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Title: FRANK-KASPER PHASES IN THE CO-NB-NI TERNARY SYSTEM AND THE ISOTHERMAL SECTION OF THE CO-NB-NI SYSTEM AT 1473K
Creator: Jia, Hui
Date: 2019
Description: The present study is to determine the Frank-Kasper phases in the Co-Nb-Ni system. Frank-Kasper phases, also called topological close packed ...
Show moreThe present study is to determine the Frank-Kasper phases in the Co-Nb-Ni system. Frank-Kasper phases, also called topological close packed (TCP) phases are one of the largest groups of intermetallic compounds. They are classified into several phases: A15, Laves, σ, µ and the M, P, R phases. In the 1200°C-isothermal section of the Co-Nb-Ni system, the µ phase and Laves phases exist over large composition ranges. In this study, the Co-Nb-Ni ternary system was investigated using optical microscopy(OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and differential scanning calorimetry (DSC). The experimental results were compared with literature data and thermodynamic calculations. XRD was used to identify the Frank-Kasper phases. In comparison with previous investigations, the large extensition of µ phase in the Co-Nb-Ni system was verified. In addition, the small phase (~67at. % Nb) shown in the alloys with ~50at. % Nb is the unstablized impurity phase. On the other hand, according to the results of experiments and thermo-calc, C15 phase was found instead of C14 phase compared with Gupta's study.
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Title: UNDERSTANDING THE FUNDAMENTAL MECHANISMS OF MESO-SCALE ENGINEERING OF GRAIN BOUNDARIES IN NI-BASE SUPERALLOYS
Creator: Alabbad, Bader
Date: 2019
Description: Polycrystalline nickel base superalloys are known to possess high temperature strength, good crack growth resistance and corrosion resistance...
Show morePolycrystalline nickel base superalloys are known to possess high temperature strength, good crack growth resistance and corrosion resistance at high temperatures. Engineering the morphology of the grain boundaries can yield to extent the high temperature resistance for polycrystalline nickel-based superalloys. Meso-scale engineering for grain boundary structures can be utilized through the development of serrated grain boundaries to improve the damage tolerance of polycrystalline nickel- based superalloys. Grain boundary precipitates induce the formation of the grain boundary serrations as precipitates protrude the grain boundary. Understanding and controlling the cooling processes from above the solvus temperature plays an important role in controlling the size and population of γ′ precipitates to modify specific microstructural features that can impact the resulting properties. Two stepped cooling process with a transition just below the γ′ solvus has the ability to form a unique microstructure as heterogeneous nucleation and growth of large γ′ precipitates along the grain boundaries during the initially slow cooling rate. The eventual transition to a standard or conventional cooling rate will then govern the formation of the secondary, intragranular γ′ precipitates. The stepped cooling process was compared to the conventional fast continuous cooling process. The effect of grain boundary misorientation on the precipitation behavior of grain boundary γ′ precipitates revealed coarse grain boundary γ′ precipitates reside along high angle grain boundaries. Mechanical testing was carried out to investigate their mechanical properties to compare and benchmark to those produced via conventional continuous cooling to quantify the extent by which serrated boundaries can be used to improve the properties where the modified microstructures led to great high temperature properties.Nickel-base superalloy 718Plus is a recently developed alloy that is being considered for use in a number of advanced gas turbine engine applications. A constructed heat treatment was proposed to drive the precipitation of fine η precipitates along grain boundaries. Lamellar η phase precipitates were able to induce the formation of serrated grain boundaries. Discontinuous cellular precipitation reaction led the formation of the lamellar η phase precipitates. The characteristic growth of lamellar η phase precipitates was influence by the grain boundary structure. Creep tests were carried out to provide a better understanding of how serrated grain boundaries formed using η phase precipitates can impact the creep properties of ATI 718Plus.
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Title: SYNTHESIS OF HIGH PERFORMANCE SILICON ANODES AND SURFACE-MODIFIED NMC CATHODES FOR LI-ION BATTERIES
Creator: Ashuri, Maziar
Date: 2019
Description: Lithium-ion batteries (LIBs) have revolutionized the portable electronic devices and electric vehicles (EV) and because of this huge demand,...
Show moreLithium-ion batteries (LIBs) have revolutionized the portable electronic devices and electric vehicles (EV) and because of this huge demand, it is important to meet high power, high specific energy, and long cycle life. All mentioned characteristics are directly related to the choice of anode and cathode electrodes. Currently, graphite is used as anode, while lithium cobalt oxide serves as cathode dominantly. Although graphite can deliver ~ 370 mA h g-1 capacity without significant capacity decay for several cycles, however it is not enough to fulfill the requirements for many applications. Silicon with the theoretical capacity of about 10 times higher than graphite is a promising anode. However, this material suffers from huge volume expansion during cycling in addition to its intrinsic low conductivity. From the cathode viewpoint, the need for materials with less cobalt content is necessary. The resources for cobalt element is very limited while the price of cobalt increasing. Furthermore, cobalt is known as toxic element. Therefore, substitution of cobalt with other elements such as manganese and nickel is necessary. Lithium nickel manganese cobalt oxide (NMC) cathode family materials are introduced following this idea.Here in this thesis, two different approaches are introduced to harness the problems associated with silicon anodes. The first approach is the core/shell design and the second one is the silicon/graphite nanocomposite with tailored structure and engineered voids. Both of these designs can be synthesized easily without complicatedsynthesis steps and harmful chemicals. They have the potential of being commercialized and they do not need expensive equipment. The silicon anodes have been tested successfully in the half-cell coin cells.As for the cathode side, two different members of NMC family materials (NMC333 and NMC532) have been tested. To enhance their electrochemical properties and rate capabilities, a facile surface modification using phosphoric acid was employed. This technique resulted in the formation of thin lithium phosphate coating around the particle. The electrodes performed very well in half-cell configuration. It is expected by utilizing the proposed cathode and anode materials in full-cell set up, a high performance battery with fast charge ability is obtained.
