Reaction Kinetics in the Hydrothermal Oxidation of Hf

The hydrothermal oxidation of Hf was studied using metal powders and chips to change the surface‐area effect. Fine monoclinic HfO₂ powders (32 to 34 nm) were formed through two parallel reaction paths, one the oxidation of Hf with H₂O and the other the consecutive reaction via formation of Hf hydride. Based on the experimental results at 500°C under 100 MPa, three reaction rate constants were determined for two kinds of starting materials. The relative widths of two paths were proved to be independent of the size of starting materials, and the pulverization and oxidation of both metal powders and chips could be explained by the same mechanism, where the formation of hydride was a driving force in pulverization.