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The transition to sustainable mobility is critical to the world. Dealing with the transition is difficult because people holding distinct worldviews may vary in environmental attitudes and travel patterns. However, current mobility choice models fail to consider diverse worldviews. This would limit their use for addressing complex, debated issues such as the mode switch from private car to public transport. This research attempts to incorporate worldviews into the study of sustainable mobility transitions. Three cultures or worldviews defined by Cultural Theory are utilised. Egalitarianism highlights social equity and environmental rights. Hierarchy values order and conformity. Individualism endorses individual autonomy and economic growth. They represent three rationalities about social life and sustainability. In this research, a behavioural and decision making framework that can capture the influence of worldviews on mobility choices is proposed. The framework is implemented by an agent-based model, the London Mobility Model, to explore the transition from private car to public transport in London. This model requires the outputs of two models. First, the British Worldviews Model measures worldviews in Great Britain and groups British people into three types of worldview adherents. It shows that on average, egalitarians have the highest environmental awareness of car use problems; hierarchists, have the medium; and individualists, have the lowest. Second, a spatial microsimulation model, the London Worldviews Model, estimates worldviews across space and populations in London. The model produces a synthetic population that allocates Londoners to this city, illustrating distinct residential patterns of the three worldview groups. Built on these results, the London Mobility Model views London as a complex system. It portrays Londoners as the three types of adherents, who vary in population and contextual characteristics. The results identify distinct transition pathways to sustainability, demonstrating that worldviews have the potential to link various aspects of sustainable mobility and urban complexities.

The elastoplastic and damage analyses, which serve as key indicators for the nonlinear performances of engineering structures, have been extensively investigated during the past decades. However, with the development of advanced composite material, such as the functionally graded material (FGM), the nonlinear behaviour evaluations of such advantageous materials still remain tough challenges. Moreover, despite of the assumption that structural system parameters are widely adopted as deterministic, it is already illustrated that the inevitable and mercurial uncertainties of these system properties inherently associate with the concerned structural models and nonlinear analysis process. The existence of such fluctuations potentially affects the actual elastoplastic and damage behaviours of the FGM structures, which leads to the inadequacy between the approximation results with the actual structural safety conditions. Consequently, it is requisite to establish a robust stochastic nonlinear analysis framework complied with the requirements of modern composite engineering practices. In this dissertation, a novel uncertain nonlinear analysis framework, namely the machine leaning aided stochastic elastoplastic and damage analysis framework, is presented herein for FGM structures. The proposed approach is a favorable alternative to determine structural reliability when full-scale testing is not achievable, thus leading to significant eliminations of manpower and computational efforts spent in practical engineering applications. Within the developed framework, a novel extended support vector regression (X-SVR) with Dirichlet feature mapping approach is introduced and then incorporated for the subsequent uncertainty quantification. By successfully establishing the governing relationship between the uncertain system parameters and any concerned structural output, a comprehensive probabilistic profile including means, standard deviations, probability density functions (PDFs), and cumulative distribution functions (CDFs) of the structural output can be effectively established through a sampling scheme. Consequently, by adopting the machine learning aided stochastic elastoplastic and damage analysis framework into real-life engineering application, the advantages of the next generation uncertainty quantification analysis can be highlighted, and appreciable contributions can be delivered to both structural safety evaluation and structural design fields.

Femtosecond two-dimensional infrared spectroscopy in combination with isotope labeling and molecular dynamics simulation has been used to investigate the structures and dynamics of nonfolding peptides and collagen peptides. Homopolymeric peptides are simple yet important, serve as model systems for investigating the intrinsic propensity of protein folding, especially for those disordered or unfolded peptides in aqueous solution. Here the full structure of (Ala)5 has been studied. Two different isotope-labeled peptides, Ala-(13C)Ala-(13C,18O)Ala-Ala-Ala, and Ala-Ala-(13C,18O)Ala-(13C)Ala-Ala were strategically designed to simplify the four-oscillator system into three two-oscillator systems. By utilizing the different polarization dependence of diagonal and cross peaks, coupling constant β and angle θ between transition dipoles has been extracted through spectral fitting. The coupling constant is around 4 cm-1 and angle around 100. These parameters were related to the dihedral angles characterizing the peptide backbone structure through DFT calculated maps. The derived dihedrals are all located in the polyproline-II region. These results were compared to the conformations sampled by hamiltonian replica-exchange MD simulation with 3 different CHARMM force fields: C22, C36 and Drude. The C22 force field predicted too high α-helix population, whereas C36 predicted that polyproline-II is the dominate conformation, consistent with experimental findings. The Drude model predicted dominating beta-sheet. Since the 2D-IR derived results were obtained from fitting to a single set of structural parameters, the effect of structural fluctuation within one conformation and structural transition between different conformations was also discussed. The C36 MD trajectories were used to simulate 2D IR spectra using the sum-over-state method and the time-averaging approximation (TAA) method. Reasonable agreement with the experimental data was achieved. Collagen is the most abundant protein in mammal. Its structural properties are important for biological functions. Here, the structure and thermal melting of a model collagen peptide, (PPG)10, has been investigated. The temperature dependent linear IR spectra of the unlabeled peptide and two isotopomers (either 13C-16O or 13C-18O labeled on the 4th glycine residue) showed that the triple helix unravels with increasing temperature and leads to greater solvent exposure. With some adjustments of calculation parameters according to linear IR spectra, 2D IR spectral simulation based on MD trajectories and TAA reasonably reproduced experimental spectra taken at the parallel and perpendicular polarization conditions. Further refinement of models and parameters are needed to improve the simulation. Our results for model nonfolding peptides and collagen peptides contribute to the fundamental understanding of peptide structure and dynamics, and to the further development of theoretical models for simulating 2D IR spectra.

The purpose of this study was to exam if Chinese students perceive different levels of social presence than their American student counterparts in a text-based online learning environment and, if so, what cultural differences are associated with different levels of social presence. A survey was conducted to collect data on social presence from both Chinese and American students. Additional open-ended questions were asked to gain more insight into the perceptions of online classes both from Chinese and American students. The results suggest that Chinese students perceived lower level of social presence than American students. They also suggest that expected response time, expectation of building relationships, learning background and English language ability may effect the Chinese students' perception of the online learning environment. The study recommends that online instructors take cultural issues into consideration to help Chinese students acclimate to the online learning environment. The suggestion is made that students need to be prepared for the obstacles that they may face that environment

5Additive Manufacturing (AM) is defined in the American Society for Testing and Materials (ASTM) standard F2792 as “a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies. It provides an advanced method for building complex geometries and parts for high performance with a significant cost savings. 55It’s advantages include the reduced need for tools and molds commonly used in manufacturing, a large reduction in wasted material, much shorter manufacturing cycles for the building of hardware, and its uniquely inherent ability to produce much more complex shapes. Polymers, metals, ceramics, and composites can all be built using some method of AM. The use of standardized vendor parameters for additive manufacturing builds has resulted in numerous defects in the as-built parts. This study looked at HR-1 products built on an EOS M290 DMLS 3D printer. The builds were monitored using an EOS in-situ monitoring system to identify when “problems” began to occur and it compares the “problems” with the results of post build computed tomography inspections. It also looked at the defects produced and evaluated them versus the additive manufacturing process parameters.

