ABSTRACT

NON-TECHNICAL SUMMARY: Materials which are stable to very high temperatures are needed for applications in aerospace, energy, and other technologies. Many such materials contain rare earth oxides, critical materials in potential short supply. Novel experimental and computational methods are studying the structure and stability of such rare earth oxides. The methodology includes diffraction (determination of crystal structure) and calorimetry (measurement of heat effects associated with melting and other reactions) on laser heated samples levitated in a gas stream and not contaminated by contact with other materials, as well as theoretical calculations. Such studies offer a unique opportunity to obtain fundamental understanding of structures, phase transitions, and melting properties and applications to technological problems. The project will also advance general experimental and computational techniques for high temperature research. It will offer opportunities for both undergraduates and graduate students in materials science, chemistry, physics, and engineering to take part in state-of-the-art research in both university and national laboratory settings. Rare earth oxides are "critical materials" in the sense of being technologically indispensable, but with limited supply. Developing energy-efficient, and environmentally-friendly processes "from cradle to grave" or, better still "from cradle to cradle" (involving recycling) is essential to sustainable management of resources and technology.

TECHNICAL DETAILS: Rare earth oxides are critical materials essential to many important technologies yet their high temperature properties, needed for such applications, are poorly known. Their structure and thermodynamics above 2000°C are being studied using a combination of novel experimental and computational methods. Aerodynamic levitation and laser heating are being used for in situ X-ray diffraction at the Advanced Photon Source, in situ neutron diffraction at the Spallation Neutron Source and for drop calorimetry at the UC Davis Peter A. Rock Thermochemistry Laboratory. Enthalpies of solid state phase transitions and fusion are being measured by calorimetry, and volume changes on phase transitions and thermal expansion of high temperature phases are being determined by diffraction. High temperature thermal analysis complements drop calorimetry. Fusion enthalpies are required for reliable calculations of eutectics of multi-component systems containing rare earths. Ab initio computations of high temperature heat capacities, thermal expansion, and temperatures and enthalpies of phase transitions and fusion are being performed at Brown University using post-density functional theory (DFT) methods, such as hybrid functionals and DFT+U, in conjunction with statistical mechanics techniques such as cluster expansion, lattice dynamics analysis and molecular dynamics. Computed thermal expansion and enthalpies of phase transitions and fusion are being compared to experimental measurements and calculations are being extended to high temperature properties of rare earth oxides not yet accessible experimentally. Thermodynamics are central to optimizing the processes for the rare earth oxides; as such this project contributes to sustainability.

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