A change in the chemical bonding strength and high-temperature creep resistance in Al₂O₃ with lanthanoid oxide doping

Abstract

High-temperature creep resistance in polycrystalline Al₂O₃ is highly improved by doping with 0.05 mol∼ lanthanoid (Ln) oxide (Ln = Sm, Eu, Tm or Lu) at 1250°C. The improvement in creep resistance probably occurs as a result of retardation of the grain-boundary diffusion in Al₂O₃ due to grain-boundary segregation of dopant cations. The change in chemical bonding state in grain boundaries with the segregation of Ln cations is examined by a first-principles molecular orbital calculation using the discrete variational-Xα method based on [Al₅O₂₁]²⁷− model cluster. The result of the calculation indicates that the ionic bonding between Al and O ions, and the covalent bonding between Al and the surrounding cation, are strengthened by the presence of Ln ions. A correlation is found between the creep resistance and product of net charges of the constituent ions. The improved creep resistance must be explained in terms of a change in chemical bonding strength around the Al ion.