Materials Science & Engineering Theses and Dissertations

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    (2022) Wang, Haotian; Rubloff, Gary; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Li-ion battery (LIB) is a popular energy storage device that predominates the market of microelectronics due to its high energy density and light weight. In the recent trend of electrification of vehicles, LIBs also showed promise in the application of electric vehicles but the energy density of current LIBs with graphite electrode doesn’t suffice the need of long driving range. Replacing graphite electrode with alloying type electrodes that have almost ten times higher energy density is thus a necessary route to improve the energy density of LIBs. However, alloying type electrodes, such as Si and Sn, typical undergo enormous volume change (up to 310%) during Li insertion and extraction, which lead to various mechanical problems such as cracking, delamination, and pulverization. These mechanical issues eventually cause catastrophic capacity fading in LIBs and thus, are central topics for the application of alloying type electrodes in next generation LIBs. This dissertation presents a three-phase experimental study of stress development in Si electrodes and Si based solid state batteries. In the first phase, ex-situ stress characterization in single-c Si electrode was performed to validate Raman spectroscopy as a promising stress characterization technique for Si electrode. In the second phase, in-situ stress characterization in patterned poly-c Si electrode with confocal micro-Raman setup was performed, to investigate the correlation between complex geometries and stress distribution in crystalline Si electrode and the critical size effect. In the last phase, a solid-state battery (SSB) platform device with lateral layout was proposed and validated for stress characterization in Si based SSBs. The platform device can also serve as a versatile testbed for electrochemistry study of bulk SSB components and interfaces. Overall, this dissertation demonstrates a methodology that combines Raman spectroscopy, novel design of electrochemical devices, and computational modeling as a powerful tool for electrochemo-mechanics study of alloying type electrodes and SSB systems.
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    Radiation-Induced Modification of Aramid Fibers: Optimizing Crosslinking Reactions and Indirect Grafting of Nanocellulose for Body Armor Applications
    (2022) Gonzalez Lopez, Lorelis; Al-Sheikhly, Mohamad; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The goal of this dissertation was to design, synthesize, and analyze novel aramid fibers by covalently grafting nanocellulose through electron beam irradiation. These nanocellulose functionalized fibers showed enhanced strength and larger surface areas, which improves their performance and applicability in fiber-reinforced composites. Unmodified aramid fibers have smooth and chemically inert surfaces, which results in poor adhesion to many types of resins. Nanocellulose was chosen as the ideal filler to functionalize the fibers due to its reactive surface and high strength-to-weight ratio. Aramid fibers were further modified by radiation-induced crosslinking reactions as a means to avoid scission of the polymeric backbone and to further increase the fiber strength.An indirect radiation-induced grafting approach was used for synthesizing these novel nanocellulose-grafted aramid fibers while avoiding the irradiation of nanocellulose. The fibers were irradiated using the e-beam linear accelerator (LINAC) at the Medical Industrial Radiation Facility (MIRF) at the National Institute of Standards and Technology (NIST). After the irradiation, the fibers were kept in an inert atmosphere and then mixed with a nanocellulose solution for grafting. The grafted fibers were evaluated by gravimetric analysis, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) spectroscopy. The mechanical properties of the synthesized fibers were studied by single fiber tensile tests. Aramid fibers were also irradiated at the MIRF in the presence of acetylene gas and triacrylate solution as a means to induce crosslinking reactions. These fibers were irradiated at both low doses and high dose rates at room temperature. A mechanism for the crosslinking of aramid fibers was proposed in this dissertation. Mechanical testing of the fibers after crosslinking showed an increase in the strength of the fibers of up to 15%. Ultra-high molecular weight polyethylene (UHMWPE) fibers were also studied, but due to an issue of entanglement of the fibers during the grafting process, their mechanical properties could not be analyzed. Future work will focus on using a better set up to avoid entanglement of these fibers. To complete the study of the radiation effects on polymers, this thesis explored the radiation-induced degradation of aromatic polyester-based resins. The composition of the resins studied included phenyl groups and epoxies, which complicate radiation-induced grafting and crosslinking reactions. Unlike aramid and polyethylene fibers, polyester-based resins have a C-O-C bond that is susceptible to degradation. The resins were irradiated at high doses in the presence of oxygen. The scission of the polymeric backbone of the polymers was studied using Electron Paramagnetic Resonance (EPR) analysis. EPR showed the formation of alkoxyl radicals and C-centered radicals as the primary intermediate products of the C-O-C scissions. The degradation mechanisms of the resins in the presence of different solvents were proposed. Changes in the Tg of the polymers after irradiation, as an indication of degradation, were studied by Dynamic Mechanical Analysis (DMA). The results obtained from this work show that irradiation of these resins results in continuous free radical-chain reactions that lead to the formation of recyclable oligomers.
