Abstract
The structure and crystallization process resulting from electron beam irradiation of porous anodic films formed on aluminum in an Ematal bath have been investigated by high resolution electron microscopy. Ion-thinned films showed a duplex structure consisting of an anion-free inner layer with a width 1/6 that of the cell wall and an anion-incorporated outer layer. Voids, which had been produced on the metal ridges at the triplicate cell connections during anodizing, were detected. Crystal nucleation occurred preferentially in the vicinity of voids under the usual operating condition of the microscope. During the growth of initial crystals, further nucleation proceeded in the cell boundaries (inner layer) and at last in the outer layer. It is supposed that the oxide structure in the vicinity of voids is different from the typical amorphous phase of the cell walls, and that the difference is produced by the effects of a strong electric field, and by heating due to electric breakdown. The crystallization rate of the films studied here was higher than that of films formed in phosphoric acid and sulfuric acid solutions, and lower than that of films formed in chromic acid solutions. High resolution lattice images revealed that the crystallized film consisted of microcrystals in the cell walls and an amorphous layer near the pore walls. Electron diffraction patterns obtained from the films after irradiation indicated a mixed phase of γ-, η-, θ- and α- alumina.
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