Friction stir processing (FSP) is an energy efficient solid-state material processing technique for microstructure modification of commercial high-strength Al alloys. Many variant techniques were developed in recent years that enabled light-weight and high-strength structure fabrication. Identifying relationship among process conditions, microstructures, and mechanical properties is of critical importance to facilitate the practical implementation of these new techniques. The research in the dissertation focusses on developing two main techniques of the FSP: a) friction stir back extrusion (FSBE) of 6063 aluminum alloy for tube making and b) FSP of 7075 aluminum alloy from powder feedstock. FSBE fabricated Al 6063 alloy tubes with refined grains in the inner walls and less orientation asymmetry, which is beneficial to the thin-walled light-wight structure building. The local concentrated precipitation and the grain-size gradient throughout the wall thickness illustrated the non-uniform straining applied by the current FSBE design. Such non-uniformity can be utilized for a selective microstructural modification in the extrusion process. FSP from premixed powder realized complete in-situ Al3Ni formation in Al 7075-Al3Ni composite. Nano-scale Ni additive in the start material produced fine-sized Al3Ni, which generated more effective hardness enhancement than the micron-scale Al3Ni reinforcement by the Orowan strengthening effect. The Ni addition also introduced precipitation kinetics change in Al 7075 alloy including Cu depletion from Al matrix and promoted T- Al2Mg3Zn3 precipitation. The current FSP set up on powder enabled fabrication of large-scale consolidated composite with homogenous microstructure. The FSP on Al 7075 powder produced fully consolidated, highly recrystallized grain structure comparable to the solid pieces after FSP. The microhardness was mainly governed by the resultant morphology and content of the strengthening intermetallics. However, the excessive Fe-rich intermetallics (Al7Cu2Fe and Al23Fe4Cu) were introduced possibly by the tool’s wearing. The Fe-rich inclusion as well as the other possible residue features in powder triggered further grain refinement and reduction of main strengthening η'/η phases in the post heat treatment. The hardness reduction of the material from powder in the post heat treatment indicated the necessity of modifying material preparation and the post-process for the perfection of this FSP techniques.

A worldwide increase in energy demand and a latent crisis in the fossil fuel supply have spurred broad research in the renewable energy. Currently, most renewable energy resources (e.g., hydro, wind, solar, tide) face supply challenges as they are known to be intermittent, unstable, and locally shackled, which calls for urgent development in energy storage and conversion. Hydrogen is regarded as an ideal energy carrier with its advantages (e.g., high energy density, environmentally friendliness, and low weight). In practice, the proton exchange membrane electrolyzer cell (PEMEC) is considered to be one of the optimal hydrogen production and energy storage devices with its superior compact design, high efficiency, preeminent hydrogen purity, and great compatibility with PEM fuel cells (PEMFCs). Although the advantages of PEMECs are apparent, the high cost holds back its large-scale application. Therefore, increasing cell efficiency, cutting down catalyst loadings, and simplifying the electrode manufacturing process are effective strategies to reduce a PEMEC’s cost and boost its commercial application. The main achievements of this dissertation include: (a) Optimize catalyst deposition method and ionomer ratio for catalyst-coated membrane (CCM). (b) Develop and compare surface treatment methods for electrode substrate. (c) Propose Ir CCLGDLs for OER in PEMECs and study CCLGDL pattern impacts. (d) Fabricate in-situ grown PtNW electrode for HER in PEMECs, indicating advantages of nano-featured catalyst structures in improving cell performance. (e) Propose all-in-one bipolar electrode concept for simplifying the fabrication process and reducing the weight, volume of the cell. (f) Synthesize in-situ grown molybdenum sulfide on carbon fiber paper for HER in PEMECs, indicating the feasibility of low-loading non-precious metal catalyst. (g) Apply thin LGDL as the GDE substrate in anion exchange membrane (AEM) electrolyzers successfully, indicating a wider application for all SPE electrochemical devices with electrode design and fabrication improvement. This research guides the future electrode design of PEMECs and other energy storage devices. Moreover, the low-cost and high-efficiency electrode designs contribute to the application and commercialization of large-scale energy storage and hydrogen production systems.

Fused Deposition Modeling (FDM) can be purchased for under five hundred dollars. The availability of these inexpensive systems has created a large hobbyist (or maker) community. For makers, FDM printing is used numerous uses. With the onset of the COVID-19 pandemic, the needs for Personal Protective Equipment (PPE) skyrocketed. COVID-19 mitigation strategies such as social distancing, businesses closures, and shipping delays created significant supply shortfalls. The maker community stepped in to fill gaps in PPE supplies. In the case of 3DP, optimization remains the domain of commercial entities. Optimization is, at best, ad-hoc for makers. With the need to PPE supplies and COVID-19 related supply delays, optimization techniques would be of great value to makers. The objective functions in this research is throughput and cost with quality factored into both. There are several parameters common to both throughput and surface roughness, including layer thickness, print speed, infill density, raster width, and wall thickness. This research will utilize a 2-level fractional factorial design, in which process parameter had a specified upper (+1) and lower (-1) level. By using the upper and lower limits, this study will more closely align with the common maker workflow. The design will have a total of 16 trials, no main effect or 2-factor interactions are confounded with any other main effect or 2-factor interactions, this will allow the parameters to be estimated separately from one another without the requirement for conducting a full factorial (32 trials). Least Squares Regression (OLS) will be completed on throughput and cost independently. Quality will be considered a component of both. For example, an OLS will be completed for the throughput to determine the respective effects of the process parameters on throughput. Using a 95% confidence interval, a process parameter with a P-value smaller that .05 will show that the process parameter has a significant effect on the throughput. Upon completion of each OLS model -Contraint methodology will be used to jointly optimize the process parameters. Validation trials will be completed to test the optimized process parameters. The results will be documented and discussed.

Optically active materials are used in many biomedical applications ranging from medical imaging to light therapies. Investigating the effects of differing nanostructure configurations on the optical performance of these materials can improve tunability, efficiency, and practicality for their respective applications. This work utilizes pulsed laser deposition (PLD) to develop nanostructured thin films and determines their optical performance for applications in computed radiography for medical imaging and in LEDs which can be used in biomedical applications such as photobiomodulation. In computed radiography, scattering of the stimulation light by the storage phosphor crystal grain boundaries in imaging plates negatively impacts spatial resolution. Storage phosphor plates with thinner phosphor layers have been developed to reduce scattering distance and increase spatial resolution, although at the expense of reduced x-ray absorption. A transparent or translucent nanostructured film, containing a much higher percentage of storage phosphor crystals achievable in bulk glass-ceramic materials made by conventional methods, may have acceptable photostimulated luminescence efficiency and imaging performance characteristics greater than commercial imaging plates. In an attempt to achieve a nanostructured film with superior performance in x-ray imaging, a glass-ceramic imaging plate for computed radiography was synthesized via PLD for the first time. The imaging plate was comprised of Eu-doped BaCl2 crystallites and an amorphous matrix. Nanolayered films comprising of BaF2, Eu2O3, and Al2O3 were synthesized via PLD with differing layered configurations to manipulate the coordinate surrounds of the europium dopant and determine its effects on optical properties. TEM cross-section analysis was conducted to verify the desired nano-layering. Different post-deposition heat treatments were investigated, and the films were evaluated for applications as a phosphor layer for UV-pumped white light LEDs which can be used for solid-state lighting and biomedical light therapies. A Mn dopant was added to europium to discover the threshold for the amount of manganese necessary to optically influence the nanolayered films. Although Mn/Eu co-doping did not prove advantageous for white light LEDs, all nanostructures of Eu-doped films have the potential for the desired application. Nanoscale control of optically-active thin films was demonstrated using pulsed laser deposition. Determining the effects of differing nanostructures on optical properties can lead to improvements in certain biomedical applications.

