GROWTH OF ALPHA-AL2O3 PLATELETS IN THE HF-GAMMA-AL2O3 SYSTEM

In the presence of a fluorine mineralizer, highly aggregated, <5 mu m alpha-Al2O3 platelet particles form by vapor transport during the ther mal transformation of gamma-alumina. Platelet aggregation was determin ed to occur by platelet intergrowth and by edge nucleation on primary alpha-Al2O3 platelets. The addition of greater than or equal to 10(10) alpha-alumina seed particles/cm(3) gamma-Al2O3 resulted in the develo pment of discrete particles during the initial stage of transformation . Impingement of the growing platelets during the latter stage of tran sformation, however, resulted in intergrowth, a process which was not changed by seeding. Particle size distribution broadening was observed to increase with increasing HF and H2O concentrations because vapor r eactant supersaturation increases the degree of edge nucleation. When initially low HF and H2O concentrations were used in seeded systems, h owever, essentially aggregate-free alpha-Al2O3 platelets of 10-15 mu m were obtained.