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Title: MICROSTRUCTURE EVOLUTION OF TWO KINDS OF HIGH-TEMPERATURE RESISTANCE FE-CR-NI ALLOYS
Creator: Pan, Weiqi
Date: 2019
Description: In this research, two different as-cast Fe-Cr-Ni alloys (35Cr-45Ni, 25Cr-35Ni) were aged at different times (1h, 10h, 100h, 1000h) and...
Show moreIn this research, two different as-cast Fe-Cr-Ni alloys (35Cr-45Ni, 25Cr-35Ni) were aged at different times (1h, 10h, 100h, 1000h) and temperature (700℃, 800℃, 900℃, 1000℃). These different aged alloys were studied by optical microscope, scanning electron microscope (SEM), Energy-dispersive spectroscopy (EDS), X-ray diffractometer, micro Vickers hardness tester. The aged sample showed a higher hardness than the as-cast one. The hardness also changed as time and temperature increased because diffusion result on phase transformation and precipitation. Both as-cast 35Cr-45Ni and 25Cr-35Ni have MC (NbC) and M7C3 (Cr7C3) phase transformation during aging. The long term high-temperature aging caused changes carbides and silicide phase. For both alloys during aging, MC carbides transformed to G-phase, and Cr7C3 transformed to Cr23C6.
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Title: Superior Reversible Hydrogen Storage of the LiBH4 + MgH2 System Enabled by High-Energy Ball Milling with In-Situ Aerosol Spraying
Creator: Ding, Zhao
Date: 2019
Description: The prospect of LiBH4 + MgH2 mixture has been limited by its sluggish kinetics, despite its excellent hydrogen storage capacity theoretically....
Show moreThe prospect of LiBH4 + MgH2 mixture has been limited by its sluggish kinetics, despite its excellent hydrogen storage capacity theoretically. We have designed a novel process termed as high-energy ball milling of MgH2 at ambient temperature along with aerosol spraying of LiBH4 dissolved in tetrahydrofuran (THF) solution (BMAS) to improve the thermodynamic and kinetic performance of LiBH4 + MgH2 hydrogen storage materials. Through this BMAS process, we have demonstrated that, for the first time, the reaction between LiBH4 + MgH2 can take place near ambient temperature, and the in-situ formation of LiH and MgB2 during BMAS is achieved through a new reaction pathway in which nano-LiBH4 decomposes to Li2B12H12 first and the newly formed Li2B12H12 reacts with MgH2 to form LiH and MgB2.Using the newly designed automated BMAS apparatus, we have successfully produced a BMAS mixture containing 1 mole of MgH2 + 0.5 mole of LiBH4, i.e., with 25% LiBH4 in the mixture for the stoichiometric reaction. The BMAS powder with 25% LiBH4 can release and absorb ~5.7 wt.% H2 at 265 oC, which is the highest one ever reported for the LiBH4 + MgH2 system at temperature ≤ 265 oC. It is found that the unusually high reversible hydrogen storage is accomplished through two parallel reaction pathways. One is nano-LiBH4 decomposes to form Li2B12H12 and H2 first and then Li2B12H12 reacts with MgH2 to form MgB2, LiH and H2. The other is nano-MgH2 decomposes to form Mg and H2 first and then Mg reacts with LiBH4 to form MgB2, LiH and H2. These reaction pathways become possible because of the presence of nano-LiBH4 and nano-MgH2 and their intimate mixing, enabled by the BMAS process. We have also revealed that the solid-state dehydrogenation kinetics of the BMAS powder with 25% LiBH4 at 265 oC is nucleation-and-growth controlled. The rate-limiting step for dehydrogenation via the two parallel reaction pathways has been identified through examination of the elementary reactions as the nucleation and growth of reaction products LiH and MgB2. Given the significantly improved hydrogen storage capacity for the LiBH4 + MgH2 system obtained via BMAS, investigation on increasing the LiBH4 content in the BMAS powder from 25% to 50% is performed. It is shown that Mg(BH4)2 can be produced during the BMAS process and it contributes to H2 release at temperature ≤ 265 oC. Three parallel H2 release mechanisms have been identified from the BMAS powder. These include (i) nano-LiBH4 decomposes to form Li2B12H12 and H2 first and then Li2B12H12 reacts with MgH2 to form MgB2, LiH and H2, (ii) nano-Mg(BH4)2 decomposes to form MgH2, B and H2, and (iii) nano-MgH2 decomposes to Mg and H2. Together these three mechanisms result in 4.11 wt.% H2 release in the solid state at temperature ≤ 265 oC. Furthermore, the predicted property of Fe3B in absorbing more H2 than releasing it is confirmed experimentally for the first time in this study. Varied models have been identified to describe the kinetic of solid-state dehydrogenation of the BMAS powder with 50% LiBH4 at 265 oC with increasing cycles. Additionally, the geometries of the solid particles involving with the dehydrogenation have also been estimated.
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Title: FRANK-KASPER PHASES IN THE CO-NB-NI TERNARY SYSTEM AND THE ISOTHERMAL SECTION OF THE CO-NB-NI SYSTEM AT 1473K
Creator: Jia, Hui
Date: 2019
Description: The present study is to determine the Frank-Kasper phases in the Co-Nb-Ni system. Frank-Kasper phases, also called topological close packed ...
Show moreThe present study is to determine the Frank-Kasper phases in the Co-Nb-Ni system. Frank-Kasper phases, also called topological close packed (TCP) phases are one of the largest groups of intermetallic compounds. They are classified into several phases: A15, Laves, σ, µ and the M, P, R phases. In the 1200°C-isothermal section of the Co-Nb-Ni system, the µ phase and Laves phases exist over large composition ranges. In this study, the Co-Nb-Ni ternary system was investigated using optical microscopy(OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and differential scanning calorimetry (DSC). The experimental results were compared with literature data and thermodynamic calculations. XRD was used to identify the Frank-Kasper phases. In comparison with previous investigations, the large extensition of µ phase in the Co-Nb-Ni system was verified. In addition, the small phase (~67at. % Nb) shown in the alloys with ~50at. % Nb is the unstablized impurity phase. On the other hand, according to the results of experiments and thermo-calc, C15 phase was found instead of C14 phase compared with Gupta's study.
