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 Processing and structural characterization toward all-cellulose nanocomposites(2021) Henderson, Doug A; Briber, Robert M; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cellulose is the most abundant biopolymer on the planet and is used in a variety of industry sectors including paper, coatings, medicine, and food. A deep understanding of cellulose is important for its development as an alternative polymer to those based on petroleum. This work focuses on two cellulose systems. The first of these, cellulose nanofibers, are the basic structural elements of naturally-occurring cellulosic materials; they exhibit excellent mechanical characteristics due to high crystallinity and a dense network of hydrogen bonding. These fibers can be separated from bulk cellulose via a TEMPO oxidation reaction followed by mechanical homogenization into a suspension in water. In this work, the production of these fibers is investigated by monitoring the change in structure of cellulose as a function of TEMPO reaction time and mechanical homogenization using small angle neutron scattering, atomic force microscopy, and optical microscopy. The second cellulose system is a molecular solution of cellulose formed using a binary solvent mixture consisting of ionic liquid and an aprotic solvent. Cellulose is difficult dissolve due to a dense hydrogen bonding network, and ionic liquids have been shown to be an effective alternative to more hazardous and energy-intensive dissolution methods for cellulose currently used in industry. The phase behavior of these solutions has been investigated using small angle neutron scattering as a function of temperature. The process of regenerating cellulose from these solutions is also investigated, as dense gels of cellulose and ionic liquid were produced with a unique multiscale ordered structure. The ultimate goal of this work is to combine cellulose nanofibers and molecular cellulose solutions in order to create all-cellulose nanocomposite films. These films are characterized using tensile testing, atomic force microscopy, and water uptake measurements in order to understand the interaction between cellulose nanofibers and molecular cellulose solutions, water resistance and tunability of mechanical properties.Item A Generalized Methodology to Characterize Composite Materials for Pyrolysis Models(2016) McKinnon, Mark; Stoliarov, Stanislav I; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The predictive capabilities of computational fire models have improved in recent years such that models have become an integral part of many research efforts. Models improve the understanding of the fire risk of materials and may decrease the number of expensive experiments required to assess the fire hazard of a specific material or designed space. A critical component of a predictive fire model is the pyrolysis sub-model that provides a mathematical representation of the rate of gaseous fuel production from condensed phase fuels given a heat flux incident to the material surface. The modern, comprehensive pyrolysis sub-models that are common today require the definition of many model parameters to accurately represent the physical description of materials that are ubiquitous in the built environment. Coupled with the increase in the number of parameters required to accurately represent the pyrolysis of materials is the increasing prevalence in the built environment of engineered composite materials that have never been measured or modeled. The motivation behind this project is to develop a systematic, generalized methodology to determine the requisite parameters to generate pyrolysis models with predictive capabilities for layered composite materials that are common in industrial and commercial applications. This methodology has been applied to four common composites in this work that exhibit a range of material structures and component materials. The methodology utilizes a multi-scale experimental approach in which each test is designed to isolate and determine a specific subset of the parameters required to define a material in the model. Data collected in simultaneous thermogravimetry and differential scanning calorimetry experiments were analyzed to determine the reaction kinetics, thermodynamic properties, and energetics of decomposition for each component of the composite. Data collected in microscale combustion calorimetry experiments were analyzed to determine the heats of complete combustion of the volatiles produced in each reaction. Inverse analyses were conducted on sample temperature data collected in bench-scale tests to determine the thermal transport parameters of each component through degradation. Simulations of quasi-one-dimensional bench-scale gasification tests generated from the resultant models using the ThermaKin modeling environment were compared to experimental data to independently validate the models.Item SYNTHESIS OF ZEOLITE@MOF NANOPOROUS COMPOSITES AS BIFUNCTIONAL CATALYSTS(2014) Zhu, Guanghui; Liu, Dongxia; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As nanoporous materials, zeolite and metal organic framework (MOF) share common characteristics of high surface areas and uniform micropores and differ in thermal/mechanical stability and structural flexibility. The integration of MOF and zeolite into composite particles is expected to produce useful hybrid nanoporous materials where inorganic zeolite and organic MOF components impart the advantages of high thermal, mechanical and structural stability of zeolites and specific functionality and high flexibility of MOFs. This thesis work addresses the synthesis of zeolite@MOF composites and the exploration of their applications as bifunctional catalysts in one-pot cascade reaction. Zeolite@MOF core-shell composites have been synthesized by solvothermal growth of MOFs on the surface of ZSM-5 particles. The acidity from framework Al-O(H)-Si sites in ZSM-5 and basicity from amine groups