The electrical conductivity of single-crystal alumina

The electrical conductivity of alumina crystals with varying chromium content has been measured in air by a two-terminal d.c. method over the temperature range 20-850°C. When first heated all specimens showed peaks and pulses in the conduction current similar to those reported by Tucker and Gibbs for clear sapphire. After several heating and cooling cycles the variation of conductivity with temperature became reproducible with an activation energy averaging about 0.5 eV in the range 200-450°C and about 1.7 eV in the range 450-850°C. Conductivities at 850°C of 1 × 10 -9 ohm -1 cm -1 for clear sapphire and of 2 × 10 -10 ohm -1 cm -1 for pink ruby are compatible with the values obtained by Harrop and Creamer, but their suggestion that trace impurities of iron control the conductivity is not wholly substantiated.