Determination of the field strength and realization of the high-field anodization of aluminum.

Previous studies suggested that high-field anodization of aluminum can be realized given that aluminum is anodized at a high voltage just below the breakdown value. However, increasing the applied voltage cannot guarantee the enhancement in electric field strength across the barrier layer of porous anodic alumina (PAA) due to a concurrent increase of the barrier-layer thickness. Here, we report comparative studies of aluminum anodization in a highly concentrated (0.75 M) and a dilute (0.1 M) oxalic acid solution. Special attention is given to the field strength during anodization. To calculate the field strengths, the barrier-layer thickness of PAA was determined by a re-anodizing technique. Electrochemical impedance spectra (EIS) were also used to measure the barrier-layer thickness to improve the reliability of the field strength measurements. Both routes can yield the same conclusion: the barrier-layer thickness increases linearly with the applied voltage. For 0.75 M oxalic acid electrolyte, the resulting regression line has a positive intercept. In this instance the field strengths can be enhanced by increasing the applied voltage. Conversely, for 0.1 M oxalic acid solution, the regression line has a negative intercept and the field strengths decrease with the applied voltage. There are different linear dependences between the barrier-layer thickness and the applied voltage that determine the change in the field strength during anodization. In the highly concentrated oxalic acid electrolyte, the high-field anodization of aluminum can be realized by enhancing the applied voltage, but cannot in the dilute acid solution. Moreover, the ordering qualities of PAA films increase with increasing field strength instead of the applied voltage. The present results may provide a decisive step towards a thorough understanding of the PAA film.