Fabrication and Characterization of Single Phase α-Alumina Membranes with Tunable Pore Diameters

Nanoporous and single phase alpha-alumina membranes with pore diameters tunable over a wide range of approximately 60-350 nm were successfully fabricated by optimizing the conditions for anodizing, subsequent detachment, and heat treatment. The pore diameter increased and the cell diameter shrunk upon crystallization to alpha-alumina by approximately 20% and 3%, respectively, in accordance with the 23% volume shrinkage resulting from the change in density associated with the transformation from the amorphous state to alpha-alumina. Nevertheless, flat alpha-alumina membranes, each with a diameter of 25 mm and a thickness of 50 mu m, were obtained without thermal deformation. The alpha-alumina membranes exhibited high chemical resistance in various concentrated acidic and alkaline solutions as well as when exposed to high temperature steam under pressure. The Young's modulus and hardness of the single phase alpha-alumina membranes formed by heat treatment at 1250 degrees C were notably decreased compared to the corresponding amorphous membranes, presumably because of the nodular crystallite structure of the cell walls and the substantial increase in porosity. Furthermore, when used for filtration, the alpha-alumina membrane exhibited a level of flux higher than that of the commercial ceramic membrane.