Materials Science & Engineering Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2792

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End date: 2019Subject: search.filters.subject.Engineering

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    First Principles Computational Design of Solid Ionic Conductors through Ion Substitution
    (2019) Bai, Qiang; Mo, Yifei; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Solid ionic conductors are key components of energy storage and conversion devices. To achieve high efficiency in these energy devices, solid ionic conductors should demonstrate high ionic or electronic conductivity. While pristine materials often suffer from poor conductivity, substituting ions in materials can tailor their electronic and ionic transport to fulfill requirements of transport properties in energy devices. In this dissertation, I applied first-principles computational techniques to elucidate the effect of ion substitution on electronic and ionic transport properties of solid materials. Therefore, three representative materials SrCeO3, La2-x-ySrx+yLiH1-x+yO3-y, and Li6KTaO6 are investigated as model systems to elucidate how ion substitution can affect the transport of electron, anion, and cation, respectively. I studied SrCeO3 as a model material to uncover the effects of B-site dopants on electronic transport. Based on theoretical calculations, I confirmed a polaron mechanism, including polaron formation and hopping, contributed to the electronic conductivity of SrCeO3. I found different dopants exhibit distinct capabilities for localizing electron polarons, and therefore result in different electronic conductivities in doped SrCeO3. The study demonstrated the capabilities of first principles computation to design new materials with desired polaron formation and migration. I studied La2-x-ySrx+yLiH1-x+yO3-y oxyhydrides as a model material to investigate H- diffusion mechanism in a mixed anion system and its relationship with the cation substitution of Sr2+ to La3+. I found the substitution of Sr2+ to La3+ can alter the H- diffusion mechanism from 2D to 3D pathways. Increasing H- vacancies through Sr2+ to La3+ substitution can also expedite the H- conductivity of the oxyhydrides. Based on the new understanding, a number of promising dopants in Sr2LiH3O were predicted to enhance H- transport. Fast Li-ion conductor materials as solid electrolytes are crucial for the development of all-solid-state Li-ion batteries. I systematically studied Li+ diffusion mechanisms in Li6KTaO6 predicted by our computational study. I found that different carrier defects such as Li vacancies or interstitials can induce distinct Li+ transport mechanisms. In addition, I developed a computational workflow to predict a wide range of materials in a prototype structure. By employing the workflow, I computationally predicted a group of Li superionic conductors with good stabilities by substituting the Li6KTaO6 structure.
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    NEXT GENERATION ANODES FOR LITHIUM ION AND LITHIUM METAL BATTERIES
    (2019) Pastel, Glenn; Hu, Liangbing; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Engineering of specific battery components can yield incremental gains in performance, but sustained advancements are derived from an understanding of charge transfer, interphase formation, and ion storage in the system. In this dissertation, the next generation of lithium-ion and alkali metal anodes are integrated with promising flame retardant electrolyte systems for safe and energy-dense portable storage devices. The intent of this research is to bring safe lithium ion batteries to the market without compromising performance and, more specifically, volumetric energy density. The first part of this dissertation describes the invention and optimization of a silicon-based additive which employs a solution-based process to functionalize silicon nanoparticle precursors. The additive is thoroughly characterized by chemical and electrochemical methods and the electrolyte interphase is improved by the attachment of partially reduced graphene oxide and sacrificial additive species. The design principles developed for the silicon-based system deviate significantly from those used for other conventional intercalation and host electrodes. As a result, in the second part of this dissertation, three chemically separable electrolyte systems, selected for their flame retardant properties, are individually investigated and tailored for energy-dense pouch cells. The bulk transport and interfacial properties of each electrolyte system are adapted to meet the industry standards of portable electronic devices. Insights into the preferred species for stable solid electrolyte interphase formation are discussed with an emphasis on the impact of fluorinated solvents and sacrificial additives. In the last part of this dissertation, alkali metal hosts are also proposed for chemistries beyond lithium ion. Novel synthesis methods including rapid joule heating are explored to form the innovative host architectures which greatly mitigate the coulombic inefficiency of metal stripping and plating in half and full cell configurations. The design principles outlined in this dissertation reveal how to successfully engineer the charge transfer, interphase formation, and ion storage of high capacity electrodes with safe electrolyte for state-of-the-art portable energy storage devices.
