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
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Item Orientation-dependent Surface Energy Characterization of Magnetostrictive Alloys for Abnormal Grain Growth Modeling(2018) Van Order, Michael Norman; Flatau, Alison B; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High sensitivity semiconductor, optical, and sensing devices require single crystal metals for their isotropy which allows for unique material properties. However they are expensive and difficult to produce. By using abnormal grain growth (AGG) techniques, our group can produce single-crystal-like materials that achieve ~90% performance of true single-crystals at ~5% of the cost. Fully understanding AGG mechanisms is crucial to the growth of high quality, cost effective alloy fabrication. Many advances have been made to understand key parameters of AGG, and we postulate that the final piece lies in understanding the surface energy of our alloys. While several surface energy measurement techniques have been developed for low-energy plastic surfaces, high-energy metal surfaces have largely been ignored due to the complexity of sample preparation and experimentation. This dissertation investigates three measurement techniques targeted for high-surface-energy iron-alloy crystal facets. The first of these techniques, the gallium drop contact angle method, examined a droplet of liquid metal gallium resting on our metal sample. By recording the shape of this droplet, a value for surface energy of targeted crystal orientations is acquired using our derived thermodynamically-based mathematical model. This study experimentally confirmed trends that are predicted in theoretical models, but identified that oxide formation on the sample surface interferes with acquisition of accurate quantitative results. This revelation led to a more robust study that expands on classic drop shape analysis techniques and eliminates complications associated with oxide layer formation. For this, multiple oxide removal procedures were performed and analyzed using X-ray photoelectron spectroscopy. The most promising procedures are polishing in an inert atmosphere and ion bombardment cleaning. Immersing the sample in an oil environment isolates this unstable iron-alloy surface from air and prevents oxidation. While in this environment, samples are probed with a deionized water droplet and a shape analysis is performed to calculate surface energy values using the Schultz method. This dissertation describes modifications to this method that utilize a technique previously only used on plastics to prevent water from spreading. I hypothesize that patterning sample surfaces with an ion mill will stabilize droplets during shape measurements, thus generating reliable surface energy calculations. Success of each technique could allow metallurgists to finally experimentally measure surface energy for any metal surface, thus providing confirmations of theory and sparking new ideas of how grain growth in metals can be controlled and even manipulated.Item CHARACTERIZING THE QUASI-STATIC AND DYNAMIC RESPONSE OF A NON-CONTACT MAGNETO-ELASTIC TORQUE SENSOR(2017) Muller, Brooks Richard; Flatau, Alison; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Advances in the development of rolled-sheet magnetostrictive materials led to testing of a prototype wireless magneto-elastic torque (WiMET) sensor using the iron alloy Galfenol. As torque was applied to a shaft, stress-induced changes in the magnetic state of Galfenol that was bonded to the shaft were proportional to the applied torque. Building on that work, this thesis investigates strategies to improve both repeatability and the signal to noise ratio of WiMET sensor output. Multi-physics models of WiMET stress and magnetic states under applied torques are used to improve understanding of sensor operation. Testing to validate simulations is performed using Galfenol and Alfenol, a newer rolled-sheet alloy, for torsional loads of 0 – 200 in-lb, and under quasi-static and dynamic (0 – 2000 RPM) loading conditions. The experimental results presented support the potential of WiMET sensor use for dynamic torque measurement and health monitoring of drive train systems.Item CHARACTERIZATION AND MODELLING OF MAGNETO-AUXETICITY, THE MAGNETICALLY INDUCED AUXETIC BEHAVIOR, IN GALFENOL(2015) Raghunath, Ganesh; Flatau, Alison B; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Iron-Gallium alloy (Galfenol) is a magnetostrictive smart material (λsat ~400 ppm) with potential for robust transduction owing to good magneto-mechanical coupling and useful mechanical properties. In addition, Galfenol exhibits a highly negative Poisson’s ratio (denoted by ν) along the <110> crystallographic directions on {100} planes with ν values of as low as -0.7 under tensile loads. Consequently, their samples become wider when elongated and narrower when compressed (aka auxeticity). This is an anisotropic, in-plane and volume conserving phenomenon with compensating contractions and expansions in the third (out of plane) direction. Since there is good magneto-elastic coupling in Galfenol, a negative Poisson’s ratio is expected to be observed under application of magnetic fields even under zero stress conditions. This work deals with systematically studying the magneto-elastic contributions in Galfenol samples between 12 and 33 atomic percent Ga as a non-synthetic (no artificial linkages, unlike foams) ‘structural auxetic’ material, capable of