Irradiation-Driven Restructuring of UO2 Thin Films: Amorphization and Crystallization

Abstract

Combustion synthesis in uranyl nitrate-acetylacetone-2-methoxyethanol solutions was used to deposit thin UO2 films on aluminum substrates to investigate the irradiation-induced restructuring processes. Thermal analysis revealed that the combustion reactions in these solutions are initiated at ∼160 °C. The heat released during the process and the subsequent brief annealing at 400 °C allow the deposition of polycrystalline films with 5-10 nm UO2 grains. The use of multiple deposition cycles enables tuning of the film thicknesses in the 35-260 nm range. Irradiation with Ar2+ ions (1.7 MeV energy and a fluence of up to 1 × 1017 ions/cm2) is utilized to generate a uniform distribution of atomic displacements within the films. X-ray fluorescence (XRF) and alpha-particle emission spectroscopy showed that the films were stable under irradiation and did not undergo sputtering degradation. X-ray photoelectron spectroscopy (XPS) showed that the stoichiometry and uranium ionic concentrations remain stable during irradiation. The high-resolution electron microscopy imaging and electron diffraction analysis demonstrated that at the early stages of irradiation (below 1 × 1016 ion/cm2) UO2 films show complete amorphization and beam-induced densification (sintering), resulting in a pore-free disordered film. Prolonged irradiation (5 × 1016 ion/cm2) is shown to trigger a crystallization process at the surface of the films that moves toward the UO2/Al interface, converting the entire amorphous material into a highly crystalline film. This work reports on an entirely different radiation-induced restructuring of the nanoscale UO2 compared to the coarse-grained counterpart. The preparation of thin UO2 films deposited on Al substrates fills an area of national need within the stockpile stewardship program of the National Nuclear Security Administration and fundamental research with actinides. The method reported in this work produces pure, robust, and uniform thin-film actinide targets for nuclear science measurements.