Digital radiography (DR) is an important two-dimensional imaging technique in the field of medicine that utilizes x-rays to form a digital image. DR employs a flat panel detector that converts incident x-rays, that have passed through the subject, to an electrical signal, which is used to create a digital image. The conversion from x-rays to electrical signals can be done either directly or indirectly. The direct method involves the x-rays being converted to an electrical signal via an array of semiconductors. The indirect method utilizes scintillators to absorb the x-rays and produce light in the visible spectrum, which is then collected by photodiodes and converted to an electrical signal. One issue that many modern flat panel detectors have is reduced efficiency due to a significant percentage of the incident x-rays passing through the detector without being absorbed. In order to raise the efficiency of these flat panel detectors, rare earth-doped scintillating glass ceramics have been proposed to replace the traditional non-scintillating borosilicate substrates that support the detector electronics. Two series of glass-ceramic scintillators were investigated for this purpose. The first series investigated the potential of calcium fluoride as a scattering center, as well as how the calcium fluoride luminescence behaved when doped with europium. The second series investigated the use of cerium as a sensitizer to improve the scintillating efficiency of terbium. Both series showed that the scattering and luminescent properties of the materials can be controlled. Scintillating glass ceramic materials have the potential to become viable replacements for the un-doped borosilicate substrates that are used in modern flat panel detectors.

This work describes research aimed at adapting advanced engineering systems for plant biotechnology. The droplet interface bilayer (DIB) is a robust and versatile platform for replicating model cell membranes, providing a bottom-up approach for synthetic cell- and tissue-like structures. In this work, a microfluidic device featuring five inlets, one for the continuous oil phase and four discrete aqueous channels for droplet generation was designed. Droplet production rates were controlled by adjusting the applied pressure of each inlet; and thus, altering the droplet sequence for capturing linear DIB networks in a downstream hydrodynamic trapping array. This microfluidic system provides a high-throughput method for generating DIB networks of complex droplet patterning for the purpose of studying higher-order combinatorial networks and signal cascades. Further, this work characterizes pressure-driven flow sensitivity to changes in microchannel resistance and differences in hydrodynamic trapping behavior compared to mechanically-driven flow. In addition, this research work developed a custom laser system to image plant canopies expressing a wide variety of fluorescent protein genes in leaves for the purpose of phenotyping transgenic plants expressing multiple constitutive or inducible fluorescent proteins. The fluorescence-inducing laser projector (FILP) system is a critical instrument for accessing the efficacy of environmental phytosensors, which are plants engineered to detect and report environmental stimuli.

With magnetization saturation roughly twice that of iron oxide nanoparticles, metallic iron nanoparticles (also termed zero-valent iron nanoparticles) have desirable properties for use as a magnetic resonance imagining (MRI) contrast agent as well as a medium for hyperthermia treatment of cancer. Metallic iron nanoparticles, however, are difficult to synthesize and maintain due to their high degree of reactivity and proclivity for oxidation. The main goal of this study was to investigate how ambient oxidation affects the chemical composition and structural properties of metallic iron nanoparticles initially synthesized through a facile reduction reaction of iron (III) chloride with sodium borohydride. A metallic iron nanoparticle with tunable oxidation would combine the biocompatibility of iron oxide with the magnetic strength of metallic iron. Metallic iron nanoparticles were examined via transmission electron microscopy (TEM), scanning electron microscopy (SEM), dynamic light scattering (DLS), powder X-ray diffraction (XRD), and Mössbauer spectroscopy to determine their morphology and structure. Relaxometry experiments were conducted to investigate the potential of as-made metallic iron nanoparticles as an MRI contrast agent. Imaging data revealed nanoparticles in the range of 10-80 nm that are arranged as either spheroids or sintered aggregates. X-ray diffraction confirmed the presence of metallic iron, while Mössbauer measurements revealed core-shell nanoparticles containing a metallic iron core covered by amorphous iron and iron oxides. Oxide percentage increased as nanoparticles were left to age under ambient conditions. Oxidation rate slowed once an adequate passivation layer was formed. Polyethylene glycol (PEG) coating of nanoparticles retarded oxidation rate, thereby preserving the metallic iron content and desirable magnetic properties of the nanoparticles.

All-vanadium redox flow batteries (VRFBs) are a promising technology for grid-level energy storage, however, there are still several limitations in the forms of durability, efficiency, and overall costs, which are barriers to its commercial viability. With both bulk electrolyte flowing through its porous matrix and species flux at the solid-electrolyte interface, electrodes are the component of VRFB systems which host electrochemical reactions and facilitate contact between the liquid phase electrolyte and the electronically conductive solid phase. While the more limiting electrode in VRFB systems is dependent on the material, for polyacrylonitrile (PAN)-based carbon felts, the anode constitutes a larger portion of the total overpotential than the cathode. In-situ characterization of modified felts can pro-vide both a path towards understanding the source improvements to the anode but also their transient behavior, thereby creating a path to higher voltage and energy efficiencies and higher commercial viability.The primary experimental components of this work are the symmetric cell con-figuration and electrochemical impedance spectroscopy (EIS). The symmetric cell is an in-situ experimental configuration, which utilizes a single electrolyte at both half-cells of a reactor. The symmetric cell allows an experiment to maintain a constant state of charge, incur no net-crossover and isolate a single redox couple.Electrochemical impedance spectroscopy is the focal experimental diagnostic technique in this work, owing to both its minimal perturbation of the state of a cell and to the insights into electrochemical interfaces it provides. The combination of these two techniques is utilized in this work to both characterize several material modifications and to enhance the understanding of the sensitivity of this diagnostic for the purposes of model selection. A novel experimental configuration is also developed, which combines a cell-in-series approach with the symmetric cell in order to continuously characterize VRFB electrodes as a function of state of charge.The outcomes of this work elucidate methodologies for in-situ characterization of anode modifications from an experimental perspective and also frameworks for characterizing porous electrochemical interfaces, investigating sensitivity, and elucidating systematic approaches to separate electrochemical phenomena occurring at a VRFB anode interface.

