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Browsing by Author "Thompson, Gregory"

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    Design Strategies for Improving the Oxidation Resistance of Multi-Principal Element Alloys
    (University of Alabama Libraries, 2024) Pavel, Michael; Weaver, Mark
    Since the early 1940’s, nickel-based superalloys (SA) have been the cornerstone of hightemperature structural alloys due to their unique yield strength, creep life, and oxidation re-sistance when subject to harsh environments exceeding 1000 °C. While they have withstoodthe increasing operation temperatures seen in turbine engines with use of intricate coolingand coatings, they have reached their core metallurgical limit. As engineering componentsrequire the ability to operated past 1000 °C, new alloys must be developed to replace thetraditional SAs. Multi-principal Element Alloys (MPEAS) are a new innovative class ofmaterials which generally contain at least 4 major alloying additions exceeding 10 At. % ofthe overall composition and exist in the central portions of the ternary phase diagram.The high entropy of mixing within the materials is said to suppress the formation of deleterioustopologically close packed (TCP) phases and stabilize simple microstructures while allowingfor heavy alloying to generate significant solid solution strengthening, slowed diffusivities,and highly tailorable microstructures. Not only are these materials useful for their potentialreplacement of SAs, but they also provide a framework for understanding the fundamentalstructure – property relationships of non-dilute alloys. This research has shown that new ageMPEAs are able to compete with traditional SAs in terms of oxidation resistance as well ascompressive strength. Several new High Entropy Superalloy (HESAs) were developed usingthe CALPHAD approach, particularly to study the impact of microstructure and chemistryon high temperature oxidation behavior. The volume fraction and precipitate size were foundto have a significant impact on scale formation and the resulting parabolic oxidation rates ofthese alloys. While these studies and those it compares to make use of arc-melting for syn-thesizing small quantities of new materials, it was determined that the abnormally large andtextured grain structure from the as-cast materials, even after homogenization, contributedgreatly to increased oxidation variability. The use of hot thermomechanical processing wasemployed and stable processing conditions were determined for HESA processing in gramquantities. The refined microstructures led to reduced variability in the thermogravimetric(TGA) results and also a total decrease in parabolic oxidation rates. Finally, the oxida-tion behavior of a newly developed, alumina forming, HESA and an ODS variant were fullycharacterized from 900 to 1200 °C.
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    Effect of Crystal Growth on Atomic Ordering and Structure-Activity Relationships in Metallic and Bimetallic Nanoparticles
    (University of Alabama Libraries, 2023) Acquaye, Francis Yaw; Street, Shane C
    Atomically ordered bimetallic (intermetallic) nanoparticles (NPs) are auspicious candidates for increasing the activity of these materials in technologically relevant applications. Typically, solution phase syntheses result in chemically disordered structures which must then be annealed and so by atomic diffusion form chemically ordered structures. Unfortunately, under most conditions, annealing sinters the NPs and reduces the active surface area. There is therefore interest in developing methods for directly achieving ordered phases by solution phase synthesis without annealing. We hypothesize that a relatively slow rate of crystal formation allows metal atoms in near proximity to nucleated seeds to take up thermodynamically favored positions in the growing nanocrystal at room temperature, and that this has consequences for the activity of the resulting NPs. We have shown that slowing the rate of crystal growth forms the thermodynamically stable face-centered cubic (fcc) structure in Ag nanoparticles with the NPs being selective for carbon dioxide reduction reaction (CO2RR) to CO. However, fast crystal growth produces a heterophase of a stable fcc and unstable 4H stacking faults Ag NPs, which has high selectivity for CO2RR to CH3OH and CO at the 4H/fcc interface, demonstrating the distinctness of their activity because of difference in structure. In a bimetallic Cu-Pt system, we demonstrated that cubic (D7) thermodynamically stable/ordered CuPt7 NPs can be observed by slow crystal growth in solution phase synthesis at room temperature. Fast crystal growth produces a disordered CuPt structure. The consequence of these structural differences in their activity is that the expanded lattice (7.766 Å) of CuPt7 gives a lesser oxygen reduction reaction (ORR) activity via a 4-electron transfer pathway. Conversely, the contracted lattice (3.809 Å) of CuPt enhances the ORR activity of CuPt via a 2-electron transfer pathway. In a magnetic bimetallic Fe-Pt system, both slow and fast crystal growth produces the disordered fcc structure. However, the slowly grown NPs show distinctively lower magnetic moment and higher coercivity relative to the NPs formed by fast growth. NPs from both methods are superparamagnetic with some contribution from a ferromagnetic phase.
