Measurements and model of UV-induced oxidation of aluminum

Abstract

We present measurements and a model of aluminum oxidation induced by ultraviolet (UV) radiation. Spots of oxide were grown by focusing synchrotron radiation onto a polycrystalline aluminum membrane in the presence of water vapor at pressures from 3 × 10−8 to 1 × 10−4 mbar (3 × 10−6 to 1 × 10−2 Pa). The UV radiation passed through a sapphire (Al2O3) window, which ensured that the UV interacted with only the aluminum metal and not the oxide overlayer. The oxide profile of each spot was then determined by measuring the membrane's transmission at a wavelength of 17.5 nm. The model combined descriptions of photoemission from the Al metal, electron-phonon scattering in the oxide, Al3+ ion transport in the oxide, and the adsorption and ionization of H2O on the oxide surface. It also accounted for UV-induced desorption of H2O and the effect of the Al3+ ion flux on the surface reactions. The model's five free parameters were fit to the laboratory measurements of UV-induced oxidation. Then, using those values, the model was used to describe and understand the oxidation of similar aluminum membranes that were used to filter extreme ultraviolet on the Solar Dynamics Observatory, a sun-observing satellite. This understanding will help prevent similar problems on future satellites. These results are the first experimental confirmation of a model of UV-induced oxidation.