Gas Permeability of Oxide Ceramics at Ultra-High Temperatures

Abstract

To evaluate the capability of candidate materials such as alumina (Al2O3) and yttria stabilized zirconia (YSZ) ceramics for use as environmental barrier coatings (EBCs) on C/C composites, the oxygen and water permeability through each oxide ceramic were measured in dry and/or wet atmospheres at ultra-high temperatures. The oxygen permeability constant of Al2O3 was much smaller than that of YSZ, and was found to be in good agreement with previous values. The surface morphology of Al2O3 after the test displayed significant dependence on the oxygen potential gradients during the test. Deep grain boundary grooves were formed on those surfaces exposed to lower Po2 ; however, on the opposite surface under a higher Po2, the grain boundaries rose, forming ridges. The dependence of the Al2O3 oxygen permeability constant on the Po2 in the testing environment displayed p-type semiconductivity. Therefore, the unique morphological change may be related to the migration of aluminum ions from the lower to the higher Po2 side by the substitution of cation vacancies that segregate at the grain boundaries. The water permeability constant of Al2O3 was under the detection limit (10-10mol/(m&middots) at the temperature of 1923K.