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NanoPorous Anodic Oxides for Functionalization of Metal Surfaces

Final Report Summary - NANOXID (NanoPorous anodic oxides for functionalization of metal surfaces)

Porous anodic oxides are of increasing interest for nanotechnological applications, such as photonic crystals, sensors and solar cells, as well as of continuing importance for protection and functionalisation of metal surfaces, e.g of aluminium in aerospace, packaging and electronics, where energy reduction and environmental compliance of processes are critical considerations. Despite significant previous research, the understanding of porous oxide growth is still subject of much debate.

The present programme has enabled in-depth probing of the mechanism of porous oxide growth involving collaboration between the University of Manchester (UK) and the University of Paris (France), combining complementary expertise in respectively HREM and IBA analysis. Key experiments have involved the use of 18O as a tracer species in the study the transport of oxygen in the growing oxide films.

Since the film morphology changes significantly as the film evolves from an initial non-porous oxide to a fully porous oxide, microscopy of the oxide at the various stages of film growth is vital to the interpretation of the tracer data. A further key feature of the study is the use of multispectrum analysis to provide confidence in quantification of nuclear data, using Spaces, SIMNRA and RUMP software, with analyses of oxides by non-resonant and resonant nuclear reactions and elastic scattering, including RBS and MEIS (Daresbury Lab. UK).

The overall experimental methodology was developed from extensive studies of anodising of aluminium in phosphoric acid electrolyte (PAE). The first studies explored the optimum conditions for carrying out 18O tracer experiments, in terms of 18O concentration in the electrolyte, thickness of oxide and conditions of anodizing (cell type, current density). The preferred method involved formation of a thin non-porous oxide in 18O-enriched electrolyte, followed by various times of anodizing in non-enriched electrolyte.

The findings revealed a major re-distribution of the 18O as the porous film evolves, which correlated with the evolution of the porous structure. The latter initially comprised a region of high porosity (60 %), with a pore morphology related to the texture of the original surface of the aluminium, followed by emerging of the major pores, of low porosity (10 %) with a size related to the anodizing voltage. The 18O was distributed during film growth between the highly porous material, which remained at the film surface, the walls of the major pores, and also the thin layer of non-porous oxide that remains at the base of the film.

Notably, the dependence of the oxygen contents of the films and the film thickness on the time of anodizing indicated increasing efficiencies of film growth and rates of film growth as the film progresses from the initial non-porous type to the fully porous morphology. The major pores appear to be generated by flow of anodic oxide. However, it is possible that the initial pores develop by a differing mechanism, which is currently being explored by using a tungsten tracer to distinguish whether flow of oxide or dissolution of oxide is the principle cause.

Following successful establishment of the methodology with PAE, other anodising electrolytes were investigated (chromic and sulphuric acid, borax), which reveal significant differences in film formation compared with PAE in terms of distribution of 18O and kinetics of oxide growth. In parallel with the studies, work has been also been carried out on porous oxide formation on Ti, Ta, Zr and stainless steel.

The overall findings of the programme contribute significantly to understanding of porous oxide growth by anodising, which will be of benefit to future development of improved porous oxides for a range of new applications that are currently being researched, as well as underpinning the use of the oxide in more traditional areas where more economic and environmentally friendly processes are sought, as well as enhanced film performance.