The origin of nucleation and development of porous nanostructure of anodic alumina films

The galvanostatic growth of porous anodic alumina films during the initial transient stages and starting region of steady state was studied by chronopotentiometry, a suitably modelled mass balance method, a high field kinetic model for ions transport in solid oxide and SEM. The results postulate that the nucleation of pores towards the end of the first transient stage results from recrystallization of rare oxide lattice produced in metal|oxide interface, accommodated with Al lattice, to the surface of initially flat film forming nanocrystallites of denser oxide and crack-like holes nuclei of pores between crystallites. During the second transient stage the successive transformation nuclei → pockets → channel-like pores results from simultaneous continuing production of oxide at a slightly declining rate, gradual localization of charge transfer processes from the flat surface to their bottom, recrystallization of oxide in the barrier layer establishing finally a stable gradient of density rising to electrolyte and conversion of barrier layer from flat to close packed shell shaped units accommodating these changes. The enhanced recrystallisation and density rise towards the pore base and wall surface and the necessity for equilibration of stresses inside each barrier layer unit and between neighbouring units yield hemispherical barrier layer units, self-ordering of porous structure approaching the highest possible 2D hexagonal symmetry in finite surface domains and permanent growth of structure in steady state.