Controlling Factor of Self-Ordering of Anodic Porous Alumina

The controlling factor of self-ordering of anodic porous alumina was investigated by focusing on the current density during film growth. The homogeneity of cell size was improved with increasing formation voltage accompanied by the exponential increase in current density. The maximum anodizing voltage for proceeding uniform oxide growth while avoiding extremely high current accompanied by gas evolution was identical with the previously established self-ordering voltage. With the increase in formation voltage up to the self-ordering voltage, the ratio of pore diameter to cell diameter lowered and converged to ∼0.3 regardless of the electrolyte type. Moreover, domains of highly self-ordered pore arrays were found in the film formed during burning, where extremely high current was locally concentrated. This suggests that the condition inducing film growth under high current density, i.e. , high electric field strength is the key controlling factor of self-ordering. Based on the above knowledge, a new self-ordered porous alumina with a 600 nm pore interval was fabricated in citric acid just under the critical voltage of burning.