Self-diffusion activation energies in α-Al2O3 below 1000°C -: measurements and molecular dynamics calculation

Results from impedance spectroscopy measurements at temperatures between 400 and 1000 degrees C, for single crystal and highly pure and dense polycrystalline alpha-Al2O3 samples with well-defined grain size, are compared with that from molecular dynamics calculation. Between 650 and 1000 degrees C, the measured activation energy for conductivity is 1.5 eV for the single crystal, and increases from 1.6 to 2.4 eV as the grain size decreases from 15 to 0.5 mu m. The molecular dynamics calculation leads to the conclusion that the self-diffusion activation energy is about 1.5 eV for O and 1.0 eV for Al in single crystal alpha-Al2O3. The much higher mobility of O ions makes the O ions responsible for the conductivity of the single crystal oxide. It seems that the grain boundary leads to an increase in the activation energy. However, the quantitative influence of grain boundary still needs to be explained. Between 400 and 650 degrees C, the measured activation energy is about 1.0 eV and independent of the grain size.