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Probing Barrier-Type Anodic Alumina Films on Nano-Patterned Substrates
[Thesis]. Manchester, UK: The University of Manchester; 2010.
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Abstract
The growth of barrier-type anodic alumina films formed by anodizing relatively rough substrates has been shown to proceed by high field ionic conduction. As a result of the ionic transport and the induced plasticity, smoothing of the oxide surfaces and the metal / oxide interfaces arises. However, such a smoothing model was deduced from topographical observations and, therefore little insight was gained about the transport mechanism leading to the flattening of the anodized specimens. Recently, the development of porous anodic alumina has been demonstrated to proceed by coupled ionic migration and material flow resulting from the field-induced mechanical stress. For rough metal surfaces, the electric field distribution is non-uniform across the specimen surface. Considering the square-dependence of the electrostrictive stress on the electric field and the distribution of the electric field across surface, a significant gradient of mechanical stress may arise across the anodic oxide layer during anodizing. As a result, stress-driven transport may participate, in addition to high field ionic conduction, to the smoothing of the specimen surface. Transport mechanisms were investigated during anodizing of patterned superpure aluminium specimens, by examination of the distributions of incorporated species, used as markers and tracers. The nature of the migration processes have been determined in correlation with the changes in the concentration of the tracer profiles as well as the variations in the anodic oxide film compositions.