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

Abstract

Results from impedance spectroscopy measurements at temperatures between 400 and 1000°C, for single crystal and highly pure and dense polycrystalline α-Al₂O₃ samples with well-defined grain size, are compared with that from molecular dynamics calculation. Between 650 and 1000°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 µm. 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 α-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°C, the measured activation energy is about 1.0 eV and independent of the grain size.

Acknowledgements

The financial support from the Swedish Foundation for Strategic Research (SSF) and Kanthal AB is greatly acknowledged. The authors wish to thank Dr Z. Zhao at the department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, for kindly providing the well-defined Al₂O₃ samples and for valuable discussions. Computations were performed using the facilities of the National Supercomputer Centre in Linköping, Sweden. The authors are thankful to Dr W. Smith at the CSE department in Daresbury in UK, for the DL_POLY software package.