Mass-Transfer Mechanism of Alumina Ceramics under Oxygen Potential Gradients at High Temperatures

Abstract

The oxygen permeability of an undoped polycrystalline α-Al₂O₃ wafer that was exposed to oxygen potential gradients was evaluated at temperatures up to 1973 K. Oxygen preferentially permeated through the grain boundaries of α-Al₂O₃. The main diffusion species, which were attributed to oxygen permeation, depended on oxygen partial pressures (P_O2), forming oxygen potential gradients. Under oxygen potential gradients generated by P_O2 below about 1 Pa, oxygen permeation occurred by oxygen diffusing from regions of higher P_O2 to regions of lower P_O2. By contrast, under oxygen potential gradients generated by P_O2 above about 1 Pa, oxygen permeation proceeded by aluminum diffusing from regions of lower P_O2 to regions of higher P_O2. In other words, O₂ molecules were adsorbed onto a surface at higher P_O2 and subsequently dissociated into oxygen ions (forming Al₂O₃), while oxygen ions on the opposite surface at lower P_O2 were desorbed by association into O₂ molecules (decomposition of Al₂O₃). The grain-boundary diffusion coefficients of oxygen and aluminum as a function of P_O2 were determined from the oxygen permeation constants.