bearing loads. This investigation addresses the profound gap in understanding this atypical behavior using empirical data supported by analytical modeling from first principles to predict the Poisson’s ratio at magnetic saturation, multi-physics finite element simulations to determine the trends in the strains along the <110> {100} directions and magnetic domain imaging to explain the mechanical response from a magnetic domain perspective. The outcome of this effort will help comprehend the association between anisotropic magnetic and mechanical energies and hence the magnetic contributions to the atomic level interactions that are the origins of this magneto-auxetic characteristic. Also, it is well established that a number of mechanical properties such as shear resistance and toughness depend on the value of Poisson’s ratio. There is a slight increase in these mechanical properties with non-zero ν values, but as we enter the highly auxetic regime (n<-0.5), these values increase by magnitudes. Hence, the possibility of n values approaching -1.0 under applied magnetic fields at zero stress is extremely intriguing, as these properties can be much larger than is possible in conventional materials. This has potential for several novel applications where the value of Poisson’s ratio can be magnetically tuned to keep it near -1 under applied stresses.Item Development of a Bio-Inspired Magnetostrictive Flow and Tactile Sensor(2012) Marana, Michael Adam; Flatau, Alison; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A magnetostrictive sensor was designed, constructed, and evaluated for use as flow or tactile sensor. Vibrissa-like beams (whiskers) were cut from sheets of the magnetostrictive iron-gallium alloy, Galfenol. These beams were cantilevered, with the fixed end of the whisker attached to a permanent magnet to provide the whisker with a magnetic bias. The free portion of the whisker was quasi-statically loaded, causing the whisker-like sensor to bend. The bending-induced strain caused the magnetization of the whisker to change, resulting in a changing magnetic field in the area surrounding the whisker. The change in magnetic field was detected by a giant magnetoresistance (GMR) sensor placed in proximity to the whisker. Therefore, the electrical resistance change of the GMR sensor was a function of the bending in the whisker due to external forces. Prototype design was aided using a bidirectionally coupled magnetoelastic model for computer simulation. The prototype was tested and evaluated under tactile loading and low speed flow conditions.Item MAGNETIC AND STRUCTURAL CHARACTERIZATION OF Fe-Ga USING KERR MICROSCOPY AND NEUTRON SCATTERING(2010) Mudivarthi, Chaitanya; Flatau, Alison B; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fe--Ga alloys belong to a class of smart materials called magnetostrictive materials. Magnetostrictive materials show dimensional (magnetostriction) and magnetization changes in response to magnetic and elastic fields. These effects can be utilized for transduction purposes. Most widely used magnetostrictive materials like Tb-Dy-Fe (Terfenol-D) show giant magnetostriction (∼2000 ppm) but suffer from low modulus of elasticity, low tensile strength and are extremely brittle, limiting their usage to applications involving only axial loads. Fe--Ga alloys have recently been discovered to show an extraordinary enhancement in magnetostriction (from 36 ppm to 400 ppm) with the addition of the nonmagnetic element, Ga. Though their magnetostriction is less than that of Terfenol-D, they boast superior properties such as ductile-like behavior, high tensile strengths (&sim 400 MPa), low hysteresis, and low saturation fields (&sim 10 mT). Understanding the origin of the magnetostriction enhancement in these alloys is technologically and scientifically important because it will aid in our quest to discover alloys with higher magnetostriction (as Terfenol-D) and better mechanical properties (as Fe--Ga). With the goal of elucidating the nature of this unusually large magnetostriction enhancement, Fe--Ga solid solutions have recently been the focus of intense studies. All the studies so far, show the existence of nanoscale heterogeneities embedded in the cubic matrix but the experimental means to correlate the presence of nanoscale heterogeneities to the magnetostriction enhancement is lacking. In this work, Fe--Ga alloys of various compositions and heat treatments were probed at different length scales - lattice level, nano-, micro-, and macro-scales. Neutron diffraction was used to probe the alloy at the lattice level to identify the existence of different phases. Small-Angle Neutron Scattering (SANS) experiments were used to study the nanoscale heterogeneities and their response to the applied magnetic and elastic fields. Ultra small-angle neutron scattering (USANS), magnetic force and Kerr microscopy were used to investigate the response of magnetic domains under externally applied magnetic and elastic fields. Piecing the results from lattice level, nano-, micro-, and macro-scales together with the macroscopic magnetostriction measurements, the nature of the magnetostriction in Fe--Ga alloys was uncovered. No evidence could be found that directly relates the presence of heterogeneities to the enhanced magnetostriction. Further, it was found that the observed heterogeneities were possibly of DO3 phase and are detrimental to the magnetostriction.Item Experimental Investigation of the Mechanical Properties and