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    ATOMIC LAYER DEPOSITION OF LEAD ZIRCONATE-TITANATE AND OTHER LEAD-BASED PEROVSKITES
    (2019) Strnad, Nicholas Anthony; Phaneuf, Raymond J; Polcawich, Ronald G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Lead-based perovskites, especially lead zirconate-titanate (PbZrxTi1-xO3, or PZT), have been of great technological interest since they were discovered in the early 1950s to exhibit large electronic polarization. Atomic layer deposition (ALD) is a thin-film growth technique capable of uniformly coating high aspect-ratio structures due to the self-limited nature of the precursor chemisorption steps in the deposition sequence. In this thesis, a suite of related processes to grow lead-based perovskites by ALD are presented. First, a new process to grow ferroelectric lead titanate (PbTiO3, or PTO) by ALD using lead bis(3-N,N-dimethyl-2-methyl-2-propanoxide) [Pb(DMAMP)2] and tetrakis dimethylamino titanium [TDMAT] as the lead and titanium cation precursors, respectively, is discussed. A 360-nm thick PTO film grown by ALD displayed a maximum polarization of 48 µC/cm2 and remanent polarization of ±30 µC/cm2. Second, a new process (similar to the ALD PTO process) to grow PZT by ALD is demonstrated by partial substitution of TDMAT with either tetrakis dimethylamino zirconium or zirconium tert-butoxide. The 200 nm-thick ALD PZT films exhibited a maximum polarization of 50 µC/cm2 and zero-field dielectric constant of 545 with leakage current density < 0.7 µA/cm2. Third, a new ALD process for antiferroelectric lead hafnate (PbHfO3, or PHO) is presented along with electrical characterization showing a field-induced antiferroelectric to ferroelectric phase transition with applications for capacitive energy storage. Finally, ALD lead hafnate-titanate (PbHfxTi1-xO3, or PHT), considered to be an isomorph of PZT, is demonstrated by combining the process for PTO and PHO. The thin-film PHT grown by ALD is shown to have electronic properties that rival PZT grown at compositions near the morphotropic phase boundary (MPB). The processes for both ALD PZT and PHT are shown to yield films with promising properties for microelectromechanical systems (MEMS) actuators and may help to dramatically increase the areal work density of such devices.
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    Biomimetic polymer based composites with 1-D titania fillers for dental applications
    (2018) Mallu, Rashmi Reddy; Lloyd, Isabel K; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The aim of this study was to develop acrylic matrix composites reinforced with one-dimensional (1-D) titanium dioxide (TiO2) micro and nano fillers that mimic the structure of enamel. To accomplish this, 1-D TiO2 was synthesized without surfactants or templates using a sol-gel assisted hydrothermal process. Two different approaches were investigated. One used titanium metal powder and yielded TiO2 rutile microrods. The other used titanium tetraisopropoxide (TTIP) and created TiO2 anatase nanorods. TiO2 morphology (size, aspect ratio and state of agglomeration) was affected by glycolic acid concentration and phosphate ion concentration for the titanium metal-based powders, and NaOH concentration for TTIP based powders. Composites were made with silanized TiO2 micro- and nano-rods in a 50:50 BisGMA:TEGDMA matrix. Organized composites made by injection molding or centrifuging and settling had more uniform mechanical properties (hardness, strength, Young’s modulus and toughness) than unorganized composites. Curing the composites under pressure reduced porosity enhancing mechanical behavior.
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    CHARACTERIZATION OF MECHANICAL PROPERTIES AND DEFECTS OF SOLID-OXIDE FUEL CELL MATERIALS
    (2018) Stanley, Patrick; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Solid-oxide fuel cells (SOFCs) have the potential to help meet global energy demands by efficiently converting fuel to electricity. The technology currently requires high temperatures and has reliability limitations. A critical concern is the structural integrity of the cell after redox cycling at operating temperatures. As new materials are developed to reduce operating temperatures and improve redox stability, the effect of the environment on the mechanical properties must be studied. Ceria-based systems have allowed the operating temperature to be decreased to the 600℃ range. For this reason, a three-point bend apparatus was developed which could test materials up to 650℃ in reducing environments. Using this apparatus, it was shown how pore geometry and amount affected strength of porous gadolinium doped ceria (GDC) at 650℃ with lower aspect ratio pores, leading to higher fracture strength due to crack tip blunting. The strength of Ni-GDC/GDC half-cell coupons showed no dependence on loading orientation at elevated temperatures in air, but were 47% weaker when the electrolyte was placed in tension under H2 as compared to when the electrolyte was placed in compression. It was also determined that a reduced Ni-GDC/GDC coupon could be exposed to air for an extended period of time and reheated under H2 with no effect to the strength, allowing for more options when processing and preparing cells. A new anode material, SrFe0.2Co0.4Mo0.4O3-δ (SFCM), was investigated for chemical expansion, oxygen non-stoichiometry, and mechanical properties. SFCM maintains phase purity under reducing conditions, with little changes to lattice parameter between oxidation and reduction, but under oxidation, SFCM forms Sr2Co1.2Mo0.8O6 impurities. SFCM supports a large degree of non-stoichiometry, up to δ = 0.176 at 600℃, due to a low enthalpy of formation for oxygen vacancies of 44.3 kJ mol−1. Fracture toughness of SFCM was determined to be (0.124 ± 0.023) MPa√m in air at room temperature and (0.286 ± 0.038) MPa√m at 600℃. The strength of SFCM-GDC half-cells increased by 31% upon heating to 600℃, after which reduction decreased strength by 29%. Reduction and redox cycling were shown to only decrease the characteristic strength, not alter the structural flaw distribution, as microcracks uniformly grew.