Different chemical species have unique line spectra that contain information pertaining to their physical properties such as temperature, pressure, and species concentration. Line spectra must be calibrated with a high-temperature absorption cell in order to validate experimentally measured spectra with simulated spectra. The University of Tennessee Space Institute uses a high-temperature, single-zone absorption cell for spectral calibrations. Tunable diode laser absorption spectroscopy (TDLAS) is the diagnostic technique used in this research to measure temperature and species concentrations of CO and CO2 mixtures in the absorption cell. However, TDLAS takes the path-average measurement of the line-of-sight, which is problematic because the absorption cell suffers from a nonuniform temperature distribution across the heated path. Thermal insulation is applied to the absorption cell in order to reduce that temperature gradient. The research presented in this thesis studies the effects that a reduced temperature gradient has on accuracy of the measured temperature and species concentration for a 3% CO/97% N2 and 1% CO2/99% N2 mixture. Results show that insulation was effective at improving the overall temperature measurement accuracy of CO by reducing the percentage difference of measured to known values from 21% - 59% to 0.70% 8.6%; however, CO2 temperature measurement accuracy only improved half of the time. Unfortunately, the measurement system was unable to produce meaningful results for CO species concentrations, but CO2 species concentrations produced reasonable results that demonstrated an increase in percentage difference of measured to known values from 55% - 155% to 87% - 161% once the insulation was applied. These results indicate that insulation showed improvement in measurement accuracy for some cases, and that the data contained inconsistencies and non physical results. Therefore, a better correction system for the nonuniform temperature distribution is required.

The goal of the research presented in this dissertation is to develop iron nanoparticles suitable for use as Magnetic Resonance Imaging (MRI) contrast agents, for enhancement of Laser Induced Thermal Therapy (LITT), and to combine these capabilities for Magnetic Resonance guided Laser Induced Thermal Therapy (MRgLITT). In order to accomplish this, small metallic iron nanoparticles were to be produced with a biocompatible coating. Synthesis methods employed are reduction and thermal decomposition of iron (ii) chloride, iron pentacarbonyl, and iron sulfate. Several biocompatible coatings were investigated to determine their performance as a protective layer. Characterization techniques used include DLS, TEM, Mössbauer Spectroscopy, NMR, and cytotoxicity.Reduction of iron (ii) chloride resulted in metal iron and iron oxide nanoparticles. Iron metal nanoparticles had not oxidized completely after three years of storage in air. Thermal decomposition of iron pentacarbonyl in organics produced iron oxide nanoparticles with an iron metal core. The reaction proved to be very reproducible. Iron sulfate reductions created metal and amorphous iron nanoparticles.

This dissertation presents the development of an instrument based on a confocal fluorescence microscope for feedback-driven trapping of a single molecule or nanoparticle in three-dimensions as it undergoes Brownian diffusion within an aqueous medium. Such trapping enables prolonged observation of a molecule while untethered and free from collisions with surfaces, which is needed to improve various studies, such as investigations of protein folding dynamics, molecular heterogeneities, and interactions. In the experiment, a dilute solution (~100 pM) of fluorescent nano-objects is inserted into a microfluidic device, which achieves trapping by control of electroosmotic flows in two crossed channels. The geometry, which is designed using COMSOL Multiphysics, funnels the flows to achieve sufficient electroosmotic speed to counteract Brownian diffusion while maintaining a 4:1 width-to-depth for wide-angle light collection by the microscope objective from the center of the crossing region. A fluorescence excitation volume centered at this point is defined by four overlapping focused laser beams, each with ~0.5 μm beam waist but with centers offset in a tetrahedral arrangement. The beams are derived from a mode-locked laser using a series of beam splitters with the pulses in each beam delayed to provide pulse-interleaved excitation at 304 MHz. Fluorescence is collected through a pinhole and split to two single-photon detectors, which provide signals for an FPGA (Field Programmable Gate Array) for time-gated counting into four channels synchronous with the pulses in each of the laser beams. The FPGA also bins the counts and applies an algorithm to estimate the direction of the position offset of the nano-object and to adjust four voltages. These are applied at the four fluid inlets of the microfluidic cross-channel to electroosmotically drive the fluid to keep the nano-object at the midpoint of the four foci. Movies of camera imaging of trapped nano-objects were acquired. Results show trapping of 40 nm FluoSpheres for ~4 minutes, 20 nm FluoSpheres for ~25 seconds, and 5 nm protein molecules of Streptavidin-Alexa Fluor™ 647 for ~1.5 seconds. In addition, Maximum Likelihood Estimation of positions from binned photons was conducted for the FluoSphere experiments to estimate effective spring constants of the trap.

Scalable and open architecture of redox flow batteries (RFBs) is a promising solution for large-scale energy storage. Among many chemistries developed for RFBs, all-vanadium redox flow batteries (VRFBs) currently show great potential for widespread commercialization. VRFBs utilize vanadium ions with different oxidation states in the negative and positive electrolytes; this characteristic frees them from irreversible capacity decay as a function of electroactive species transport through the membrane (i.e. crossover). However, crossover of vanadium ions and water during the charge/discharge cycling not only results in a lost discharge capacity, but also has real-time influence on the cell performance.Several parameters affect solute and solvent crossover during cycling. In this dissertation, experimental data along with multiscale computational modeling tailored to quantify the contributions to capacity decay stemming from ion-exchange membrane properties (e.g. equivalent weight and degree of reinforcement), flow field design, electrolyte properties, and operating conditions. A major focus has been to understand the effect of the electrode/membrane interface on the capacity decay and contact resistance. Novel ex-situ conductivity cells have been devised to assess ionic conductivity of the ion-exchange membranes along with electrolytes leading to details on the impact of interfacial phenomena on ionic conductivity and crossover.To quantify the long-term influence of crossover, a unique set-up (we call it IonCrG: Ionic Crossover Gauge) was built and fabricated enabling real-time measurement of the ionic transport across the polymeric membrane using ultraviolet-visible (UV/Vis) spectroscopy. The IonCrG enables separation of contributions to crossover emerging from concentration and electrostatic potential gradients. To investigate the instantaneous impact of crossover on the performance, a real-time current density distribution diagnostic has been implemented for measuring the in-plane current density distribution.The insights gained from this suite of experimental diagnostics and multiscale modeling have inspired design of systems with enhanced performance and greatly decreased crossover losses. Novel cell topologies along with asymmetric electrolyte compositions were designed and engineered for mitigating the ionic crossover during the operation of VRFBs. The cell architecture as well as the electrolyte configuration proposed in this dissertation provides an inexpensive and passive solution for retaining capacity during extended cycling of aqueous RFBs.

Nanoparticles of Fe3O4 (magnetite) and gamma-Fe2O3 (maghemite) have been studied by Mössbauer spectroscopy. These nanoparticles have applications as contrast agents for Magnetic Resonance Imaging. At room temperature, they are superparamagnetic and the magnetic hyperfine fields were averaged to zero. The spectra of Fe3O4 were composed of two lines corresponding to two crystal sites, identified from their isomer shifts as Fe3+ on the tetrahedral A site and Fe2.5+ (mixed Fe2+ and Fe3+) on the octahedral B site. The relative intensity of the two lines shows that the samples are almost stoichiometric with the formula Fe3-xO4, where x is less than 0.04 at room temperature. The x that was obtained was compared to those obtained in a magnetic field and at room temperature. The lines are broad and the measurements as a function of temperature were analyzed using Néel’s theory of superparamagnetism to yield values of the relaxation times. The same nanoparticles that have oxidized into gamma-Fe2O3 have been studied for comparison.

