Formation mechanism of porous channels and evolution of surface stripes during constant current anodization
Traditional theories are incapable of explaining the formation mechanism of surface stripes and channels in porous anodic alumina (PAA) . In order to elucidate the formation process of surface stripes and PAA channels,the limitations of the traditional"field-assisted dissolution theory"and the equilibrium theory of oxide growth and dissolution were meticulously analyzed by integrating the voltage-time curve and SEM images of the anodic oxidation process. The growth rate of PAA pore oxides is experimentally verified to be 113 nm/min,which is significantly greater than the dissolution rate of alumina in a 60℃ phosphoric acid solution of 5.8 nm/min. The experiment demonstrate that there is no equilibrium between oxide growth and dissolution during the formation of PAA channels. The kinetics of Al anodizing is explained by the theory of ionic current and electronic current. Ionic current leads to the growth of barrier oxide,while electronic current results in the decrease of oxide growth efficiency and the generation of oxygen bubbles. The oxygen bubble mold effect gives rise to the formation of cylindrical channels and hemispherical bottoms of PAA. The surface stripes are caused by the chemical dissolution of the electrolyte and the volume expansion of oxygen bubbles under the anion contaminated layer,rather than by self-organizing process. This paper is beneficial to the structural regulation of widely used TiO2 and ZrO2 nanotubes.
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