Aluminium anodizing in selenic acid: electrochemical behaviour, porous structure, and ordering regimes

Porous anodic alumina films are of great practical importance in membrane science and nanotechnology due to their unique structure possessing the arrays of aligned cylindrical channels. The most promising functional properties are observed for anodic alumina with an ordered porous structure. However, the pore ordering occurs only in a narrow range of anodizing conditions, which are commonly chosen by the empirical search. Here, recently proposed kinetic approach is applied to a directed search of self-ordering anodizing conditions in selenic acid electrolyte. We have developed a new self-ordering regime at the anodizing voltage from 60 to 100 V, which covers a novel interpore distance interval from 120 to 160 nm. Anodic alumina films obtained in this voltage range are free of cracks and possess a highly ordered porous structure, where more than 75% of pores have hexagonal coordination. Current efficiency of aluminium anodizing, anodic alumina formation efficiency, mass fraction of selenate impurities, interpore distance, thickness-to-charge density ratio, and volume expansion factor are determined as a function of anodizing voltage.