Redox flow batteries (RFBs) are a promising grid energy storage technology with the potential to solve numerous problems arising from anticipated increases in renewable energy integration. These systems involve a number of key design tradeoffs, many involving the movement of liquid electrolyte through the system, which impact the overall system efficiency.This work examines the design of the fluid network throughout RFB systems and how design tradeoffs impact the system efficiency. Insights are gained that inform future research and design decisions. Initially, this work focuses on single cell concerns, primarily flow field design. Two common designs from the literature, serpentine and interdigitated, are analyzed to uncover underlying transport characteristics. These findings are used to develop and evaluate two new flow field designs, the Equal Path Length (EPL) and aspect ratio designs. All four designs are evaluated with regard to a number of metrics including viability to be scaled up to larger sizes.To assess the viability of these conclusions on a commercial system, a short four cell stack was developed and evaluated for efficiency and mass transport performance. Additional efficiency concerns specific to stack design such as shunt current were explored using a novel in-situ shunt current measurement technique. The relative impact of this self-discharge phenomenon is investigated and its relationship to operating conditions is evaluated.Finally, a cost model taking into account earlier findings of this dissertation is presented. This model provides context for these findings in terms of real world applications and financial metrics. This model allows for the exploration of which design factors are key cost drivers and provides a guideline for where future research should focus to reduce cost.The primary outcome of this dissertation is a new framework for the design of RFBs rooted in the principle that there may not be one panacea solution for every design problem. Unique solutions dependent upon operating conditions and applications are necessary to maximize performance and minimize cost. Using the conclusions presented in this work will guide researchers and system engineers alike, allowing for improved efficiency and minimized costs as RFBs become a viable grid energy storage solution.

Hydrogen is considered as an environmentally friendly storage medium for renewable energies and has attracted extensive attention, since it can provide 2.5 times as much energy per unit mass as fossil fuels, and provide hazard-free products. As an effective method for hydrogen production, the proton exchange membrane electrolyzer cell (PEMEC) provides a number of advantages: rapid response to the power input, compact system design, high purity product (up to 99.995%), high operation current density (above 2 A/cm2), and high operation pressure (up to 350 bar). Due to insufficient performance and high cost of PEMEC stacks, the use of hydrogen as large-scale energy storage with PEMECs has been limited so far. Thus, low-cost and high-efficiency PEMECs are highly desirable to establish a clean and sustainable energy infrastructure. To bridge the gap between lab-scale and industrial hydrogen productions, the main objectives in this research include: (1) Exploring the possibility of application of metallic additive manufacturing (AM) on PEMECs. (2) Integrating the components of PEMECs into one part with better performance and compact system design by using AM technologies. (3) Bipolar plate development with AM and protective coating for durable and high-efficiency PEMECs. (4) Combination of AM non-conductive bipolar plate with thin film liquid/gas diffusion layers for low-cost hydrogen production. (5) Developing conductive mesoporous bi-material catalysts with ultra-high reaction sites and efficiency for oxygen evolution reaction in PEMEC by using advanced manufacturing. Those bipolar plates and membrane electrode assemblies (MEAs) with excellent electrochemical performance and low cost will open a pathway for developing widely available water electrolyzers or other energy conversion devices, such as batteries, solar cells, and fuel cells.

For this work, a fluorochlorozirconate thin film storage phosphor was developed in a two-step process. In the first step, the synthesis and characterization of the glass portion of the storage plate was completed. The second step incorporated crystalline and dopant materials into the glass matrix to develop the glass-ceramic thin film. After synthesis was completed, characterization was conducted, including measurement of photostimulated emission after prior x-ray irradiation indicating the ability of the thin film to act as a storage phosphor plate.The presence of reduced zirconium in fluorozirconate (FZ) glasses is highly unfavorable due to its detrimental effect on glass quality. Previous researchers have relied upon the use of a fluorine-containing processing gas to prevent the reduction of zirconium in FZ glasses deposited as thin films by pulsed laser deposition (PLD). However, the use of a fluorine-containing processing gas as an oxidizing agent is disadvantageous, due to its toxicity. Eliminating the need for the processing gas would lead to a significantly safer and simpler process. The approach presented within this thesis is to incorporate indium, which is multivalent, into the PLD ablation target in order to stabilize the zirconium and remove the need for a processing gas. Using a fluoride glass target, FZ glass films were successfully deposited on fused silica substrates by PLD, without the need for any processing gas.In computed radiography applications using conventional granular storage phosphor materials, scattering of the stimulating laser light during read-out leads to decreased spatial resolution. The desire for high spatial resolution has led to imaging plates with thinner phosphor layers in order to reduce this scattering. The films produced in this study consist of multiple transparent layers of orthorhombic phase barium chloride nanocrystals doped with europium, separated by nanoscale layers of fluoride glass. The transparent nature of these films reduces blurring of the image due to scattering of the stimulating light during read-out. The films are deposited on fused silica substrates via pulsed laser deposition, utilizing a multicomponent target. This method allows for a higher concentration of luminescent centers compared to glass-ceramic imaging plates synthesized in bulk form.

Lacking the energy dissipative mechanics such as plastic deformation to rebalance localized stresses, similar to their ductile counterparts, brittle material fracture mechanics is associated with catastrophic failure of purely brittle and quasi-brittle materials at immeasurable and measurable deformation scales respectively. This failure, in the form macroscale sharp cracks, is highly dependent on the composition of the material microstructure. Further, the complexity of this relationship and the resulting crack patterns is exacerbated under highly dynamic loading conditions. A robust brittle material model must account for the multiscale inhomogeneity as well as the probabilistic distribution of the constituents which cause material heterogeneity and influence the complex mechanisms of dynamic fracture responses of the material. Continuum-based homogenization is carried out via finite element-based micromechanical analysis of a material neighbor which gives is geometrically described as a sampling windows (i.e., statistical volume elements). These volume elements are well-defined such that they are representative of the material while propagating material randomness from the inherent microscale defects. Homogenization yields spatially defined elastic and fracture related effective properties, utilized to statistically characterize the material in terms of these properties. This spatial characterization is made possible by performing homogenization at prescribed spatial locations which collectively comprise a non-uniform spatial grid which allows the mapping of each effective material properties to an associated spatial location. Through stochastic decomposition of the derived empirical covariance of the sampled effective material property, the Karhunen-Loeve method is used to generate realizations of a continuous and spatially-correlated random field approximation that preserve the statistics of the material from which it is derived. Aspects of modeling both isotropic and anisotropic brittle materials, from a statistical viewpoint, are investigated to determine how each influences the macroscale fracture response of these materials under highly dynamic conditions. The effects of modeling a material both explicitly by representations of discrete multiscale constituents and/or implicitly by continuum representation of material properties is studies to determine how each model influences the resulting material fracture response. For the implicit material representations, both a statistical "white noise" (i.e., Weibull-based spatially-uncorrelated) and "colored noise" (i.e., Karhunen-Loeve spatially-correlated model) random fields are employed herein.

