Theses and Dissertations at Montana State University (MSU)
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Item Broken-symmetry phases of matter and their effects on electronic and magnetic properties(Montana State University - Bozeman, College of Letters & Science, 2023) Peterson, Sean Fahlman; Chairperson, Graduate Committee: Yves U. Idzerda; This is a manuscript style paper that includes co-authored chapters.Physical symmetries inherent to a material are often reflected in its electronic and magnetic properties. The in-plane four-fold rotational symmetry of thin-film ferromagnets inherent to their tetragonal lattice is also exhibited by their cubic anisotropy. The magnetization as a function of applied magnetic field can be calculated via the Stoner- Wohlfarth model. These calculated hysteresis loops were fit to measured hysteresis loops to determine anisotropy constants consistent with known values. An electronic nematic state reduces the in-plane four-fold rotational symmetry of materials by inducing a structural transition from tetragonal to orthorhombic/monoclinic, with two-fold symmetry. This reduced symmetry persists in the electronic thermal transport. Nematicity enhances nearest-neighbor hopping along one axis and reduces it along the other. This results in a deformed Fermi surface compressed (elongated) along the axis of stronger (weaker) electron hopping. This drags van Hove singularities through the Fermi level, affecting quasiparticle lifetimes. Calculating conductivity from the Boltzmann kinetic equation, nematicity enhances thermal transport along one axis and diminishes it along the other. Additionally, s-wave superconductivity coexisting with nematicity creates a feedback on the superconducting gap with a d-wave instability, which can lead to gapless excitations. In the case of weak feedback, nematic superconductors behave like fully-gapped superconductors along both axes, where transport decreases exponentially with temperature. Once gapless excitations form, transport along both axes becomes T -linear at low-T . Similarly, striped antiferromagnetism (AFM2 and AFM3) reduces the rotational symmetry of a square unit cell to a larger two-fold symmetric magnetic cell. Modeling the band structure with a tight- binding model and considering a smaller periodicity in momentum-space, gaps the Fermi surface along one axis. Calculating conductivity reveals diminished transport along one axis and enhanced thermal transport along the other. Considering d-wave superconductivity in this model results in two cases. One has highly anisotropic transport with greatly enhanced T -linear transport along one axis and diminished transport decreasing exponentially with temperature along the other. The second has weakly anisotropic transport with diminished T -linear conductivity along both axes. The symmetry of a material's properties, such as magnetic anisotropy and thermal transport, are intrinsically linked to their crystalline, electronic, and magnetic symmetries.Item Hall effect and electrical conductivity studies of some MHD and fuel cell related materials(Montana State University - Bozeman, College of Letters & Science, 1978) Snyder, Stuart CodyItem Electrical conductivity of MHD coal slags to 2025 K(Montana State University - Bozeman, College of Letters & Science, 1978) Westpfahl, David JohnItem Sr 2-x VMoO 6-Y double perovskites : a new generation of solid oxide fuel cell anodes(Montana State University - Bozeman, College of Letters & Science, 2013) Childs, Nicholas Brule; Chairperson, Graduate Committee: Richard J. Smith; Cameron Law, Richard Smith, Stephen Sofie, Camas Key, Michael Kopcyzk and Michael Lerch were co-authors of the article, 'Electronic current distribution calculation for a NI-YSZ solid oxide fuel cell anode' in the journal 'Fuel cells' which is contained within this thesis.Fuel cells are an attractive power source due to their ability to efficiently convert chemical energy stored in fuel directly into electricity. The ability of Solid Oxide Fuel Cells (SOFCs) to reform hydrocarbons at the anode provides for fuel flexibility, an advantage over other types of fuel cell technologies. The primary goals of this dissertation were to investigate the limitations of the currently used anode cermet material, synthesize a double perovskite material (Sr ₂₋xVMoO ₆₋y) without these limitations and investigate the electrical conduction properties of this mixed ionic and electronic conductor (MEIC) in a SOFC anode environment. The electronic current density limitation of a Ni-YSZ anode was determined through the development of a computer simulation and use of experimental data. The electronic current density distribution for nickel particles in a Ni-YSZ anode was calculated via a Monte-Carlo percolation model. Experiments were performed to determine the failure current densities of thin nickel wires in a SOFC anode environment. The results show a current density limitation of Ni-YSZ anodes that is not expected with MEIC anodes. A MEIC anode material, Sr ₂₋xVMoO ₆₋y, was synthesized and characterized using a variety of techniques. The expected MEIC nature of this perovskite material eliminates a potential anode limitation, while adding other benefits over Ni-YSZ. X-ray diffraction (XRD) was used to verify crystal structure. In contrast to the trace amounts of secondary insulating phases found through XRD, XPS shows a high percentage (85-90%) of these secondary phases at the surface. The electrical conductivity of Sr ₂₋xVMoO ₆₋y was found to exceed that reported for Ni-YSZ anodes in a typical SOFC anode environment. Polycrystalline Sr 1.9VMoO ₆₋y'' samples exhibited higher electrical conductivity than that reported for SrMoO ₃ polycrystalline samples, making it a candidate for being the highest electrical conducting oxide known. These conduction values were only measured after specific thermal treatments in a reducing atmosphere. These treatments reduced secondary surface phases, Sr ₃V ₂O ₈ and SrMoO ₄, into their more conducting counterparts, SrVO ₃ and SrMoO ₃. Vanadium and molybdenum valence state XPS fitting parameters for primary and secondary phases are reported.