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    Effective Mitigation of Polysulfide Shuttle Effect Through Surface-Engineered and Hetero-Structure-Engineered Cathodes for Metal-Sulfur Batteries
    (University of Alabama Libraries, 2023) Wei, Zhen; Wang, Ruigang
    Environmental pollution caused by the continuous consumption and burning of fossilfuels is harmful to human health. Wind energy and solar energy cannot be extensively applieddue to their intermittent nature. Therefore, renewable, clean, and sustainable energy storagesystems such as rechargeable batteries are in urgent need in order to meet the increasingrequirements of the large-scale production of portable electronic devices and electric vehicles. Inaddition to lithium-ion batteries, lithium–sulfur (Li-S) batteries are an important focus ofacademic and industrial energy storage research owing to their higher theoretical energy density(2,600 Wh kg−1) and the use of low-cost materials. The sulfur cathode with favorablecharacteristics such as natural abundance and environmental friendliness makes Li-S batteries apromising next-generation energy storage technology. According to similar electrochemicalconversion mechanisms, the low-cost sulfur cathode can also be coupled with a wide range ofmetallic anodes, such as sodium (Na), potassium (K), magnesium (Mg), and aluminum (Al).These new "metal–sulfur" battery systems have demonstrated promising potential in loweringthe production cost and/or producing high energy density. The current state of the researchdemonstrates that metal–sulfur batteries are now at the transitioning point from laboratory-scaledevices to a more practical energy-storage application. However, practical commercialization isgreatly hindered by the notorious technical challenge known as the polysulfide shuttle effectinducing a huge loss of active material and rapid capacity decay. The overall objective of thisdissertation aims to effectively mitigate polysulfide shuttling and improve the long-termelectrochemical performances of rechargeable metal-sulfur batteries. To achieve that, we proposetwo effective technical concepts, which are surface engineering and heterostructure engineeringof sulfur-based cathodes in metal-sulfur batteries.
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    Going Beyond Shockley-Queisser Limit Perovskite Chalcogenides Tandem Solar Cell
    (University of Alabama Libraries, 2023) Gokul Menon, Harigovind; Yan, Feng; Daniewicz, Steven
    The efficiencies of single junction solar cell (SJ SC) technologies like silicon and cadmium telluride have been rapidly increasing and are nearing their fundamental Shockley Queisser (SQ) efficiency limit. New novel methods must be developed to go beyond the limitations of SJ SCs. A promising approach is the combination of an efficient narrow bandgap (NBG) absorber with a wide bandgap (WBG) absorber to form a tandem solar cell that can effectively utilize the solar spectrum. The aim of this work is the realization of mechanically stacked perovskite-antimony selenide (Sb2Se3) and perovskite-cadmium telluride (CdTe) 4T tandem solar cells. Perovskite materials with their excellent performance and tunable bandgap make it an ideal candidate for tandem solar cells. Sb2Se3 with a bandgap of 1.2eV and a theoretical efficiency limit of over 30% and CdTe with a bandgap of 1.5eV and an efficiency over 22% make them potential NBG absorbers. In this work, we first simulated 4T tandem cells pairing a 1.6eV WBG Perovskite solar cell (PSC) with a 1.2 eV NBG Sb2Se3 cell and a 1.6eV WBG PSC with a 1.5 eV NBG CdTe cell using Solar Cell Capacitance Simulator (SCAPS). We obtained a simulated tandem PCE of 23.14 % for the perovskite Sb2Se3 tandem and a PCE of 23.37% for the perovskite CdTe tandem. We then developed two WBG perovskite top cells with a bandgap of 1.6eV and 1.77eV to study the impact of bandgap on the tandem architecture. We also developed a CdTe solar cell with Cu doping which had a PCE of 17.94% and a Sb2Se3 solar cell with a PCE of 5.76%. As a substitute for the opaque metal back electrode for the top cell, we developed an indium tin oxide (ITO) transparent electrode using DC sputtering to fabricate a semitransparent top cell. By mechanically stacking the sub-cells, we obtained an excellent 4T tandem PCE of 16.13% for the perovskite-Sb2Se3 tandem and a tandem PCE of 19.41% for the perovskite-CdTe tandem. The experimental results show promising tandem cell performance and pave the way to go beyond the SQ limit.
