Formation and control of agglomerates in alumina powder

High-purity alumina powder with submicrometer particle size was colloidally dispersed and classified in either water or ethanol to remove agglomerates from as-received powder, After the slurries were dried, fine alumina particles cohered to form agglomerates, which were ''hard'' when formed in a water slurry, and ''soft'' in an ethanol slurry, Firing of the powder compacts with ''hard'' agglomerates resulted in inhomogeneous microstructures, and homogeneous microstructures were formed with ''soft'' agglomerates, The reasons for their formation are discussed and experimentally confirmed, In the case of water slurry, a small amount of transition alumina reacted with water at low pH, After drying, alumina gels were formed and acted as a strong binder between cr-alumina particles to form ''hard'' agglomerates.