Yttrium doping effect on oxygen grain boundary diffusion in α-Al₂O₃

The yttrium doping effect on grain boundary diffusion was directly estimated using bicrystal experiments. For this purpose, pristine and yttrium-doped α-Al₂O₃ bicrystals with the same geometrical configuration were fabricated. The grain boundary oxygen diffusion coefficients were measured by the isotopic tracer profiling technique using secondary ion mass spectrometry. The grain boundary diffusion coefficients of the pristine and yttrium-doped boundary were best described as δ D_gb = 8.4 × 10^{- 6} exp fenced(- 627 [kJ/mol] / RT) and δ D_gb = 6.5 × 10^{- 4} exp fenced(- 729 [kJ/mol] / RT), respectively. It was thus found that yttrium doping retards grain boundary diffusivity by approximately 10 times compared to the pristine crystals, while their activation energies were not greatly different. On the other hand, the simultaneously obtained volume diffusion coefficient showed similar values to previously reported results, indicating that extrinsic diffusion occurred in the grain interior. Taking these facts into account, the yttrium effect can be explained by a "site blocking" mechanism or a "swamp out" mechanism, or by both of these.