Doctoral Dissertations
Permanent URI for this collection
Browse
Recent Submissions
- Practical Pathways to Efficient MRAM: Spin-Orbit Torques, Low-Damping, and Anomalous Hall Conductivity in Polycrystalline MaterialsMaizel, Rachel Emily (Virginia Tech, 2026-02-26)There are multiple pathways forward to next generation magnetic random access memory. In this thesis we explore two simple solutions with industry implementation in mind. The first is a low-damping (α< 5×10⁻³) ferromagnetic single-layer with modest anti-damping spin-orbit torque (SOT), $θ_{DL} ≈ 0.05. Here, we investigate an alternative approach to the traditional heavy metal/ferromagnet bilayer to produce SOTs, which suffers from high-damping that is detrimental to energy-efficiency. Instead, of breaking inversion symmetry at the interface we continually break symmetry along the thickness axis by creating an intentional compositional gradient that is purely ferromagnetic and maintains low damping. Crucially, we find that a compositional gradient is not necessary to achieve large damping-like SOTs, instead finding direct evidence from grazing-incidence x-ray diffraction for a strain gradient. The next pathway investigated is an easy-to-grow, polycrystalline alternative to non-collinear antiferromagnets which require high temperature growth (>400°C). We find that sputter-grown γ-FeMn with no post-annealing, has a small non-zero net magnetization (≈(0.02-0.07)μB/atom) and perpendicular magnetic anisotropy only slightly larger than those found in non-collinear antiferromagnets like Mn₃Sn while still exhibiting a large anomalous Hall conductivity of 14 S/cm at room temperature. We show that these unique magnetic and transport properties are the result of pinning at the grain boundaries which can be tuned to enhance the anomalous Hall conductivity.
- Scalable Surrogates for Counts and Computer ExperimentsBarnett, Steven D. (Virginia Tech, 2026-02-26)Data collected by the Interstellar Boundary Explorer (IBEX), recording counts of heliospheric energetic neutral atoms (ENAs), exhibit a phenomenon that has caused space scientists to revise hypotheses about the physical processes, and computer simulations under those models, that are in play at the boundary of our solar system. Providing estimates and associated uncertainty quantification (UQ) of the rate at which ENAs are generated is vital to theory development and validation. Gaussian processes (GPs) constitute an excellent nonparametric regression tool that can provide accurate out-of-sample prediction and UQ. But GPs are unconventional for modeling non-Gaussian observations, are inefficient on large training data, and struggle to model nonstationary response surfaces, all issues present in the IBEX application. To address this gap, I propose a fully Bayesian, Vecchia-approximated, Poisson deep GP surrogate model. I demonstrate its improved predictive capability over competitors through multiple simulated examples. Further, I develop a novel, fully Bayesian framework for solving Bayesian inverse problems, coupling a Poisson response with a Vecchia-approximated GP surrogate of an expensive simulator with high-dimensional output. I demonstrate the utility of this new framework via simulated scenarios in terms of recovering the "true" computer model parameters and enhancing prediction over models that rely exclusively on physical observations. I apply these new technologies to IBEX satellite data and associated computer models developed at Los Alamos National Laboratory.
