Modeling the steady-state growth of porous anodic alumina

Porous anodic alumina films are produced by anodic oxidation in various acidic electrolytes. The self-organized structure consists of an evenly spaced hexagonal array of mutually parallel pores. Interpore distances range from ten to several hundred nanometers. The film is easily produced with control over the pore geometry, and it can be extremely well ordered. These features have found use in various applications including the production of nanomaterials. Recently, porous films have been shown on other metals. An improved mechanistic understanding would aid exploration of other systems and lead to new applications.;Results based solely on ionic migration within the electric field were evaluated with respect to the requirement that steady-state geometries have time invariant interface evolution profiles. Two models were developed to simulate the processes occurring during steady-state pore growth. The first used the assumption of no space charge in the oxide, and the second applied current continuity. Both were coupled to high-field ionic conduction. Neglecting space charge in the oxide yielded unrealistic behavior with highly nonuniform interface motion, suggesting the importance of space charge. In contrast, interface motion predicted by the current continuity model was uniform, except in a localized region near the convex ridges of the metal-film interface between neighboring pores. Ionic conduction alone is unable to fully rationalize the porous structure.;The current continuity model was expanded to include transport by stress-driven material flow in addition to ionic migration. This phenomenon is indicated by experimental tracer studies as well as measurements of accelerated pore growth relative to interfacial progression into the metal. Direct simulation of experimental tracer experiments revealed quantitative results that are in excellent agreement. The flow is driven by compressive stress in the pore base near the film-solution interface. This compressive stress is largely attributed to electrostriction. The overall stress distribution also depends on the volume change at the metal-film interface during oxidation and the nonlinear current-electric field relationship governing ionic conduction. The stress distribution at the metal-film interface generated by flow suggests interface diffusion toward the ridges, which is consistent with observations involving dilute aluminum-gold alloys. This interface diffusion can qualitatively explain the time invariance of the metal-film interface near the ridges.