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ItemChirality control of tailored one-dimensional polysaccharide nanocrystals(Georgia Institute of Technology, 2024-04-27) Bukharina, DariaOne-dimensional polysaccharide nanocrystals, derived from living organisms, can self-organize into complex structures that possess long-range hierarchical order making them great candidates for high-performance structural composites with multifunctional capabilities. Their abundance in nature and biodegradability makes them excellent candidates as sustainable materials of the future. However, a greater fundamental understanding of how these nanoscale building blocks organize into functional microstructures is needed to push the boundaries of mechanical and photonic metamaterials for the future. The goal of this thesis is to uncover the intrinsic mechanisms behind self-assembly phenomenon in natural systems, understand the critical forces and parameters required for their successful hierarchical organization into chiral nematic structure and with those insights manipulate the surface chemistry to create self-assembly templates for use in photonic films for optical filters, chiral encryption, smart coatings, or biosensors. In this thesis, we first provide fundamental insight into how chiral interactions in 1D polysaccharide systems emerge, using cellulose nanocrystals (CNCs) as an example. Then, we show how CNCs interactions can be tuned and controlled via their surface modification. By functionalizing them with single stranded DNAs we show the possibility for CNCs chiral complexation through DNA-guided assembly. A nanoscale-controlled strategy to induce stimuli responsiveness and dynamic chirality. The challenges of this process and strategies to overcome them are discussed. Lastly, a top-down 3D printing approach to engineer chiral CNC-based photonic crystals with unique optical activities is developed here. This method shows how thin films capable of controlled pre-programmed circularly polarized absorbance and emission can be constructed from CNC-composites for future smart coatings, optical encryption, or optical filters. Overall, the goal of this work is to inspire applicational implementation of bioderived nanocrystals by demonstrating how their properties can be controlled and tailored based on the application. This work advances fundamental understanding of the assembly of polysaccharides nanocrystals in nature and creates a toolset to aid in the design and engineering of future metamaterials.
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ItemCharacterization and effects of heterogeneities on shock compression properties in high-solids loaded additively manufactured polymer composites(Georgia Institute of Technology, 2024-01-11) Wagner, Karla BrookeHigh-solids loaded polymer composites contain several hierarchies of heterogeneities and are of interest for use as ceramic green bodies and energetic crystals embedded in a polymer matrix. The recent and rapid growth of additive manufacturing (AM) and the engineering need for more complex geometries and individualized products has led to a surge of interest in fabricating high loading particle composites via AM. In particular, Direct Ink Write (DIW) extrusion involving layer-by-layer deposition of a composite paste made of a high loading of solids and a curable polymer binder is used to fabricate such composites in different geometries and forms. However, DIW-AM introduces further complexity in composites due to formation of process-inherent heterogeneities such as particle aggregation or porosities, which can be random, directional, or stochastic. The structure and composition of such materials vary across several length scales, resulting in processing and mechanical behavior that is difficult to predict or understand. Shock-compression of heterogeneous particle-filled polymer composites often involves complex interactions, which can make it difficult to predict their dynamic mechanical properties. The shock compression behavior is often dominated by mesoscale defects (including porosity) or interactions of the shock wave with interfaces and particulates. Traditional diagnostic methods, such as velocity interferometry, enable temporally resolved measurements, but are limited in spatial resolution and generally provide volume averaged responses. Spatially resolved measurements are therefore also necessary to provide sufficient information regarding the mesoscale processes which dominate performance of such materials. X-ray phase contrast imaging, a spatially and temporally resolved technique, in conjunction with traditional velocimetry, can enable observation of the effects of hierarchical heterogeneities on shock compression response. In this work, the effect of print geometry and porosity (process-inherent heterogeneities) on the shock compression response of an additively manufactured high-solids loaded composite is studied. The composite contains three reinforcing phases: two inorganic particles and one organic particle, all with differing size distributions and morphologies. They are surrounded by a UV curable polymer binder. In order to investigate the effect of these process inherent heterogeneities on shock response, the high-solids loaded composite’s microstructure is first quantitatively characterized via microcomputed tomography imaging and computational analysis in three dimensions. Next, the composite undergoes plate-impact experiments at Argonne National Laboratory’s Advanced Photon Source’s Dynamic Compression Sector, with X-ray PCI used as an in-situ and in-material diagnostic. This is combined with PDV for validation. The phase contrast images are analyzed in order to measure shock and particle velocities directly from the translation of the shock wave and particles over time. Finally, the effects of print geometry, impact direction relative to print orientation, and porosity are studied by combining the aforementioned structural characterization with the shock response of the material determined via X-ray PCI. This reveals that print geometry does result in differing macroscale shock response (quantified with EOS), and that print geometry, impact orientation, and pore morphology all have an effect on microscale shock response (quantified with pore collapse velocity). We expect that these factors, only studied on a relatively small scale in this work, will become more exaggerated as sample size and therefore quantity of heterogeneities grows.
