Materials Science & Engineering Theses and Dissertations

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

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Scalable Rapid Fabrication of Low-Cost, High-Performance, Sustainable Thermal Insulation Foam for Building Energy Efficiency
(2024) Siciliano, Amanda Pia; Hu, Liangbing; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Bio-based thermal insulation materials offer a promising path towards energy savings in the buildings sector. However, these materials face competitiveness challenges against conventional petroleum-based alternatives due to issues with inferior insulation performance, poor compressive strength, and limited manufacturing scalability. Various fabrication methods such as freeze drying, thermal bonding, and chemical treatment have been proposed to enhance the material’s internal structure by introducing additional pores, creating a more complex path for heat transfer, and improving insulation efficiency. Despite advancements, the manufacturing scalability of these methods and their integration into industrial production remain unachieved.This thesis aims to bridge the gap between laboratory experiments and large-scale production by developing low-cost, sustainable cellulose-based thermal insulation. By investigating both aqueous and non-aqueous-based processing strategies, this work proposes several different fabrication techniques, leading to significant savings in energy, time, and cost. Establishing a comprehensive understanding of the interactions among the fabrication process, insulation foam, manufacturing scalability, and intended product application is imperative. This understanding accounts for variations in processing parameters (e.g., pretreatments, binders, temperature, time) and their impact on the insulation foam’s internal structure and overall performance. By examining the relationship between processing parameters and material structure, this thesis not only advances the fundamental understanding necessary for optimizing fabrication but also provides strategic guidance for selecting and designing scalable bio-based thermal insulation foams. Studying and characterizing commercially viable methods that seamlessly integrate with current industrial infrastructures is crucial for facilitating the transition from small-scale laboratory experimentation to large-scale industrial production. Through various technical strategies, this work illustrates how our understanding can be utilized to offer direction for fabrication method selection, design, and processing, ultimately optimizing the scalable rapid fabrication of low-cost, high-performance sustainable thermal insulation materials for building energy efficiency.
INTERFACES IN THIN-FILM SOLID-STATE BATTERIES
(2024) Castagna Ferrari, Victoria; Rubloff, Gary GWR; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The lack of a diagnostics approach to monitor interface kinetics in solid-state batteries (SSBs) results in an incomplete knowledge of the mechanisms affecting device performance. In this study, a new protocol for process control of SSB interface formation and their evolution during operation is presented. Thin-film SSBs and diagnostic test devices that are composed by a permutation of isolated layers were simultaneously fabricated using sequential sputtering deposition and in-situ patterning using shadow masks. Physics-based electric circuit models were designed for deconvolution of impedance profiles, which enabled an evaluation of bulk properties and space-charge layers at interfaces individually and during operation under different states-of-charge. Relative permittivity values of fundamental battery components (cathode, electrolyte and anode) were calculated as a function of the frequency and the applied voltage. Interfacial impedances, as well as space-charge layers formed at heterojunctions during charge and discharge processes, were successfully deconvoluted using the diagnostic test devices and electric circuit modeling. The cathode-electrolyte interphase was kinetically stable under a voltage window of 0 – 3.6 V vs Cu, and it had an estimated ionic conductivity of the order of 10-9 S/cm, hence it is a localized limiting factor for Li+ transfer. The anode-electrolyte interphase was thermodynamically stable upon completion of the fabrication process, but it became kinetically unstable during charge and discharge cycles. Hence, the proposed diagnostics protocol enlightened the necessity of implementing interfacial engineering on these interphases in the future for improvement of cyclability and stability of SSBs and ionic devices.
DIRECT INK WRITING SOLID-STATE LI+ CONDUCTING CERAMICS FOR NEXT GENERATION LITHIUM METAL BATTERIES
(2024) Godbey, Griffin Luh; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The global pursuit of safer and higher-capacity energy storage devices emphasizes the crucial link between the microstructures of electrochemically active materials and overall battery performance. The emergence of solid-state electrolytes featuring multi-layered, variable porosity microstructures presents fresh opportunities for developing the next generation of rechargeable batteries. However, this advancement also brings forth novel challenges in terms of device integration and operation. In this dissertation, solid-state Li-ion conducting electrolytes were 3D printed to enhance the performance of porous electrolyte layers within porous-dense-porous trilayer systems.LLZO-based garnet electrolyte scaffolds were fabricated via 3D printing using direct ink writing (DIW), facilitating the generation of scaffolds with minimal tortuosity and constriction in comparison to previous porous scaffolds manufactured through tape casting. Rheological techniques, including stress and time sweep tests, were employed to characterize the DIW inks and discern their conformal and self-supporting properties. The analysis focused on ink characteristics critical for Direct Ink Writing (DIW), emphasizing properties essential for achieving high aspect ratio printing and minimal constriction in 3D structures. Drawing from this ink research, two distinct 3D architectures, columns and grids, were fabricated. Column structures were utilized in assembling Li-NMC622 and Li-SPAN cells, with detailed discussions highlighting improvements in printing and sintering outcomes. Notably, NMC622, characterized by larger particle sizes, demonstrated complete infiltration within 3D printed porous networks, yielding a promising specific capacity of 169.9 mAh/g with minimal capacity fade. Further optimization involved integrating a porous 3D scaffold to facilitate SPAN infiltration in Li-SPAN cells, resulting in a specific capacity of 1594 mAh/g, albeit with significant capacity fade. The Li-S was implemented into a grid structure achieving 763 mAh/gS with less than 0.25% capacity loss over 16 cycles. Lastly, comprehensive morphology analysis was conducted to evaluate the effectiveness of our optimal DIW structures and to inform future enhancements of such cells.
