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It was found by electron microscopy that three kinds of oxides were formed on aluminium films by heat oxidation in air in addition to amorphous oxide forming a protective layer. They were morphologically different: (A) flower-like amorphous oxide, (B) plates of a single crystal of γ-Al 2 O 3 , and (C) dendrite crystals of γ-Al 2 O 3 . At 400°C type-A, at 500°C type-A and type-B, and at 600°C type-C as well as type-B were formed. They all appeared preferentially at grain boundaries of the aluminium films. It is supposed that surface irregularities of the metal play an important role in the formation of these oxides.
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Suito et al. (1966) studied this question.
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