Theses and Dissertations
Permanent URI for this collection
Browse
Search Collection
1 - 5 of 34077
-
DissertationConstruction of microstructures in electrodes for enhancement of energy density for lithium-ion batteries.( 2025-05)Lithium-ion batteries (LIBs) technology has advanced rapidly over the past few decades with various applications in the market from portable electronic devices to electric vehicles (EVs). As one of the promising renewable energy sources, the demand for LIBs has been steadily increasing to accelerate the transition from traditional fossil fuel to sustainable energy. Therefore, the development of high-performance electrodes is the key to bridge the increasing energy density to the market requirement. Among the strategies of enhancing energy density of LIBs, increasing the mass loading of electrochemically active material is considered a practical and straightforward way. However, the increased thickness of electrodes presents challenges in fabrication and leads to a higher tortuosity of electrodes because of the randomly distributed solid-state material particles. Consequently, lithium-ion diffusion is hindered, causing performance loss at high cycling rate. In this dissertation, two types of manufacturing methods, acoustic-field particle patterning and phase-inversion, are investigated to aid the building of microstructures in the electrodes with high areal capacity (> 3 mAh/cm2) to facilitate ion transport. As the first research in implementing acoustic-field particle patterning in electrodes fabrication for LIBs, we successfully constructed the ordered structures to boost electrochemical performance while maintaining low-cost and efficient processing. Additionally, this manufacturing method has demonstrated its versatility in producing electrodes with various chemistries, including LiFePO4 (LFP), Li4Ti5O12 (LTO) and sulfur. On the other hand, we achieved a higher compressive modulus and reduced tortuosity of thick electrodes by adjusting the composition of non-solvent for the phase-inversion process. Furthermore, we explored the integration of both acoustic-field patterning and phase-inversion methods to regulate micro-channels generated during fabrication process. With all the outcome in the research, this dissertation provides novel approaches to design and construction of low-tortuosity structures in the high-performance electrodes for enhancement of energy density for LIBs.
-
ThesisReproductive characteristics of female alpacas: A retrospective analysis of a North American alpaca herd( 2025-05)The alpaca (Vicugna pacos), a South American camelid domesticated for fiber, meat and cultural value, has become a livestock species of interest in the U.S. because of their sustainable production of premium fiber. With approximately 13,123 alpacas, the U.S. hosts the world’s fourth-largest alpaca population. Distinct environmental and management conditions have likely differentiated this North American population from the South American population that is more plentiful. While South American alpaca reproduction has been extensively studied, the North American herd remains understudied, creating a critical research gap as demand for sustainable alpaca products rises. This thesis characterizes reproduction parameters in a single North American Huacaya alpaca herd over an 11-year period (2011–2022). A total of 874 breeding records from 268 females was used to generate herd descriptive statistics. All but 2.2% of females conceived during the 6-month period encompassing spring and summer. The average number of services per conception (SPC) was 1.86 1.5, and 80.7% of females conceived with only one or two matings. The mean gestation length (GL) was 344.0 ± 10.7 days, with a range of 316-374 days. The average cria birthweight (BW) was 8.10 ± 1.2 kg, with a range of 4.2-12.2 kg. Data were organized for further analysis into two subsets consisting of only first-parity females (n = 132). The first subset contained only first-parity records (n=132), whereas the second subset included records from the same females from all parities including the first (n = 433). Factors that could potentially influence the response variables of GL, SPC, and BW were examined; those factors included season of conception [S; summer(May, June, July, August), fall (September, October, November, December)], dam age at first successful conception (ADFS; first-parity females only), dam age at conception (second and later parities), cria fleece color, dam fleece color, calving interval (CI), and parity. Season of conception influenced GL of first-parity females. The GL of females that conceived in fall season had a shorter (P<0.001) gestation length (336.0 ± 2.95 days) than females that conceived in the summer season (350.4 ± 0.89 days). Cria birthweight (BW) was significantly influenced by GL. Longer mean GL was associated with heavier mean cria BW. For SPC, dams that conceived at 25 months of age required more services (2.33 0.21) than females of other ages. For the analysis of multi-parity records, GL was affected by S, age at conception and CI. The GL was 7 days longer (P<0.0001) for females that conceived in the summer season (350.18 1.66 days) versus the fall season (343.47 2.11 days). Females that conceived at 15-24 months of age had longer (P<0.0001) GL (353.40 3.60 days) than females of older ages. The BW was affected by GL (used as a covariate), S, and parity. Crias born to females that conceived in summer were heavier (8.34 0.1 7kg; P<0.001) than those born to dams that conceived in fall (7.86 0.22 kg). Crias born to first parity females were lighter (7.38 0.1 kg; P<0.001) than those born to multiparous females. This study establishes performance indicators for female alpaca reproduction in North America, emphasizing the roles of seasonality, parity, and herd management in optimizing productivity. By addressing critical knowledge gaps, this thesis highlights opportunities for future research, such as multi-farm studies to enhance reproductive outcomes.