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Title: STRUCTURAL DEFECT CONTROL TO IMPROVE SOLID-STATE REACTION AND ELECTROCHEMICAL PERFORMANCE OF NaCrO2 CATHODE FOR SODIUM-ION BATTERIES
Creator: Luo, Mei
Date: 2018
Description: NaCrO2 has been studied lately as a promising cathode material for Na-ion batteries. Consequently, this study was conducted to investigate how...
Show moreNaCrO2 has been studied lately as a promising cathode material for Na-ion batteries. Consequently, this study was conducted to investigate how high-energy ball milling before the high temperature reaction influences the synthesis reaction and electrochemical performance of NaCrO2 cathodes for Na-ion batteries. In-situ synchrotron X-ray diffractometry is employed for the first time to provide a comprehensive understanding of the critical reaction temperatures and reaction pathway. It is found that high-energy ball milling at room temperature can result in significant changes in the synthesis reaction of NaCrO2 when compared to reactants without high-energy ball milling. These changes include a decreased onset temperature for the formation of O3-NaCrO2, an increased reaction kinetics, an alternation of the reaction pathway, and a complete reaction at 900℃ to form thermally-stable O3-NaCrO2 phase. In contrast, without high-energy ball milling the reaction product at 900℃ is a highly impure NaCrO2 with a poor thermal stability. Equally important, it is found that mechanical activation enhances the bulk ionic conductivity as well as the conductivity at the interface of NaCrO2 particles; moreover, high-energy ball milling before reaction can lead to higher specific discharge capacities of NaCrO2 half cells and contribute to the best capacity retention over 50 cycles among all the reported NaCrO2 without coatings. All these improvements of NaCrO2 cathodes for Na-ion batteries are ascribed to the mechanical activation induced by high-energy ball milling before high temperature reaction.
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Title: Structural Studies of Degradation Mechanism of Lithium Rich Manganese, Nickel, and Iron Based Cathodes
Creator: Aryal, Shankar
Date: 2018
Description: Layered oxide compounds are superior with respect to discharge voltage and discharge specific capacity compared to other families of cathodes....
Show moreLayered oxide compounds are superior with respect to discharge voltage and discharge specific capacity compared to other families of cathodes. Therefore, LiCoO2 and LiMnxNiyCozO2, are the most commonly used cathodes since the commercialization of lithium ion battery. Recently, Li rich Ni, Mn, and Co oxide composite cathodes have been introduced with some improvements. As Co is toxic and expensive, attempts have been made to replace Co with cheap and environmentally friendly Fe. This dissertation reports that comparable discharge specific capacity and discharge voltage can be achieved by replacing Co with Fe and optimizing the composition of Mn, Ni, and Fe. However, the capacity and voltage fading on cycling are still remaining challenges. Structural change on electrochemical cycling is the main reason behind this fading. X-ray absorption spectroscopy (XAS), the specific element probe technique to study local structure and X-ray diffraction (XRD) to study the crystallographic phase information are utilized to understand the degradation/aging mechanism. A series of Li rich Mn, Ni, and Fe oxide composite cathode materials Li1.2Mn(0.30+x)Ni(0.40-x)Fe0.10O2 for x = 0, 0.05, 0.10, 0.15, 0.20 and 0.25 were prepared using a sol-gel synthesis method. Rhombohedral and monoclinic crystal phases are found in Li rich Mn, Ni, and Fe composite oxide materials, but pure rhombohedral phase cannot be obtained without excess Li in the stoichiometric LiMO2 form. The pure monoclinic phase Li2MnO3 is also synthesized to confirm its presence in the composite oxide cathodes. Particle size and surface morphology are studied with scanning electron microscopy. The composite cathodes are cycled to over 100 cycles at 0.3C, for C = 250 mAhg-1 rate. XAS before and after 100 electrochemical cycles of Li rich Mn-Ni-Fe based cathodes is reported for the first time. The determination of fractional contents of monoclinic and rhombohedral phases in the composite oxide cathodes is not possible by powder XRD analysis, however, Li2MnO3 content decreases on decreasing Mn content and on increasing Ni content. The composition with higher Ni content has a higher degree of cation mixing. The synergistic effect of rhombohedral and monoclinic phases in Li rich Mn, Ni, and Fe based cathode is critical for stable electrochemical performance. The Li1.2Mn0.50Ni0.20Fe0.10O2 cathode showed the most stable cyclability performance (194 mAhg-1 first discharge capacity with 94 % capacity retention after 100 cycles at 0.3C rate) however, Li1.2Mn0.40Ni0.30Fe0.10O2 (220 mAhg-1 first discharge capacity with 57 % capacity retention) and Li1.2Mn0.55Ni0.15Fe0.10O2 (241 mAhg-1 first discharge capacity with 68 % capacity retention) cathodes showed higher 1st discharge capacity but poor cyclability under the same charge/discharge cycling.The XAS at Mn K-edge is used to explain the mechanism of Li2MnO3 activation for the improved electrochemical performance of Li rich Mn, Ni, and Fe oxide composite cathode, however Li2MnO3 contributed differently in different compositions. Synchrotron XRD and XAS measurements probed the lattice size expansion, which decreases the chemical potential of Li ions in the cathode on cycling leading to lower discharge voltage after cycling.
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Title: Enhancing Charge/Discharge Cycle Stability of NaCrO2 Cathode for Na-ion Batteries via Carbon Coatings
Creator: Shi, Zhepu
Date: 2018
Description: In this study, we report the effects of carbon coating on the electrochemical cycle stability of Na-ion batteries made of NaCrO2 cathodes....
