The effect of lutetium dopant on oxygen permeability of alumina polycrystals under oxygen potential gradients at ultra-high temperatures

The oxygen permeability of undoped and lutetia-doped polycrystalline alumina wafers that were exposed to oxygen potential gradients was evaluated at temperatures up to 1973 K. The main diffusion species during oxygen permeation through the alumina grain boundaries were found to depend on oxygen partial O2 pressures (PO2), which created oxygen potential gradients. Under oxygen potential gradients generated by PO2 values below 10-3 Pa, where oxygen permeation occurred by oxygen diffusion from regions of higher to lower PO2, segregated lutetium at the grain boundaries suppressed only the mobility of oxygen in the wafers, without affecting the oxygen permeation mechanism. By contrast, under oxygen potential gradients generated by PO2 values above 103 Pa, where oxygen permeation proceeded by aluminum diffusion from regions of lower to higher PO2, lutetium had little effect on aluminum diffusion and migrated together with aluminum, resulting in precipitation and growth of Al5Lu3O12 particles on the higher PO2 surface.