Morphological Instability Leading to the Formation of Self-Ordered Porous Anodic Oxide Films

Porous anodic oxide (PAO) films are grown by electrochemical polarization of Al, Ti, Zr, Nb, Hf, and W in baths that dissolve the oxide. Procedures to grow films with highly ordered arrangements of nanoscale pores have led to the extensive use of PAO films as templates for nanostructured devices. The porous film geometry may be controlled precisely via the film formation voltage and bath composition (1). Recently, tracer studies and modeling showed that transport in the amorphous oxide involves both electrical migration and plastic flow (2,3). The oxide seems to behave as an incompressible material during steady-state growth of the porous film. Linear stability analysis incorporating the assumption of incompressibility predicted important features of PAO (4). These include the constant ratio of interpore distance to anodizing voltage on Al for any electrolyte composition; narrow ranges of oxidation efficiency (the fraction of oxidized metal atoms that remain in the oxide) producing ordered PAO films on Al and Ti; and the inability to produced ordered films composed of divalent metal oxides. However, the analysis did not predict the observed onset of instability at a critical oxide thickness, and the observed dependence of the interpore distance on the electrolyte composition.