The Y-doping effect upon grain-boundary diffusion was directly estimated by using bicrystal experiments. For this purpose, pristine and Y-doped α-Al2O3 bicrystals having the same geometrical configuration were fabricated. The grain-boundary O diffusion coefficients were measured via the isotopic tracer profiling technique using secondary ion mass spectrometry. The grain-boundary diffusion coefficients of the pristine and yttrium-doped samples (figure 20) were best described by:

δDgb = 8.4 x 10-6exp[-627(kJ/mol)/RT]

and

δDgb = 6.5 x 10-4exp[-729(kJ/mol)/RT]

respectively. It was thus found that Y-doping retarded grain-boundary diffusivity by approximately 10 times as compared to pristine crystals, while their activation energies were not greatly different. On the other hand, the simultaneously obtained volume diffusion coefficients had similar values to previously reported results, indicating that extrinsic diffusion occurred in the grain interior. Taking these facts into account, the Y-effect could be explained by a site-blocking mechanism or a swamp-out mechanism, or by both of these.

Yttrium Doping Effect on Oxygen Grain Boundary Diffusion in α-Al2O3. T.Nakagawa, I.Sakaguchi, N.Shibata, K.Matsunaga, T.Mizoguchi, T.Yamamoto, H.Haneda, Y.Ikuhara: Acta Materialia, 2007, 55[19], 6627-33

Figure 20

Grain-Boundary Diffusivity of O in Al2O3

(pure material)