What controls the pore spacing in porous anodic oxides?

In this paper, we use energy-based perturbation criteria to examine whether strain or electrostatic energy acts as a driving force for porosity initiation in anodic oxides. By doing so, we also succeeded to rationalise the dependence of pore spacing on anodising conditions. Our experimental approach consists of measuring in-situ the internal stress in anodic oxide films grown galvanostatically on aluminium in phosphoric acid, and to correlate these data with the measured pore spacing of the obtained porous films. Our results indicate that the possibility of a strain energy-induced surface instability is unlikely, as for this case the constitutive dependence of pore spacing on internal stress was not verified. Instead, the measured pore spacing, electric field and barrier oxide thickness obtained on our anodic alumina films indicate that electrostatic energy is the main driving force for pore initiation, as well as the factor controlling the pore spacing. Corroborative quantitative evidence for this novel electrostatic-based scaling law is provided by data compiled from the literature for a range of other anodic oxide systems, including nanoporous alumina and nanotubular titania films. (C) 2010 Elsevier B.V. All rights reserved.