Transport phenomena in undoped and chromium or yttrium doped-alumina

Abstract

Diffusion coefficients of ⁵¹Cr, ⁵⁹Fe and ⁶³Ni are measured from 1200° up to 1700°, in undoped alpha-alumina single crystals in air. Results indicate that “extrinsic” diffusion is preponderant and that cationic species diffusion occurs more rapidly than oxygen diffusion. By treating the intensity vs. penetration depth curves, it is possible to separate lattice diffusion from diffusion by dislocations.

The influence of the amount of doping by a homovalent cation Cr⁺³ or Y⁺³ (molar concentration 800 and 8000 ppm Cr₂O₃, 1000 ppm Y₂O₃) on the electrical conductivity and diffusion in α-alumina single crystals is also studied in the same temperature range and for oxygen partial pressures higher than 10⁻¹⁵ atm. The electrical conductivity is found to increase at both high and low oxygen partial pressures, these variations being more pronounced in the case of the yttrium-doped sample. Furthermore, one observes that the electrical conductivity decreases when the concentration of chromium increases.

From the electrical conductivity measurements performed during the equilibration of the gas-oxide system we have deduced the variation of the chemical diffusion coefficient for the yttrium-doped α-A1₂O₃. The results obtained, under air, indicate the presence of a second phase at temperatures lower than 1650°.