Show moreIn this study, we report the effects of carbon coating on the electrochemical cycle stability of Na-ion batteries made of NaCrO2 cathodes. Various coating approaches and conditions have been investigated for 10-h high energy ball milled NaCrO2. It is shown that mixing the carbon source with NaCrO2 particles before the high-temperature carbonization reaction is a critical step. The solution-based mixing of the carbon source with NaCrO2 leads to the best carbon coating uniformity. Furthermore, carbonization treatment should be limited to 10 min at 650 ℃ in order to prevent the reaction between carbon and NaCrO2 to form chromium carbides. Uniform carbon coating can improve the capacity retention of NaCrO2 over charge/discharge cycles and the best capacity retention achieved in this study is 70% after 50 cycles. Furthermore, once the coating is uniformly distributed, NaCrO2 exhibits a very high specific capacity (140 mAh/g) which is significantly higher than the typical value of 110 mAh/g reported in the literature. The unusually high specific capacity observed is attributed to the enhancement of Na-ion intercalation and de-intercalation rates at the electrode/electrolyte interface because of the presence of the carbon coating.
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Title: IRREGULAR GROWTH AND INTERFACIAL EFFECT IN THIN FILM MULTILAYER STRUCTURES FOR USES IN PHOTOCATHODE APPLICATIONS
Creator: Lee, ZhengRong
Date: 2021
Description: Improving photocathode performance by increasing the electron density while lowering the angular spread of emitted electrons can improve...
Show moreImproving photocathode performance by increasing the electron density while lowering the angular spread of emitted electrons can improve particle accelerator performance, expanding the reach of both fundamental and applied science. Materials science expertise is needed to design new photocathodes with these desired properties. Nemeth, et al, determined that a multilayered photocathode structure consisting of MgO/Ag/MgO could be engineered for higher brightness and lower dispersion [Nemeth, et al, Phys. Rev. Lett. 104, 046801 (2010)]. The dispersion of the surface bands impacts the angular spread of the emitted beam, and the model predicted that the bands could be tuned by precisely controlling the layer thicknesses of the multilayer structure. We synthesized and probed this MgO/Ag/MgO system experimentally. We measured the work function, emittance, and quantum efficiency of multilayer photocathodes with different MgO layer thicknesses to compare with theoretical predictions. We observed that although the general trend was as predicted, the measurements and the model were not in exact agreement [Velasquez, et al, Appl. Surf. Sci. 360, 762 (2016)]. In this work, we have undertaken a study of the electronic structure of the interfaces to explore how these observed deviations may have originated. It is possible that the fabrication process leads to non-ideal interfaces compared to those constructed in the simulations. To study how the fabrication affects the interfaces, hard X-ray photoemission spectroscopy(HAXPES) was used to probe the chemistry of the buried interfaces within the thin film multilayer structure of Ag and MgO. In these multilayer structures, we observed that the silver layers were predominantly metallic. A small high binding energy (ΔE = 0.69 eV) peak was also observed in the Ag 3d core level in the samples. This peak is shifted in the opposite direction of the binding energy shift in silver oxides, suggesting that this peak is not due to the formation of silver oxides at the interfaces with the MgO. Two possible explanations for the origin of this peak then are charge transfer at the interface from the Ag to the oxide monolayer or the formation of silver nanoparticles during the growth process. Based upon simple depth profiling analysis, we postulate the former is the more likely explanation. In addition, the O 1s and Mg 1s core level indicated the presence of Mg(OH)2. The MgO layers react with H2O in the vacuum chamber or ideal gas used as a buffer during sample transfer. Since the theory predicts a strong dependence upon the number of MgO layers surrounding the Ag, the formation of Mg(OH)2 likely contributes to the non- ideal behavior, even given the similarity in the electronic structure to MgO (large band gap insulator) and Mg(OH)2. The speed at which this reaction occurs would significantly limit the lifetime and the utility of the MgO/Ag multilayer photocathodes. In order to custom engineer multilayer photocathodes, complete control over the growth process will be needed to ensure that the ideal surfaces are formed. Using non-reactive materials would greatly increase the lifetime of the engineered photocathodes.
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Title: INVESTIGATION OF STRUCTURE AND PROCESSING EFFECTS ON THE ELECTROCHEMICAL PERFORMANCE OF COBALT-FREE, LITHIUM- AND MANGANESE-RICH LAYERED OXIDE CATHODE FOR LIBS
Creator: Kucuk, Kamil
Date: 2021
Description: Rechargeable Li-ion batteries (LIBs) have been widely used in a diverse range of energy storage systems because of their high energy and power...
