Atmospheric-pressure plasma oxidation of aluminum – mechanism and elementary film properties – “AppoxAl”

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The large and ever-increasing role played by aluminum and its alloys in everyday products is made possible by their kinetic resistance against oxidation and corrosion - in spite of a thermodynamic instability with respect to oxide formation under ambient conditions. The kinetic stability of aluminum (alloys) is due to a native, a few nanometer thick oxide film, formed spontaneously at ambient oxygen partial pressures and low temperatures. For a range of optical, electrical, electrochemical or automotive applications, however, it would be of interest to further enhance the oxide thickness using an eco-friendly low-cost process. The advent of atmospheric-pressure plasma processes may offer a viable solution to achieve this goal. In the proposed project the state of knowledge regarding atmospheric-pressure plasma-enhanced oxidation of aluminum by dielectric barrier discharges, poorly developed so far, shall be expanded substantially. Roles played by oxidizing species formed in DBDs such as oxygen atoms, ozone, or nitrous oxide as well as effects of physical factors like the plasma emission in the VUV region or the contact with streamer microdischarges in filamented DBDs shall be elucidated.In order to achieve these objectives an experimental approach will be used, along with a simplified modeling of plasma- or photochemical gas-phase kinetics, allowing to estimate the average densities or fluxes of potentially relevant species and to correlate model results with observed oxide growth. To validate the models, the results will also be compared with ozone or nitrous oxide concentrations in effluents from the discharges and with optical emission measurements, using Xe as an actinometer.Oxidation experiments will be conducted with three types of reactors: (1) a 2D-gradient DBD reactor allowing a combinatorial strategy using multiple combinations of temperature and oxygen concentrations in a single experiment, (2) a single-filament DBD reactor allowing to localize streamer microdischarges on the Al surface, and (3) a VUV reactor enabling irradiation of the oxide surface in the presence of controlled gas phases with photon energies below or above the oxide band gap. The experiments will be preceded or accompanied by electrical, optical, and analytical discharge characterization. The resulting oxide films will routinely be characterized with respect to thickness and composition. Single-filament experiments will be evaluated with respect to thickness distribution and oxide composition. As aluminum is a prototype passivating metal, scientific results obtained in this project are of interest for other passivating metals, too. Due to economical and ecological advantages of DBD processes, compared with other manufacturing methods, project results are also of substantial practical interest for applications of Al. Therefore, first indications of the technical usability of plasma-generated oxide films will be obtained by electrical and electrochemical tests.