Grain boundary structures and O tracer diffusivities (table 6) were studied in transparent ceramics whose compositions varied from a 2% excess of Y2O3 to an 0.5% excess of Al2O3. In the former case, a second phase (yttrium aluminium perovskite) which contained Si was found in the grain boundaries. In the case of an Al2O3 excess, both Al-rich particles (alumina) and a Si-rich segregated layer were found in the grain boundaries. It was found that volume diffusion of the O tracer was little affected by the excesses. On the other hand, the grain boundary diffusion of O (table 7) was suppressed in Y2O3-excess specimens, as compared with Al2O3-excess specimens. These differences were attributed to the effect of chemical reactions between second phases and intergranular liquid during sintering.
I.Sakaguchi, H.Haneda, J.Tanaka, T.Yanagitani: Journal of the American Ceramic Society, 1996, 79[6], 1627-32
Table 6
Arrhenius Parameters for Bulk O Diffusion in Al5Y3O12
Condition |
Addition |
Do (m2/s) |
Q (kJ/mol)
|
non-annealed |
2%Y2O3 |
5.8 x 10-10 |
259.6 |
non-annealed | 1%Y2O3 | 7.7 x 10-11 | 223.9 |
non-annealed | stoichiometric | 4.3 x 10-8 | 304.1 |
non-annealed | 0.5%Al2O3 | 3.9 | 519.4 |
annealed | 2%Y2O3 | 2.1 x 10-7 | 348.2 |
annealed | 1%Y2O3 | 2.2 x 10-9 | 286.1 |
annealed | stoichiometric | 2.6 x 10-10 | 265.8 |
annealed | 0.5%Al2O3 | 2.3 x 10-5 | 408.5
|
Table 7
Arrhenius Parameters for Grain Boundary O Diffusion in Al5Y3O12
Condition |
Addition |
Do (m2/s) |
Q (kJ/mol)
|
non-annealed |
stoichiometric |
2.1 x 10-10 |
393.8 |
non-annealed | 0.5%Al2O3 | 124 | 688.8 |
annealed | stoichiometric | 1.1 x 10-8 | 463.9 |
annealed | 0.5%Al2O3 | 5.6 x 10-9 | 459.2
|