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    Nanomaterials for Garnet Based Solid State Energy Storage
    (2018) Dai, Jiaqi; Hu, Liangbing; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Solid state energy storage devices with solid state electrolytes (SSEs) can potentially address Li dendrite-dominated issues, enabling the application of metallic lithium anodes to achieve high energy density with improved safety. In the past several decades, many outstanding SSE materials (including conductive oxides, phosphates, hydrides, halides, sulfides, and polymer-based composites) have been developed for solid-state batteries. Among various SSEs, garnet-type Li7La3Zr2O12 (LLZO) is one of the most important and appealing candidates for its high ionic conductivity (10-4~10-3 S/cm) at room temperature, wide voltage window (0.0-6.0V), and exceptional chemical stability against Li metal. However, its applications in current solid state energy storage devices are still facing various critical challenges. Therefore, in this quadripartite thesis I focus on developing nanomaterials and corresponding processing techniques to improve the comprehensive performance of solid state batteries from the perspectives of electrolyte design, interface engineering, cathode improvement, and full cell construction. The first part of the thesis provides two novel designs of garnet-based SSE with outstanding performance enabled by engineered nanostructures: a 3D garnet nanofiber network and a multi-level aligned garnet nanostructure. The second part of the thesis focuses on negating the anode|electrolyte interfacial impedance. It consists of several processing techniques and a comprehensive understanding, through systematic experimental analysis, of the governing factors for the interfacial impedance in solid state batteries using metallic anodes. The third part of the thesis reports several processing techniques that can raise the working voltage of Li2FeMn3O8 (LFMO) cathodes and enable the self-formation of a core-shell structure on the cathode to achieve higher ionic conductivity and better electrochemical stability. The development and characterization of a solid state energy storage device with a battery-capacitor hybrid design is included in the last part of the thesis.
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    DEVELOPMENT OF FUNCTIONAL METAL OXIDE THIN FILMS VIA HIGHTHROUGHPUT PULSED LASER DEPOSITION FOR ADVANCED ENERGY APPLICATIONS
    (2018) Liang, Yangang; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    High-throughput methodologies are effective for rapid exploration of new materials with enhanced physical properties. In this thesis, we combine highthroughput pulsed laser deposition (HT-PLD) synthesis with rapid characterization techniques (X-Ray Diffraction, Atomic Force Microscopy, Electrochemical Impedance Spectroscopy, etc.) to quickly optimize metal oxide materials for energy conversion devices. The solid oxide fuel cell (SOFC) is one of the most promising energy conversion technologies. Despite years of concerted efforts by the research community, widespread commercialization of SOFCs is hindered by their high operating temperature requirements (>800 °C). Currently, there are limitations on the performance of electrolyte and cathode materials, which prevent a significant reduction in this operating temperature. To this end, we developed all-thin-film SOFC structures to probe fundamental transport properties via out-of-plane measurements in epitaxial electrolyte films with idealized interfaces. A highly conducting and thermally stable bottom electrode is combined with a library of top microelectrodes (30𝜇𝑚 ≤ 𝑑 ≤500𝜇𝑚), in a Cox and Strack-like geometry, which enables a direct and highspatial- resolution investigation of the intrinsic transport properties of the model electrolyte Sm0.2Ce0.8O2-δ (SDC20). This work demonstrated the utility of prototypical out-of-plane all-thin-film heteroepitaxial electrochemical devices as a model platform which can be extended to high-throughput investigations. We have used the high-throughput thin film formalism to develop a fundamental understanding of surface oxygen reduction reaction (ORR) mechanisms in mixed-conducting cathode materials by fabricating thin-film microelectrode arrays of La0.6Sr0.4Co1-xFexO3-δ (0≤x≤1) on a YSZ (100) substrate. The electrochemical properties of these microelectrode stacks are investigated via scanning impedance spectroscopy, and reveal that electrochemical resistance is dominated by surface oxygen exchange reactions on the electrode through a two-phase boundary pathway. A monotonic increase in electrochemical resistance is observed in La0.6Sr0.4Co1-xFexO3-δ from x =0 to x =1 along with a decrease in chemical capacitance corresponding to a decrease in oxygen vacancy concentration. A 𝑝𝑂( dependence of 𝑅* and 𝑘, for the whole spread film with the 𝑚 in a range of 0.5 to 0.75 is