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ItemUnderstanding Electrode-Electrolyte Interactions for Increased Energy Density in Supercapacitors for Aerospace Applications(Georgia Institute of Technology, 2023-04-26) Allen, Julia MadelineIn the design for these electrochemical double layer capacitors, the electrodes are made with vertically-aligned carbon nanotube forests. This forest is functionalized with a coating of titania in an attempt to increase the energy density of the supercapacitor by adding pseudocapacitive redox reactions between the coating and the electrolyte. Functional alumina coatings are also investigated to improve the ability to understand and control the electrode-electrolyte interactions. The electrode materials investigated will be shown to form different morphologies depending on the presence of alumina as a base layer and the vertical location within the forest. In samples where the alumina is present, the coating forms a conformal shell around the individual carbon nanotubes. However, the alumina coating only forms near the top of the forest. In regions where the alumina is not present, either because the deposition process was not able to penetrate that far into the forest, or the sample did not have an alumina base layer applied at all, titania coatings form a non-continuous coating of discrete titania nanoparticles attached to the nanotube walls. There is no change to these coatings after 1,000 charging and discharging cycles observed. In addition to fabrication of these devices and samples, a set of novel ionic liquids are synthesized with a methyl-carbonate(trifluoromethylsulfonyl)imide anion, an asymmetric anion. Asymmetric anions are theorized to have superior properties to symmetric anions. The structure of these ionic liquids is characterized for confirmation of successful synthesis, and the melting and degradation temperatures are determined experimentally. Many properties of ionic liquids impact device performance, although, not all of the effects have been well characterized. Some of the most important properties for energy storage, are the electrochemical stability window, conductivity, and melting temperature. There is not a database containing electrochemical stability window data, but electrical conductivity and normal melting temperature data have been compiled. Therefore, a machine learning algorithm is developed and used to create predictive models for the electrical conductivity. Models like these can be used to enhance the selection and testing process. They also offer potential to predict completely new ionic liquids with optimal combinations of several properties for use in specific applications. Devices are made using these electrode materials and room temperature ionic liquid electrolytes and characterized using a variety of electrochemical techniques to evaluate the capacitance, series resistance, specific energy, specific power, cycling stability, and pseudocapacitance. Supercapacitors utilizing carbon nanotube forests with pseudocapacitive coatings are confirmed to exhibit signs of pseudocapacitive reactions occurring. Cyclic voltammetry results indicate that these pseudocapacitive reactions are occurring through surface redox reactions. Supercapacitors using a base layer of alumina beneath a layer of titania demonstrate improved performance (1.01 mF) over supercapacitors fabricated without the alumina layer (0.82 mF). The supercapacitors with no coatings added to the carbon nanotubes have an average capacitance of 0.67 mF. Galvanostatic charge/discharge testing results also indicate that the supercapacitors with alumina and titania coatings exhibit pseudocapacitance, while those without coatings do not. However, the shape for the discharge curve indicates that there are intercalation or intercalation with partial redox occurring as well. The supercapacitors with alumina and titania coatings have the lowest resistance and highest capacitance on average. A study of supercapacitor performance at different scan rates is performed to gain a better understanding of the reactions occurring within the devices. This method is used to separate the current into non-faradic and faradaic components. Faradaic reactions are observed to contribute to the device capacitance in the case of supercapacitors fabricated with pseudocapacitive coatings. The intercalation processes, identified with galvanostatic charge/discharge testing are responsible for the faradaic current measured in this technique.
