Self-ordering Regimes of Porous Alumina:  The 10 Porosity Rule

Transmission electron microscopy analysis of self-ordered porous alumina obtained by electrochemical anodization shows that self-ordering requires a porosity of 10%, independent of the specific anodization conditions. This corresponds to a volume expansion of alumina to aluminum of about 1.2. We propose that self-ordering of porous alumina with any interpore distance is possible if the applied potential, which mainly determines the interpore distance, and the pH value of the electrolyte, which mainly defines the pore radius, match the 10% porosity rule.