Dielectric Properties of Single Crystals of Al2O3 and Al2O3 Doped with Chromium or Vanadium

Abstract

Dielectric constant (K), loss (tan δ), and conductivity (σ) of single crystals of Al2O3, Al2O3: Cr (0.03 and 2 mol%) and Al2O3:V (2 mol%) have been measured as a function of frequency in the range 102 to 107 Hz and at temperatures between 30 and 450 °C, (where the electric field is always perpendicular to the optic axis). The K value of Al2O3, at 30 °C is 9.4 and frequency-independent. Doping by chromium or vanadium produces no detectable change in the K value of Al2O3. The dielectric loss of all these samples at 30 °C is below detection level (tan δ < 0.0005) throughout this frequency region. In these crystals, the dielectric constant increases with temperature exhibiting a frequency dependence from about 200 °C; the changes in K with temperature being larger at lower frequencies (similar behaviour is exhibited by the dielectric loss). This behaviour of K is attributed to space charge polarisation due to crystal defects. The plots of log σ vs. 1/T at different frequencies for these crystals do not coincide indicating the conduction in this temperature region is due to impurities. The activation energies for conduction — in the high temperature region — are calculated to be 0.77, 0.69, 0.64, and 0.62 eV respectively for the above mentioned solids. The lower activation energy values for conduction in doped crystals may be attributed to a larger charge carrier concentration in them (compared to Al2O3).