HIGH-TEMPERATURE HEAT-CAPACITIES AND ELECTRICAL CONDUCTIVITIES OF BORON CARBIDES

The heat capacities and the electrical conductivities of B(x)C (x = 3, 4, 5) were measured by means of direct heating pulse calorimetry in the temperature range from 300 to 1500 K. The heat capacities of B(x)C increased with increasing x value. This increase in the heat capacity is probably related to the change of the lattice vibration mode originated from the reduction of the stiffness of the intericosahedral chain accompanied with a change from C-B-C to C-B-B chains. A linear relationship between the logarithm of sigma-T (sigma-T is the electrical conductivity and T is the absolute temperature) of B(x)C and the reciprocal temperature was observed, indicating the presence of small polaron hopping as the predominant conduction mechanism. The electrical conductivity of B(x)C also increased with increasing x value (from 4 to 5) due to an increase of the polaron hopping of holes between carbon atoms at geometrically nonequivalent sites, since these nonequivalent sites of carbon atoms were considered to increase in either B11C icosahedra or in icosahedral chains with increasing x. The electrical conductivity of B3C was higher than that of B4C, which is probably due to the precipitation of high-conducting carbon. The thermal conductivity and the thermodynamic quantities of B4C were also determined precisely from the heat capacity value.