Auxetic Behavior of Iron-Gallium Alloys(2009) Schurter, Holly Marie; Flatau, Alison B; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Iron-gallium alloys (known as Galfenol) are a unique material that have shown great potential for numerous applications. They exhibit a strong magneto-mechanical coupling, otherwise known as magnetostriction, which lends itself very well to transducer applications, from the nano-scale to macro scale. In addition, Galfenol is one of only a few metal alloys known to exhibit large auxetic or negative Poisson's ratio behavior. In order to develop any Galfenol-based applications, it will be necessary to understand its mechanical behavior. The goal of the research presented in this thesis therefore is to measure the elastic properties of Galfenol for a range of compositions in order to create a database, as well as present trends in the elastic properties. This is achieved through tensile testing of single-crystal Galfenol dogbone-shaped specimens and through resonant ultrasound spectroscopy (RUS) of small parallelepiped samples.Item Development and Validation of a Bidirectionally Coupled Magnetoelastic FEM Model for Current Driven Magnetostrictive Devices(2009) Graham, Frank; Flatau, Alison; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A bidirectionally coupled magnetoelastic model (BCMEM) has been extended to include electric currents in its magnetic finite element formulation. This enables the model to capture the magnetoelastic behavior of magnetostrictive materials subjected to elastic stresses and magnetic fields applied not only by permanent magnets but also by current carrying coils used often in transducer applications. This model was implemented by combining finite element solutions of mechanical and magnetic boundary value problems using COMSOL Multiphysics 3.4 (Finite Element Modeling software) with an energy-based non-linear magnetomechanical constitutive model. The BCMEM was used to simulate actuator load lines and four point bending results for Galfenol, which were then compared to experimental data. The model also captured the ΔE effect in Galfenol. The BCMEM can be used to study and optimize the design of future current driven magnetostrictive devices.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.Item Combinatorial Investigation of Magnetostrictive Materials(2007-08-24) Hattrick-Simpers, Jason Ryan; Takeuchi, Ichiro; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Combinatorial materials synthesis is a research methodology, which allows one to study a large number of compositionally varying samples simultaneously. We apply this technique in the search for novel multifunctional materials. The work presented here will discuss the combinatorial investigation of novel magnetostrictive materials. In particular, binary Fe-Ga and the ternary Fe-Ga-Al, Fe-Ga-Pd systems are studied. Magnetron co-sputtered composition spread samples of the alloys have been fabricated to study composition dependent trends in magnetostriction. Magnetostriction measurements on all systems studied here have been carried out by optically measuring the deflection of micro-machined cantilever arrays. Measurements of the magnetostriction on binary Fe-Ga thin-films show similar compositional trends as had been reported in bulk systems. The maximum value of magnetostriction observed is 220 ppm, which is comparable to bulk values. A previously unreported minor maximum in magnetostriction as a function of composition has been found for Ga contents of about 4 at%. It is believed that the origin of this minor maximum is related to a peak in the magnetic moment of Fe atoms in Fe-Ga alloys at this composition. We have mapped the Fe-Ga-Pd and Fe-Ga-Al ternary systems. Large regions of the phase diagrams have been mapped out in a single experiment, and the observed magnetostrictive dependence on Ga content matches trends seen in bulk. It was found that the trend of magnetostriction deviated from that of bulk with the inclusion of as little as 1 at% Pd. The addition of up to 10 at % Al to Fe70Ga30 was possible without severe degradation of its magnetostriction.Item Design and Testing of a Galfenol Tactile Sensor Array(2006-12-20) Hale, Kathleen; Flatau, Alison; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The smart material Galfenol, Fe(100-x)Ga(x), where 15<x<28, offers a unique combination of mechanical and magnetostrictive properties that are expected to lead to its use in new sensor and actuator concepts. This thesis seeks to determine if Galfenol can be used to develop a 2-dimensional array of force sensors as part of a 3-D magnetic circuit that, if properly scaled, could mimic the tactile force sensing capabilities needed for use in robotic grippers, prosthetic devices, and robotic surgery. This concept takes advantage of the fact that Galfenol is not brittle and its permeability has high sensitivity to mechanical loads. The hypothesis is that applying stress or force to one or more of the Galfenol rods will produce changes in Galfenol's permeability which will produce changes in the flux density distribution in the magnetic circuit that can be used to determine information about both the load's magnitude and location. The studies performed demonstrated that the decrease in permeability of a loaded rod results in complex changes in magnetic flux. Results from this thesis include recommendations for modifications to better match the rod flux density to the applied load levels and prevent rod top separation.