The incorporation and mobility of chromium species in anodic alumina films

By anodizing of a thin layer of Al-0.5 at.% Cr alloy, ca. 54 nm thick, superimposed on electropolished superpure aluminium, chromium species are incorporated into the anodic alumina as a thin band ca. 3 nm thick. This allows the relative mobility of chromium species, with respect to aluminium ions, to be determined, providing key information to assist development of insight into ionic transport processes in amorphous anodic films under high electric fields. During anodizing of the thin alloy film, chromium atoms are not oxidized and are accumulated in a layer of alloy, ca. 2 nm thick, immediately beneath the anodic film as a consequence of prior oxidation of aluminium. When the alloy film is totally consumed by anodizing, all the chromium atoms in the enriched layer are oxidized and incorporated immediately into the anodic film, forming a thin chromium species-containing band of ca. 3 nm thickness. With further anodizing, the incorporated chromium species band migrates outwards without widening. From chemical sectioning of the anodic film and XPS surface analysis, the chromium species are present as Cr3+ ions in the anodic film. The relative migration rate of Cr3+ ions, with respect to Al3+ ions, determined from the precise location of the chromium species-containing layer, using transmission electron microscopy and Rutherford backscattering spectroscopy, is 0.74. (C) 1997 Elsevier Science Ltd.