph of Fig. 7, the pressure being applied statically and having a value of at least 100 kilobars, the temperature having a maximum value of 3327 DEG C.; the specimen then being returned to ambient atmospheric conditions and boron nitride having the specified hardness recovered. The product may be cubic boron nitride or alternatively it may have a wurtzite structure with a density of 3.43 gm./c.c., an optical index of refraction for red light of 2.22, and a hardness of 10 on Mohs scale, the lattice constants being a0 = 2.55 A, and c0 = 4.70 A at 25 DEG C. The temperature and pressure conditions of the process may lie in the region above the line M in Fig. 7, when the product is essentially cubic boron nitride. A catalyst is defined in the Specification as meaning at least one of the following: an alkali metal, an alkaline earth metal, Sn, Pb, Sb, and nitrides thereof. The initial boron nitride may be doped with Si, Ge, Se or Be, when semi-conducting crystals are produced.