Preparation and characterisation of uranium oxides with spherical shapes and hierarchical structures

Abstract

An easy way of preparation, based on the precipitation of U⁴⁺ or UO₂²⁺ cations by urea in the presence of PEG at T = 90–120 °C, was set up to prepare shape-controlled spherical uranium oxides. The parametric study of heating time and temperature allowed us to tailor the size of the mesocrystals obtained, which varied from 50 to 250 nm. For U(IV)-based samples, advanced characterization by the combination of SEM and HR-TEM observations and PXRD and SAXS measurements revealed a hierarchical organization of the powder with three different levels. The first one corresponded to small crystallites of about 3 nm, which grouped into spherical agglomerates of 15–20 nm and then reaggregated to produce the bigger spheres observed (up to 200 nm in diameter). On the other hand, U(VI)-bearing spherical aggregates were found to be more likely in a metastable form, evolving towards the precipitation of crystalline metaschoepite. In the last step, the samples prepared at low temperature were fired between 700 and 1000 °C under various atmospheres in order to tailor the final O/M ratio in the resulting oxides. In spite of the important chemical modifications associated, the precursors were generally found to present pseudomorphic conversion towards the final high temperature oxides and still exhibited a spherical form, provided the conditions of calcination were properly selected.