Michele Manuel

In situ and ex situ transmission electron microscopy observation of small Kr bubbles in both single-crystal and polycrystalline UO2 were conducted to understand the inert gas bubble behavior in oxide nuclear fuel. The bubble size and volume swelling are shown as weak functions of ion dose but strongly depend on the temperature. The Kr bubble formation at room temperature was observed for the first time. The depth profiles of implanted Kr determined by atom probe tomography are in good agreement with the calculated profiles by SRIM, but the measured concentration of Kr is about 1/4 of the calculated concentration. This difference is mainly due to low solubility of Kr in UO2 matrix and high release of Kr from sample surface under irradiation.

In situ and ex situ transmission electron microscopy observation of small Kr bubbles in bothsingle-crystal and polycrystalline UO2 were conducted to understand the inert gas bubblebehavior in oxide nuclear fuel. The bubble size and volume swelling are shown as a weakfunction of ion dose but strongly depend on the temperature. The Kr bubble formation at roomtemperature was observed for the first time. The depth profiles of implanted Kr determined byatom probe tomography are in good agreement with the calculated profiles by SRIM, but themeasured concentration of Kr is about 1/3 of calculated one. This difference is mainly due to lowsolubility of Kr in UO2 matrix, which has been confirmed by both density-functional theorycalculations and chemical equilibrium analysis.

Fission products, such as krypton (Kr), are known to be insoluble within UO2, segregating toward grain boundaries and eventually leading to a lowering in thermal conductivity and fuel swelling. Recent computational studies have identi?ed that differences in grain boundary structure have a signi?cant effect on the segregation behavior of fission products. However, experimental work supporting these simulations is lacking. Atom probe tomography was used to measure the Kr distribution across grain boundaries in UO2. Polycrystalline depleted UO2 samples were irradiated with 0.7 MeV and 1.8 MeV Kr-ions and annealed to 1000C, 1300C, and 1600C for 1 h to produce a Kr-bubble dominated microstructure. The results of this work indicate a strong dependence of Kr concentration as a function of grain boundary structure. Temperature also influences grain boundary chemistry with greater Kr concentration evident at higher temperatures, resulting in a reduced Kr concentration in the bulk. Although Kr segregation takes place at elevated temperatures, no change in grain size or texture was observed in the irradiated UO2 samples.

We present a model to study the electrochemical effects of voids in oxide materials under equilibrium conditions and apply this model to uranium dioxide. Based on thermodynamic arguments, we claim that voids in uranium dioxide must contain oxygen gas at a pressure that we determine via a Kelvin equation in terms of temperature, void radius and the oxygen pressure of the outside gas reservoir in equilibrium with the oxide. The oxygen gas within a void gives rise to ionosorption and the formation of a layer of surface-charge on the void surface,which,in turn, induces an influence zone of space charge into the matrix surrounding the void. Since the space charge is carried in part by atomic defects, it is concluded that, as a part of the thermodynamic equilibrium of oxides containing voids, the off-stoichiometry around the void is different from its remote bulk value. As such,in a uranium dioxide solid with a void ensemble, the average off-stoichiometry level in the material differs from that of the void-free counterpart. The model is applied to isolated voids in off-stoichiometric uranium dioxide for a wide range of temperature and disorder state of the oxide.

Atom probe tomography (APT) provides the ability to detect subnanometer chemical variations spatially, with high accuracy. However, it is known that compositional accuracy can be affected by experimental conditions. A study of the effect of laser energy, specimen base temperature, and detection rate is performed on the evaporation behavior of uranium dioxide (UO2). In laser-assisted mode, tip geometry and standing voltage also contribute to the evaporation behavior. In this investigation, it was determined that modifying the detection rate and temperature did not affect the evaporation behavior as significantly as laser energy. It was also determined that three laser evaporation regimes are present in UO2. Very low laser energy produces a behavior similar to DC-field evaporation, moderate laser energy produces the desired laser-assisted field evaporation characteristic and high laser energy induces thermal effects, negatively altering the evaporation behavior. The need for UO2 to be analyzed under moderate laser energies to produce accurate stoichiometry distinguishes it from other oxides. The following experimental conditions providing the best combination of mass resolving power, accurate stoichiometry, and uniform evaporation behavior: 50 K, 10 pJ laser energy, a detection rate of 0.003 atoms per pulse, and a 100 kHz repetition rate.

