Electrical Conduction in Single‐Crystal and Polycrystalline Al ₂ O ₃ at High Temperatures

The electrical conductivity and ion/electron transference numbers in Al 3 O 3 were determined in a sample configuration designed to eliminate influences of surface and gas‐phase conduction on the bulk behavior. With decreasing O 2 partial pressure over single‐crystal Al 2 O 3 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 O 2 (1 atm) the conductivity above 1400°C was σ≃3×10 3 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 Al 2 O 3 and MgO‐doped Al 2 O 3 showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas‐phase conduction above 1200°C increased drastically with decreasing O 2 partial pressure (below 10 −10 atm).