Hyperstoichiometric Uranium Dioxides: Rapid Synthesis and Irradiation-Induced Structural Changes

Abstract

Uranium dioxide (UO2), the primary fuel for commercial nuclear reactors, incorporates excess oxygen forming a series of hyperstoichiometric oxides. Thin layers of these oxides, such as UO2.12, form readily on the fuel surface and influence its properties, performance, and potentially geologic disposal. This work reports a rapid and straightforward combustion process in uranyl nitrate–glycine–water solutions to prepare UO2.12 nanomaterials and thin films. We also report on the investigation of the structural changes induced in the material by irradiation. Despite the simple processing aspects, the combustion synthesis of UO2.12 has a sophisticated chemical mechanism involving several exothermic steps. Raman spectroscopy and single-crystal X-ray diffraction (XRD) measurements reveal the formation of a complex compound containing the uranyl moiety, glycine, H2O, and NO3 – groups in reactive solutions and dried combustion precursors. Combustion diagnostic methods, gas-phase mass spectroscopy, differential scanning calorimetry (DSC), and extracted activation energies from DSC measurements show that the rate-limiting step of the process is the reaction of ammonia with nitrogen oxides formed from the decomposition of glycine and uranyl nitrate, respectively. However, the exothermic decomposition of the complex compound determines the maximum temperature of the process. In situ transmission electron microscopy (TEM) imaging and electron diffraction measurements show that the decomposition of the complex compound directly produces UO2. The incorporation of oxygen at the cooling stage of the combustion process is responsible for the formation of UO2.12. Spin coating of the solutions and brief annealing at 670 K allow the deposition of uniform films of UO2.12 with thicknesses up to 300 nm on an aluminum substrate. Irradiation of films with Ar2+ ions (1.7 MeV energy, a fluence of up to 1 × 1017 ions/cm2) shows unusual defect-simulated grain growth and enhanced chemical mixing of UO2.12 with the substrate due to the high uranium ion diffusion in films. The method described in this work allows the preparation of actinide oxide targets for fundamental nuclear science research and studies associated with stockpile stewardship.

Spontaneous Crystallization for Tailoring Polymorphic Nanoscale Nickel with Superior Hardness

“…The specific surface area of the Ni nanoparticles was measured by the Brunauer–Emmet–Teller (BET) method using an ASAP 2020 instrument (Micromeritics). Helios Nanolab 600 (Thermo Fisher Scientific) DualBeam scanning electron microscope (SEM) was employed to investigate the microstructure of materials and prepare thin (100 nm) slices lifted from the sintered samples for transmission electron microscopy (TEM) imaging. , Titan 300 TEM (FEI) and aberration-corrected Spectra 30-300 (Thermo Fisher Scientific) scanning TEM microscopes were used to investigate the structure of amorphous and crystalline materials.…”

“…Helios Nanolab 600 (Thermo Fisher Scientific) DualBeam scanning electron microscope (SEM) was employed to investigate the microstructure of materials and prepare thin (100 nm) slices lifted from the sintered samples for transmission electron microscopy (TEM) imaging. 47,48 Titan 300 TEM (FEI) and aberration-corrected Spectra 30-300 (Thermo Fisher Scientific) scanning TEM microscopes were used to investigate the structure of amorphous and crystalline materials. The Vickers hardness (VH) of the sintered samples was measured by a microhardness testing station (M-400, Leco).…”

High-Performance Material for the Effective Removal of Uranyl Ion from Solution: Computationally Supported Experimental Studies

“…UO 2 was the main ion to be used in experiments, which cannot stand still while oxidizing or deoxidizing reactions happened. 31 The U 3 O 7 phase could be evaluated as oxygen deficient from the U 3 O 8 viewpoint, while it is an oxygen gaining phase from the UO 2 viewpoint, which can be concluded as reactions as follows or The U 2 O 5 phase, shown by stars, with a JCPDS file #43-0111, is one of the most common compounds, seen after adsorption, and also can be concluded as oxygen gaining from the UO 2 side while it is oxygen deficient from the UO 3 side as shown by possible reactions as follows or …”

“…UO 2 was the main ion to be used in experiments, which cannot stand still while oxidizing or deoxidizing reactions happened. 31 The U 3 O 7 phase could be evaluated as oxygen deficient from the U 3 O 8 viewpoint, while it is an oxygen gaining phase from the UO 2 viewpoint, which can be concluded as reactions as follows…”

Advances in actinide thin films: synthesis, properties, and future directions

“…Their contribution also includes growth on several different substrates using magnetron sputtering. Further advances in this field since then include reactive sputtering on LSAT (110) [53,54], magnetron sputtering on YSZ(100) [55], DC sputtering on SrTiO 3 (110) [56], PAD of UO 2+x by Zhang et al [57], and spin coat combustion on aluminum sheets by Roach et al [58]. U x Th 1−x O 2 thin film synthesis was achieved by Cakir et alto study surface reduction using ice [59].…”