ELECTROLYTE AND INTERFACE DESIGNATION FOR HIGH-PERFORMANCE SOLID-STATE LITHIUM METAL BATTERIES
(2024) Zhang, Weiran; Wang, Chunsheng; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The demand for advanced battery technology is intensifying as electric energy becomes the foundation of modern technologies, such as smart devices, transportation, and artificial intelligence. Batteries play a crucial role in meeting our increasing energy demands and transitioning towards cleaner and more sustainable energy sources. However, range anxiety and safety concerns still hinder the widespread application of battery technology.Current Li-ion batteries, based on graphite anode, have revolutionized battery technology but are nearing the energy density limits. This necessitates the development of metal batteries, employing lithium metal as anode which eliminates host materials that do not contribute to capacity, thereby offering 10 times higher specific capacity. Recent research on lithium metal batteries has seen a significant surge, with growing knowledge transitioning from Li+ intercalation chemistry (graphite) to Li metal plating/stripping. The electrolyte, which was previously regarded as an inert material and acting as a Li+ ion transportation mediator, has gradually attracted researchers’ attention due to its significant impact on the solid electrolyte interphase (SEI) and the Li metal plating/stripping behaviors. Compared to the traditional liquid electrolytes, solid-state lithium metal batteries (SSLMB) have been regarded as the holy grail, the future of electric vehicles (EVs), due to their high safety and potential for higher energy density. However, there are notable knowledge gaps between liquid electrolytes and solid-state electrolytes (SSEs). The transition from liquid-solid contact to solid-solid contact poses new challenges to the SSLMB. As a result, the development of SSLMB is strongly hindered by interface challenges, including not only the Li/SSE interfaces and SSE/cathode interfaces but also SSE/SSE interfaces. In this dissertation, I detailed our efforts to highlight the role of electrolytes and interfaces and establish our understanding and fundamental criteria for them. Building on this understanding, we propose effective and facile engineering solutions that significantly enhance batterie metrics to meet real-world application demand. Rather than simply introducing new compositions or new designations, we are dedicated to introducing our understanding and mechanism behind it, we hope the scientific understanding, the practical solution, and the applicability to various systems can further guide and inspire the electrolyte and interface designation for next-generation battery technology.
INTERFACE AND STRUCTURES IN LITHIUM-GARNET QUASI-SOLID-STATE BATTERIES
(2024) Gritton, Jack Evans; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
A confluence of adoption of the internet of things, mobile electronics, electric vehicles, and shift towards adoption of intermittent green energy sources has led to a need for rapid improvement in battery technology in metrics ranging from rate capability and energy density to safety. While significant strides have been made through traditional liquid-based lithium-ion batteries, these oft-conflicting demands require fundamental shifts in battery chemistry, especially enabling safe incorporation of lithium metal anodes. Given their high conductivity, non-flammability, wide electrochemical stability window, and stability to lithium metal, lithium-stuffed garnets of the family LLZO provide one of the most promising alternative electrolytes to replace traditional flammable electrolytes. Two of the largest factors holding back these ceramic electrolytes are interfacial compatibilities and the interplay between processing and electrolyte mass. While drastic improvements have been made in the interface between garnet and lithium metal to improve rate capability, similar jumps in full cells have not been observed for rate and capacity. Using a varied cathode loading and a combination of EIS and DRT, we showed that garnet-catholyte interface was the main contributor to resistance in quasi-solid-state batteries of reasonable cathode loadings utilizing Pyr14TFSI based catholyte. Two methods were then used to improve this interface: modification of the garnet structure interfacing with catholyte, and modification of catholyte composition. Through the use of these methods, rate capabilities and capacity were drastically improved from the baseline system, both at elevated and room temperature. In addition to reducing interfacial resistance, cell polarization can be reduced through using thinner electrolytes. Given its higher mass density and lower conductivity in comparison to liquid electrolytes, garnet has historically had to rely on its greater stability to higher energy density electrodes to maintain competitive energy densities or utilize thin-film procedures that reduce mass but result in orders of magnitude lower conductivity than bulk produced garnets. To balance conductivity, ease of processing, and cell mass, a new combination of bulk-derived processing has been developed that allows for thin free-standing cubic garnet and thin, flexible, porous garnet. Cells using these new thin garnets achieved high cycling rates, and significant capacities.
