Results from impedance spectroscopy measurements between 400 and 1000C, for single crystal and highly pure and dense polycrystalline α-A12O3 samples with well-defined grain size, were compared with that from molecular dynamics calculation. Between 650 and 1000C, the measured activation energy for conductivity was 1.5eV for the single crystal, and increased from 1.6 to 2.4eV as the grain size decreased from 15 to 0.5μm. The molecular dynamics calculation led to the conclusion that the self-diffusion activation energy was about 1.5eV for O and 1.0eV for Al in single crystal α-Al2O3. The much higher mobility of O ions made the O ions responsible for the conductivity of the single crystal oxide. It seemed that the grain boundary led to an increase in the activation energy. However, the quantitative influence of grain boundary still needs to be explained. Between 400 and 650C, the measured activation energy was about 1.0eV and independent of the grain size.

Self-Diffusion Activation Energies in α-Al2O3 below 1000°C - Measurements and Molecular Dynamics Calculation. Pan, J., Öijerholm, J., Belonoshko, A.B., Rosengren, A., Leygraf, C.: Philosophical Magazine Letters, 2004, 84[12], 781-9