Boron carbide is a hard and durable material that is widely used in industry. It is well known for its hardness, low density and neutron absorption cross-section, among others. Its synthesis can be carried out by combustion or carbothermal methods. The carbothermal method usually involves the use of boric acid and carbon as raw materials, and a high-temperature reduction reaction in an electric arc furnace. The combustion synthesis method can be improved by pre-pressing the boron-carbon mixture, to remove air and gases, thus improving the consistency of the product.
Carbothermal reduction is a simple and economical process to synthesize boron carbide, as it is based on the conversion of solid boron oxide into gaseous boron suboxides, followed by nucleation growth of solid boron carbon. The fractional carbon conversion kinetics can be described by a non-linear expression, where ko is the nucleation rate constant and ea is the activation energy of the reactants. The kinetics of boron carbide synthesis by carbothermal reduction is strongly dependent on the ratio of the carbon to boron content in the precursor, as well as on the temperature and the heating rate of the carbon-boron oxide mixture.
The boron carbide powders obtained from rapid carbothermal reduction (RCR) were densified using spark plasma sintering at different temperatures and dwell times to compare sintering behavior with commercial samples. The RCR powders showed smaller particle size, lower free carbon and higher density of twin boundaries compared to commercial samples. The X-ray diffraction results demonstrated that the resulting boron carbide powders had good stoichiometry and crystalline structure.