Thermal Conductivity and Heat Capacity of the Monocarbide, Monophosphide, and Monosulfide of Uranium

Abstract

The heat-transport properties and heat capacity of UC, UP, and US were determined between room temperature and 600°C. These properties were measured by a transient technique with a laser as a heat pulse source. The thermal conductivity was calculated from the product of the thermal diffusivity, heat capacity, and density. At room temperature the thermal conductivity of the three compounds corrected to theoretical density was found to be 0.057, 0.033, and 0.021 cal sec−1 cm−1°C−1 for UC, UP, and US, respectively. At 600°C the value for UC decreased to 0.041 cal sec−1 cm−1°C−1. The thermal conductivity of UP remained equal to its room-temperature value and that of US increased to 0.029 cal sec−1 cm−1°C−1. The room-temperature heat capacity values for the three compounds investigated were all slightly above 12 cal mole−1°K−1. At 600°C the heat capacity of UC increased to 15.2 cal mole−1°K−1, whereas no appreciable changes were found for UP and US at this temperature. The experimental heat-capacity values were used to formulate expressions of the type Cp=a+bT+cT−2 with the aid of a computer program, and enthalpies and entropies of the compounds were calculated from 400° to 1000°K. The electronic contributions to the thermal conductivities of the three compounds were calculated by means of the Wiedemann-Franz law with the Lorenz constant for metallic conductors. The limitations of this approach are discussed and an alternate method of separating the phonon from the electronic part of the thermal conductivity is presented. A value of 0.02 cal sec−1 cm−1°C−1 for the lattice conductivity alone was obtained. This value is close to the 0.01 to 0.02 cal sec−1 cm−1°C−1 obtained by using the Wiedemann-Franz law on UP and US. The thermal conductivity of UC above 300°C can be ascribed to electrons if the traditional Lorenz constant is used. However, comparison of the thermal and electrical conductivity for the three compounds indicates that the Lorenz number when applied to UC should be lower, possibly because of the presence of optical scattering. It was found that the lattice resistance to the flow of heat in this group of compounds is essentially temperature independent and that calculations based on the classic models do not give good agreement with measured values.