Critical factors in the nucleation and growth of alpha alumina from beohmite gels

The transformation of {dollar}rmtheta{dollar}-{dollar}rm Alsb2Osb3{dollar} to {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar} in unseeded and {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar}-seeded boehmite-derived alumina powders was studied in dry air and water vapor. Seeding accelerated the transformation to {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar} by providing low energy nucleation sites for the transformation. Water vapor accelerated the transformation to {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar} by increasing the concentration of aluminum vacancies in the surfaces and boundaries of the {dollar}rmtheta{dollar}-{dollar}rm Alsb2Osb3{dollar} and {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar} crystals. The combination of seeding and water vapor allowed complete transformation to {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar} in 1 hour at 925{dollar}spcirc{dollar}C, approximately 225{dollar}spcirc{dollar}C lower than an unseeded powder heated in dry air.; The transformation to {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar} was also studied in boehmite-derived alumina thin films. In contrast to the behavior of the powders, the transformation to {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar} was inhibited in the water vapor. The morphology also changed from the characteristic 'vermicular' morphology in dry air to a hexagonal platelet morphology in the water vapor. The adsorption of a high concentration of hydroxyl groups preferentially poisons growth of the basal plane of the {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar}, leading to the hexagonal morphology. The transformation to {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar} was inhibited due to the loss of aluminum vapor species from the films heated in water vapor.; The effect of water vapor on the microstructural evolution in boehmite-derived alumina pellets was also investigated. The water vapor accelerates a non-densifying transport process, resulting in inhibited densification of the pellets. The pellets reached maximum densities of {dollar}sim{dollar}60% when heated in the water vapor. The hexagonal morphology observed in the films was also present in both the unseeded and seeded pellets. The platelets in the unseeded pellets were larger than in the seeded pellets due to the greater distance between nucleation events and correspondingly larger volume available for growth.; The nucleation mechanism of {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar} from {dollar}rmtheta{dollar}-{dollar}rm Alsb2Osb3{dollar} was studied in {dollar}rmtheta{dollar}-{dollar}rm Alsb2Osb3{dollar} produced from both pseudoboehmite and well-crystallized boehmite. The nucleation of {dollar}rmalpha{dollar}-{dollar}rm Alsb2Osb3{dollar} occurs by diffusional nucleation. Nucleation does not occur by the shear of {dollar}rmtheta{dollar}-{dollar}rm Alsb2Osb3{dollar} crystals that have reached a 'critical crystal size'.