Dielectric Properties of Alumina at High Temperatures

The dielectric constant and dissipation factor of both single‐crystal and polycrystalline alumina were measured in air over the frequency range lo 2 to 3 × 10 5 cycles per second, in the temperature interval 900° to 1300° C. The three‐electrode guard‐ring method was used. The observed dielectric losses were due predominately to free‐electron conduction mechanisms. The volume conductivity of single‐crystal and polycrystalline alumina was calculated from the high‐frequency data. The temperature dependences of the volume conductivity for two single crystals of different thicknesses were similar. The large thermal activation energy indicated intrinsic conductivity; the energy band gap of alumina was calculated to be approximately 7.3 electron volts. The bulk conductivities of the polycrystalline alumina were considerably higher than the single‐crystal values, presumably because of impurities in the polycrystalline material. The results were compared with values previously reported in the literature.