PRODUCTION OF ULTRAFINE ALPHA-ALUMINA POWDERS AND FABRICATION OF FINE-GRAINED STRONG CERAMICS

Hydrous alumina powders, pure, seeded with alpha alumina, containing ammonium nitrate and containing both ammonium nitrate and seeds, were prepared by hydroxide precipitation. Their crystallization and sintering behaviour were investigated and mechanical properties of the ceramics were tested. Pure hydrous alumina transformed to alpha alumina crystals, with a size of ca. 200 nm, at 1200 degrees C, after undergoing the usual metastable phase changes during heat-treatment. The powder needed to be sintered at 1600 degrees C to achieve a high density. The ceramic had an average grain size of ca. 9 mu m. Seeding lowered the transformation temperature to ca. 1120 degrees C and caused the transformation to begin at ca. 600 degrees C. The material could be sintered at 1500 degrees C and had a grain size of 2 mu m. The nitrate, predominantly present as ammonium nitrate, lowered the transformation temperature to ca. 1150 degrees C and altered the proportion of the intermediate phases. However, the materials still had to be sintered at 1500 degrees C to achieve > 97% density. When both seed particles and nitrate ions were present the material almost completely transformed at 950 degrees C to uniform crystals of alpha alumina with a size < 60 nm that sintered to > 99% theoretical density at 1450 degrees C. The final ceramic had a uniformly grained ( < 1.0 mu m) microstructure and exhibited strength up to 800 MPa.