Heat-induced local structural variation in porous-type amorphous alumina prepared by anodization

Porous-type anodic amorphous alumina is an important functional material in the current chemical industry and the future fields of nanotechnology and glass science. To identify their heat-induced local structural variations, anion-free, phosphate-, oxalate-, and sulfate-incorporated amorphous alumina were investigated. The glass transition temperature of the all amorphous alumina was determined to be similar to 470 degrees C. In the phosphate-incorporated alumina, two-step crystallization process involving an anion-free inner layer and anion-incorporated outer layers were identified. In addition, the sensitive local structural variation in anion-free and -incorporated alumina was clarified. In the anion-free sample, the average coordination number (NAl-O) increases slightly just before the crystallization temperature due to nucleation. In the oxalate- and sulfate-incorporated samples, the average NAl-O gradually decreased and reached at value close to that of the anion-free sample due to gradual cleavage in the chemical bonds between the anions and aluminum ions. For the phosphate-incorporated sample, the average NAl-O decreased and exhibited a value substantially different from that of the anion-free sample because of the gradual formation of chemical bonds between phosphate and aluminum ions. These findings contribute to the development of an intrinsic understanding of the changes in physical and chemical properties with heat treatment for anodic alumina and the structural features of intermediate oxides that cannot be glass.

Automated summary

Porous-type anodic amorphous alumina is an important functional material with heat-induced local structural variations. The incorporation of different anions (phosphate, oxalate, sulfate) in amorphous alumina resulted in distinctive changes in the average coordination number. The findings contribute to the understanding of physical and chemical property changes and structural features of intermediate oxides that cannot form glass.