LIQUID-PHASE-ASSISTED TRANSFORMATION OF SEEDED GAMMA-ALUMINA

alpha -Al//2O//3-seeded, boehmite-derived gamma -Al//2O//3 was transformed in the presence of Y//2O//5, resulting in a 205 degree C decrease in the alpha -Al//2O//3 transformation temperature and a 74% reduction in the apparent activation energy for the gamma to alpha -Al//2O//3 transformation at temperatures greater than 850 degree C. These changes are attributed to the lowered energy barrier for nucleation by seeding and the lowered activation energy for material transport through the liquid relative to the unseeded, solid-state transformation. Growth of the transforming alumina yielded fine-grained alpha -Al//2O//3 particles which exhibited a highly faceted morphology. It is proposed that the combined control of both nucleation and growth during liquid-phase-assisted transformation provides a potentially powerful technique for tailoring powder characteristics in many material systems which undergo nucleation and growth processes.