- Assessing the Effects of Exoskeletons on Physical Demands, Trip and Slip Risks, and User Perceptions in Manual Mining TasksAkinwande, Feyisayo Alexander (Virginia Tech, 2026-02-25)Work-related musculoskeletal disorders (WMSDs) are a major health concern worldwide in the mining sector and are associated with frequent exposure to risk factors prevalent in manual mining tasks. Occupational exoskeletons (EXOs) are a promising ergonomic intervention to mitigate WMSD risk, by reducing physical demands and improving work performance. The purpose of this dissertation was to assess the potential benefits of using EXOs for addressing health and safety challenges encountered by miners, while also examining the limitations associated with EXO use, as a means of providing new evidence to guide the effective selection and application of passive arm-support exoskeletons (ASEs) and back-support exoskeletons (BSEs), help avoid unintended/preventable side effects resulting from this technology, and aid in maximizing the benefits of EXO use in mining. The first study identified and assessed the opportunities for and feasibility of implementing EXOs in mining, through an online survey with industry stakeholders. Miners reported potential benefits of EXOs for lifting and overhead work and shared concerns about EXO use. They also emphasize the need to ensure task compatibility, comfort, and affordability to ensure safe and effective adoption in mining. The second study quantified the potential benefits and risks of using ASEs and BSEs for diverse manual mining tasks using controlled lab-based simulations. Both ASE and BSE effects were device- and task-specific. BSEs significantly reduced peak trunk extensor activity during lifting and overhead installation tasks, although perceptions of exertion and discomfort differed by device: soft BSE reduced perceived upper-back exertion, whereas rigid BSE increased waist/hip discomfort. ASEs also differed in their effects on total shoulder muscle activity across tasks, but their use reduced perceived exertion across most body regions with minimal reported discomfort. The third study assessed the effects of BSEs on trip and slip risks during load carriage on different surface slopes. Using both BSEs differentially altered minimum foot clearance (MFC) and required coefficient of friction (RCoF). Rigid BSEs increased right foot MFC and RCoF, whereas the soft BSE largely preserved baseline gait mechanics, with no significant effects on objective slip or trip risk metrics. Overall, we found that the efficacy of ASEs and BSEs are highly device- and task-dependent. These results provide critical insights to inform evidence-based guidelines for the safe implementation of occupational EXOs in mining and other physically demanding industries.
- Overcoming Challenges in Reversible Addition–Fragmentation Chain Transfer Polymerization using Photoinduced Electron/Energy Transfer CatalysisBaker, Jared Galen (Virginia Tech, 2026-02-24)The development of polymerization methodologies is discussed, with an emphasis on addressing two limitations in reversible addition–fragmentation chain transfer (RAFT) polymerization. Both methods employ photoinduced electron/energy transfer (PET) catalysis to generate radicals within the polymerization system. PET catalysis was selected as the initiation pathway because the rate of radical introduction is tunable across a wide range of conditions, including, but not limited to, photocatalyst identity, photocatalyst concentration, wavelength of light, light intensity, and temperature. The first limitation that is discussed is the incorporation of a single monomer unit at a defined position within the backbone of a polymer chain. Previously, a single monomer unit could be incorporated only at the beginning or end of a polymer chain in reversible-deactivation radical polymerization (RDRP), or otherwise, single units could undergo multiple incorporations. Using expansive condition screening with PET-RAFT polymerization, a set of conditions was identified that resulted in a single-unit monomer insertion (SUMI) within the polymer backbone. The reaction depended on polymer concentration, monomer concentration, and temperature, and resulted in no detectable double- or higher-order insertions. The second limitation addressed was the blocking order requirements associated with both RAFT polymerization and RDRP. This limitation had previously been studied in the field, but it either exhibited termination reactions or applied only to specific systems. Using in-depth kinetic experiments and computational studies, we identified unique conditions that enabled us to overcome the blocking-order requirements associated with RDRP. Polymer and photocatalyst concentrations were crucial to the success of the reaction. Using this method, the impact of blocking order on material properties was evaluated and found to affect material behavior significantly. The method yielded a novel high-molecular-weight thermoplastic elastomer that retained its shape. Expanding on the second method, we sought to maximize chain end fidelity and further elucidate the underlying rates of the technique. By coupling the development of new characterization methods to quantify rates within the system and extensive kinetic experiments, the ratio of the rate of trithiocarbonate activation to the rate of trithiocarbonate termination could be measured. By optimizing conditions to achieve a high ratio of trithiocarbonate activation to termination, a significant increase in chain end fidelity relative to the previously identified conditions was achieved and led to a deeper understanding of which conditions are vital to the method. Lastly, the impact of PET-RAFT polymerization on the uniformity and properties of polymer networks is discussed. In this study, PET-RAFT polymerization yielded controlled networks initially, but could not yield controlled chain extended networks, resulting from decreased chain mobility in the networks. However, PET-RAFT polymerization enabled access to tunable properties, resulting in changes to the hydrophobicity and swelling ratios of the networks.
