Structure of porous anodic films formed on aluminum and their crystallization by electron beam irradiation

The structure of porous anodic films formed on aluminum in chromic acid solution and their crystallization behavior during electron beam irradiation have been investigated by using high resolution electron microscopy. Crystals nucleated preferentially at triplicate cell boundaries but occasionally in the vicinity of pore walls. Subsequent crystal growth proceeded rapidly and smoothly. The crystallization was completed within 10min under the ordinary operating condition of the microscope. The crystals were large in size and small in number. This is explained by the structural uniformity of cell walls which is due to the small amount of incorporated species in the film except on the cell boundaries and in the vicinity of pore walls. The structure of the oxide on triplicate cell boundaries was different from the typical amorphous phase of cell walls. The difference is attributed to both the high electric field and the heat of electric breakdown between pore vases and metal ridges at the triplicate connections of semispherical cell vases. The featherlike pore walls of the film is explained by the drilling and healing effect of breakdown. High resolution lattice images revealed that the crystallized film consisted of three areas, i.e., highly crystallized area, incomplete crystalline area and hardly crystallized area near pore walls.