Abstract
Fine monoclinic HfO2 powders were prepared from Hf metal chips by reaction with high-temperature high-pressure water. Dependence of the reaction on temperature-pressure-time was examined in both closed and open systems. Hydrothermal oxidation of Hf proceeded through three stages: the surface oxidation of Hf metal chips with H2O, the reaction of the Hf with released H2 to form hydrides, and the succeeding oxidation of the hydrides with H2O to form HfO2. Most of the fine HfO2 powders were, however, considered to be formed by the latter two reactions after the cracking and pulverizing of metal chips associated with hydriding. The difference in the reaction rates between the closed and open systems was explained by taking into account the fugacity relations of H2 and H2O in the respective systems.
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