Abstract
When guards are used to eliminate surface and gas conduction in emf measurements, thermocouple effects of the electrodes and guards must be taken into account, and unwanted gradients in gas composition must be eliminated. Results for single‐crystal and polycrystalline Al2O3 are given. Single crystals have an ionic transference number, t̄l of ≅1, with a possible small decrease as a result of electronic (hole) conduction at T>1450°C. Polycrystalline samples exhibit ti values of tel(=1‐tl) increasing with decreasing temperature and increasing O2 pressure. The electronic conduction results from movement of holes along grain boundaries.

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