Development of Uranium Fluoride Microstructures

Uranium microstructured materials with controlled size and shape have found applications in several branches of the nuclear industry including as targets for medical isotopes production, fuels for nuclear reactors, standards for analytical measurements, and energy sources for space exploration. Morphological studies of actinide materials are also highly relevant to nuclear forensics-related work. Other potential applications include information storage, catalysis, sensors, and luminescent devices. Most studies on uranium microstructured materials have focused on binary oxides, nitrides, carbides, and fluorides, with the spherical shape being the dominant morphology. Prior to the work reported in this dissertation, aside from UF4 microspheres (ms) and UF4(H2O)2.5 microrods (mr), uranium halide or oxyhalide micromaterials with controlled morphologies (microspheres, microrods, microplates (mp)) have not been reported. This paucity provides an opportunity for the development of novel materials with tunable catalytic and spectroscopic properties, as well as new nuclear fuels with increased gas retention and target materials with specific release rates of isotopes. Due to the preponderance of uranium and fluorine in the nuclear industry, investigation into the preparation of uranium fluoride micromaterials is of interest. A promising method to tailor micromaterials with controlled chemical composition, morphology, and size is chemical transformation. Chemical transformations are reactions performed on microscale materials to selectively modify their chemical composition, crystal structure and/or morphology. In this work, the chemical transformation investigated is the replacement of oxygen by fluorine atoms in oxide materials using a solid-gas reaction. The elaboration of uranium fluoride micromaterials from the fluorination of uranium oxides (U3O8 and UO2) microspheres, microrods, and microplates in autoclaves with in-situ HFg (produced from the thermal decomposition of silver bifluoride and ammonium bifluoride) has been fully demonstrated. By chemically transforming these uranium oxides into uranium (IV) and uranium (VI) fluorides while retaining morphology, several new micromaterials (i.e., (NH4)3UO2F5 ms/mr/mp, (NH4)xUFx+4 ms, UO2F2 mr/mp, and UF4 mp) were prepared for the first time. The materials were characterized by powder x-ray diffraction and scanning electron microscopy. The successful preparation of uranium fluorides ms, mp and mr presented here can pave the way for the development of other f-element fluoride micromaterials.