Show moreRechargeable Li-ion batteries (LIBs) have been widely used in a diverse range of energy storage systems because of their high energy and power density, low self-discharge, and tolerable memory effect, compared to the conventional alkaline, lead acid, and nickel-cadmium (Ni/Cd) batteries. [2] Since not only cathodes materials control the energy density of a cell, but also the capacity of cathode material characteristically restricts the cell capacity (as well as about 40% of the cell cost results from the cost of cathode raw materials), the majority of studies on LIBs have been carried out on developing alternative cathodes with higher energy, lower cost, and more environmentally friendliness. [2], [3] From this perspective, both Co-free and lithium- & manganese-rich (LMRO) layered oxide MNF cathodes, Li1.2(MnxNiyFez)O2, have recently attracted great attention in lithium-ion battery (LIB) research for electric vehicles and energy storage devices due to their high capacities of over 250 mAhg−1 and being eco-friendly and inexpensive compared to the cobalt-based Li-rich Li1.2(NixMnyCoz)O2 and Ni-rich Li(NixMnyCoz)O2 (NMC), and LiCoO2 commercial cathodes. Replacing toxic and expensive Co in the LMRO cathodes with environmentally friendly and much cheaper Fe element has been extensively studied over the last two decades. It was suggested by Aryal, S. et. al., in 2018. [4] that the Li1.2(Mn0.50Ni0.20Fe0.10)O2 (MNF502010) Co-free LMRO MNF cathodes seem better in terms of capacity-retention with higher discharge capacity and less voltage fade compared to other MNF compositions. However, the MNF502010 cathode still suffers from its lower experimental capacity, compared to its expected theoretical capacity (270-455 mAhg−1), as well as capacity decay, voltage fade, poor rate capability, and thermal instability. In this dissertation, it is reported that comparable specific discharge capacity with less amount of voltage fading and capacity decay can be achieved by fluorine doping, synthesizing materials in large amounts (0.1 mol synthesis at least) with two-step firing, and then washing the obtained nanocomposites with H3PO4 to create Li3PO4 layer on the surface of bulk MNF composites. The specific discharge capacity and cycling performance of the Co-free MNF502010 cathodes were studied and enhanced by using and optimizing these approaches in this work for the first time. However, voltage fading and capacity decay are still remaining challenges, even if they are remarkably mitigated by applying these approaches. Structural changes due to layered to spinel transformation, less amount of monoclinic phase activation leading to structural deformation occurring after 1st charge, dissolution of the transition metals (TM), and oxygen release (loss of lattice oxygen) from the MNF material upon following electrochemical cycling at higher voltage (≥ 4.5V ) seem the main reasons behind these challenges, specifically the voltage fading and capacity decay.A series of fluorine-doped/undoped, Co-free MNF502010 nanocomposite cathode materials (Li1.2(Mn0.50Ni0.20Fe0.10)O2(1−x)F2x, briefly F-doped MNF) were synthesized by using a sol-gel technique. Firstly (Chapter 4), the fluorine was substituted for oxygen in the parent MNF compound in different fractions (0.00, 0.025, 0.05,0.075, 0.10, which means 0%, 2.5%, 5.0%, 7.5%, and 10%), in order to optimize the amount of fluorine for better performance; secondly (Chapter 5), a large batch (0.1mol, 10 times more than the previous batch) of 5%F-doped material was prepared by a modified sol-gel synthesis which is modified by heating at 700 ◦C for different time-periods; 7.5 hours (7.5h), 15 hours (15h, two-step firing, 7.5h + rest for 12h + 7.5h), instead of heating directly 15 hours (d15h), as done in the first chapter; finally (Chapter 6), H3PO4 treatment resulting in a non-uniform Li3PO4 layer on the bulk surface. These approaches were respectively applied on doped/undoped MNF502010 nano-composites, in order to overcome the challenges already mentioned above. Finally, the effects of these approaches on the structural, morphological, and electrochemical properties of MNF cathode materials were investigated by means of powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) with energy dispersive X-ray (EDS) analysis, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), galvanostatic charge-discharge cycling, and X-ray absorption spectroscopy (XAS, an element specific probe technique). Specifically, ex-situ XAS was performed at the Mn, Ni, and Fe K-edge and used to detect the changes both in the oxidation state of the transition metal (TM) ions and their local environments in order to get a better understanding of the improved performance of the composite materials, as well as their failure mechanism. Moreover, the EXAFS data were modeled to gain insight into the influence of these approaches on the electrochemical performance of both pristine (uncycled) and cycled electrodes (after the 100th discharge). From correlating the electrochemical performance of the modified/unmodified MNF nano-composite cathodes to their XANES and EXAFS analysis, the ability to achieve higher specific capacity is strongly dependent on the formation of a well-ordered layered structure and the amount of monoclinic component (Li2MnO3) activation resulting in higher redox-activity of the Mn cations. The long-term cyclability or capacity retention can be enhanced by heating the resulting powders with a two-step firing (instead of directly 15 hours) and washing them with 1wt%H3PO4 solution to create a Li3PO4 conductive and protective layer.
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Title: Effects of the Silicon Content on the Dimensional Changes of Electrodes for Lithium-ion Cells: An Electrochemical Dilatometry Study
Creator: Rodrigues Prado, Andressa Yasmim
Date: 2021
Description: The continuous growth of the electric vehicle market has significantly increased the demand for Li-ion batteries (LIBs). However, state-of-the...
Show moreThe continuous growth of the electric vehicle market has significantly increased the demand for Li-ion batteries (LIBs). However, state-of-the-art LIBs are not yet able to meet the EV industry demand for high energy density and long cycle life rechargeable batteries, prompting efforts to improve the performance of Li-ion cells. In this context, silicon became the most promising next-generation active material for LIBs negative electrodes, especially because Si can significantly increase the lithium storage capacity of the commonly available anodes. Nonetheless, commercialization of Si-based electrodes has been hindered by the poor electrochemical performance of these electrodes, which is mainly attributed to the severe volumetric changes in the silicon particles related to the electrochemical reactions with Li. Since the electrodes are composites with a complex combination of various materials interspaced by pores, the electrode-level swelling may differ significantly from the particle-scale expansion. Furthermore, an increase in electrode thickness due to silicon expansion can have a direct effect on how Li-ion cells are designed, as the accommodation of electrode dilation requires additional cell space to prevent significant dynamic stresses. Thus, the actual volumetric energy density of a LIB cell depends on the electrode swelling, since the higher the magnitude of the electrode expansion, the lower the gains in energy density. Monitoring the electrode dilation is just as important as the electrochemical evaluation when designing cells with Si-based anodes.In this work, we use high-resolution operando electrochemical dilatometry to quantify the (de)lithiation-induced expansion/contraction of silicon, blended silicon-graphite and graphite electrodes, upon electrochemical cycling. We evaluate the relationship between electrode capacity and dilation and observe that while the lithiation capacity improved with increasing the silicon content, the electrode swelling is highly aggravated. For silicon-rich anodes, the electrode dilation can be higher than 300%, and the expansion profile consists of a combination of slow swelling at low levels of lithiation followed by an accelerated increase at higher lithium contents. This non-linear dilation allows for narrowing the swelling by limiting the electrode capacity. In addition, we investigate how electrode properties, such as porosity, affect the dilation profile, and quantify the irreversible expansion of the electrodes. Finally, we discuss some of the challenges associated with the dilatometry technique and suggest experimental approaches for obtaining consistent and reliable data.
