Tadayuki Nakayama1 and Tatsuya Koizumi2
1School of Science and Engineering, Waseda University, Tokyo
The present authors have studied the structure of thin oxide films formed on the Zr surface under various conditions. The following results were obtained by electron microscopy and electron diffraction. (1) The oxide films formed by dry oxidation, e.g., heating in air at 300° ∼ 500°C or dipping in a fused salt bath (NaNO3 50 g, KNO3 50 g, MnO2 1 g: 400°C), consisted of only the monoclinic ZrO2. (2) A number of regularly aligned lines of about 2500 Å in length was observed in some oxide films formed on the surfaces smoothed by cloth-polishing. (3) The tetragonal ZrO2 as well as the monoclinic ZrO2 was detected in the Zr oxide films produced by wet oxidation such as immersing in boiling conc. HNO3. (4) On the hydrogen-absorbed surfaces by either cathodizing in 5%H2SO4 or immersing in 35%HCl, the oxide films composed of the tetragonal and the monoclinic ZrO2 crystals were produced by dry oxidation. (5) When the Zr specimen was quenched in water after heating in air at 400°C, the surface was covered with both the tetragonal and the monoclinic ZrO2 crystals. (6) The tetragonal ZrO2 in the oxide films produced by wet oxidation transformed to the monoclinic ZrO2 by heating in air at above 1150°C. (7) These results indicate that the meta-stable tetragonal ZrO2 crystals are produced in the oxide films partly due to the existence of water or hydrogen in oxidizing atmosphere.
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