- Beyond Oil Wealth: Resiliency of the Aliyev Administration in AzerbaijanSullu, Yagiz (Virginia Tech, 2026-02-24)In political science literature, many scholars highlight how natural resources, particularly oil, have often been more of a curse than a blessing for developing countries. The Dutch Disease phenomenon highlights that the rapid development of the natural resource sector leads to a decline in other sectors, primarily the manufacturing and agricultural industries. It also leads to currency appreciation, which makes a country's exports more expensive and less competitive in the global markets. Politically, developing countries with rich natural resources tend to be more authoritarian, and oil wealth contributes to the durability of authoritarian regimes. Many scholars have predicted that oil-dependent regimes would be vulnerable to oil price shocks and periods of declining production. However, examining the political trajectories of oil-dependent regimes during this period reveals that while some have experienced political instability and regime breakdown, others continue to remain in power. The literature on the politics of oil offers some insights into why certain regimes persist while others collapse during oil price shocks. However, the terminal decline in oil production is an emerging concept, and its political impact has received limited attention in the literature. In a case study of Azerbaijan, this dissertation aims to explain the durability of Ilham Aliyev's oil-dependent regime amid a terminal decline in oil production that started in 2010. Drawing on theories of how political institutions contribute to regime durability, this dissertation will highlight the role of the regime's ruling New Azerbaijan Party and its coercive institutions in maintaining power during this challenging time.
- Kinetics of propylene disproportionation over a cobalt oxide-molybdena-alumina catalystLewis, Michael Justin (Virginia Polytechnic Institute, 1968)
- The thermodynamic behavior of polyethylene at its melting pointWaldman, Nathan (Virginia Polytechnic Institute, 1967)
- Structures and incentives in the public provision: a comparative analysis of finances between community college systems of Virginia and North CarolinaClark, Jeff Ray (Virginia Polytechnic Institute and State University, 1974)
- The metabolism of cellulose dextrins by Cellvibrio gilvusVessal, Mahmood Ismail (Virginia Polytechnic Institute, 1967)
- Cyclic loading and crack propagation--an elastoplastic finite element studyCandogan, Ali (Virginia Polytechnic Institute and State University, 1974)
- Investigation of the turbulent properties of the wake behind self-propelled, axisymmetric bodiesChieng, Ching-Chang (Virginia Polytechnic Institute and State University, 1974)
- A method of approximation for the determination of the transient response of nonlinear feedback control systemsVarner, George Clifford (Virginia Polytechnic Institute, 1967)
- Transients in a monotube boiler: a digital simulationGardiner, Arthur Wilfred (Virginia Polytechnic Institute and State University, 1973)
- Electronic spectra and structure of some electroactive organotransition metal complexesBares, Leo Allan (Virginia Polytechnic Institute and State University, 1974)
- Alum recovery from water and wastewater treatment plant sludgesChen, Ben Hsueh Hai (Virginia Polytechnic Institute and State University, 1973)
- A stochastic model for irreversible, two-step, first-order, biochemical, consecutive reactionsBartley, Sandra Grace (Virginia Polytechnic Institute and State University, 1973)
- Economic strategies for beef cattle production applied to the Shenandoah area of VirginiaChiang, Ying-Chih (Virginia Polytechnic Institute and State University, 1973)
- Nonlinear stability of a thin liquid film adjacent to a compressible streamBordner, Gary Lee (Virginia Polytechnic Institute and State University, 1973)
- The development of a stochastic model of the atmosphere between 30 and 90 km to be used in determining the effect of atmospheric variability on space shuttle entry parametersCampbell, Janet W. (Virginia Polytechnic Institute and State University, 1973)
- The interaction of a turbulent flat-plate boundary layer with the wake of a circular cylinder mounted normal to the plateArimilli, Rao V. (Virginia Polytechnic Institute and State University, 1973)