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Title: Understanding the effect of temperature, strain and grain size on the stress relaxation behavior of Ni-base and steel alloys
Creator: Reny, Julia
Date: 2021
Description: Stress relaxation research is very valuable as it provides information on how materials relieve stress under a constant strain. In...
Show moreStress relaxation research is very valuable as it provides information on how materials relieve stress under a constant strain. In applications where parts are submitted to a constant deformation, it is important to know that regions containing notches or stress concentrations will be able to relax enough to avoid the formation of cracks during operations. In this study, the stress relaxation behaviors of experimental Ni-base superalloy RRHT5 and Pyrowear 53 steel alloy were investigated. Tests were conducted under different conditions, varying temperature, initial strain, and grain size to explore the influence of each of these factors. The stress relaxation test data was used to calculate apparent activation volumes which can be correlated with plastic deformation mechanisms governing the stress relaxation. Understanding these mechanisms and identifying how specific factors influence them is essential to further control and optimize the design and processing of materials to obtain the best properties. Depending on the test conditions, the stress relaxation underwent one or two stages dominated by different mechanisms. These governing mechanisms were considered to be either dislocation-dislocation interactions, dislocation rearrangement, dislocations spreading from localized slip bands or dislocation-precipitate interactions.
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Title: Synthesis and Processing of NaSICON Membranes with High Ionic Conductivity and Good Mechanical Strength
Creator: Chiang, Shan-Ju
Date: 2019
Description: Natrium super ion conductors (NaSICONs), Na1+xZr2SixP3-xO12 (0 ≤ x ≤ 3) are compounds that commonly used as solid electrolytes and membranes...
Show moreNatrium super ion conductors (NaSICONs), Na1+xZr2SixP3-xO12 (0 ≤ x ≤ 3) are compounds that commonly used as solid electrolytes and membranes of sodium based batteries, or in gas sensors and fuel cells due to their high sodium ion conductivity, low thermal expansion, and ability to accommodate ions in the lattice. However, NaSICON with high relative density (> 97%) and minimum impurity phases is found to be very difficult to obtain. Furthermore, the cost of the general synthesis methods is a serious drawback. Multi-high-temperature heating procedures is often employed to increase the density and to attain the single phase NaSICON because the particle size and free ZrO2 are better reduced. This research explores the possibility of densification and synthesis of NaSICON in one high-temperature reaction through a novel process termed Integrated Mechanical and Thermal Activation (IMTA) and the co-sintering behavior as well as the NaSICON composite membranes from tape casting. The sintering temperature of NaSICON was decreased by mechanical activation at room temperature using high-energy ball milling. Sintered NaSICON-based materials showed highest total ionic conductivity of 1.45 × 10-3 S cm-1 at room temperature and high density of 3.155 g cm-3 (96.5%). An alternative to obtaining full densification (99%) of NaSICON ceramics was developed utilizing traditional solid-state reaction. This sintered NaSICON without any sintering aid exhibited the total conductivity, 6.59 × 10-4 S cm-1 at 25 °C, and the highest density of 3.238 g cm-3, a better than 2.6% enhancement from the original samples.The second part of the work has comprised of successful fabrication of NaSICON/polymer composite membranes and bi-layered NaSICON/stainless steel membranes to enhance the mechanical flexibility of pure NaSICON films. The effect of different particle sizes of stainless steel on the sintering behavior and shrinkage rate were studied systematically. The effect of solid content in the slurry was also studied to control the density of both support layer and NaSICON body. The affect structural ratios have on co-sintered tapes along with ionic conductivity was investigated using Electrochemical Impedance Spectroscopy (EIS). The co-sintered membrane exhibited a total conductivity as high as 4.580 × 10-4 S/cm at room temperature. EIS results showed the high Na-ions conductivity strongly depends on the feature of grain boundary and the high densification of NaSICON layer.
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Title: DATA-DRIVEN FIRST-PRINCIPLES STUDY OF ORDERING PHENOMENA IN COMPLEX ALLOYS
Creator: Kim, George
Date: 2021
Description: Determining the chemical (dis)ordering behavior in materials such as high entropy alloys (HEAs), and ternary Laves phases is fundamental to...
Show moreDetermining the chemical (dis)ordering behavior in materials such as high entropy alloys (HEAs), and ternary Laves phases is fundamental to developing structure-property relations that can be used as guiding principles for alloy design. A common obstacle in materials engineering is that an improvement of a material property comes at the expense of some other desirable properties. For example, trade-offs may be made between strength and ductility, or strength and density, etc. The large compositional and configuration space of possible HEAs, and Laves phases contain potential candidate materials with a balance of optimized properties and tunable structural and functional properties. However, fully exploring the large compositional and configurational space with experimental or even high-throughput Density Functional Theory (DFT) approaches is infeasible, and as of yet, predictive rules for phase stability and chemical (dis)order in HEAs, and Laves phases are still open questions.In this thesis, a HEA with chemical disorder, Al0.3CoCrFeNi, was studied using complementary experimental, DFT, and ML methods. The chemical disorder within the HEA resulted in a severely distorted lattice leading to a reduction in stiffness. Temperature dependence of chemical ordering behavior is studied in NbTaTiV and NbTaTiVZr HEAs using Monte Carlo (MC) simulations, which predicts short-range ordering (SRO) as well as short-range clustering (SRC) behavior in both HEAs. The compositional dependent behavior of substitutional ordering in two ternary Laves phases is evaluated and compared using cluster expansion (CE) models and Monte Carlo (MC) simulations.
