SELF-DIFFUSION IN ALPHA-AL2O3 .4. OXYGEN GRAIN-BOUNDARY SELF-DIFFUSION IN UNDOPED AND YTTRIA-DOPED ALUMINA POLYCRYSTALS

Oxygen self-diffusion coefficients were determined in polycrystals of a-alumina either 'undoped' or doped with 500 wt.ppm (225 molppm) Y2O3 in the temperature range 1460-1720 degrees C, with O-18(2) and seconda ry-ion mass spectrometry. Oxygen grain-boundary diffusion coefficients obey the relations D'O-->Al2O3(cm(2)s(-1)) = 1.6 x 10(16)exp(- 921(kJ mol(-1))/ RT) for 'undoped' alumina and D'(O)--Al2O3:Y2O3(cm(2)s(-1)) = 7.0 x 10(10)exp(- 800(kJ mol(-1))/RT) for 'yttrium-doped' alumina. The comparison between the two materials indicates that yttrium additi on decreases the oxygen grain-boundary diffusion. For both materials, it was observed that the activation energy of grain-boundary diffusion is greater than the activation energy of the bulk diffusion. Similar results were obtained in single crystals for diffusion in the bulk and in subboundaries. These results were interpreted as being due to impu rity segregation along boundaries and led to a model.