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    Understanding the effect of fabrication conditions on the structural, electrical, and mechanical properties of composite materials containing carbon fillers
    (2022) Morales, Madeline Antonia; Salamanca-Riba, Lourdes G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Carbon structures are commonly used as the reinforcement phase in composite materials toimprove the electrical, mechanical, and/or thermal properties of the matrix material. The structural diversity of carbon in its various forms (graphene, carbon nanotubes, graphite fibers, for example) makes it a useful reinforcement phase, as the properties of the composite material can be tailored for a specific application depending on the structure and properties of the carbon structure used. In this work, the incorporation of graphene/graphitic carbon into an aluminum metal matrix by an electrocharging assisted process (EAP) was investigated to create a composite material with enhanced electrical conductivity and yield strength. The increased electrical conductivity makes the composite suitable for application in more efficient power transmission lines. The increased strength makes it useful as a lightweight structural material in aerospace applications. The EAP involves applying a direct current to a mixture of molten aluminum and activated carbon to induce the crystallization of graphitic sheets/ribbons that extend throughout the matrix. The effect of processing conditions (current density, in particular) on the graphitic carbon structure, electrical properties, and mechanical properties of the composite material was investigated. The effect of porosity/voids and oxide formation was discussed with respect to the measured properties, and updates to the EAP system were made to mitigate their detrimental effects. It was found that the application of current results in some increase in graphitic carbon crystallite size calculated from Raman spectra, but many areas show the same crystallite size as the activated carbon starting material. It is likely that the current density used during processing was too low to see significant crystallization of graphitic carbon. There was no increase in electrical conductivity compared to a baseline sample with no added carbon, most likely due to porosity/voids in the samples. The mechanical characterization results indicated that the graphitic carbon clusters formed by the process did not act as an effective reinforcement phase, with no improvement in hardness and a decrease in elastic modulus measured by nanoindentation. The decreased elastic modulus was a result of compliant carbon clusters and porosity in the covetic samples. The porosity/voids were not entirely eliminated by the updates to the system, thus the electrical conductivity still did not improve. Additionally, a multifunctional composite structure consisting of a carbon-fiber reinforced polymer (CFRP) laminate with added copper mesh layers was investigated for use in aerospace applications as a structural and electromagnetic interference (EMI) shielding component. The CFRP provides primarily a structural function, while the copper mesh layers were added to increase EMI shielding effectiveness (SE). Nanoindentation was used to study the interfacial mechanical properties of the fiber/polymer and Cu/polymer interfaces, as the interfacial strength dictates the overall mechanical performance of the composite. Further, a finite element model of EMI SE was made to predict SE in the radiofrequency to microwave range for different geometry and configurations of the multifunctional composite structure. The model was used to help determine the optimum design of the multifunctional composite structure for effective shielding of EM radiation. It was found from nanoindentation near the fiber/polymer and Cu/polymer interfaces that the carbon fibers act as an effective reinforcement phase with hardness in the matrix increasing in the interphase region near the carbon fibers due to strong interfacial adhesion. In contrast, the Cu/polymer interface did not exhibit an increase in hardness, indicating poor interfacial adhesion. The EMI SE model indicated that the combination of CFRP layers, which primarily shields EMI by absorption, and Cu mesh, which predominantly shields by reflection, provided adequate SE over a wider frequency range than the individual components alone. Further, it was found that the SE of the CFRP layers were improved by including multiple plies with different relative fiber orientations.