in MOFs obtained by pre-/post-synthetic modification endow zeolite@MOF composites bifunctionality in two-step cascade catalytic reactions.Item A Multi-Scale Approach for Characterizing the Mechanical Behavior of Pin-Reinforced Composite Sandwich Structures with Digital Image Correlation(2013) Brandveen, Bianca Renee; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the last 10 years, pin–reinforced composite sandwich structures have become an interesting research topic in aerospace and naval engineering because of their low weight and high compressive properties. Current models lack rigorous physical understanding of the mechanics of these structures and do not accurately predict their performance. This hybrid numerical–experimental research approach investigates the compressive and flexural mechanical behavior of these materials and also characterizes and models the mechanical response in the form of full–field displacements and strains using Digital Image Correlation (DIC). This thesis establishes an experimental mechanics characterization approach spanning several length scales, including: single pins, representative volume elements, contoured beams, and cylindrical shells with 6” radius of curvature. The previously assumed deformation response of pins within the composite was substantiated with 2D and 3D DIC and extrapolated to the macroscale for both straight and contoured composites.Item Thermomechanical Behavior of Polymer Composite Heat Exchangers(2011) Robinson, Franklin Lee; Bar-Cohen, Avram; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Industrial cooling with seawater, particularly at elevated temperature and salinity, shortens the useful lives of conventional metallic heat exchangers. Cost effective, corrosion-resistant heat exchangers are required to fully utilize available saline water resources. Polymer composites, which use carbon fibers for thermal and mechanical reinforcement, are a promising material for such heat exchangers. The present thesis provides a characterization and thermomechanical analysis of heat exchangers fabricated using thermally conductive polymers. The change in mechanical properties resulting from exposure to saltwater at elevated temperature is characterized for raw and reinforced polymers. Then, thermal performance of such heat exchangers is compared to that of heat exchangers fabricated from conventional corrosion-resistant materials. Finally, the mechanical and combined thermomechanical response of such heat exchangers to conditions typical of LNG operations is studied and compared to that of heat exchangers fabricated from conventional corrosion-resistant materials.Item POLYMER COMPOSITES FOR SENSING AND ACTUATION(2011) Kujawski, Mark Paul; Smela, Elisabeth; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis concerns materials for polymer actuators and mechanical sensors. Polymer actuators are a class of artificial muscle with promising actuation performance; however, they are currently limited by the materials used in their fabrication. The metal-foil type mechanical strain gauges are commercially available and well understood; however, typically have gauge factors less than 5.5 [1], cannot be patterned into custom shapes, and only monitor small areas. New materials provide opportunities to improve the performance of both polymer actuators and mechanical sensors. The aim of this research was to develop, characterize, and implement such materials. Specifically, this thesis describes novel composites of exfoliated graphite (EG) blended with elastomeric hosts. The mechanical and electrical properties of these composites were tailored for two specific applications by modifying the EG loading and the elastomer host: compliant electrodes and strain gauges. Compliant electrodes were demonstrated that had ultimate tensile strains greater than 300% and that could withstand more than 106 strain cycles. Composites fabricated with polydimethylsiloxane (PDMS) exhibited conductivities up to 0.2 S/cm, and having tangent moduli less than 1.4 MPa. This modulus is the lowest reported for loaded elastomers above the percolation threshold. Conductivity was increased to more than 12.5 S/cm by fabricating composites with polyisoprene (latex) elastomers, and the tangent moduli remained less than 5 MPa. Actuation strains of polymer actuators were increased 3 fold using the composites as electrodes, compared to using carbon-grease electrodes. This was due to the composites ability to be spincoated with thin insulating layers of PDMS, allowing 30% higher electric fields to be applied. Strain gauges fabricated with these composites exhibited gauge factors (GFs) > 27,000, to the authors knowledge this is the highest GF ever reported. The effects of humidity, temperature and strain were investigated.Item Experimental Detection and Quantitative Interrogation of Damage in a Jointed Composite Structure(2008-06-24) Gentzen, Vanessa; Wereley, Norman; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The aerospace field has concentrated substantial attention toward the development of structural health monitoring (SHM) systems for multidisciplinary applications. Research is motivated by catastrophic failures of operational systems which may have been avoided with the prior implementation of a successful damage prediction method. This research utilizes a smart sensor array to collect sensing information over a variety of damaged scenarios on a composite lap-joint assembly. Damage was implemented as bolt torque loss within the joint. A damage index was used as the key diagnostic feature to interrogate damage within the structure. Pattern recognition of the damage index, in addition to a rule-based, statistical discrimination method was employed to detect, localize and quantify damage due to bolt torque loss in the structure. The methodology accurately detected the presence of damage within the joint, localized the damage within the structures four quadrants, and assessed the level of torque loss.