Stoichiometric limits of carbon-rich boron carbide phases

The stoichiometric boron-to-carbon ratios of various carbon-rich boron carbide phases were investigated. Many powders or sintered pellets were analysed and the total boron, total carbon, free-boron and free-carbon concentrations were measured for each specimen before calculating the stoichiometric ratio. The methods described were applied to a large number of analyses and the standard deviation of the results was evaluated. The stoichiometric boron-to-carbon ratio for carbon-rich boron carbide phases was found to depend on the synthesis conditions. Boron dissolves up to 21.6 at.% C at low temperatures and up to 24.3 at.% C near the melting point. Free carbon is deposited as the result of a solid state phase transformation. The stoichiometry of boron carbide prepared by the magnesothermal reaction is B3.63C (approximately B11C3). Boron carbide produced by arc melting boron oxide and carbon is a solid solution. This single carbide melts congruently at a temperature of approximately 2450°C with a carbon content of 18.4 at.% (B4.45C).