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 thermal transformation of gamma-alumina. Platelet aggregation was determined 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 development of discrete particles during the initial stage of transformation. Impingement of the growing platelets during the latter stage of transformation, 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 reactant supersaturation increases the degree of edge nucleation. When initially low HF and H2O concentrations were used in seeded systems, however, essentially aggregate-free alpha-Al2O3 platelets of 10-15 mu m were obtained.