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

摘要

The oxygen permeability of an undoped polycrystalline alpha-Al_2O_3 wafer that was exposed to oxygen potential gradients was evaluated at temperatures up to 1973 K. Oxygen preferentially permeated through the grain boundaries of alpha-Al_2O_3. The main diffusion species, which were attributed to oxygen permeation, depended on oxygen partial pressures (P_(O_2)), forming oxygen potential gradients. Under oxygen potential gradients generated by P_(O_2) below about 1 Pa, oxygen permeation occurred by oxygen diffusing from regions of higher P_(O_2) to regions of lower P_(O_2). By contrast, under oxygen potential gradients generated by PQ, above about 1 Pa, oxygen permeation proceeded by aluminum diffusing from regions of lower PQ, to regions of higher P_(O_2). In other words, O_2 molecules were adsorbed onto a surface at higher Al_2O_3, and subsequently dissociated into oxygen ions (forming Al_2O_3), while oxygen ions on the opposite surface at lower P_(O_2) were desorbed by association into O_2 molecules (decomposition of Al_2O_3). The grain-boundary diffusion coefficients of oxygen and aluminum as a function of Al_2O_3 were determined from the oxygen permeation constants.