Morphological development of oxygen bubbles in anodic alumina

The morphological development of oxygen bubbles in amorphous, anodic alumin a is examined during anodizing an Al-1 atom % Cu alloy. Using transmission electron microscopy, a sequence of stripped films reveals the dynamic evolu tion of numerous bubbles in the thickening film. The formation of the bubbl es is associated with the enrichment of copper in the alloy due to growth o f the anodic oxide. Evidently the alumina exhibits a liquid-like behavior u nder the electric field, assisted by inherent free-volume, which accommodat es nucleation, growth, clustering, and coalescence of bubbles, without dama ge to the film. Bubbles nucleate throughout growth of the film, close to th e 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 equilibr ium, with typical pressures of several hundred megapascals. The bubbles, wi th dimensions up to a few tens of nanometers, are obstacles to ionic transp ort, which leads to nonuniformity of film thickness and interfacial roughne ss. (C) 2000 The Electrochemical Society. S0013-4651(99)10-073-9. All right s reserved.