Early stages of UO2+x sintering by in situ high-temperature environmental scanning electron microscopy

Abstract

Early stages of various uranium oxides sintering were monitored in situ through High Temperature Environmental Scanning Electron Microscopy. Systems composed of two UO2+x microspheres were heated up to 900−1200 °C under PO2 ranging from 10−10 to 25 Pa. The oxide phases stabilized were assessed through thermodynamic calculations and powder X-ray diffraction. In all the conditions tested, the formation and the development of a neck was evidenced and image processing led to quantitative data describing the morphological changes.The evolution of the sintering degree was fitted using an exponential law and allowed the evaluation of the activation energy. When PO2 led to stabilize hyper-stoichiometric UO2+x oxides, the values obtained increased with x, typically in the 200−400 kJ mol−1 range. Under air atmosphere, the stabilization of U3O8 led to EA = 260 ± 40 kJ mol−1. Finally, the main diffusion mechanism driving neck formation was found to be volume diffusion, independently from the oxide stoichiometry.