Crystallization in anodic oxide films

The factors which influence the structure of barrier anodic oxides have been reviewed and it is seen that high internal compressive stresses are conducive to the formation of crystals. These stresses arise when new oxide is formed mainly at the oxide-metal interface and oxygen ion transport dominates the growth process. For some oxides, e.g. ZrO 2 and HfO 2, the oxygen ion transport fraction is close to one at most rates of growth and the anodic oxides are usually crystalline. For others, e.g. Al 2O 3, Nb 2O 5, Ta 2O 5 and TiO 2, increasing metal ion transport at high rates of growth is associated with the formation of glassy material as the compressive stresses diminish. Incorporation of impurity ions in sufficient quantities may modify the ionic transport fractions and hence the stresses and the degree of crystallinity. Many of the concepts developed for anodic films seem applicable to passive film and oxides grown at high temperatures.