Dr. Aleksandra Baron-Wiechec
UK Atomic Energy Authority, Abington
Anodic films on aluminum and its alloys have been extensively studied for many decades. This is partially due to the fact that they are important for protection against corrosion. Similarly, the constant miniaturization of electronic devices contributes towards a renaissance of research into porous anodic alumina films, specifically for nanotechnological applications.
A controlled morphology of porous alumina films is currently achieved empirically. The process involves the electrochemical conversion of Al surfaces (anodizing) in liquid electrolytes, by applying a mixture of either current or potential. Porous alumina films typically consist of an inner barrier region and a much thicker, outer porous region. The growth of the films and the formation of the pores have usually been explained by a field-assisted dissolution mechanism. In this model, the anodic alumina is formed at the metal/film interface due to migration of O2- ions across the barrier layer under a high electric field and a generation of pores by dissolution of the alumina at the pore base. However, dissolution rates revealed by oxygen isotopes studies are too small to support this mechanism. Recent evidences have led to a re-evaluation of the mechanism of pores formation. Using a new tracers approach, it was shown that the pore growth can be explained by the flow of oxide within the barrier layer, with field-assisted dissolution having a minor or negligible role under many conditions of film growth. The ﬂow arises from the plasticity of the ﬁlm material in the presence of ion migration and to the stresses associated with ﬁlm growth.
The tracer approach is currently being developed to examine pore formation under a range of conditions of film growth, including anodizing in borax and chromic acid which results in a different pore morphology to the one typically achieved in phosphoric acid. I will present results obtained by a combination of high resolution electron microscopy and accelerator-based ion beam techniques, supporting a field-assisted flow of oxide mechanism in porous formation.