Composition and structure of boron carbides prepared by CVD

Abstract

Single crystals of boron rich binary B-C compounds, in which interesting compositional and structural effects were “frozen in”, and pure single crystals of the well-known rhombohedral boron carbide of composition B 13C 2 to B 13C 3 were prepared by CVD on to Ta and BN substrates. The tetragonal boron carbides B 51C [ ▪B 48B 2(BC)] and B 25C [ ▪B 48B 2C 2] were formed under normal pressure at substrate temperatures between 1150 and 1200°C, from the starting materials BBr 3, CH 4 and H 2. Under a pressure of 10 -3 Torr the reactants BI 3 and CI 4 yielded a tetragonal phase B 49C 3[ ▪B 48(BC)C 2] at 1050–1100°C and an orthorhombic phase B 8C [ ▪B 192C 24] at 900–950°C. Our preparative and structural studies on the thermodynamically stable rhombohedral B-C phase led to the conclusion that only the boron rich boron carbide B 12+ x C 2, with x very nearly 1.0 [ ▪B 13C 2], and a carbon content of 14.6 wt%, exists in a completely ordered structure and can be prepared with a satisfactory stoichiometry; i.e. the more stable rhombohedral boron carbide has the structure chemical composition B 12CBC and its rhombohedral unit cell contains a linear CBC chain and not a C 3 chain along [111]. The incorporation of carbon into the B 13C 2 lattice occured only up to a composition B 13C 3 (20.4 wt% C). The B 4C compound with a carbon content of 21.7 wt% was never formed.