Direct synthesis of fine boron carbide powders using expanded graphite

Abstract

Abstract The paper presents a simple approach to obtain boron carbide powder by direct synthesis from elements. For the first time expanded graphite (EG) powder, which is a reactive form of carbon with a unique structure and microstructure, was used as a carbon source for the synthesis of this carbide. As a boron source, fine-grained amorphous boron powder was used. The synthesis was carried out in excess of boron, i.e.; the B to C ratio was 10:1 in mass. The simplicity of the synthesis was based on the fact, that the substrates did not require homogenization because a layer of boron powder was placed over a layer of expanded graphite in a closed graphite crucible. The reaction chamber was heated at temperatures from 1550 to 1650\u202f°C for 2\u202fh in argon. As a result, a fine-grained boron carbide powder (crystallite size of 20–40\u202fnm) which did not require further chemical and mechanical treatment (aggregate size about 1\u202fμm) was prepared. The plausible mechanism of the boron carbide synthesis was the transport of carbon towards boron via carbon monoxide (vapour phase) and its subsequent reaction leading to carbide formation.