CATALYST DEVELOPMENT FOR NON-OXIDATIVE METHANE UPGRADING TOWARDS HYDROCARBONS AND HYDROGEN PRODUCTION
(2024) LIU, ZIXIAO; Liu, Dongxia; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Methane, the main constituent of natural gas and biogas is deemed to be an alternative source to replace crude oil in the production of chemicals and fuel. Non-oxidative methane conversion enables methane coupling or splitting to produce hydrogen and more significant hydrocarbons, but catalyst deactivation has been a challenge in past research. This dissertation addresses catalyst deactivation issues in non-oxidative methane conversion by inventing novel catalyst systems. For direct non-oxidative methane coupling, a pathway for methane upgrading into hydrogen, olefin, and aromatic products, the silica-supported catalysts were synthesized by flame fusion of a mixture of quartz silica and metal silicate precursors. Compared to the cristobalite silica-supported catalysts reported previously, vitreous silica-supported catalysts have disordered Si-O bonds and structural defects, enabling better metal dispersion and more vital metal-support interaction. The as-prepared vitreous silica-supported iron catalyst had a shorter induction period in methane activation and lower coke yield. The increase in iron concentration elongated the catalyst induction period and promoted aromatics and coke formation. Among different transition metal catalysts, the cobalt supported by vitreous silica had the best methane conversion, hydrocarbon product yield, and catalyst stability. For catalytic methane pyrolysis, a pathway producing COx-free hydrogen and valuable carbon product, a siliceous zeolite-supported cobalt catalyst was invented. In comparison to the methane pyrolysis catalysts in literature, the siliceous zeolite support in the invented catalyst has limited Brönsted acidity and increased mesoporosity, which limited the acid-catalyzed deactivation mechanism and facilitated the mass transport, and thus significantly increased the catalyst lifetime. The cobalt sites change the cluster sizes and coordination structures with the loading concentrations in the zeolite support, which leads to carbon products with different properties.
LOW TEMPERATURE PLASMA-METAL INTERACTIONS: PLASMA-CATALYSIS AND ELECTRON BEAM-INDUCED METAL ETCHING
(2024) Li, Yudong; Oehrlein, Gottlieb G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Low-temperature plasma can generate different types of chemically reactive species at gas temperatures far below what is required to form such species from thermal excitation. Interactions between these reactive plasma-generated species and material surfaces have great potential for various applications, such as semiconductor etching or gas conversion. Synergistic effects, where the production rate with two inputs is greater than the sum of the consequences of each individually, have been demonstrated by combining the plasma with other energy inputs such as heat or kinetic energy from ions or electrons. Understanding the mechanisms by which these species interact with relevant surfaces is vital for the future development of plasma processing, chemistry and physics. In this work, we focus on the interaction of long-lived plasma species, particularly neutrals, with metal. A remote plasma-surface configuration was applied, where the plasma itself does not directly contact the surface. Two examples of plasma-metal interactions will be discussed, one taking place at atmospheric and the other at low pressure. The first case is plasma-assisted catalytic oxidation of methane (CH4) using a nickel (Ni) catalyst at atmospheric pressure, implemented by combining a remote plasma jet. The interrelation of real-time measurements of reaction products and surface adsorbates and plasma diagnostics allowed the identification of atomic oxygen as the key plasma-generated species that drives the synergistic plasma-catalytic reaction. The in-situ characterizations of the surface and gas phase reactions reveal the possible key reaction pathways for the plasma-catalysis reactions. We also observed the activation of the catalyst resulting from long-lasting catalyst surface modification induced by plasma species interaction. The second case is the damage-free etching of refractory metals, ruthenium (Ru) and tantalum (Ta), at low pressure. This was implemented by combining a remote plasma source (RPS) with an electron beam (EB) source. We investigated the effects of CF4 and Cl2 additions to Ar/O2 RPS effluents and we find that Ar/O2 with Cl2 addition induces the highest Ru etch rate (ER) and best removal selectivity over Ta. The surface chemistry characterization by spatially-resolved XPS reveals the possible mechanism of the electrons and neutrals induced materials etching. We also proposed a model that considers the fundamental aspects of the etching reaction and successfully predicts the major features of the electron and neutral induced etching reactions.
