Abstract
Early stage oxidation of a dilute‐depleted uranium‐molybdenum alloy was analysed in situ under ultra‐high vacuum conditions by AES and XPS. At the equivalent of less than 300 ns at 1‐atm O2, U‐5Mo oxidizes to form stoichiometric UO2. No molybdenum oxidation is observed. After an oxygen dose of approximately 39 L, the oxide layer approached a limiting thickness of approximately 2.4 nm. The oxidation kinetics followed a logarithmic rate law, with the best fit to the experimental data for the oxide thickness, d, being given by d = 1.26 log(0.12t + 0.56). Changes in oxygen KLL and 1s peak positions associated with transformation from chemisorbed oxygen to metal oxide were observed at similar oxygen doses of 2.3 and 2.6 L O2 by AES and XPS, respectively, which opens up the possibility of using well‐characterized XPS chemical information to inform Auger peak shifts.
Highlights
The uranium-oxygen system exhibits one of the most complex chemistries for any metal.Thermodynamics predicts the formation of numerous stoichiometries between UO2 and UO31,2
All of the proposed uranium peaks were shown to shift to a lower kinetic energy with oxidation, indicating the potential for extracting chemical state information with
Oxygen peak shifts associated with transformation of chemisorbed oxygen to that within a metal oxide were observed to be in the order of 1.5 eV at between 2.3 and 2.6 L O2 for the two techniques
Summary
The uranium-oxygen system exhibits one of the most complex chemistries for any metal. Uranium dioxide has a Pilling-Bedworth ratio close to 2, which describes the volume ratio between the oxide and underlying metal and is such that compressive stresses lead to defects in the oxide layer. This initially permits anionic diffusion to the metal surface, allowing further oxidation and cracking and spalling of the oxide. In situ studies on U-Nb alloys at UHV by XPS and AES found early stage oxidation to occur initially by the formation of UO2, followed by successively higher niobium oxides; NbO, NbO2 and Nb2O5 15,16. Alloying with molybdenum has been shown by gravimetric methods and x-ray diffraction (XRD) to improve corrosion performance compared to unalloyed uranium, not to the same level as for niobium 18–20. In this work we use oxidation at extremely low oxygen partial pressures (pO2 < 10-8 mbar) to study the very early stages of oxidation of the candidate U-5Mo alloy by AES and XPS
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