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    High-Efficiency and Stable Carbon-Based Planar Perovskite Solar Cells
    (University of Alabama Libraries, 2023) Sankaranarayanan Nair, Vijayaraghavan; Daniewicz, Steve
    Perovskite solar cells (PSCs) have attracted much attention both in research and industrial domains. The power conversion efficiencies (PCE) of PSCs have been improved from 3.8% to 25.8% in just over a decade, rivaling that of silicon solar cells. Hybrid perovskite materials have exceptional optoelectrical properties and can be processed using cost-effective solution-based methods. In contrast, the fabrication of silicon solar cells requires high-vacuum, high-temperature, and energy-intensive processes. The combination of excellent optoelectrical properties and cost-effective manufacturing makes hybrid perovskite a winning candidate for solar cells.As the PCE of PSCs improves and their long-term stability increases, one of the crucial hurdles to be addressed is the cost-effective scalable fabrication and its long-term stability. Most high-efficiency solar cells require an energy-consuming method of depositing the cathode to complete the cell. These high-efficiency devices also require highly expensive noble metal electrodes. Herein, following recommendations from the literature, carbon-based nanomaterials are utilized, and their effects on the efficiency, fill factor (FF), open-circuit voltage (Voc), and short circuit density (Jsc) are analyzed. The PSCs and carbon counter electrodes are fabricated at low temperatures (~100 degrees Celsius). As per the literature, carbon-based devices exhibited an efficiency of >19%. This endeavor further buttresses the fact that carbon nanomaterials are promising candidates for the future of low-cost, high-performance, and scalable production of PSCs. Thus, complex vacuum deposition of expensive cathodes to complete the cells might be eliminated.In this thesis, I focus on several novel interface engineering techniques, that can reduce the surface roughness of the carbon electrode, and improve the interface it forms with the underlying layer. These interfacial engineering techniques improved the charge collection efficiency of the carbon, and thereby reduced the recombination happening at the interface. As a result, the PCE could be enhanced from ~13% to >18%.In addition to these techniques, a high-quality narrow band gap perovskite layer was developed with low dimensional 2D perovskite passivation to further improve the device performance. Together with the improved perovskite film quality and optimized film composition, along with interface engineered carbon electrode, an impressive PCE of >21% was attained.
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    In Situ Resource Utilization for Metal-Composites Formed by Solid-State Additive Manufacturing
    (University of Alabama Libraries, 2023) Lopez, Jessica; Thompson, Gregory; Allison, Paul
    In-situ resource utilization is a concept envisioned for repurposing and using natural materials available on-site. Items manufactured from these materials on-site reduce the cost of transporting them to construct end use needs. To that end, Additive Friction Stir Deposition (AFSD) provides a low energy consumption additive manufacturing method to fabricate these materials into build products. In this research, both land debris (silica and lunar regolith) and carbonaceous material were repurposed by incorporating them into aluminum feedstock while additively manufacturing the mixtures as a ceramic metal composite. The dispersion of the secondary phase was quantitatively characterized as a function of the deposition process and related to the mechanical strength. Outcomes of this work revealed that these additions increase the strength of the AFSD aluminum material when no such additions are present. Furthermore, the particulate morphologies and location evolution within the build structure were dependent on stirring characteristics of AFSD. This research demonstrates the feasibility of AFSD in processing such composites with the underlying science of how they, in addition to the aluminum matrix, evolve under AFSD.
