Nanoporous α-Alumina Membranes with Pore Diameters Tunable over Wide Range of 30-350 nm
In this study, we investigated the optimum conditions for the fabrication of α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm by the anodizing of aluminum and subsequent heat treatment. The morphological and structural change of the porous alumina membrane during heat treatment was also evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), and thermogravimetry-mass spectrometry (TG-MS). Because the pore diameter could be adjusted by changing the anodizing conditions including the electrolyte species and formation voltage, anodizing of high-purity aluminum sheets was conducted in sulfuric acid, oxalic acid and phosphoric acid at each specific voltage. Here, we applied two-step anodizing process to improve the regularity of the pore arrangement of the anodic film. As a result, the deformation of alumina membrane such as warp and cracking during heat treatment for crystallization was effectively suppressed [1, 2]. By optimizing the conditions for anodizing, subsequent detachment, and heat treatment, nanoporous and single phase α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm were successfully fabricated. Even in the case of anodic porous film with short pore interval of approximately 60 nm, which was formed at 25 V in sulfuric acid, α-alumina membrane that maintained straight channels was obtained. The α-alumina membranes exhibited high chemical resistance in various concentrated acidic and alkaline solutions as well as when exposed to high temperature steam under pressure . These results open a new route to technological and scientific applications.
 T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Electrochemistry, 82, 448-455 (2014)
 T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Materials, 8, 1350-1368 (2015)