Dopant effect on grain boundary diffusivity in polycrystalline alumina

The densification behavior during sintering in 0.1 mol% MgO-, MnO-, SrO-, LuO 1.5-, TiO 2-, ZrO 2- or PtO 2-doped Al 2O 3 was investigated at the sintering temperature of 1300–1500 °C in order to systematically examine the dopant effect on grain boundary diffusivity in Al 2O 3. 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 Al 2O 3 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 Al 2O 3; a lower energy level of the dopant element’s outer shells provides a higher value of diffusivity in the divalent or tetravalent cation-doped Al 2O 3. 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 Al 2O 3. The ionic bond strength in the vicinity of the grain boundaries dominates the high-temperature grain boundary diffusion in polycrystalline Al 2O 3.