Correlation between self-organized pore formation behaviors and the current-time characteristics in porous anodic alumina: A quantitative analysis of the voltage dependence of pore morphological changes

We report the self-organized pore formation behaviors of porous anodic alumina (PAA) by investigating the correlation between morphological changes at the metal/oxide interface and the current-time I-t characteristics during anodization. PAA is fabricated by anodizing aluminum at voltages of 20-60 V in 0.3 M oxalic acid. In comparing the I-t curves of PAA anodized at various voltages, we found that all curves showed similar shapes, regardless of voltage, but that the values and times at the maximum and the minimum points of the curves were different depending on the voltages. In order to explain the correlation between the morphological changes and the I-t curves, we categorized the curves into four regions based on their common features: Regions I, II, III, and IV correspond to barrier layer formation, pore initiation, steady-state pore formation, and self-arrangement of pores, respectively. Then, we proposed an equation consisting of four formulas, one corresponding to each region. We found that the pore size increased and its density decreased during the anodization process of regions I-III, whereas they remained constant after the anodization of region III. The size, density, and arrangement of pores were also governed by the anodizing voltage. These changes could be explained using variations of coefficients in the formulas.