Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Identification of atomic and microstructural responses in anhysteretic Fe-based ferromagnetic shape memory alloys(2018) Steiner, Jacob; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Very soft ferromagnets are commonly amorphous so that no magnetocrystalline anisotropy energy density contributes to the coercivity. This thesis focuses on a class of Fe-based alloys softer than amorphous ferromagnets but crystalline in structure, exhibiting linear, isotropic, as well as totally hysteresis-free magnetization. This class includes Fe-Ga, Al, Ge, Si and Pd alloys, and the majority of the experimental studies focused on FeGa alloys with compositions between 17 and 26 at. % Ga. FeGa has seen much research since the discovery of its large saturation magnetostriction, e||-⊥, up to 310 ppm, reported by Clark et al. in 2000. Our studies probed the magnetic, magnetostrictive, and structural characteristics of these alloys to elucidate the origin of its anomalous magnetic and magnetoelastic properites. The magnetostriction we observe defies classical theories established by Joule in 1847, which pertain only single phase, crystalline materials. Magnetic anisotropy measurements demonstrate the FeGa alloys possess both cubic and uniaxial symmetry, indicating the presence of more than one phase, and a measured soft anisotropy constant of 1000 J/m3 for the cubic symmetry deviates from conventional proportionality between large magnetostriction and magnetic anisotropy for the majority of materials. Selected area diffraction and nanoelectron diffraction performed in a transmission electron microscope confirm the multi-phase nature of the FeGa alloys' microstructure, including disordered A2, ordered D03, and 6M D03 martensite phases. High resolution images show the microstructure is comprised of ~5 nm crystallites, even for alloys manufactured to be single-crystalline. Novel in situ field measurements were carried out in the microscope to probe the structure as a function of field, and these results demonstrate that the volume fraction of D03 appears to vary in response to the field. It is also shown that the magnetic and structural characteristics of FeGa alloys change with repeated cycles of thermal and magnetic measurements. The Fe82Ga18 alloy studied exhibited increased e||-⊥ from 300 to 600 ppm, increased signal for uniaxial magnetic anisotropy, and increased D03 and 6M volume fraction. These results have signicant implications for future modelling of magnetostrictive behavior that takes into account varying phase content of multi-phase alloys, and the results also highlight the significance of processing and kinetics in the Fe-Ga system.Item Characterization of Bending Magnetostriction in Iron-Gallium Alloys for Nanowire Sensor Applications(2008-11-21) Downey, Patrick; Flatau, Alison; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This research explores the possibility of using electrochemically deposited nanowires of magnetostrictive iron-gallium (Galfenol) to mimic the sensing capabilities of biological cilia. Sensor design calls for incorporating Galfenol nanowires cantilevered from a membrane and attached to a conventional magnetic field sensor. As the wires deflect in response to acoustic, airflow, or tactile excitation, the resultant bending stresses induce changes in magnetization that due to the scale of the nanowires offer the potential for excellent spatial resolution and frequency bandwidth. In order to determine the suitability for using Galfenol nanowires in this role, the first task was experimentally characterizing magnetostrictive transduction in bending beam structures, as this means of operation has been unattainable in previous materials research due to low tensile strengths in conventional alloys such as Terfenol-D. Results show that there is an appreciable sensing response from cantilevered Galfenol beams and that this phenomenon can be accurately modeled with an energy based formulation. For progressing experiments to the nanowire scale, a nanomanipulation instrument was designed and constructed that interfaces within a scanning electron microscope and allows for real time characterization of individual wires with diameters near 100 nm. The results of mechanical tensile testing and dynamic resonance identification reveal that the Galfenol nanowires behave similarly to the bulk material with the exception of a large increase in ultimate tensile strength. The magnetic domain structure of the nanowires was theoretically predicted and verified with magnetic force microscopy. An experimental methodology was developed to observe the coupling between bending stress and magnetization that is critical for accurate sensing, and the key results indicate that specific structural modifications need to be made to reduce the anisotropy in the nanowires in order to improve the transduction capabilities. A solution to this problem is presented and final experiments are performed.