observed, indicating that the oxygen vacancy transport to surface-adsorbed oxygen intermediates is the ratedetermining step for mixed conducting cathodes. This study demonstrates the rich insights obtained via high-throughput methodologies and the promise of applying such techniques to discover highly active solid-state cathode materials. We have also looked at PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) as a doubleperovskite cathode material, which exhibits the combined conduction of e-, O2-, and H+. The high capacity of PBSCF to adsorb H2O at high temperature (Proton concentration: 1.7 mol% at 600 °C) and its excellent ORR performance can facilitate the cathodic electrochemical reaction in proton conducting SOFCs (p-SOFCs). A thinfilm library was used to investigate the ORR mechanism for PBSCF by systematically varying the size of the microelectrode arrays. By combining a chemically stable electrolyte, BaZr0.4Ce0.4Y0.1Yb0.1O3 (BZCYYb4411) with a thin dense PLD PBSCF interface layer between the cathode material and the electrolyte, we have demonstrated breakthrough performance in p-SOFCs with a peak power density of 548 mW/cm2 at 500 °C and an unprecedented stability under CO2. The behavior of this p-SOFC can compete with that of high performance oxide-ion-conducting SOFCs. Such performance can create new avenues for incorporating fuel cells into a sustainable energy future. We have further developed a high-throughput pulsed laser deposition approach to grow phase pure and high quality crystalline V1-xWxO2 (0 ≤ x < 4%) thin films on different substrates, which is challenging because of the complex phase diagram of vanadium oxides where there are many polymorphs of VO2. We systematically study how tungsten doping affects the poorly-understood phase transition hysteresis via a composition-spread approach. We have demonstrated for the first time that a composition of V1-xWxO2 (x ≈ 2.4%) satisfies unique ‘cofactor conditions’ based on geometric nonlinear theory. Our findings inform a strategy for developing more reliable vanadium dioxide materials. In addition, the potential application of V1-xWxO2 thin films in lithium-ion rechargeable batteries were systematically studied based on the tungsten concentration dependence of electrical properties of V1-xWxO2.
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    EVALUATION AND IMPROVEMENT OF MECHANICAL AND CHEMICAL RESILIENCE OF INTERMEDIATE-TEMPERATURE SOLID OXIDE FUEL CELL ANODES
    (2017) Hays, Thomas; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Solid oxide fuel cells are in the process of reaching maturity as an energy generation technology, but a number of technical challenges exist, namely mechanical and chemical resilience, that hinder the realization of their full potential and widespread deployment. As more research and development work has been performed on intermediate temperature SOFCs based on gadolinium doped ceria, there persists a number of gaps in the understanding of the behavior of these devices. The mechanical properties of component material and SOFC structures in non-ambient conditions, the nature and degree of damage caused by sulfurized hydrocarbon fuels, and the potential for leveraging produced thermal energy are not satisfactorily characterized for GDC-based SOFCs. Mechanical testing of porous GDC and anode supported SOFC coupons from room temperature to 650°C was performed in air and reducing conditions using a test system designed and built for this application. Spherical porosity was determined to result in the higher strength compared to other pore geometries and a positive linear dependence between temperature and strength was determined for SOFC coupons. Additionally, placing the electrolyte layer in compressive stress resulted in higher strengths. Standard SOFCs were operated while exposed to hydrogen and methane containing ppm level hydrogen sulfide concentration. An infiltration technique was used to deposit a fine layer of GDC on the inner surfaces of some cell anodes, and the results of sulfur expose was compared between infiltrated and unmodified cells. GDC infiltration allowed cells to operate stably for hundreds of hours on sulfurized fuel while unmodified cells were fatally damaged in less than two days. A primary and a resulting secondary degradation mechanism were identified and associated with sulfur and carbon respectively through surface analysis. A novel technique for measuring thermal power output of small-scale SOFCs operating on a variety of fuels was developed and used to evaluate electrical and thermal outputs while operating on simulated anaerobic digester biogas. These findings were used to propose a multi-utility generation system centered on a nominal 10 kW SOFC unit fed by anaerobic digesters and capable of producing clean water and electricity for 50 individuals through direct contact membrane distillation driven by captured waste heat from the SOFC.
