Investigations into the Solution Combustion Synthesis of Uranium Oxides and Their Mechanisms

Herein we explore the application of solution combustion synthesis (SCS) in the production of bulk uranium oxide nanoparticles and thin films. These studies evaluated the method’s ability to reduce hexavalent uranium (uranyl nitrate hexahydrate) to its various oxides. The investigation of a UO₂(NO₃)₂ + glycine + H₂O system showed SCS as a simple and viable method for the rapid, low energy production of U₃O₈ and hyperstoichiometric UO₂ (UO_2.12). Spin coating of precursor solutions onto aluminum backings and subsequent heat treatment produce uniform thin films. The films show durability under irradiation by an ion beam, demonstrating unique defect-simulated grain growth. Therefore, SCS also offers a unique method for the preparation of robust thin films for applications in nuclear physics measurements and uranium oxide studies.

Thermal analysis of the system shows active participation by an intermediate uranyl – glycine – nitrate complex in the heat release mechanism of the combustion reaction. To further probe such participation by intermediate complexes, a UO₂(NO₃)₂ – acetylacetone system was examined in both aqueous and 2-methoxyethanol solvents. Isolated compounds from aqueous solutions yield two unique uranyl monomers that upon heating dimerize to form an identical dimer. Further heating in the presence of oxygen results in an exothermic decomposition mechanism. However, combustion of aqueous systems is not achieved due to the retention of solvent at elevated temperatures. 2-methoxyethanol systems yielded a uranyl dimer that upon heating decomposed exothermally even under inert atmosphere. Due to the structures high thermal stability relative to the ignition temperature, the compound may participate it the bulk combustion mechanism. Such studies on the role of intermediate complexes can improve our understanding of mechanistic processes involved within SCS.

Expansion of the SCS process to transuranic actinides was investigated through the use of europium nitrate as an analogue for late trivalent actinides. Combustion of an Eu(NO₃)₃ – acetylacetone – 2-methoxyethanol system produced mixed phase cubic/monoclinic Eu₂O₃. Increases in the fuel to oxidizer ratio in air show a controlled increase to monoclinic phase composition. Combustion under inert N₂ atmosphere hinders such phase growth. Dilution of precursor gels with Eu₂O₃ product shows success in decreasing the concentration of the monoclinic phase produced. These results give a unique opportunity in the study of rare earth oxide polymorphism. As such, SCS could offer a new manner in which higher energy rare earth oxide phases are produced. Electrospraying of reactive solutions generate thin uniform oxide films. Such a process can be implemented in the production of transuranic thin films for nuclear physics experiments or actinide materials research.