Dopant effect on grain boundary diffusivity in polycrystalline alumina

The densification behavior during sintering in 0.1 mol% MgO-, MnO-, SrO-, LuO1.5-, TiO2-, ZrO2- or PtO2-doped Al2O3 was investigated at the sintering temperature of 1300-1500 °C in order to systematically examine the dopant effect on grain boundary diffusivity in Al2O3. The densification behavior was monitored from room temperature to the sintering temperature using a laser-scanning system, which allows in situ, non-contact measuring of the specimen's dimensions. The grain boundary diffusivity in Al2O3 is sensitively affected by the dopant cation which segregates at the grain boundaries. The dopant effect on the grain boundary diffusivity is related to the ionicity in Al2O3; a lower energy level of the dopant element's outer shells provides a higher value of diffusivity in the divalent or tetravalent cation-doped Al2O3. A first-principle molecular orbital calculation revealed that the grain boundary diffusivity correlates well with the net charge of the Al and O ions in the cation-doped Al2O3. The ionic bond strength in the vicinity of the grain boundaries dominates the high-temperature grain boundary diffusion in polycrystalline Al2O3.