Control of Oxygen Permeability in Alumina under Oxygen Potential Gradients at High Temperature by Dopant Configurations

The oxygen permeability of polycrystalline α‐alumina wafers, which served as models for alumina scales on alumina‐forming alloys, under steep oxygen potential gradients (ΔPO2) was evaluated at 1873 K. Oxygen permeation occurred by the grain‐boundary (GB) diffusion of oxygen from the higher‐oxygen‐partial‐pressure (PO2) surface to the lower‐PO2 surface, along with the simultaneous GB diffusion of aluminum in the opposite direction. The fluxes of oxygen and aluminum at the outflow side of the wafer were significantly larger than at the inflow side. Furthermore, Lu and Hf segregation at the GBs selectively reduced the mobility of oxygen and aluminum, respectively. A wafer with a bilayer structure, in which a Lu‐doped layer was exposed to a lower PO2 and an Hf‐doped layer was exposed to a higher PO2, decreased the oxygen permeability. When the sign of ΔPO2 was reversed, however, the oxygen permeability of the wafer was comparable to that of a nondoped wafer. Co‐doping with both Lu and Hf markedly increased the oxygen permeation, presumably because the Lu‐stabilized HfO2 particles that were segregated at the GBs acted as extremely fast diffusion paths for oxygen through the large number of oxygen vacancies introduced by the solid solution of Lu in the particles.