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Far-From-Equilibrium Processing of Materials under Extreme Conditions

This research project aims to acquire fundamental insight into the formation and properties of material phases formed under far-from-equilibrium conditions, including mechanochemical synthesis and exposure to highly energetic ion beams. The innovative strategy of this project is the application of advanced neutron total scattering experiments, coupled with physical property measurements, to investigate the nature of induced structural disorder in complex oxides over a range of length-scales. This approach presents a significant improvement over traditional long-range characterization techniques, utilizing x-ray and electron probes, that are insensitive to anion sublattices and the unique aperiodic, short-range structural features produced by extreme processing conditions. Our preliminary work has revealed that disorder in milled and ion irradiated ceramics is more complex than previously thought, with locally ordered structural motifs that are specifically arranged such that the average, long-range structure does not represent the actual local atomic configuration. The formation process of this disorder appears to be decoupled across length scales, proceeding locally at different rates than over longer length scales. This finding is of importance as disorder is inherent to many energy-related applications under which materials must perform in harsh environments. Using pyrochlore oxides as a model system, this project will entail mechanochemical synthesis and ion irradiation to produce well-defined non-equilibrium phases with a range of disorder and defect structures. Systematic analysis of the structural behavior with coupled experiments and modeling will identify the underlying processes that drive the formation of unconventional disorder across length scales. The outcome of this research project will not only help to build a robust atomic-scale understanding of far-from-equilibrium phases with high levels of disorder accessible through extreme environment processing, but will also enable the use of both processing techniques as a means to tailor physical properties (e.g., oxygen transport) to enhance functionality in technological applications (e.g., solid oxide fuel cells).

Materials Science of Actinides

The research performed within this project is part of a Department of Energy funded Energy Frontier Research Center (Materials Science of Actinides). The mission is to conduct transformative research in actinide sciences in order to understand and predict the performance of nuclear materials that must function in a reactor (e.g., nuclear fuel) or must safely encapsulate radionuclides over millennia (e.g., nuclear waste forms). We perform studies on damage formation and recovery mechanisms, defect behavior, phase transformations and nanoscale structural evolution in materials under irradiation, pressure, and/or temperature.

(Courtesy of Cameron Tracy)

For example, energetic radiation can cause dramatic changes in the physical and chemical properties of actinide materials, degrading their performance in fission-based energy systems. We have studied, by means of X-ray scattering experiments, the physical principles underlying radiation damage production in chemically complex actinide and surrogate materials (UO2, CeO2, and ThO2) and found that the redox behavior of the actinide materials plays a critical role in the observed radiation response under fission fragment-like ion irradiation (C.L. Tracy et al., Nature Communications 2015). This knowledge is important to design advanced nuclear fuels and predict their performance in engineering systems.

In addition to nuclear fuel materials, we also investigate the defect behavior in ceramics, such as pyrochlores. Complex oxides are candidate materials for a wide range of engineering applications, such as solid electrolytes for fuel cells, host materials for nuclear waste containment, and thermal barrier coatings for gas turbine jet engines. Predicting the transport of radionuclides is important for their safe use as nuclear waste forms and requires a detailed knowledge of how the atomic structure responds to self-irradiation. When subjected to extreme environments such as high temperatures or highly ionizing radiation, many of these compounds partially lose their long-range crystal structure. This means that A-site and B-site cations were thought to randomly mix in the material. Neutron total scattering experiments, however, revealed the cations and oxygens in the materials are not randomly arranged at the atomic level, but only appear so when sampling over longer scales (J. Shamblin et al., Nature Materials 2016). This discovery indicates that discrepancies may arise when extrapolating the materials structure from the microscale to the atomic scale (or vice versa) and an accurate description of the local defect structure is needed in order to accurately model waste form properties and degradation phenomena.

Radiation Effects in Alloys and Metals: A New Forensic Signature

This is a Department of Homeland Security funded project, which addresses the pressing need to develop and produce new nuclear forensic signatures to rapidly identify interdicted nuclear materials. In collaboration with UT’s Institute of Nuclear Security (Radiochemistry Center of Excellence), we perform research on different alloys and metals to correlate the build-up of structural modification from self-irradiation, environmental factors (e.g., oxidation), and temperature exposure (e.g., grain-size changes) with the aging of the material. Four-probe measurements are used to characterize the defect behavior in these materials through the detection of changes in electrical resistivity. Complementary to these measurements, we utilize broadband dielectric spectroscopy to assess the frequency-dependent resistivity in damaged materials. This provides additional information on the defect kinetics at elevated temperatures, which may be an independent signature of the damage accumulation process in nuclear materials and the associated elapsed time. In the future, the observed changes in physical properties (i.e., electrical conductivity) due to radiation effects will be linked to the underlying defect structure using X-ray and neutron scattering measurements. By carefully quantifying and correlating physical property measurements with structural properties, we aim to provide an additional route to age-date nuclear materials.

Actinide Materials under Extreme Conditions

The investigation of the behavior of materials under extreme conditions has become an important focus in materials science and energy research. Through affiliation with a Carnegie/Department of Energy Alliance Center, we investigate the behavior of actinide compounds, e.g., U3O8, UO3, (U, Th)O2, UO2+x, and (U, Th)C, under a range of coupled extreme conditions. These materials exhibit diverse and unique phase transformations and unusual physical properties under varying degrees of pressure and temperature. To contribute to the understanding of this unique class of materials under extreme conditions, we investigate the largely unexplored phase space of a number of actinide compounds under energetic heavy ion irradiation, high pressure, and/or high temperature. This work, coupled with X-ray and neutron scattering and impedance spectroscopy studies, will yield fundamental insights regarding defect formation, defect annealing, and defect-driven phase transformations under extreme conditions, including the correlation of kinetics and thermophysical properties to changes in the electronic structure of the materials. This knowledge will accelerate the discovery of novel actinide materials that may have unanticipated properties, such as improved radiation tolerance. This research offers a unique opportunity to synthesize and tailor the properties and functionality of innovative forms of matter through manipulation of localization and delocalization of 5f electrons in actinide elements.