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Anodic oxidation of high purity aluminum in sulfuric acid bath produces colorless oxide layers by DC, but yellow oxide layers by AC. Using a rectangular AC (Va=+20V, Vc=-10V, 50-10, 000Hz), a light brown colored oxide layer was formed accompanied by a faint electroluminescence. The color depends on the AC frequency used. The integrally colored oxide layer showed also a faint photoluminescence by UV irradiation. From these results the authors propose the following scheme of the coloration: At the moment of the polarity inversion, the trapped electrons inside the barrier layer are excited to the conduction band of the oxide and are accelerated by high electric field induced by the dielectric polarized space charge. These high energy electrons (2-5eV) then collide with the incorporated sulfate ions, which are eventually converted into the elementary sulfur.
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