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    Study of thermoelectric generators and perovskite solar cells for renewable energy applications
    (University of Alabama Libraries, 2020) Ouyang, Zhongliang; Li, Dawen; University of Alabama Tuscaloosa
    This dissertation aims at explorations of two promising renewable energy devices: one is thermoelectric generators (TEGs) and the other is perovskite solar cells (PVSCs). The first half of this dissertation (Chapter 2 & 3) focuses on the simulation study of TEGs while the second half (Chapter 4 & 5) concentrates on the experimental study of PVSCs. Chapter 1 serves as an overall introduction of TEGs and PVSCs. Chapter 2 investigates simulation of segmented TEGs with various state-of-the-art thermoelectric (TE) materials between 300 K and 1000 K. The influence of thermal radiation, electrical and thermal contact effects have been studied. The results show that these effects, if well-regulated, do not prevent segmented TEGs from achieving high efficiency and output power density. In Chapter 3, segmented TEGs have been further modelled to find out the best cost-performance ratios. The results reveal that successful segmentation of TE materials can offer a cost-performance ratio of ~0.86 $ W-1, less than commercially desired cost-effectiveness of 1 $ W-1, while maintaining an efficiency of 17.8% and delivering a power density over 3 Watt cm-2. These results predict the commercial feasibility and competitiveness of segmented TEGs in the same dollar per watt metrics as other renewable energy devices. Chapter 4 presents a rapid layer-specific annealing on perovskite active layer enabled by ultraviolet (UV) light-emitting diode (LED) and efficiency close to 19% is achieved in a simple planar inverted structure. These results justify that if the UV dosage is well-managed, UV light is capable of annealing perovskite into high-quality film rather than simply damaging it. Moreover, the layer-specific photonic treatment allows accurately estimating the deposition energy required to form perovskite film at device quality level. Chapter 5 exhibits an effort towards scalable manufacturing of perovskite solar panels. Perovskite mini-modules have been demonstrated with blade-coating and rapid thermal processing (RTP) in ambient environment. Mini-modules with an active area over 2.7 cm2 exhibit a champion efficiency of 17.73%. These results pave the way for large-scale production of PVSCs through high-speed roll-to-roll printing. Chapter 6 summarizes the conclusions and proposes a possible future work.
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    Understanding the Processing, Microstructure, and Mechanical Properties Relationship in a Thick Gauge High Strength Niobium-Microalloyed Line Pipe Steel
    (University of Alabama Libraries, 2024) Hossain, Afm Monowar; Kumar, Nilesh
    Natural gas, oil, and in near future, increasingly hydrogen and CO2 are expected to be carried through steel pipelines at higher pressures for increased productivity, demanding the development of thick gauge high strength–high toughness line pipe steel plates. The present research explores the relationship among processing, microstructure, and mechanical properties in Nb-microalloyed API Grade X70 line pipe steel. The study investigates how microstructural, mechanical, and fracture characteristics vary through the thickness of 22-mm thick hot-rolled steel obtained from hot-band coils manufactured in an industrial facility for spiral pipe. The analysis was conducted at different depths (surface, ¼th (quarter), and ½nd (center)) of the thick plate. Microstructural analysis demonstrates that the variations in phase structure, grain size, dislocation, precipitation, and texture components, probably due to temperature and plastic strain variations from surface to the center of a 22 mm plate during thermo-mechanical controlled processing (TMCP) of the microalloyed steel.The mechanical properties at different layers exhibit differences arising from the through-thickness inhomogeneity of microstructure. Surface location showed less sensitivity to orientation due to an overall weaker texture. The surface of the plate exhibited lower elastic modulus, higher work-hardening behavior, and higher plastic strain ratio compared to the other locations. The yield strength (YS) modeling from strengthening contributions demonstrates that grain size had the maximum contributions on YS at the surface, while precipitation and dislocation strengthening contributed significantly to YS in the interior (quarter and center) of the plate. Ductile-to-brittle transition temperatures (DBTT) were evaluated through experiment and prediction from microstructure. The outer part of the plate exhibited better DBTT characteristics than the core. Smaller effective grain size significantly lowers the DBTT, while microstructural heterogeneity, precipitates, and dislocations have a detrimental effect on increasing it. Fractured surface analysis shows that high-angle grain boundaries and phase structures significantly influence crack deflection, which is crucial in determining fracture behavior. Larger grains, secondary particles, and precipitates at the center location reduce resistance to crack nucleation and propagation. The understanding of microstructural, mechanical, and fracture behaviors developed here can be applied to develop thicker gauge line pipe steels and metallic materials with high strength, ductility, and toughness, ultimately leading to better and safer pipelines.