Thermal conductivity of the gadolinium calcium silicate apatites: Effect of different point defect types

The gadolinium calcium silicates having the apatite crystal structure can accommodate a wide range of point defects, including oxygen and cation vacancies, as well as anti-site defects, depending on the Gd/Ca ratio. Compositions having only cation or oxygen vacancies were identified and the thermal diffusivity and conductivity were measured up to 1000C. All the compositions, including the stoichiometric composition, exhibit low thermal conductivities from room temperature to high temperature with the defect-containing compositions having even lower thermal conductivities. The high temperature thermal conductivity, at temperatures below the onset of significant radiative heat transport, decreases with the inverse square root of the cation and anion vacancy concentration, consistent with simple defect scattering models. Based on the data, it is concluded that the oxygen vacancies are slightly more effective in reducing thermal conductivity. * Now at School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124.