Self-diffusion was studied, at temperatures ranging from 1460 to 1720C, in polycrystalline samples of the -phase which were undoped, or doped with 225ppm[mol] of yttria. This was done by using secondary-ion mass spectrometry and 18O2. It was found that the O grain-boundary diffusivity (table 7) obeyed the relationship:
D(cm2/s) = 1.6 x 1016exp[-921(kJ/mol)/RT]
in the case of undoped samples, and the relationship:
D(cm2/s) = 7.0 x 1010exp[-800(kJ/mol)/RT]
in the cased of yttria-doped samples (table 8). A comparison of the results suggested that yttrium additions decreased the O grain-boundary diffusivity. In the case of both materials, it was noted that the activation energy for grain-boundary diffusion was greater than the activation energy for bulk diffusion (table 9). Similar results were obtained in single crystals, for both bulk and sub-boundary diffusion. These results were explained in terms of impurity segregation along boundaries.
D.Prot, M.Le Gall, B.Lesage, A.M.Huntz, C.Monty: Philosophical Magazine A, 1996, 73[4], 935-49
Table 7
Diffusion of O in the Grain Boundaries of Undoped Alumina
Temperature (C) | D (cm3/s) | D (cm2/s) |
1499 | 8.9 x 10-19 | 8.9 x 10-12 |
1540 | 9.7 x 10-18 | 9.7 x 10-11 |
1580 | 1.7 x 10-17 | 1.7 x 10-10 |
1629 | 3.1 x 10-17 | 3.1 x 10-10 |
1660 | 1.8 x 10-16 | 1.8 x 10-9 |
1689 | 6.0 x 10-16 | 6.0 x 10-9 |
1718 | 1.5 x 10-15 | 1.5 x 10-8 |
Table 8
Diffusion of O in the Grain Boundaries of Doped Alumina
(225ppm[mol] of Y2O3)
Temperature (C) | D (cm3/s) | D (cm2/s) |
1460 | 5.3 x 10-21 | 5.3 x 10-14 |
1505 | 1.9 x 10-18 | 5.0 x 10-13 |
1550 | 2.1 x 10-20 | 2.1 x 10-13 |
1580 | 7.2 x 10-20 | 7.2 x 10-13 |
1614 | 1.6 x 10-18 | 1.6 x 10-11 |
Table 9
Diffusion of O in the Bulk of Doped Alumina
(225ppm[mol] of Y2O3)
Temperature (C) | D (cm2/s) |
1460 | 1.3 x 10-16 |
1505 | 1.5 x 10-16 |
1550 | 1.8 x 10-16 |
1580 | 1.1 x 10-15 |
1614 | 2.7 x 10-15 |