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    (2022) Ostrovskiy, Yevgeniy; Wachsman, Eric; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Ceramics have a wide variety of applications for energy conversion and other industries because of their unique properties. Conduction of multiple charged species simultaneously enables their use as membranes, electrodes, and more. Perovskites especially, have highly tunable features, and can be modified through doping, surface coating, and microstructure. In this work, each of those approaches was used to improve and/or characterize ceramic components for either proton conducting membranes or solid oxide fuel cells (SOFCs). In the case of membranes, perovskites have limited electronic conductivity, which reduces their ability to permeate hydrogen. Through changing the dopants used in existing perovskite compositions, the electronic conductivity was improved dramatically allowing its use as an n-type conductor. This was achieved by using Pr as a dopant, which introduces electronic conductivity due its multivalent nature. It also has a favorable ionic radius for proton conduction, which is required for hydrogen permeation. In the case of fuel cells, both performance and stability need to be improved for their widespread adoption. The surface chemistry and physical properties of two major cathode materials were evaluated, La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and Sm0.5Sr0.5CoO3 (SSC). Both are susceptible to the formation of unwanted secondary phases during operation as SOFC cathodes. By studying the surface chemistry of LSCF it was possible to better understand the mechanism of cathode degradation. In the case of LSCF, it was found that electrostatic forces that result from a chemical potential difference between the bulk and surface, promote the segregation of Sr cations from the bulk which is responsible for the degradation. However, the behavior of SSC was more difficult to determine. In SSC, cation segregation was far more dependent on the grain orientation than LSCF and therefore was more difficult to quantify, and the techniques used for improving stability in LSCF were unsuccessful when applied to SSC. Additionally thin films deposited through atomic layer deposition (ALD) were tested as a means of enhancing the performance of LSCF. Due to the challenges of using ALD on porous substrates, the role of variables in the deposition process that can be widely implemented were studied, with a focus on oxygen vacancies. It was found that the choice of oxidizer and the addition of an annealing step can dramatically improve the effectiveness of thing film electrode coatings. Although ALD may not be practical for modifying SOFC electrodes, these are process steps that can be easily implemented by other researchers to improve their existing approaches. Finally, the role of microstructure was addressed as well. Tuning the porosity of SOFC anodes is essential for large scale fabrication of fuel cells and improving their performance and reliability. A microstructure featuring a hierarchal porosity was able to improve the performance of SOFC anodes, especially at lower temperatures and fuel ratios. Improvements in microstructure will allow the fabrication of larger scale SOFCs that are more reliable and mechanically stronger, with minimized performance losses associated with using a thicker anode. The primary scientific merit of this research is demonstrated in the work on cathode degradation and coatings. There the focus was on using a methodology based on a fundamental material property, oxygen vacancies, which are essential to many applications of metal oxides. With this type of approach, it is possible to apply similar techniques to other areas of research involving metal oxides or thin films. The main engineering merit of this research is evaluating the relationship between microstructure, SOFC performance, and large SOFC production. Commercialization of SOFCs requires that they are as effective as possible outside of ideal conditions (pure fuel, high temperatures). Hierarchal porosity has been shown to improve performance under both conditions and can also be applied to cathodes.
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    Investigating Aluminum Nitride As A Protection Layer For Lithium Germanium Thiophosphate Solid Electrolytes
    (2022) Klueter, Sam; Rubloff, Gary; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Lithium germanium thiophosphate (LGPS) is an attractive solid-state electrolyte material due to its exceptionally high ionic conductivity, which rivals organic liquid electrolytes. Despite this potential, other properties have impeded its adoption into solid-state batteries, particularly the poor voltage stability of the material at potentials near that of high voltage cathodes or lithium metal. Aluminum nitride (AlN) can serve as an anodic protection interlayer between LGPS and lithium metal, enhancing cell performance. AlN is grown via plasma-enhanced ALD at 250 °C using TMA and both N2 and NH3, but the deposited films show significant oxygen contamination originating from the plasma and lack crystallinity. Galvanostatic cycling and electrochemical impedance spectroscopy show that LGPS-coated cells perform better than bare cells, with expected lifetimes >3x greater in certain cases. Finally, XPS line scans highlight the slow room-temperature reactivity between AlN and evaporated lithium, and a computational model is built to aid further XPS experiments.