INTEGRATION OF SUPERCONDUCTORS INTO WIDE BANDGAP SEMICONDUCTOR ENVIRONMENTS FOR DEPLOYABLE SINGLE PHOTON DETECTORS
(2024) Drechsler, Annaliese Grace; Christou, Aristos; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Superconducting nanowire single photon detectors (SNSPDs) are the photon detecting devices of the future. These devices offer exceptional detecting capabilities over a wide range of wavelengths, which will enable next generation systems for optical communications, light detection and ranging, quantum key decryption, and astronomy among others. There are substantial materials, fabrication, and device development challenges that need to be addressed before these devices are ready for large scale deployment in arrays. This dissertation demonstrates novel approach to SNSPD development by monolithically integrating superconducting materials with wide bandgap semiconductor systems to scale these devices. Specifically, this work explores the integration of niobium nitride (NbN) with multi-channel aluminum gallium nitride (AlGaN)/gallium nitride (GaN) superlattice devices to leverage the benefits of materials similarity and lattice matching to provide high quality detector performance in the proposed system. The multichannel superlattice device selected for this work, the superlattice castellated field effect transistor (SLCFET) utilizes a novel δ-doping approach to generate conducting channels. Epitaxial structures were studied between 300K and 4K. This structure exhibits a substantial reduction in epitaxial resistance, determined to be a result of mobility improvement to 4151.5 cm2/Vs through Hall effect analysis. Phonon scattering modelling indicates that the device is limited by polar optical phonon scattering at high temperatures and interface roughness between the channels at cryogenic conditions. Field effect transistors fabricated from this epitaxial structure were tested and shown to exhibit exceptionally high performance at low temperatures, proving feasibility of device integration. A production-scalable NbN deposition process was developed for SNSPD fabrication. Thorough analyses determined the relationship between deposition parameters and the resultant crystallinity, defectivity, and surface morphology. Analysis of ultra-thin films determined that the NbN films grow through a step-flow growth mechanism. This data was used to develop a temperature-dependent empirical model of the kinetics of the surface morphology and growth mechanism evolution based on the Avrami equation. Fabrication processes were developed using these films to pattern SNSPDs with narrow linewidths down to 50 nanometers composing the meander structure for long wavelength performance. Thorough analysis of the impact of electron beam lithography write conditions were conducted to propose ideal fabrication conditions. Methods were proposed and implemented to address defectivity by reducing the impact of elasto-capillary forces on line collapse including chemical surface modification using hexamethyldisilazane and resist thinning using polymethyl methacrylate (PMMA) and ZEP and implementing charge dissipation layers. Additional processes were proposed and implemented to enable integration into the SLCFET fabrication flow. The SLCFET devices and NbN structures were tested and determined to be functional, thus demonstrating the feasibility of integration. An initial integrated device was designed and modelled by combining a SLCFET with NbN SNSPDs, using the RF output as a readout approach. The devices were successfully fabricated using the processes developed within this dissertation. Testing of the devices showed a 30dB signal difference between the normal and detecting states, thus demonstrating the first device of its kind, representing a substantial contribution to the field. This will open the door for full-scale array development using novel on and off chip signal processing approaches proposed in this work.