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ItemInvestigating the Interfacial Evolution of Lithium Metal in Anode-Free Batteries with Liquid and Solid-State Electrolytes(Georgia Institute of Technology, 2023-04-25) Sandoval, Stephanie ELithium-ion batteries (LIBs) have powered consumer electronics for decades, and continued improvements have enabled the rapid growth of the electric vehicles (EV) market. However, increased energy density and specific energy are necessary to facilitate further expansion of EV market share and for emerging technologies such as electric flight. A promising avenue to achieve this is by replacing conventional graphite anodes in LIBs with lithium (Li) metal anodes. The use of Li metal in Li metal batteries (LMBs) is of great interest due to its high theoretical capacity (3,860 mAh g-1) and low redox potential (-3.04 V vs SHE). However, the uncontrollable growth of Li throughout cycling limits the electrochemical performance of LMBs. A battery configuration in which Li metal is plated onto a bare current collector during the first charge, known as the “anode-free” architecture, is particularly promising in that this increases volumetric energy density by ~85% compared to conventional LIBs. The main challenge of the anode-free architecture arises from limited Li+ inventory in the cell and uncontrolled morphological evolution of Li. Nonetheless, these cells are potentially safer and easier to manufacture than LMBs since they can be assembled in their fully discharged state like LIBs. To enable anode-free batteries, it is important to gain a fundamental understanding of Li nucleation and growth mechanisms as well as subsequent stripping mechanisms on bare current collectors. This thesis work focuses on understanding the cyclic evolution of Li in anode-free batteries using both liquid and solid electrolytes, while exploring methods, such as the use of alloy interlayers, to spatially control Li deposition/stripping behavior. The nucleation and growth of Li on bare current collectors is first explored in liquid electrolyte systems by combining electrochemical methods with operando optical microscopy. We find that silver interlayers enable improved Coulombic efficiency (CE) for Li cycling in multiple electrolyte systems compared to bare current collectors or other alloy layers. Operando optical microscopy reveals reduced growth of dendritic Li on silver-coated current collectors at high current densities compared to bare current collectors, as well as different dendrite growth and stripping dynamics. Unlike liquid electrolyte systems, there is little work in understanding the cyclic evolution of Li in solid-state batteries (SSBs). I first focus on investigating Li evolution on bare current collectors using cryogenic focused ion beam (cryo-FIB) and X-ray computed tomography (CT) imaging paired with electrochemical methods. It is demonstrated that substantial amounts of Li can be deposited on bare current collectors at relatively high current density in SSBs, thus showing that deposition is not the limiting process in anode-free SSBs. Instead, we find that Li stripping from bare current collectors causes accelerated short circuiting that limits cycle life. The use of nanoscale alloy interlayers is then explored as a method to improve cell performance. Here, we investigate the cyclic evolution of Li on bare and alloy interfaces in SSEs by leveraging cryo-FIB and plasma FIB methods paired with in situ electrochemical impedance spectroscopy (EIS). These methods provide insight into the Li cycling dynamics that occur at the solid-solid interface. We find that Li deposits non-uniformly in bare current collectors and upon stripping, there are clear electrochemical signatures of contact loss across the solid-solid interface. In contrast to bare current collectors, alloy coated electrodes enable uniform deposition and stripping, thus improving overall cell performance. Finally, we explore operando Li deposition and stripping dynamics in anode-free batteries through X-ray CT. Together, these datasets provide evidence that cyclic deposition/stripping performance can be significantly improved through the use of thin alloy interlayers, and mechanisms are proposed describing the dynamic action of these interlayers on electrochemical behavior.
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ItemRapid evaluation of cyclic performance using small-volume metal samples through spherical indentation(Georgia Institute of Technology, 2023-04-25) Johnson, Camilla E.This thesis develops and implements high-throughput indentation protocols which require small sample volumes to rapidly characterize the cyclic behaviors of metals through extraction of intrinsic material properties. First, novel spherical microindentation protocols are developed on wrought Ti-6Al-4V (Ti64) samples then extended to laser powder bed fusion processed Inconel 718 (IN718) samples. In these studies, repeatable load-displacement loops are successfully converted into hysteresis stress-strain loops. Hysteresis energy density plots allowed the indentation stress at which significant plastic deformation began to be pinpointed. Peak indentation stress and strain values are used to create cyclic stress-strain curves which showed great agreement to that from conventional uniaxial cyclic tests of material from the same sample blocks. The study on IN718 samples captured changes in the cyclic response as a function of post-processing heat treatments and anisotropy of the as-printed samples. Finally, cyclic spherical nanoindentation protocols are developed and demonstrated on titanium alloy Ti-6Al-2Sn-4Zr-2Mo (Ti6242) to evaluate the cyclic behavior as a function of grain orientation of the primary-alpha phase and morphology between the globular primary-alpha and dual-phase alpha -beta basketweave grains. Due to the small length scales of this study, unique features tied to dislocation motion were captured in the load-displacement and stress-strain loops as well as the final cyclic indentation stress-strain curves. Outcomes from this work contribute to material design rules and provide reliable mechanical data useful for refining crystal plasticity models. These case studies critically evaluate the effectiveness of the newly developed high-throughput cyclic response screening methods. It is seen that the protocols have a large applicability across multiple materials and length scales and contribute to expediting materials design, innovation, and characterization of advanced materials.