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    IN-OPERANDO ELECTRON MICROSCOPY AND SPECTROSCOPY OF INTERFACES THROUGH GRAPHENE-BASED MEMBRANES
    (2017) Yulaev, Alexander; Leite, Marina S.; Kolmakov, Andrei; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Electron microscopy and spectroscopy (EMS) techniques enable (near-) surface and interfacial characterization of a variety of materials, providing insights into chemical/electrochemical and morphological information with nanoscale spatial resolution. However, the experimental realization of EMS in liquid/gaseous samples becomes problematic due to their incompatibility with high vacuum (HV) conditions. To perform EMS under elevated pressure conditions, electron transparent membranes made of thin C, SiO2 or/and Si3N4 are implemented to isolate a liquid/gas sample from HV environment. Nevertheless, even a few ten nanometer thick membrane deteriorates signal quality due to significant electron scattering. The other challenge of EMS consists in inaccessibility to probe solid state interfaces, e.g. solid-state Li-ion batteries, which makes their operando characterization problematic, limiting the analysis to ex situ and postmortem examination. The first part of my thesis focuses on developing an experimental platform for operando characterization of liquid interfaces through electron transparent membranes made of graphene (Gr)/graphene oxide (GO). The second part is dedicated to probing Li-ion transport at solid-state-battery surfaces and interfaces using ultrathin carbon anodes. I demonstrated the capability of GO to encapsulate samples with different chemical, physical, and biological properties and characterized them using EMS methods. I proposed and tested a new CVD-Gr transfer method using anthracene as a sacrificial layer. Characterization of transferred Gr revealed the advantages of our route with respect to a standard polymer based approach. A novel platform made of an array of Gr-capped liquid filled microcapsules was developed, allowing for a wide eld of view EMS. I showed the capability of conducting EMS analysis of liquid interfaces through Gr membranes using energy-dispersive X-ray spectroscopy, photoemission electron microscopy, and Auger electron spectroscopy. Using operando SEM and AES, I elucidated the role of oxidizing conditions and charging rate on Li plating morphology in all-solid-state Li-ion batteries with thin carbon anodes. Operando EMS characterization of Li-ion transport at battery interfaces with carbon or Gr anodes will provide valuable insights into safe all-solid-state Li-ion battery with enhanced performance.
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    MORPHOLOGY OF CELLULOSE AND CELLULOSE BLEND THIN FILMS
    (2017) Lu, Rui; 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, renewable, biocompatible and biodegradable natural polymer. Cellulose exhibits excellent chemical and mechanical stability, which makes it useful for applications such as construction, filtration, bio-scaffolding and packaging. It is useful to study amorphous cellulose as most reactions happen in the non-crystalline regions first and at the edge of crystalline regions. In this study, amorphous thin films of cotton linter cellulose with various thicknesses were spincoated on silicon wafers from cellulose solutions in dimethyl sulfoxide / ionic liquid mixtures. Optical microscopy and atomic force microscopy indicated that the morphology of as-cast films was sensitive to the film preparation conditions. A sample preparation protocol with low humidity system was developed to achieve featureless smooth films over multiple length scales from nanometers to tens of microns. X-ray reflectivity, X-ray diffraction, Fourier transform infrared spectroscopy and high resolution sum-frequency generation vibrational spectroscopy were utilized to confirm that there were no crystalline regions in the films. One- and three- layer models were used to analyze the X-ray reflectivity data to obtain information about roughness, density and interfacial roughness as a function of film thickness from 10-100nm. Stability tests of the thin films were conducted under harsh conditions including hot water, acid and alkali solutions. The stability of thin films of cellulose blended with the synthetic polymer, polyacrylonitrile, was also investigated. The blend thin films improved the etching resistance to alkali solutions and retained the stability in hot water and acid solutions compared to the pure cellulose films.