Preparation of Fine Monoclinic Hafnia Powders by Hydrothermal Oxidation

Fine single‐phase monoclinic HfO₂ powders were prepared from Hf metal chips and high‐temperature high‐pressure water by hydrothermal oxidation in closed and open systems. In the closed system, Hf metal was converted to HfO₂ by treatment at ≥600°C under 100 MPa for 3 h. At 500°C half the Hf reacted to produce Hf hydride and a small amount of HfO₂. In the open system, Hf metal scarcely reacted at 500°C, but at 600°C the reaction was more rapid than the corresponding run in the closed system.