Hydrogen is a ‘zero-emission’ energy carrier, which could be an important part of environment-friendly solutions to the global energy crisis via energy storage without producing greenhouse gases. The proton exchange membrane electrolyzer cell (PEMEC) is one of the most practical and energy efficient methods for producing high purity hydrogen from renewable sources, such as wind, hydro and solar energy. Since the wide commercialization of PEMECs is still hindered by their performance, cost and durability, superior performance PEMECs with low-cost and high-efficiency are strongly desired. The membrane electrode assembly (MEA), which consists of liquid/gas diffusion layers (LGDLs), catalyst layers (CLs) and membrane, is the core component of the PEMECs. LGDLs play an important role in enhancing the performance of PEMECs. They are expected to transport electrons, heat, and reactants/products simultaneously with minimum electrical, thermal, interfacial, and fluidic losses. CLs are mainly formed by noble metals or their oxides, which has great impact on PEMEC performance, durability and cost. The objective of this research is to develop novel MEAs coupled with the titanium-based thin/tunable LGDLs (TT-LGDLs) that has the well-controlled pore morphologies. The main achievements of this research include: (a) The TT-LGDLs can achieve superior performance due to the remarkably reduced ohmic and activation losses, and the effects of pore morphologies have been identified. (b) The gold electroplating is a promising method for the PEMEC performance enhancement by surface modifications. (c) The microporous layers (MPLs) offer some improved PEMEC performance under specific conditions, but may not be required for optimum TT-LGDLs. (d) The novel GDEs with ultra-low Pt catalyst loadings have been developed, which has obtained an acceptable performance with a significantly improved catalyst mass activity. (e) The theoretical analysis is adopted to study the true electrochemical reaction mechanism in PEMECs, and a model is developed, which is used to simulate the PEMEC performance and optimize the parameters of the electrodes. The novel thin MEAs developed in this research point out a promising direction for future MEA development, and can be a guide for the high-efficiency and large scale energy storage.

Database parameters for line spectra are collected from many studies and require independent verification for use in the design and measurements of laser absorption spectroscopy systems. While many lower temperature lines are well characterized, the higher temperature and pressure lines found in studies of combustion products rely on theoretical scaling. High-temperature calibration devices are therefore desired to allow experimental validation of parameters under test conditions. For this research, the design of a high-temperature, three-zone gas cell is proposed as a replacement for the current absorption cell system at The University of Tennessee Space Institute (UTSI). This new design, a prototype of which has been built and tested at The Technical University of Denmark, allows for measurements of test gas samples at uniform temperatures of at least 1300K. A computational fluid dynamics (CFD) model is shown for the current UTSI gas cell that has

This dissertation presents theoretical, numerical, and experimental research into a technique for extending the observation time of a single molecule in solution, while also enabling measurement of its diffusion coefficient. A confocal microscope is used to observe the fluorescently labelled molecule in aqueous solution, which is confined within a nanochannel. By focusing a laser beam into the nanochannel and applying electrokinetic flow along the tube, a molecule passes through the laser beam and emits a burst of photons. The molecule then passes back and forth through the focus while the voltage is repeatedly reversed at a fixed delay after each detected burst. First, a Monte Carlo simulation of the single-molecule recycling (SMR) process is made to develop algorithms for timing the flow reversals and to study the choice of experimental parameters for diffusivity measurements. To detect fluorescence bursts from the background, a weighted sliding sum algorithm is applied, and the results show it has clear advantages over a previously used photon binning algorithm. Maximum-likelihood methods are developed to measure single-molecule diffusivities and their confidence limits from the variation in the times between detections. The simulations show that SMR can distinguish single molecules with diffusivities differing by a factor of ~1.3 or less, which is smaller than that resolvable in ensemble experiments by fluorescence correlation spectroscopy. Simulations are also developed to study both 1 and 2-photon excitation of ultrasmall CdSe quantum dots, which exhibit fluorescence intermittency or blinking. The simulation incorporates standard photophysics in such a way as to account for the known non-ergodic power-law dependence of the blinking intervals. For SMR experiments, two configurations are implemented: capillary microchannel devices are fabricated and used with a piezo system to provide motion, and nanochannels from a previous research project are used with applied voltage to give electroosmotic motion. A real-time control system that implements the weighted sliding sum and motion switching algorithms studied in the simulations is developed. The results from the experiments demonstrate SMR for 40 nm fluorescently labelled beads for hundreds of times and yield diffusion coefficients in the magnitude of 10–13 m2s–1 in a nanochannel.

All-vanadium redox flow batteries (VRFBs) are an emerging grid-scale energy storage technology; however, enhancements in terms of performance, efficiency, durability, and cost are required before it can become commercially viable. These improvements are achievable through the development of advanced materials, superior architecture, and ultimately a deeper fundamental understanding of the influence of various phenomena and operational parameters on cell performance. There currently are a lack of in-situ experimental diagnostic techniques which can help in achieving this fundamental understanding. Two separate distributed diagnostic techniques were developed in this work: in-plane current distribution, and neutron radiography. Localized current distribution measurements can identify regions of reduced performance, and provide valuable insight regarding mass transport as a function of materials and design, and potentially localized degradation. A novel interpretation of the dimensionless Damkohler number was adapted to locally assess the relative contributions of kinetic and mass transport losses across the active area within an operating cell. Neutron radiography is an effective technique for the identification and quantification of individual phases within a two-phase fluid. This technique was implemented to determine the onset voltage of unwanted side reactions in an operating VRFB. Identification of such unfavorable operating conditions and limitations for different electrode materials can lead to a wider safe operating voltage window, which can be implemented to reduce charging times and improve cell efficiency. The successful demonstration of these techniques provides the field with two validated in-situ diagnostic techniques, advanced analysis, and benchmark data that can provide feedback for engineering enhanced materials, designs, and more optimal operating conditions for these cells, ultimately leading to enhanced fundamental understanding and improved performance.

