Matrix-mediated, chemical vapor transport synthesis of alpha alumina platelet-shaped particles

Platelet-shaped {dollar}alpha{dollar}-{dollar}rm Alsb2Osb3{dollar} particles with diameters between 1 and 15 {dollar}mu{dollar}m and thicknesses between 0.5 and 1.2 {dollar}mu{dollar}m were synthesized from gamma alumina precursor powders in the presence of HF. The HF reacted to form aluminum fluoride vapor species which promoted chemical vapor transport (CVT) in the gamma alumina powder matrix. The HF mineralizer was chemisorbed to the {dollar}gamma{dollar}-{dollar}rm Alsb2Osb3{dollar} powder and the water for hydrolysis of AlF{dollar}sb3{dollar} evolved from the gamma alumina powder.; The reactant concentrations most strongly affected nucleation. At HF concentrations {dollar}geq{dollar}1.6 wt% and water concentrations {dollar}>{dollar}1.0 wt%, nucleation decreased the platelet diameters by a factor of four over lower reactant concentrations. Conversely, lower reactant concentrations yielded the lowest nucleation rate and the largest diameter platelets. For low reactant concentrations, seed particles promoted growth of {dollar}alpha{dollar}-{dollar}rm Alsb2Osb3{dollar} platelet shaped particles where the platelet diameter decreased as the seed concentration increased above 10{dollar}sp{lcub}10{rcub}{dollar} {dollar}alpha{dollar}-{dollar}rm Alsb2Osb3{dollar} particles/cm{dollar}sp3{dollar} {dollar}gamma{dollar}-{dollar}rm Alsb2Osb3{dollar}.; Both nucleation and growth kinetics were studied. Nucleation was rate-limiting below {dollar}sim{dollar} 1198 K with activation energies as large as 740 kJ/mol. Above 1198 K, processes leading to growth were rate-limiting. The growth kinetics also suggested at least two different rate-limiting processes.; Platelet-shaped particles grow in the presence of HF and the aspect ratio increases as the HF concentration increases. The proposed platelet growth mechanism is two-dimensional nucleation and growth, where preferential adsorption of HF on the basal surface kinetically determines the morphology. Both the radial and thickness growth rates decreased {dollar}sim{dollar} 6 orders of magnitude when the HF concentration was increased from 1.0 to 3.0 wt%.; Thermodynamic calculations predicted that the principal transporting species was AlF{dollar}sb2{dollar}O. The AlF{dollar}sb2{dollar}O partial pressure was {dollar}>{dollar}3 orders of magnitude larger than any other aluminum fluoride species. Furthermore, transport is driven by the aluminum fluoride activity difference between the gamma and alpha alumina phases, and AlF{dollar}sb2{dollar}O has the largest activity difference.