Oxide materials are used in numerous applications such as thermal barrier coatings, nuclear fuels, and electrical conductors and sensors, all applications where nanometer-scale stoichiometric changes can affect functional properties. Atom probe tomography can be used to characterize the precise chemical distribution of individual species and spatially quantify the oxygen to metal ratio at the nanometer scale. However, atom probe analysis of oxides can be accompanied by measurement artifacts caused by laser-material interactions. In this investigation, two technologically relevant oxide materials with the same crystal structure and an anion to cation ratio of 2.00, pure cerium oxide (CeO2) and uranium oxide (UO2) are studied. It was determined that electronic structure, optical properties, heat transfer properties, and oxide stability strongly affect their evaporation behavior, thus altering their measured stoichiometry, with thermal conductance and thermodynamic stability being strong factors.

The mechanisms of oxygen stoichiometry variation in UO2 at different temperature and oxygen partial pressure are important for understanding the dynamics of microstructure in these crystals. However, very limited experimental studies have been performed to understand the atomic structure of UO2 near surface and defect effects of near surface on stoichiometry in which the system can exchange atoms with the external reservoir. In this study, the near (110) surface relaxation and stoichiometry in UO2 have been studied with density functional theory (DFT) calculations. On the basis of the point-defect model (PDM), a general expression for the near surface stoichiometric variation is derived by using DFT total-energy calculations and atomistic thermodynamics, in an attempt to pin down the mechanisms of oxygen exchange between the gas environment and defected UO2. By using the derived expression, it is observed that, under poor oxygen conditions, the stoichiometry of near surface is switched from hyperstoichiometric at 300K with a depth around 3 nm to near-stoichiometric at 1000K and hypostoichiometric at 2000 K. Furthermore, at very poor oxygen concentrations and high temperatures, our results also suggest that the bulk of the UO2 prefers to be hypostoichiometric, although the surface is near-stoichiometric.

Crystalline uranium oxide thin films were deposited in an unbalanced magnetron sputtering system by sputtering from a depleted uranium target in an Ar + O2 mixture using middle frequency pulsed dc magnetron sputtering. The substrate temperature was constantly maintained at 500 °C. Different uranium oxide phases (including UO2-x, UO2, U3O7 and U3O8) were obtained by controlling the percentage of the O2 flow rate to the total gas flow rate (fO2) in the chamber. The crystal structure of the films was characterized using X-ray diffraction and the microstructure of the films was studied using transmission electron microscopy and atom probe tomography. When the fO2 was below 10%, the film contains a mixture of metallic uranium and UO2-x phases. As the fO2 was controlled in the range of 10–13%, UO2 films with a (2 2 0) preferential orientation were obtained. The oxide phase rapidly changed to a mixture of U3O7 and U3O8 as the fO2 was increased to the range of 15–18%. Further increasing the fO2 to 20% and above, polycrystalline U3O8 thin films with a (0 0 1) preferential orientation were formed. The hardness and Young's modulus of the uranium oxide films were evaluated using nanoindentation. The film containing a single UO2 phase exhibited the maximum hardness of 14.3 GPa and a Young's modulus of 195 GPa. The UO2 thin film also exhibited good thermal stability in that no phase change was observed after annealing at 600 °C in vacuum for 104 h.

We report on imaging subsurface grain microstructure using picosecond ultrasonics. This approach relies on elastic anisotropy of crystalline materials where ultrasonic velocity depends on propagation direction relative to the crystal axes. Picosecond duration ultrasonic pulses are generated and detected using ultrashort light pulses. In materials that are transparent or semitransparent to the probe wavelength, the probe monitors gigahertz frequency Brillouin oscillations. The frequency of these oscillations is related to the ultrasonic velocity and the optical index of refraction. Ultrasonic waves propagating across a grain boundary experience a change in velocity due to a change in crystallographic orientation relative to the ultrasonic propagation direction. This change in velocity is manifested as a change in the Brillouin oscillation frequency. Using the ultrasonic propagation velocity, the depth of the interface can be determined from the location in time of the transition in oscillation frequency. A subsurface image of the grain boundary is obtained by scanning the beam along the surface. We demonstrate this subsurface imaging capability using a polycrystalline UO2 sample. Cross section liftout analysis of the grain boundary using electron microscopy was used to verify our imaging results.