SYNTHESIS AND CHARACTERIZATION OF OXIDE THIN FILMS FOR ION TRANSPORT
(2023) Wang, Richmond Shiwei; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Metal oxides are a diverse category of materials that exhibit many material properties and applications. They can range from insulators to superconductors and have uses varying from electronic semiconductor devices to solar cells and even biomedical applications. Thin films, specifically, attract interest due to their unique properties compared to the bulk alternatives. Their small nano to micro scale size provides a large surface to volume ratio, which can affect defect density and alter the optical, electrical, or mechanical properties. The ease of fabrication and the tunability of thin films through techniques such as strain engineering or doping enables them to be widely utilized to characterize material properties and interfaces. Of these metal oxides, perovskite rare-earth nickelates exhibit intriguing electrical and optical properties, such as metal-to-insulator transitions. LaNiO3 is the only exception where the metallic phase is robust at all temperatures. Electrolyte gating can be an efficient method to manipulate the electronic behavior of LaNiO¬3 and induce an insulating phase. This work utilized ionic liquid gating with electric double-layer transistor devices to control the electronic properties of LaNiO3. The electrolyte gating leads to an insulating phase transition with an increased film resistivity by over six orders of magnitude. The electrolyte gating behaviors are found to be dependent on not only gating voltage and duration, but also the atmospheric environment. X-ray photoelectron spectroscopy (XPS) analysis revealed that ionic liquid gating promotes the removal of oxygen from the crystal to form oxygen vacancies, which also affects the Ni valence state. The insulating phase transition was attributed to enhanced electron correlation as well as an opening of the charge transfer gap due to the reduced overlap between Ni and O bands. The filling of carriers is controlled by the gate voltage. These results suggest that electrolyte gating devices can be useful for manipulating electron-electron correlation, furthering materials research in realizing exotic physics in correlated systems. Li-ion batteries are widely used in electronics, appliances, electric vehicles, and energy storage systems. The transport of the Li ions in Li-ion batteries can be affected by many factors, such as crystal structure or the formation of interfaces between the electrodes and the electrolyte used. Investigation of delithiation in a LiCoO2 cathode and the effects of Li removal on the structure demonstrated that charge compensation occurs via the formation of Co4+ and hole generation near O atoms. One drawback of conventional Li ion batteries is the flammability of the liquid electrolyte solvents. Li7La3Zr2O12 (LLZO) solid electrolytes have been considered an alternative to these liquid electrolytes. However, the reduced ionic conductivity of LLZO needs to be improved to fully realize their use in Li ion batteries. A hybrid electrolyte composed of LLZO with small amounts of liquid electrolyte can potentially resolve this issue by increasing Li conductivity between the LLZO and electrodes. This results in the formation of a solid-electrolyte interface, which can adversely affect the performance of the overall battery. To fully understand how this interface forms and how it impacts the performance, Ta-doped LLZO/Li2O multilayer films were exposed to a liquid electrolyte solution containing LiPF6. XPS analysis showed that carbon contaminants such as LiCO2 were reduced upon electrolyte exposure. Fluorinated metals such as LiF, LaF3, and ZrF4 were detected at depths up to 10 nm, suggesting that LLZO interacts more strongly with HF than other decomposition products of the electrolyte. Overcoming the formation of fluorinated metals is critical to realizing hybrid electrolytes that use the SE LLZO in combination with a LE containing LiPF6 without being inhibited by solid-electrolyte formation.
ELECTRIFIED HIGH-TEMPERATURE MANUFACTURING AND APPLICATIONS IN ENVIRONMENTAL SCIENCE
(2023) Li, Shuke; Hu, Liangbing; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
High temperature processes hold great potential for material and chemical manufacturing.On the one hand, high temperature can help overcome energy barriers and thus effectively convert precursors to desired products. On the other hand, high temperature can also boost the reaction rate and improve synthesis efficiency. Recent development of electrified high temperature technologies by our group further revealed the important role played by non-equilibrium conditions on nanomaterial and chemical syntheses. For example, Joule-heating of carbon-based materials through a programmable electrical signal can offer spatial and temporal temperature profiles, which can be used to manipulate the chemical reaction pathways. For another example, tunable heating duration and quenching rates can be used to achieve a range of compositions and structures of nanoparticles. In this dissertation, two specific applications of the electrified high temperature technology will be explored, including: (1) Thermal shock synthesis of multielemental nanoparticles as selective and stable catalysts; and (2) Efficient biomass upgrading via pulsed electrical heating. Supported nanoparticle (NP) catalysts are widely used for various reactions. However, it remains challenging to synthesize high quality NPs with accurate morphologically and structure control. In this part of the research, NP catalysts with morphology or structural design were prepared by high temperature thermal shock methods. Ultra-small and high-loading carbon supported Pt3Ni NPs: Strong electrostatic effect was introduced between metal salts and carbon particles that can largely improve anchoring and dispersion of the precursors, thereby achieving high NP loading (40 wt%) as well as small NP size and good distribution (1.66 ± 0.56 nm). This method is not only limited to bimetallic NPs synthesis or NPs on carbon black but can be extended to a range of NP compositions on various substrate materials, thus providing a general strategy for developing ultrafine and high-loading NPs as electrocatalysts for various reactions. Sustainable aviation fuels (SAFs) are essential to meet future air travel demand while reducing the carbon footprint. Among many potential feedstocks to produce SAFs, lignin stands out as it is an abundant and renewable aromatic biopolymer that is usually treated as a waste material from the paper industry. However, converting lignin to SAFs by conventional thermochemical processes has been challenging due to poor control on the reaction pathway which leads to undesired product distribution. In this study, a programmed electrified heating method was designed and used to break down large lignin molecule to small aromatic molecules with targeted product distribution. A controlled heating step offers sufficient energy input to break down lignin molecules to smaller fragments without excessive secondary reactions toward undesired species such as coke. The lignin thermal decomposition products were evaluated as potential precursor for SAFs generation. This process can be further extended to process other biomass materials such as algae and sawdust to value-added chemicals.
