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 (PO2), forming oxygen potential gradients. Under oxygen potential gradients generated by PO2 below about 1 Pa, oxygen permeation occurred by oxygen diffusing from regions of higher PO2 to regions of lower PO2. By contrast, under oxygen potential gradients generated by PO2 above about 1 Pa, oxygen permeation proceeded by aluminum diffusing from regions of lower PO2 to regions of higher PO2. In other words, O2 molecules were adsorbed onto a surface at higher PO2 and subsequently dissociated into oxygen ions (forming Al2O3), while oxygen ions on the opposite surface at lower PO2 were desorbed by association into O2 molecules (decomposition of Al2O3). The grain-boundary diffusion coefficients of oxygen and aluminum as a function of PO2 were determined from the oxygen permeation constants.