Effect of Hf 4+ Concentration on Oxygen Grain-Boundary Diffusion in Alumina

The kinetics of oxygen grain‐boundary diffusion in alumina was studied as a function of HfO2 concentration. The oxidation of Ni marker particles to NiAl2O4 spinel was utilized to delineate the position of the oxidation front. The HfO2 doping levels spanned the solubility limit, ranging from 100 to 2000 ppm (Hf:Al concentration). For each dopant level, the parabolic rate constant (k) was determined for oxidation anneals carried out at 1400°C. Relative to undoped alumina, HfO2 doping slowed the oxygen transport kinetics by a factor of approximately 3–8, depending on the dopant concentration. At 1400°C, the solubility limit of HfO2 was found to be between 100 and 200 ppm. The results showed that the level of benefit saturated at the dopant level corresponding to the solubility limit (sollim), where 100 ppm