Mass transfer through a single grain boundary in alumina bicrystals under oxygen potential gradients

The mass-transfer behavior through grain boundaries (GBs) in alumina was systematically investigated using four types of alumina bicrystals. The alumina bicrystal wafers were exposed to the constant oxygen potential gradient(ΔPO₂)generated by the combination of two different oxygen partial pressures PO₂ (II)and PO₂ (I) of 105 and 1 Pa, respectively, at 1923 K. Ridges were formed along the GBs on the surface subjected to PO₂ (II), and deep GB ditches were developed on the PO₂ (I) surface mainly during the migration of aluminum thorough GBs from the PO₂ (I) surface to PO₂ (II) surface. The surface morphology changes in the vicinity of the GBs were observed by atomic force microscopy. It was found that the surface morphology changes indicative of the aluminum GB diffusion were strongly dependent on the GB characteristics. The GB diffusion coefficients of aluminum estimated from the volume of the GB ridges showed a clear correlation to the local bonding environments of GB cores estimated from theoretical calculations reported previously.