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Title: NANOMATERIALS FOR ADVANCED BATTERY CATHODES
Creator: Moazzen, Elahe
Date: 2020
Description: Cathode materials are key components that directly determine the power density of a battery. One of the most effective ways of developing high...
Show moreCathode materials are key components that directly determine the power density of a battery. One of the most effective ways of developing high power density cathodes is bringing them into the nano-scale world, which results in many expected and unexpected properties. Some of the desired characteristics include faster charge/discharge kinetics, improved capacity retention and structural stability due to the higher surface to volume ratio and shorter ion diffusion paths. In this dissertation a number of uniquely designed nano-sized cathode materials and nanocomposites are developed and investigated for alkaline aqueous and lithium ion battery applications. Nickel hydroxide (Ni(OH)2), which is one of the most important cathode materials in alkaline batteries, suffers from low conductivity, which usually leads to inefficient discharge and incomplete utilization of the material. A series of Ni(OH)2/Co(OH)2 core/shell nanoplatelets were synthesized and systematically investigated as cathode materials. Structure-property correlations revealed that electrochemical behavior and reversibility of Co(OH)2 redox conversion depended non-linearly on the average shell thickness, with the best performance (99.6% of theoretical capacity of the composite material) achieved at shell thickness of 1.9 ± 0.3 nm. Two fundamental phenomena were suggested to be responsible for the superior performance: templated shell deposition and galvanic coupling of core and shell materials.Manganese (IV) oxide (MnO2), which is another practical cathode that has a great potential to be utilized for a variety of energy storage systems, still has some major challenges including reversible cycling in rechargeable batteries. One of the most crucial challenges is the fact that polymorphs of MnO2 have different electrochemical activities as aqueous and Li-ion battery cathodes. However, most synthetic samples contain a mixture of polymorphs, which makes the structure-property correlations more complicated. This dissertation reports on systematic studies correlating synthesis, thermal and mechanical processing, and composite formation with polymorph composition, electrochemical performance and ion intercalation mechanisms. Among all the results, several main conclusions were reached: 1) Through control of the synthesis parameters and post-processing, desired phase compositions and nanoparticle morphologies, which optimize MnO2 performance in aqueous alkaline electrolyte, can be achieved. Nanoparticles with higher fraction of the akhtenskite polymorph showed higher reversible capacities in LiOH electrolyte (~210 mAh g-1), with stable performance for over 50 cycles. The effects of sub-nanoparticle organization of MnO2 polymorphs by thermal treatment without any morphology change on cycling performance, phase activation, and charge/discharge mechanisms in LiOH electrolyte as well as the detailed mechanism of the polymorph conversion during annealing were studied and for the first time, demonstrating that the electrochemical activity of MnO2 material strongly depends not only on the lattice structure of individual polymorphs but also on the sub-nanoparticle polymorph architecture and interphases.2) Several processing strategies, including thermal and mechanical processing, and composite fabrication were utilized to develop functional MnO2 cathodes for Li-ion batteries. Improvements in capacity and cycling performance were correlated to the presence of the pyrolusite phase of MnO2 and the crystallite size. Composite fabrication by graphene oxide wrapping also provided significant performance improvements through polymorph composition control and improved conductivity.
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Title: FUNCTIONALIZED NANOSCALE MATERIALS FOR PROTEIN BIOMARKER DETECTION
Creator: Zhang, Youwen
Date: 2020
Description: Proteins are vital biomolecules in living organisms which function as the working element for many aspects of life. An abnormal expression of...
Show moreProteins are vital biomolecules in living organisms which function as the working element for many aspects of life. An abnormal expression of proteins or expression of unique proteins is often associated with certain disease. Accordingly, proteins have become valuable biomarkers for disease diagnosis and prognosis. So far, numerous methods have been developed for detections of protein biomarkers. However, most of them suffer from the lack of accuracy, sensitivity, and specificity for clinical diagnostic applications. With the rapid advancement in nanotechnology, functional nanoscale materials, which could overcome the biocompatibility and biological recognition ability, have been widely used to develop sensitive and selective biosensors.In this dissertation, two kinds of functionalized nanoscale materials-based sensing strategies are investigated for protein biomarker detection. One strategy takes advantages of graphene oxide (GO) and utilizes fluorescence resonance energy transfer (FRET) for fast and sensitive protease detection by covalent attaching fluorescently labeled protease substrate peptide to the GO surface. This type of GO-based fluorescence sensor is highly sensitive (with a detection limit of picomolar concentration) and selective (other structure similar proteases does not interfere with the target analyte detection). In addition, it could accurately analyze serum samples. With this strategy, we have successfully achieved the detection of the HIV-1 PR (HIV-1 protease, a significant biomarker for AIDS) and ADAMs (a disintegrin and metalloproteinases, a biomarker for human cancers). It could be visualized that this GO platform could be utilized to detect various proteases by only changing the peptide substrate and solution pH. In addition, by coupling multiple substrate peptides on the GO surface, we developed a multiplex GO sensing system for simultaneously profiling of the activities of a panel of MMPs/ADAMs. Under the assistance of joint entropy and programming, our sensor could identify up to 5 types of human cancers, and offers the potential to detect other cancer types by changing biomarkers.The other strategy is to utilize nanopore stochastic sensing to detect proteins, which involves measuring the ionic current modulation generated by analytes’ electro-osmotic flow through a chemical functionalized nanoscale sized pore. As a sensitive and label-free technique, nanopores have been highly recognized as one of the emerging techniques to detect analytes at the single-molecule level. Unlike DNA molecules which are uniformly charged, proteins are an isotropically charged molecules, which have low translocation probability through a nanopore. Since the protein pore-based sensing system is not suitable as deployable tools for detection of proteins due to the size limitation and fragile nature of the biological membranes. In this project, we fabricated solid-state nanopores using PET membranes followed by chemical functionalization of their inner surfaces. The modified- PET nanopore was sensitive and could detect HIV-1 protease at picomolar concentration. More importantly, the modified-nanopore sensor was selective, and could differentiate the target protein from others such as Trypsin, BSA and HSA. Furthermore, the modified PET nanopore strategy developed in this work provide a general platform for exploring fundamental protein dynamics and rapid detection of proteins at the single-molecule level
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Title: PREPARATION OF MG3SB2-XBIX THERMOELECTRIC MATERIAL AND THEIR LASER SINTERING BEHAVIOR
Creator: Xiao, Xudong
Date: 2020
Description: In the introduction part, a novel thermoelectric material, Zintl compounds were introduced due to its potential high thermoelectric...