This dissertation focuses on the development of novel generation and microwave scattering diagnostic techniques for small volume plasmas. The small volume plasmas presented in this work fall under the two generalized categories: 1) laser-induced plasmas and 2) non-equilibrium microdischarges. Chapter I presents the application of microwave scattering theory to laser-induced breakdown in air. The MIE solution to Maxwell’s equations is employed to reveal three distinct phases of the evolution of the laser-induced breakdown in air. Chapter II presents a novel method of quantifying thresholds for laser-induced breakdown. These thresholds are established via total electron number measurement from dielectric calibration of microwave scattering. Chapter III presents high-repetition-rate (HRR) nanosecond laser pulse train scheme for laser ignition. Demonstration of the ignition of combustible gaseous mixtures is shown to have an order-of-magnitude reduction in per-pulse energy using the HRR LI method over traditional laser ignition methods. Chapter IV presents ion-kinetic measurements of a laser induced plasma in sodium-argon and sodium-air gaseous mixtures. Coherent microwave Rayleigh scattering (Radar) from Resonance Enhanced Multi-Photon Ionization (REMPI) is utilized for the measurement of sodium ion neutral stabilized and cluster dissociative recombination rates. Chapter V presents rotational temperature measurements in a DC microdischarge produced in air. Radar REMPI measurements of O2 rotational temperature is performed at eight axial locations between pin-to-pin electrodes. Chapter VI presents relative concentration measurements of atomic oxygen in DC and pulsed Discharges. Relative atomic oxygen concentrations were obtained via Radar REMPI. The effects of pressures, gas composition, and discharge voltage were explored for the DC and pulsed discharges. Comparisons between two-photon absorption laser induced fluorescence (TALIF) and Radar REMPI techniques were made for atomic oxygen concentration measurements in a pulsed discharge. Chapter VII presents a method of reducing the breakdown voltage of a DC microdischarge via metal nanoparticle seeding. Reductions in the breakdown voltage were seen to be as high as 25% for a PD scaling of 40 Torr-cm from the seeding of iron and aluminum nanoparticles into the discharge gap.

In electrochemical energy devices, including fuel cells, electrolyzers and batteries, the electrochemical reactions occur only on triple phase boundaries (TPBs). The boundaries provide the conductors for electros and protons, the catalysts for electrochemical reactions and the effective pathways for transport of reactants and products. The interfaces have a critical impact on the overall performance and cost of the devices in which they are incorporated, and therefore could be a key feature to optimize in order to turn a prototype into a commercially viable product. For electrolysis of water, proton exchange membrane electrolyzer cells (PEMECs) have several advantages compared to other electrolysis processes, including greater energy efficiency, higher product purity, and a more compact design. In addition, the integration of renewable energy sources with water electrolysis is very attractive because it can be accomplished with high efficiency, flexibility, and sustainability. However, there is a lack in fundamental understanding of rapid and microscale electrochemical reactions and microfluidics in PEMECs. This research investigates the multiscale behaviors of electrochemical reactions and microfluidics in a PEMEC by coupling an innovative design of the PEMEC with a high-speed and microscale visualization system (HMVS). The results of the investigation are used to aid in revealing the electrochemical reaction mechanisms and the microfluidics behavior including bubble generation, growth and detachment, which all together play a very important role in the optimization of the design of PEMECs. The effects of operating parameters such as current density, temperature and pressure on the electrochemical reactions and the microfluidics are determined and analyzed by mathematical models of PEMECs, which also match the experimental results. Improved understanding of the electrochemical reactions and microfluidics in PEMECs can not only help to optimize their designs, but can also help advance many other applications in energy, environment and defense research fields.

This dissertation aims to characterize laser-induced plasma from a physics point of view as warm, dense matter. Use of nominal nanosecond pulsed laser radiation initiates a plasma with electron temperatures of the order of 10 electron volts and electron densities of the order of air species densities at standard ambient temperature and pressure. For laser ablation and/or optical breakdown at or near a solid surface, the electron density can amount to be 1000 times greater. Spectroscopic investigations of the plasma emissions provide a method by which the electron density, temperature, and shockwave expansion may be determined. Of particular interest are the earliest of times following ablation of an aluminum target sample. At these times, observed spectral line profiles become distorted due to self-absorption affects. Here, experimental corrections to self-absorbed Al 394.4 and 396.15 nm, ground state transitions and the hydrogen Balmer series alpha, beta, and gamma lines are calculated by retro-reflecting the plasma emission back through the plasma volume. The plasma are initiated by laser ablating an Al alloy 6061 target sample in a 90 percent hydrogen and 10 percent nitrogen gas atmosphere at 840 Torr with 1064 nm Nd:YAG 14 ns laser pulses. Electron densities are calculated by fitting the corrected profiles to Voigt profiles. The widths of the lines are used with empirically developed formulas to infer the electron density. The temperature of the plasma is found by using the Boltzmann plot method constructed from the Balmer series lines. Spatially resolved, diatomic molecular aluminum monoxide spectra are fit to theory spectra to infer the plasma temperature along the axis of the laser incidence at time delays greater than 2.5 microseconds following target ablation. Computer tomography methods, including Abel inversion, are used to find the radial profile of the inferred temperatures. Finally, a series of measurements of the plasma shockwave are presented to determine the geometry of the shock front expansion using the shadowgraph imaging technique.

In this work, fourteen ionic liquids (ILs) were assayed as potential next-generation engine oil additives. After screening for corrosion, thermal stability and oil solubility, candidate additives were subjected to friction and wear tests in both boundary and mixed regime lubrication. While each IL demonstrated friction and wear reduction compared to base oil without any additives, oil miscible ILs tetraoctylphosphonium bis 2-ethylhexyl phosphate ([P8888][DEHP]) and trioctylammonium bis 2-ethylhexylphosphate ([N888H][DEHP]) were the best performers in bench tests with a XX% and XX% improvement in wear over the base oil respectively. Each of these ILs excellent solubility and superior performance was attributed to their symmetric nature and hydrogen bonding in the case of [N888H][DEHP]. The worn specimens generated using these two ionic liquids were chosen for detailed surface analysis. Each showed the presence of an amorphous protective tribofilm dominated by phosphates and iron oxides. Each of these ILs performed well in experimentally formulated oils as well. [P8888][DEHP] was chosen for full-scale engine fuel economy testing due to its synergy with the common anti-wear additive, zinc dialkyldithiophosphate (ZDDP). The [P8888][DEHP]-containing formulated oil demonstrated better fuel economy compared to an oil of the same viscosity containing only ZDDP as the anti-wear additive. Its superior performance was well-correlated with friction and wear bench tests. These results demonstrate the effectiveness of using ILs as engine oil additives and provide a scientific basis for further work towards implementing and optimizing additive packages containing ILs.

This doctoral dissertation introduces the research in the computational modeling and simulation for the microbial fuel cell (MFC) system which is a bio-electrochemical system that drives a current by using bacteria and mimicking bacterial interactions found in nature. The numerical methods, research approaches and simulation comparison with the experiments in the microbial fuel cells are described; the analysis and evaluation for the model methods and results that I have achieved are presented in this dissertation. The development of the renewable energy has been a hot topic, and scientists have been focusing on the microbial fuel cell, which is an environmentally-friendly and promising technology. The MFC full cell is a complex system which has different reactions, coupled with mass and electrons transport in bulk liquid. Therefore, this research contains interdisciplinary fields. The methods will be adopted includes: (1) numerical methods (finite volume method/finite difference method/ parallel computation/ multiple step times etc.); (2) computational fluid dynamics method (diffusion equation, Nernst-Planck equation etc.); (3) experimental electrochemical analysis methods; (4) the biological treatment process (biofilm growth, anaerobic/aerobic bacteria etc.). The uniqueness of this work: (1) a comprehensive computational bioelectrochemical fuel cell models was firstly constructed in the research; (2) the primary physical phenomena have been systematically analyzed in both steady and transient states; (3) The simulation evaluated the MFC system which are hardly obtained directly in the experiments. The computational work in MFC modeling achieved four goals: (1) Characterized the primary factors which affect the MFC performance and used them to describe a complex microbial fuel cell model; (2) Derived a series of appropriate electrochemical /biological /chemical reactions equations for the analysis of the mechanics in MFC; (3) Applied computational methods in the model construction and built a series of sub-models for the MFC system; (4) Simulated the models and compared with the experimental results, gave the analysis for the MFC phenomena which are used for optimizing the design of the MFC system.