-
DissertationAdvancements in recycled plastic food packaging safety and suitability: Addressing chemicals of concern (CoCs), recycling policies, material processing and performance( 2025-05)The ever-growing nature of plastic production and the accompanying surge in plastic waste bring on an irresistible challenge where plastic recovery and recycling could be vital solutions to combat this catastrophe. While new regulations offer promise for battling this situation through mandatory utilization of recycled plastic in new product manufacturing, the lack of knowledge regarding their chemical safety hinders their full potential. For example, their application as raw material for direct food contact material (FCMs) is challenging without enough information on chemical safety. Therefore, exploring the challenges in post-consumer plastic recycling systems, particularly for high-value applications such as FCMs can shed some light on this field. A comprehensive evaluation of global plastic recycling policies, chemicals of concern such as phthalates and bisphenol, and the identification of intentionally and non-intentionally added substances in post-consumer recycled (PCR) plastic, especially for polyolefins, a common plastic in packaging products are an urgent need. While the urgency for safety and quality evaluation in PCR plastic is undeniable, exploring the challenges in the recycling chain/framework itself is also imperative. Developing a recycling system that assures minimal contamination, cost-effectiveness, sustainability and high-quality plastic generation is essential. The review of EPR policies and regulations provides insight on how the regulations, mandates, and policies influence recycling rates and producer accountability through fees, taxes, and landfill bans. This dissertation compares PCR plastic from material recovery facilities (MRFs) against virgin plastic and compares food and non-food application PCR plastic. The MRF recovered PCR plastic contained higher concentrations of phthalates and bisphenols than virgin plastics, although all samples complied with the Toxics in Packaging Clearinghouse (TPCH) regulations. One crucial focus of this dissertation is to identify intentionally added substances (IAS), non-intentionally added substances (NIAS), and other chemicals of concern (CoCs) and assess their impact on the safety of recycled materials. The Cramer classification system highlights a slightly higher proportion of high-toxicity Class III compounds in PCR polyolefins, especially higher in non-food application PCR polyolefins (polypropylene), emphasizing the need for regular monitoring. Several analyses have been performed to understand the different thermal, physical, and molecular properties of different sources (e.g., food and non-food applications). Another essential aspect addresses the influence of different washing techniques and the recyclability of the wash water during plastic recycling at the recycled plastic recovery/processing facilities. The findings enumerate the importance of surfactants in post-consumer plastic wash water and the implication of ultrasonication in improving phthalates and bisphenols' cleaning efficiency. At the same time, it also generates contaminated wash water with phthalates and bisphenols, underscoring the importance of adequate water treatment before reuse or disposal. Compiling restricted substance list (RSL) from publicly available RSLs provides critical insights into emerging CoCs, their regulatory thresholds, and potential health risks. This RSL would serve as a framework for converters, brand owners, and regulatory decision-makers and help analyze chemical migration risks for food application plastics. This dissertation improves our understanding of the recycling system, strategy, material composition, regulatory compliance and points towards the limitations in current recycling systems and scientific information. The findings identify the need for optimizing post-consumer plastic washing techniques, refining recycling methods, and implementing progressive policies to generate a clean recycling stream, ensuring safety, efficiency, and sustainability in FCM applications. This work is a valuable resource for researchers, manufacturers, and policymakers in reducing plastic waste, increasing utilization in direct food contact applications, and promoting a circular economy. This research's impact is understanding the CoCs in post-consumer plastics, emphasizing the need for improved plastic recycling frameworks and more effective decontamination techniques. Based on the findings of this work, future research should investigate effective washing techniques in lessening CoC contamination without affecting the material's physical properties with more sustainable surfactants to improve the quality and safety of PCR plastic streams.