COMBINATORIAL EXPLORATION OF HALF-HEUSLER (Ta0.4 Nb0.4 Ti0.2)–Fe–Sb THIN FILMS VIA HIGH-THROUGHPUT POWER FACTOR MAPPING AND FREQUENCY-DOMAIN THERMOREFLECTANCE (FDTR)
(2023) Kirsch, Dylan; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Thermoelectrics (TEs) are a class of materials capabl¬¬e of converting heat into electricity in the solid state. Their widespread application is limited by the low efficiency (≈ 5 %) for commercial modules in applications such as waste heat recovery and refrigeration. Half-Heusler (hH) TE intermetallic alloys have good electrical properties that are easily tuned by doping but are limited in commercial deployment due to high thermal conductivity (TC). This limits the achievable thermal gradient across a TE module, reducing the efficiency. One method to improve hH alloy performance is to decrease the lattice contribution to the TC through solid-solution alloying. Combinatorial synthesis approaches have the advantage of rapid sample fabrication and characterization over a wide range of material compositions. This approach can provide insights into materials systems that could be missed using conventional synthesis approaches. Several publications reported p-type NbFeSb hH alloys can accommodate off-stoichiometry, which could positively impact the TE properties similar to TC decrease observed via Ta-alloying. Combinatorial thin film co-sputter synthesis of hH alloy (Ta0.40Nb0.40Ti0.20)-Fe-Sb composition spread libraries coupled with high throughput (HiTp) characterization is utilized to produce maps of the composition-structure-property relationships as a function of Fe- and Sb-content in this system for the first time. Continuous spread composition gradient and homogeneous discrete co-sputtered combinatorial thin film synthesis methodologies are leveraged to investigate the hH stability region and TE performance in (Ta0.40Nb0.40Ti0.20)-Fe-Sb. Combinatorial thin film characterization requires specialized custom or commercial instrumentation capable of scanning across samples. Established HiTp tools were utilized to characterize the crystal structure, electrical transport properties (Seebeck coefficient and electrical resistivity), and chemical composition of the films. A scanning thin film TC measurement instrument was not available prior to this dissertation. Without this ability, the dimensionless TE figure-of-merit zT cannot be calculated. To address this need, a custom, automated Frequency Domain Thermoreflectance (FDTR) instrument was designed and constructed. FDTR TC measurements are presented on single-phase F¯4 3m off stoichiometric discrete combinatorial hH (Ta0.40Nb0.40Ti0.20)-Fe-Sb for the first time. Maximum zT values at 296 K are calculated to be 0.076 for compositions (Nb0.412Ta0.327Ti0.261)28.5Fe40.3Sb31.2 and (Nb0.418Ta0.328Ti0.254)35.0Fe31.7Sb33.3 having TC values around 2.25 ± 0.27 W m-1 K-1.
OPTIMIZATION OF PLASMA ASSISTED MOLECULAR BEAM EPITAXY GROWN NbxTi1-xN FOR EPITAXIAL JOSEPHSON JUNCTIONS
(2023) Thomas, Austin Michael; Richardson, Christopher; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This thesis is an investigation into the growth and characterization of NbNx and TiN transition metal nitrides, along with the alloy NbxTi1-xN. These materials are commonly used in many applications ranging from superconducting quantum computing, superconducting conventional computing, high kinetic inductance devices such as single photon detectors, and hard coatings for industrial applications. This thesis will begin with an overview of superconducting quantum computing and superconducting materials, then review the fabrication of Josephson junctions and highlight the need for material improvement. The goal of this work is to grow a superconducting nitride material which can be engineered to lattice match with AlN, the barrier layer in a hypothetical all-nitride, epitaxially grown superconducting quantum computing structure. The alloy NbxTi1-xN is chosen as the superconducting alloy of choice due to the range of lattice constants available, the high critical temperature of these nitrides, and the high quality of material able to be grown using PAMBE. The first aim of this thesis studies the binary transition metal nitrides NbNx and TiN to generate endpoints for various properties of the alloy NbxTi1-xN. This thesis is one of the first investigations of multi-phase growth of ε-NbN and γ-Nb4N3, and demonstrates control over the phase, crystal orientation, superconducting properties, and surface morphology by changing PAMBE growth parameters. The second aim of this thesis demonstrates the growth of NbxTi1-xN and is the first investigation of tunable material properties for this alloy by adjusting the composition. The last aim of this work is the development of a novel annealing scheme used to prepare NbxTi1-xN thin films for Josephson junction integration. The novel annealing scheme ensures excellent surface roughness of NbxTi1-xN thin films, increases the superconducting critical temperature of this alloy from approximately 14 K to 16.8 K, and improves the crystal quality by way of nitrogen incorporation and improvement of the crystal quality. The results from this work will be crucial in developing NbxTi1-xN / AlN / NbxTi1-xN Josephson junctions with smooth, uniform interfaces and low-loss, defect free nitride materials. Additionally, this thesis represents an investigation into the relationship between phases of NbNx and TiN, the role of nitrogen incorporation caused by in-situ annealing, and a useful record of control over this material using PAMBE growth conditions and alloy composition.