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ItemEffects of Redox Potential on Corrosion in Molten Fluoride Salt(Georgia Institute of Technology, 2023-04-25) Sankar, Krishna MoorthiOver the last decade, there has been an increased interest in Molten Salt Nuclear Reactors (MSR) and Concentrated Solar Power Plants (CSP) as methods of generating clean, safe, and economical energy. For MSRs, this will be accomplished by using molten salts, commonly molten fluoride salts such as FLiNaK (LiF-NaF-KF), as the coolant and heat transfer agent. A major challenge in the implementation of these new technologies is the reliable performance of structural materials that must withstand molten fluoride salts at high temperatures. Molten fluoride salts can be extremely corrosive to the structural alloys, and this corrosion is exacerbated by the presence of various oxidizing impurities in the salt. These impurities drive up the redox potential of the molten salt, which can lead to the selective dissolution of more active alloying elements from the structural materials in contact with the salt. As a higher redox potential of the molten salt is the main cause of these changes, redox control methods such as purification of salt, addition of active elements that can react with and consume the oxidizing impurities, and electrochemical cathodic protection can be effective in controlling corrosion of structural materials in the salt. However, the exact effect of different types of impurities and redox control methods on the redox potential of the salt and the behavior of structural materials in contact with the salt had not been thoroughly studied. This research was aimed at understanding the effect of various oxidizing impurities on the corrosion of structural materials and interaction of nuclear graphite with molten fluoride salts and the viability of various redox control methods to control degradation of structural materials. This work also focused on creating a methodology to reproducibly measure the redox potential of structural materials in a molten fluoride salt system. The effects of addition of impurities and application of redox control methods on the corrosion of structural materials, mechanical properties of structural materials, and behavior of nuclear graphite were systematically probed by standardized static exposure tests of candidate alloys and nuclear graphite in molten FLiNaK, with manipulation of salt compositions and exposure condition variables to answer specific research questions. The changes in redox potential due to the changes in these variables were probed using electrochemical techniques to quantifiably correlate these variables with the corrosion of the structural materials in molten fluoride salt. The knowledge gained from this research clarifies the effect and efficiency of different impurities and redox control methods on corrosion in molten fluoride salts in MSR and CSP conditions.
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ItemDevelopment of Proton-Conducting Electrolytes with Enhanced Performance and Stability for Reversible Solid Oxide Cells(Georgia Institute of Technology, 2023-04-12) Luo, ZheyuReversible solid oxide cells (ReSOCs) that efficiently operate under both fuel cell (fuel to energy) and electrolysis (energy to fuel) modes in a switchable manner are a promising technology for energy storage and conversion. Proton-conducting electrolytes are attracting increasing attention due to their promising conductivity at intermediate temperatures, enabling operation of ReSOCs with high efficiency. However, one of the reasons that they have not been widely adopted is the lack of an electrolyte material that possesses both high ionic conductivity and sufficient stability, especially against high concentrations of steam and carbon dioxide. This objective of this work is to develop novel proton-conducting electrolyte materials for high-performance ReSOCs. To achieve high proton conductivity, acceptor doping with rare earth elements is a commonly used strategy, which is critical to the formation of protonic defects. The results reveal that conductivity, ionic transference number (tion), chemical stability, and compatibility with NiO (a common fuel-electrode material) are all closely correlated with dopant size. In particular, the reactivity with NiO is found to strongly affect the properties of the electrolytes and hence cell performance. Among all compositions studied, an electrolyte with proper acceptor dopant shows excellent chemical stability and minimal reactivity towards NiO, as predicted from density functional theory (DFT)-based calculations and confirmed by experimental results. In addition, proton-conducting reversible solid oxide cells (P-ReSOCs) based on the optimized electrolyte demonstrate excellent stability and exceptional performance. Donor doping is an effective strategy for improving the chemical stability of BaCeO3-based proton conductors. However, donor-doped materials often exhibit very low conductivity. The enhanced proton conductivity of donor-doped barium cerate is demonstrated by compensating the incorporation of donor dopants with excess acceptor doping, highlighting the potential of defect chemistry engineering for enhancing conductivity and durability simultaneously. When compared to the state-of-the-art proton conductors with similar conductivity, the optimized donor-doped electrolyte materials demonstrate a significantly enhanced chemical stability, especially against high concentrations of steam, which is vital to water electrolysis for hydrogen production. While the development of new materials is the focus of this thesis, the technical approach is composed of various electrochemical techniques, surface characterization, and computational modeling to understand the rationale behind the difference in properties. It is hoped that the concepts developed in my studies can offer insights into the rational design of novel materials for chemical and energy transformation technologies.