Hydrothermal Oxidation of Hf Metal Chips in the Preparation of Monoclinic HfO₂ Powders

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.