Structural engineering of nanoporous alumina by controlling the anodization voltage during the spontaneous current oscillation in hard anodization

In aluminum anodization process the spontaneous current oscillation in certain electrolytes and anodization voltages can occur. The behavior of current in this process is called spontaneous oscillatory current otherwise we call the current behavior as non-oscillatory. The current difference between the spontaneous oscillatory and non-oscillatory conditions is appreciable (more than 50 mA/cm2) whereas to switch from spontaneous oscillatory to non-oscillatory behavior we only need to change the anodization voltage less than 3 V. The pore structure of porous anodic alumina film can be modulated by this oscillatory behavior. This effect occurs due to the variation of the anodization current which causes the variation of pore diameter along the pores. But by this procedure it is hard to control the structure of the pore as it is required, because the modulated structure mainly depends on the spontaneous current oscillation. In this article, it is shown that this spontaneous oscillatory behavior can be switched to nonoscillatory condition by changing the anodization voltage. Therefore by switching the anodization voltage between the spontaneous oscillatory and non-oscillatory behaviors, in any time interval, the pore structure of nanopore alumina can be engineered.