The Synthesis of Boron Carbide in an RF Plasma

Abstract

The increasing demand for finely powdered boron carbide together with the rapidly rising cost of this material has stimulated research into alternative methods for its production. When boron trichloride is reacted with methane‐hydrogen mixtures in a radio frequency argon plasma, boron carbides of variable B/C ratios are obtained as submicron powders, the product stoichiometry depending on the reactant composition. This reaction has been studied over a range of reactant compositions and flow rates to obtain information about the reaction mechanism involved and the growth of particles from the gas phase. The amount of reaction taking place and the product stoichiometry have been determined as functions of the reactant stoichiometry and flow rate using a factorial experimental design. The products obtained were studied by x‐ray diffraction analysis to determine any changes in crystal structure with composition. It was found that the B/C molar ratio of the products varied linearly with the B/C molar ratio of the reactants, was unaffected by the molar ratio (for ) and decreased as the reactant flow rate increased. X‐ray diffraction analysis of the products revealed that the lattice parameters of the crystalline material decreased linearly as the carbon content increased up to the composition . Samples containing excess carbon over the composition showed the presence of free graphite, while most of the samples with a carbon content of less than 13.6% (corresponding to ) also showed the presence of β‐rhombohedral boron, the product obtained in the absence of methane. Line broadening measurements suggest that the crystallite sizes of both the boron carbide and β‐rhombohedral boron lie in the 200–300Å range.