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An anodic porous alumina prepared in phosphoric acid electrolyte was heat-treated under various conditions and structural transformations were investigated. Heating the anodic porous alumina resulted in crystallization from amorphous phase to transition alumina at ∼900 °C; subsequently, aluminum phosphate was formed in the anion-incorporated layer at ∼1000 °C, and finally, phase transition from transition alumina to α-alumina occurred above 1350 °C. The aluminum phosphate particles grew, and sintering of the alumina and phase transition to α-alumina was promoted with increasing temperature and total quantity of heat. When the temperature and total quantity of heat were further increased, aluminum phosphate passed into the gaseous phase and the gas migrated through straight pores to the membrane surface along with sintering of the α-alumina. By removing the aluminum phosphate particles, the introduction of mesopores inside the pore walls and expansion of the surface area can be easily achieved.

Porous-type anodic aluminum oxide films have attracted considerable attention as a key material in the fabrication of several types of devices owing to their potential technological applications. However, conventional anodic alumina films have low chemical resistance because the anodic oxide films obtained by typical anodizing processes are amorphous. Expanding the application of alumina films necessitates enhancement of the chemical resistance of alumina. Previously, we identified the optimized anodizing conditions and the detachment method for suppressing thermal deformation, such as curving and cracking, during heat treatment of the alumina membrane (1-4). In our previous study, anodizing was conducted under an applied voltage of 25 V–185 V, which resulted in the formation of an anodic porous alumina film with pore diameters tunable over a wide range of approximately 30-350 nm. When oxalic acid was used as an electrolyte, the fabrication of a crack-free and flat α-alumina membrane with a diameter of 25 mm, a thickness of 50 μm and a pore diameter of approximately 60 nm was fabricated using the optimized anodizing conditions in oxalic acid at 40 V, followed by subsequent detachment from the substrate and heat treatment (2). In the case of phosphoric acid, the fabrication of α-alumina membrane with a pore diameter of 350 nm was achieved through optimization of various conditions (3). In the present study, the chemical composition and morphological changes of the alumina membrane formed in phosphoric acid during heat treatment were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA). The results of analyses clearly showed that the alumina membrane formed in phosphoric acid has unique characteristics such as coexistence of crystalline alumina and aluminum phosphate at high temperature region above around 1100 oC. Therefore, we focused on the effect of heat treatment conditions (e.g., temperature, heating rate and holding time) on the morphological changes of alumina cell wall in comparison with alumina films formed in sulfuric acid and oxalic acid. By controlling of heating conditions, alumina membrane with hierarchically porous structures consisted of main macropores and smaller textural mesopores in the cell walls could be fabricated. Through-hole structure of heated alumina membrane was maintained even at high temperature, and additional mesopores were formed by the selective removal of aluminum phosphate from alumina cell wall. [1] F. Rashidi, T. Masuda, H. Asoh, S. Ono, Surface and Interface Analysis, 45, 1490-1496 (2013) [2] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Electrochemistry, 82, 448-455 (2014) [3] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Materials, 8, 1350-1368 (2015) [4] H. Asoh, T. Masuda, S. Ono, ECS Transactions, 69, 225-233 (2015)