Electrical Conduction in Single-Crystal and Polycrystalline Al2O3 at High Temperatures

Abstract

The electrical conductivity and ion/electron transference numbers in Al3O3 were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O2 partial pressure over single-crystal Al2O3 at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O2 (1 atm) the conductivity above 1400°C was σ≃3×103 exp (–80 kcal/RT) Ω−1 cm−1, which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al2O3 and MgO-doped Al2O3 showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O2 partial pressure (below 10−10 atm).