Show moreIn the introduction part, a novel thermoelectric material, Zintl compounds were introduced due to its potential high thermoelectric performance in low-temperature applications as thermoelectric devices. Recent researches focused on Mg3Sb2-based Zintl Compounds was summarized in this article, and the general methods and its limitations to fabricate Mg3Sb2-based thermoelectric modules were introduced. Thus, a novel selective laser melting process was introduced, and it has enormous potential to fabricate Mg3Sb2-based thermoelectric modules in commercial applications. What’s more, the challenges of selective laser melting method were also discussed in the articles. For developing selective laser melting as the mature method to fabricate Mg3Sb2-based thermoelectric modules, many works and researches need to be done. In my project, the powder of Mg3.1(Sb0.3Bi0.7)1.99Te0.01 thermoelectric material was synthesized by using the mechanical alloying process and the proper particle size and distribution of powder using for selective laser melting was obtained by increasing the time of mechanical alloying. The thermoelectric properties of the powder prepared by mechanical alloying were measured, and it shows the powder still in a good performance after a long-time mechanical alloying particle reduction process. Finally, the obtained powder was treated by different parameters of continued wave fiber laser. The morphology and composition of the sintered area were analyzed to better understanding the process of laser sintering. More work needs to be done for using a selective laser melting method to fabricate Mg3Sb2-based thermoelectric modules in the future.
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Title: SI NANOSTRUCTURED COMPOSITE AS HIGH PERFORMANCE ANODE MATERIAL FOR NEXT GENERATION LITHIUM-ION BATTERIES
Creator: He, Qianran
Date: 2019
Description: Silicon has attracted huge attention in the last decade as the anode material for Li-ion batteries because it has a theoretical capacity ∼10...
Show moreSilicon has attracted huge attention in the last decade as the anode material for Li-ion batteries because it has a theoretical capacity ∼10 times that of graphite. However, the practical application of Si is hindered by three major challenges: large volume expansion during cycling (∼300%), low electrical conductivity, and instability of the SEI layer caused by repeated volume changes of the Si material. Our study focused on novel design and synthesis of Si anodes that can solve all the key problems of Si anodes simultaneously. The Si micro-reactors we designed and synthesized contain well-designed internal structures, including (i) nanoscale Si building blocks, (ii) the engineered void space, and (iii) a conductive carbon shell. Because of these internal structures and nitrogen doped carbon shell, these sub micrometer-sized Si particles are termed as Si micro-reactors and denoted as Si@void@C(N). According to our electrochemical results, the as-synthesized Si micro-reactors could live up to 1000 charge/discharge cycles at high current densities (up to 8 A/g) while still providing a higher specific capacity than the state-of-the-art carbonaceous anodes. Our investigation shows that the unique design of Si@void@C(N) has a relatively low specific surface area (SSA) which significantly reduces the undesired surface side reactions and increases ICE to 91%, while the engineered voids with nano-channel shape inside the structure can accommodate Si volume expansion and keep the structure and SEI layer stable. Furthermore, the porous N-doped carbon shell along with nano-channeled voids allows rapid lithiation of the Si micro-reactor without Li plating during ultrafast charging. As a result, Si@void@C(N) exhibits ultrafast charging capability with high ICE, superior specific capacity and long cycle life.
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Title: DOPING OF SODIUM CHROMIUM OXIDE CATHODE MATERIALS TO ENHANCE ELECTROCHEMICAL PERFORMANCE FOR SODIUM-ION BATTERIES
Creator: wang, ziyong
Date: 2019
Description: In this project, we investigated the effects of doping several types of metals to NaCrO2 on its electrochemical performance. The doping method...
Show moreIn this project, we investigated the effects of doping several types of metals to NaCrO2 on its electrochemical performance. The doping method is aiming to stabilize the O3-type structure by partial substituting some of Cr with other metals during intercalation/deintercalation by suppressing Cr6+ migration to alkaline slab, and thus facilitate long-term cycle performance and reversible capacity. All doped NaCrO2 powders were hereby denoted to NaMe0.1Cr0.9O2 (Me=Al, Co, Ni, Mn). To achieve metal-doped NaCrO2 powders, sodium, chromium and dopant sources were mixed with various metal oxides and then subjected to 6-hour high energy ball milling, followed by heating in flow-Ar tube at 900℃ for 1 hour. Pristine NaCrO2 powder synthesized in the same process was to make comparisons with doped ones. To understand the mechanism of doping, field emission scanning microscopy (FESM) and energy Disperse Spectroscopy (EDS), as well as X-ray diffractometer (XRD), were employed to analyze the morphology and composition of final products. Benefiting from Ni doping, NaNi0.1Cr0.9O2 cell exhibited a high reversible capacity of 132 mAh g-1 at the initial cycle in a potential region between 2.0 and 3.6 V vs. Na/Na+, and 78 % of capacity retention over 70 cycles. For NaMn0.1Cr0.9O2, reversible capacity at first discharge is about 30 mAh g-1, lower than that of Ni-doped and pristine NaCrO2, while the cycle retention stays at nearly 100% after 100 cycles. The opposite charge/discharge behaviors from Ni- and Mn-doped NaCrO2 provide us a potential method for the optimization of cathode materials with the best electrochemical performance in the future.
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