Direct synthesis of fine boron carbide powders using expanded graphite

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 °C for 2 h in argon. As a result, a fine-grained boron carbide powder (crystallite size of 20–40 nm) which did not require further chemical and mechanical treatment (aggregate size about 1 μm) 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.