The kinetics of the PuO₂ to Pu₂O₃ conversion

In an oxidizing environment, the oxide formed on plutonium (Pu) metal is composed of a plutonium dioxide (PuO₂) top layer and a thin cubic plutonium sesquioxide (Pu₂O₃) middle layer. In a reducing environment, the PuO₂ layer auto-reduces to cubic Pu₂O₃. The speed and extent of this conversion depend on the combination of temperature and time. While PuO₂ provides a strong diffusion barrier against unwanted Pu corrosion by gaseous species (like hydrogen), Pu₂O₃ does not, since its crystal structure has chains of oxygen vacancies. The kinetics of the PuO₂ reduction are, therefore, of fundamental interest and enable researchers to better protect Pu from corrosion. In this report, the oxygen-diffusion-limited kinetics of the dioxide to sesquioxide conversion were obtained by dynamically heating a PuO₂-covered Pu sample from 294 to 418 K in a high-vacuum vessel equipped with an in situ spectroscopic ellipsometer. The physical/chemical constraints in the conversion process were combined with the ellipsometry method of multi-sample analysis to track the percentage of PuO₂ and to compute the extent of Pu₂O₃ formation. The resulting diffusion coefficients were compared against and then combined with complementary literature data to produce a comprehensive set of kinetic parameters for reliably modeling oxide conversion over a larger temperature range than spanned by prior studies. The extracted thermal activation energy barrier (43.7 kJ/mol) and pre-exponential factor (5.0 × 10^-10 cm²/s) for the oxygen-diffusion-limited process can be used to accurately model the PuO₂ to Pu₂O₃ transformation in vacuum and/or inert gas applications.