Grain boundary electronic structure and high-temperature plastic flow in polycrystalline Al2O3

High temperature creep deformation in fine-grained, polycrystalline Al2O3 is suppressed by the doping of 0.1 mol% LuO1.5 or ZrO2, While that is accelerated by MgO or TiO2-doping at 1250degreesC. The difference in the creep resistance is considered to be originated from change in the grain boundary diffisivity in Al2O3 due to the grain boundary segregation of the dopant cation. Change in the chemical bonding state in the cations-doped Al2O3 is examined by a first-principle molecular orbital calculations using DV-Xalpha method based on [Al5O21](27-) cluster model. The dopant effect on the high-temperature creep resistance in polycrystalline Al2O3 is in good agreement with the change in the ionic bonding strength between Al and O. The change in the chemical bonding strength can be explained in terms of effects of both dopant cation and accompanying vacancy, which changes constitutions of molecular orbitals and the chemical bonding strength in Al2O3.