Growth thermodynamics and kinetics of nano-sized alpha-Al2O3 powders

Growth thermodynamics and kinetics of nano-sized Al2O3 Powders occur during theta- to alpha-phase transformation are examined using DTA/TG; XRD, and TEM techniques. The phase transformation experiences critical and primary crystallite size phenomena. Once the theta-crystallites coarsen to the size of similar to22 nm (critical size of phase transformation for theta-crystallites, d(ctheta)), the transformation takes place, forming alpha-Al2O3 nuclide of critical size similar to17 nm (d(ctheta)). The alpha-nuclide then grow to primary size (d(p)), similar to 45 nm by self-coalescence. Thus the primary crystallite size 45 nm can be the smallest stable size for alpha-Al2O3 powders. The duration required for the transformation from one theta-crystallite to one theta-crystallite is 1.5 min, if the heating rate employed is 10degreesC/min. Kinetic calculations of the activation energies for growth of theta-crystallite, formation of alpha-nucleus, and the coalescence growth to primary size of the alpha nuclei can be >80, similar to85, and <50 kJ/mol, respectively.