A novel marker for the determination of transport numbers during anodic barrier oxide growth on aluminium

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Abstract
Anodic oxidation of thermally oxidized aluminium leads to the local development of thin disc-shaped γ′-Al2O3 crystals in the amorphous barrier oxide layers. The possibility of utilizing these γ′-Al2O3 crystals as ‘markers’ for the determination of the transport numbers during anodic barrier oxide growth on aluminium has been assessed. An excellent agreement was observed between the cationic transport numbers obtained using the present marker and by the usual implanted xenon marker method. Thus, the inertness and immobility of the usual xenon markers has now been confirmed. Additionally, the development of γ′-Al2O3 in the growing barrier oxide gives important insight into the effects of structure of the various oxides of alumina on the ionic transport processes during anodic oxidation. For example, a significantly higher field strength is required for ionic migration through γ′-alumina than the usual amorphous anodic alumina.