In this dissertation, random inhomogeneous porous channels were generated statistically, and single- and multi-phase flow models were developed to investigate diffusion behavior of gases in porous media. Three different methods were used to simulate inhomogeneous porous flow channels. First, the path-percolation theory was adapted in diffusion studies to generate random high-tortuosity (above 1.07) porous channels with a desired porosity within a specified confidence level. Cluster labeling process was applied to simulate paths for the gas molecules, and the resulting effective porosity was investigated statistically. Second, the double-path-percolation theory was introduced to simulate low-tortuosity (between 1.0005 and 1.0700) flow channels. Using a combined void- and solid-cluster labeling process, this new model also simulates paths in both solid and void regions in the channel, hence transport analysis can be performed in both regions. Third, two dimensional slices of the micro-computed tomographies of Mitsubishi Rayon Corp. MRC-105 and Sigracet SGL-25BA gas diffusion layer samples, which are used in polymer electrolyte fuel cells, were digitized, and the effective porosities were determined statistically by cluster labeling process. A single-phase Lattice-Boltzmann model (LBM) was developed to simulate gas flow in the channels generated. Velocity distributions were obtained to evaluate the effective tortuosity in gas diffusion layer samples and different channels generated by single- and double-path-percolation theories. Furthermore, multi-phase LBMs were developed to investigate the impact of liquid formation on mass transfer in porous channels. Statistical results of porosity, effective porosity, and tortuosity of the system with different liquid volumes were investigated. Velocity distributions in porous channels with different solid-liquid-vapor combinations were analyzed. Moreover, a portion of the solid surface inside the channel was set hydrophobic, and multi-phase effects on mass transport were examined. A software was developed for a combined path-percolation – Lattice-Boltzmann model, and the performance was improved by different high-performance computing system implementations. The techniques introduced in this dissertation can be utilized in inhomogeneous porous media application involved with single- and multi-phase mass transport with surface-fluid interactions. This work is unique through its statistical approach and cluster labeling process.

Thermally driven transport of water vapor in polymer electrolyte fuel cells, also known as the heat-pipe effect or phase-change-induced flow, can transport several times the generated amount of water given enough temperature differentials. Understanding this transport process is necessary to properly engineer the water balance in the fuel cell to ensure high performance and long operational life. Channel-land architecture, diffusion media heat and mass transport properties, and operational age can all have an influence on thermally driven flow. High resolution neutron imaging was used to determine the steady-state water accumulation in various cell configurations to understand the influence of these parameters. A novel non-dimensional parameter was proposed to predict the influence of engineering parameters on water balance. The thermal transport number (TTN) compares the strength of anode and cathode thermally driven flow to determine a bias for transport to the anode. Channel-land architecture, specifically asymmetric flow field patterns with larger anode lands, was found to pump water to the anode and allow for large accumulations of water with saturation approaching 60%. Water transport was facilitated to the anode by a delta-T inversion caused by the insulating effect of the cathode gas channel located opposite of the center of the large anode land. To limit anode water accumulation, a new experimental high diffusion resistance anode diffusion media was evaluated. This material was found to be effective at reducing anode water accumulation primarily due to the increased tortuosity of the material but also due to the higher thermal conductivity reducing the thermal transport effect. Material age was shown to reduce cell water content due to increased hydrophilic nature and increased thermal conductivity that increased from inlet to outlet with greater effects on the anode. A parametric study was performed to determine what parameters have the strongest influence on thermally driven transport. It was found that porosity and tortuosity of the diffusion media, and channel-land architecture can be engineered to drive water balance in a favorable direction. Temperature and thermal conductivity primarily influence the strength of thermal transport. This work demonstrated methods to properly design thermal management for optimized and predictive water transport.

Fuel cells offer the potential for high efficiency energy conversion with only water and heat as significant products of the electrochemical reaction. For a cost-competitive product, fuel cell researchers are exploring the limits of the Pt catalyst loading in parallel with performance and durability trade-offs. A significant portion of the performance loss in low-cost PEMFCs is associated with the partial pressure of oxygen (for an air cathode) at the Pt surface. This dissertation explores the main components of oxygen transport resistance which are associated with diffusion through partially saturated porous media and the ionomer coating in the catalyst layer. Under typical proton exchange membrane fuel cell (PEMFC) operating conditions, temperature gradients through the porous gas diffusion layer (GDL) can result in product water condensation. As a result, non-uniform partial saturation of the GDL changes the local effective porosity and tortuosity encountered by oxygen diffusing to the catalyst layer. This work establishes the impact of saturation on practical fuel cell system efficiency losses related to shutdown purge time and overall stack resistance. The transport resistance is further investigated in two-dimensions using limiting current experiments with simultaneous neutron imaging. The analysis of these data results in a diffusion coefficient vs. saturation relationship for two common GDL carbon fiber substrates. A significant oxygen transport limitation also occurs near the Pt surface. This is investigated here with loading studies that fix electrode thickness and bulk properties. The impact of Pt dispersion is probed by varying the average distance between Pt particles. Results elucidate how the electrode structure impacts local transport loss. It is demonstrated that local transport loss is not fully captured with a normalized Pt area. Additional geometric considerations that account for ionomer surface area relative to the Pt particles are required to resolve performance loss at low Pt loading as electrode structure varies. Furthermore, within this ionomer layer an interfacial resistance at both the gas and Pt interfaces is required to account for performance trends observed. These results demonstrate that residual performance loss associated with low cathode Pt loading can be mitigated by minimizing oxygen flux through the gas/ionomer interface.

The development of new materials and processes have enabled defense, industrial, and research devices that operate in high temperature environments. Measurement technology must keep up with the demand of these environments. The objective of this work is to provide a correlation between the heat transfer coefficient (and Nusselt Number) and the flow Reynolds number (and Prandtl number) for a truncated cylindrical probe. The correlation provides reduced uncertainty for materials whose heat transfer coefficient is not well defined. The configuration for the experiment uses the University of Tennessee Space Institute’s (UTSI) blow down air supply system discharging into a duct and exhausting to atmosphere. This system provides dry pressurized air that is used at the supply thermally stabilized total temperature to the test section. The test article is a heated probe that is instrumented with thermocouples. Experimental data were recorded for many run conditions. These data were utilized to develop the correlation between the heat transfer coefficient and the Reynolds number. As a result of the work presented in the body of this text a correlation between the heat transfer coefficient and the Reynolds number was developed with an uncertainty of 1.24%.