Morphological development of oxygen bubbles in anodic alumina

The morphological development of oxygen bubbles in amorphous, anodic alumina is examined during anodizing an Al-1 atom % Cu alloy. Using transmission electron microscopy, a sequence of stripped films reveals the dynamic evolution of numerous bubbles in the thickening film. The formation of the bubbles is associated with the enrichment of copper in the alloy due to growth of the anodic oxide. Evidently the alumina exhibits a liquid-like behavior under the electric field, assisted by inherent free-volume, which accommodates nucleation, growth, clustering, and coalescence of bubbles, without damage to the film. Bubbles nucleate throughout growth of the film, close to the alloy/film interface, but growth of pre-existing bubbles appears favored. The bubbles, which number about 1 x 10(14) m(-2), are in pressure equilibrium, with typical pressures of several hundred megapascals. The bubbles, with dimensions up to a few tens of nanometers, are obstacles to ionic transport, which leads to nonuniformity of film thickness and interfacial roughness. (C) 2000 The Electrochemical Society. S0013-4651(99)10-073-9. All rights reserved.