Materials modeling by ab initio methods and machine learning interatomic potentials: a critical assessment
(2023) Liu, Yunsheng; Mo, Yifei; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Atomistic modeling is a crucial research technique in materials science to simulate physical phenomena based on the interactions of atoms. Density functional theory (DFT) calculation has been the standard technique for evaluating atom interactions, but their applications are limited due to high computation cost. Classical interatomic potential, as another widely adopted technique, provides an alternative by enabling large-scale simulations that are computationally inexpensive. However, the employment of classical potentials for cheap atomistic simulations is achieved in exchange of accurately evaluating atom interactions and the transferability to different chemistries. Most recently, machine learning interatomic potential (MLP) emerges as a new computation technique to bridge the gap between first-principles computation and classical potentials.I first utilized the atomistic modeling based on DFT calculations to find novel Li superionic conductor, a key component of the emerging all-solid-state Li-ion battery technology. I performed a systematic study on the Li-ion conduction of lithium chloride materials system and predicted a dozen potential Li superionic conductors. I revealed that the Li-ion migration in the materials is greatly impacted by the Li content, the cation configuration, and the cation concentrations. I further demonstrated tuning these three factors in designing new chloride Li-ion conductors. Then, I studied the atomistic dynamics predicted by MLPs in comparison to DFT calculations to answer the open question whether MLPs can accurately reproduce dynamical phenomena and related physical properties in molecular dynamics simulations. I examined the current state-of-the-art MLPs and uncovered a number of discrepancies related to atomistic dynamics, defects, and rare events compared to DFT methods. I found that testing averaged errors of MLPs are insufficient and developed evaluation metrics that better indicate the accurate prediction of related properties by MLPs in MD simulations. I further demonstrated that the MLPs optimized by the proposed evaluation metrics have improved prediction in multiple properties. I also study the performance of MLPs in assessing the elemental orderings in a large variety of phases across composition range in alloy system. Using the Li-Al alloy system as a case study, I trained MLPs using only a few phases and the trained MLP demonstrated good performances over a number of existing and other hypothetical materials in- and out- of the training data across the Li-Al binary alloy system. We developed several new evaluation metrics on energy rankings to evaluate the elemental ordering, which is critical for studying the phase stabilities of materials. I tested MLP transferability to other phases and the limits of MLP applications on commonly performed simulations. I also studied the effect of diverse training data on MLP performances. With these efforts evaluating MLP performances for a number of properties, metrics, prediction errors, and dynamical phenomena, I constructed a dataset with a large number of MLPs and their performances and performed an empirical analysis to identify the challenging properties to be predicted by the MLPs. Further, I identified pairs of properties that are challenging to predict. This series of works demonstrated that atomistic modeling is an effective computation technique for studying atomistic dynamic mechanisms, evaluating materials thermodynamics, and guiding materials discovery. As an emerging computational technique, MLPs show good performance on many materials and have great potential to enhance the materials research, but the results show that critical assessments are needed to examine their capabilities to accurately reproduce physical phenomena and understand their limits of performing reliable simulations. My thesis evaluates MLP performances on a number of critical issues related to materials simulations and provides guidance to improve MLPs for future studies.