-
DissertationThree Essays in Macroeconomics( 2025-05)This thesis comprises three essays in macroeconomics that investigate and analyze issues of critical importance to developing economies, which differ markedly from their developed counterparts in several dimensions, including, but not limited to, the level of financial market development, the availability of and accessibility to a comprehensive range of financial instruments, and the robustness of institutional frameworks. These structural disparities along with cultural and societal norms often influence households' choice of specific goods and services, which can, in turn, generate adverse macroeconomic implications for developing economies. Against this backdrop, each chapter in this thesis examines a distinct topic pertinent to developing economies, shedding light on the associated policy challenges and potential opportunities for intervention. More specifically, Chapter 2 examines gold's role as a store of value and its long-term real return in a dynamic, two-country general equilibrium model. Unlike conventional assets like equities and bonds, gold does not generate a pecuniary cash flow. However, investors derive non-pecuniary benefits from holding gold, such as when wearing it as jewelry, and in its role as a hedge against economic uncertainty. Our model incorporates the notion that agents derive utility from holding gold beyond its role as a financial asset, while also considering the agent's utility from consuming a freely tradable common perishable consumption good. We demonstrate that gold’s positive, yet real interest rate trailing return is primarily driven by the relative growth rates of global real output and the global gold stock. Additionally, we explore India’s increasing share in private global gold holdings, examining the implications of persistent gold demand and the role of gold policies, such as tariffs. This study contributes to the literature by providing a micro-founded framework for understanding gold’s economic role within household portfolios. In line with this theme, Chapter 3 explores the interaction between household preferences, portfolio choices, and the risk premium associated with gold. Using a theoretical framework that incorporates preferences over assets, we analyze how the risk premium and allocation to gold in household wealth are influenced by its dual role as a financial asset and an object of intrinsic value. The findings demonstrate that when households derive utility from gold, such as its use as personal adornment, they accept a lower risk premium and allocate a higher share of wealth to gold compared to settings where such preferences are absent. Empirical analysis, based on data from 31 countries between 2010 and 2022, provides qualified support for these theoretical predictions. In countries with a pronounced preference for gold, the risk premium is significantly lower. This result underscores the distinct role of gold in economies with incomplete financial markets, such as India, where households face limited access to formal financial instruments and rely on gold as a store of value and a hedge against economic uncertainties. The study contributes to understanding how cultural and societal preferences shape asset demand and the broader implications for financial markets and policymaking in emerging economies. Chapter 4 develops a theoretical framework to analyze optimal taxation in a small open economy with capital constraints, where the government borrows externally to finance domestic consumption and capital formation, repaying the debt within a finite horizon through proportional income taxes. Government intervention in the presence of market distortions is often welfare-enhancing under certain conditions. A well-established result in the literature is that, with appropriate policy instruments, government transfers can replicate competitive equilibrium allocations in distorted environments. We demonstrate that the optimal strategy involves borrowing entirely in the first period, followed by constant tax rates and consumption levels until the penultimate repayment period. This policy minimizes welfare losses associated with consumption fluctuations and ensures stable capital accumulation. Our findings suggest that extending the maturity of debt can be a crucial policy tool for improving welfare outcomes in constrained economies. The findings have significant implications for constrained economies, particularly in developing regions where borrowing is critical for infrastructure investment.
-
DissertationAdvancing the shifted boundary method: scalable multi-physics simulations on octree meshes for complex geometries( 2025-05)This dissertation advances the field of computational mechanics by developing, optimizing, and applying the Shifted Boundary Method (SBM) integrated with octree-based meshes for accurate and efficient simulation of large-scale problems involving complex geometries. The research is structured into four key contributions: First, the SBM framework is extended and optimized to improve the simulation of Partial Differential Equations (PDEs) on irregular domains. By shifting boundary conditions to surrogate boundaries on Cartesian meshes and correcting them with Taylor expansions, we achieve high accuracy without requiring boundary-fitted meshes. We demonstrate that the numerical error of SBM can be significantly reduced by an optimal choice of surrogate boundaries, with mathematical proofs validating optimal convergence. Second, the dissertation introduces a robust Octree-SBM framework for simulating incompressible Navier-Stokes equations. Leveraging efficient surrogate boundary construction on incomplete and adaptive octree meshes, we address the computational challenges of fluid dynamics in complex geometries. Benchmark simulations confirm the framework's scalability, accuracy, and efficiency across various flow regimes. Third, the SBM is applied to multiphysics thermal-flow simulations, coupling incompressible flow with heat transfer. Using a linearized Navier-Stokes RB-VMS formulation, the framework captures diverse thermal-flow phenomena with precision, enabling accurate enforcement of Dirichlet and Neumann boundary conditions on non-conformal meshes. Benchmark studies over a wide range of Rayleigh and Reynolds numbers validate the approach for laminar to turbulent regimes. Fourthly, the dissertation integrates SBM with adaptive mesh refinement (AMR) for incompressible flow and coupled thermal-flow problems. By employing vorticity-based adaptivity on hierarchical octree meshes, the framework dynamically resolves fine-scale features such as complex vorticity structures and steep thermal gradients while reducing computational costs. This integration enhances the capability to handle non-trivial geometries and evolving flow patterns in distributed-memory environments. Finally, the SBM framework is extended to simulate flow past open-interface geometries. By integrating SBM with octree meshes and leveraging Nitsche's method for weak enforcement of boundary conditions, this extension achieves numerical stability and accurate flow blockage without requiring boundary-fitted or excessively refined meshes. Applications to engineering scenarios confirm the framework's scalability and computational efficiency in addressing large-scale problems involving open-interface geometries. The methodologies and applications presented in this dissertation establish the Octree-SBM framework as a practical and effective tool for solving computational fluid dynamics and multiphysics problems. The framework is shown to be robust, scalable, and adaptable, addressing specific challenges such as complex geometries, diverse flow regimes, and efficient handling of large-scale simulations with accuracy and computational efficiency.