Magnetic nanoparticle inks for syringe printable inductors
(2023) Fedderwitz, Rebecca; Kofinas, Peter; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Direct Ink Writing (DIW) additive manufacturing (AM) has the transformative potential to construct complex shapes and devices with a single apparatus by exchanging the printable material at the print head. Iron cobalt (FeCo), permalloy (Ni80Fe20), and iron (II,III) oxide (Fe2O3·FeO) nanoparticles with varying magnetic properties were incorporated in resins to explore the influence of particle loading on printability and inductor device performance. It was generally found that increasing particle loading increased ink viscosity, with a loading maximum approaching 29 – 42 vol% depending on the particle type and resin mixtures due to differences in particle shape and size and resin viscosity. With more magnetic content, composites had higher magnetic permeability and inductance. Syringe printable, colloidal, aqueous magnetic inks were made using both stabilized iron oxide and MnZn doped ferrite nanoparticles with added free polymers. MnZn doped ferrite inks are printed into toroids, sintered to improve magnetic permeability and mechanical robustness, and constructed into an inductor device. Inductors with high magnetic permalloy nanoparticle content were also syringe printed into toroids and hand-wound to demonstrate their viability in fabricating three-dimensional inductors. The effect of particle size on stability and printability was observed. The research presented in this thesis investigates various methods for formulating magnetic nanoparticle inks and evaluates the contributions of particle stabilization, free polymer content, solvent composition, and particle loading on the rheological behavior required for syringe printing. Material properties and device performances were characterized using methods such as zeta potential and settling studies to observe particle functionalization and stability, rheology to study viscoelastic flow behavior, and vector network analysis to measure inductance and device efficiency to showcase the viability of this technique to manufacture passive electronic devices.
The Determination of Preferred Orientation in Rolled Electrical Steels Using Single Diffraction of Neutrons
(1963) Eugenio, Manuel; Duffey, Dick; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)
Preferred orientation in rolled electrical steels has been determined using single diffraction of neutrons from the University of Maryland pool-type nuclear reactor (DMR) operating at 10 KW thermal . X-rays are used extensively to determine preferred orientations in metallic wires and rolled sheets, but X-rays suffer the disadvantage of high absorption and cannot be used effectively on thick samples without chemical or mechanical treatment which ultimately results in the destruction of the samples. The use of reactor neutrons for this purpose is believed to offer particular advantages such as the use of thicker samples and wider beams. To this end, neutrons from the UMR were scattered directly from metallic sheet samples to obtain diffraction patterns from which preferred orientations of the crystallographic axes could be deduced. The neutron diffraction data were obtained in the form of : 1) Maxwellian curves; and 2) rocking curves. To obtain the first type of curve, the sample and neutron detector were rotated at a 1-to-2 angular ratio, respectively, and the diffraction pattern was essentially the Maxwellian neutron energy distribution. From the maximum of the Maxwellian curve, the crystallographic plane mainly responsible for the reflection was calculated; from this, the main orientation was deduced. For the second type of curve, the sample was rocked back and forth, with the neutron detector fixed, and the resulting pattern was used to infer the variation of a given crystallographic direction about its main orientation. The results of this study, particularly on grain-oriented and cube-textured silicon-iron (Si-Fe) alloy sheets demonstrate that single diffraction techniques can be used to determine preferred orientation in highly oriented materials. The results on Si-Fe sheets described as non-oriented indicate the possibility that these techniques may be applicable to ordinary rolled metallic sheets, which are not highly oriented.
Functionalized Thin-Film Shape Memory Alloys for Novel MEMS Applications
(2023) Curtis , Sabrina M.; Takeuchi, Ichiro; Quandt, Eckhard; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Nickel-titanium (NiTi) shape memory alloy (SMA) films are already implemented into microelectromechanical system (MEMS) devices such as sensors, actuators, and implantable medical devices. In this thesis, I used DC magnetron sputter deposition to study the influence of film composition, microstructure, and annealing conditions on the stability of the phase transformation for the NiTi-based SMA thin films TiNiCu, TiNiCuCo, and TiNiHf. SMAs are a type of smart material that can undergo stress or temperature-induced solid-to-solid phase transformation between two different crystalline phases. In NiTi-based SMAs, the two phases are known as martensite with a monoclinic crystalline structure and austenite with a cubic crystal structure. The temperature-induced phase transformations can be used to switch between the martensite and austenite phases, and thus switch between two sets of material properties in the SMA. For example, in NiTi-based SMAs the Young’s modulus, electrical resistivity, and coefficient of thermal expansion of the austenite phase are typically 2X larger than that of the martensite phase. The transformation temperatures, recovery strains, enthalpy of transformation, and fatigue properties of NiTi SMAs can be tuned by alloying NiTi with other elements like copper (Cu), cobalt (Co), and hafnium (Hf). For example, certain compositions of sputtered TiNiCu and TiNiCuCo are known to be ultra-low fatigue SMAs, able to reversibly undergo the phase transformation for 10+ million cycles without degradation in the mechanical or thermal properties. The primary focus of this thesis was the integration of these sputtered NiTi-based SMA thin-films into the following four novel MEMS devices: 1) TiNiCu for magneoelectric sensors, 2) TiNiHf for bistable actuators, 3) TiNiCuCo for stretchable electronics and 4) thin-film SMA stretchable auxetic structures for wearable and implantable medical devices. The shape memory effect was observed in TiNiCu and TiNiHf films when the film thickness and lateral dimensions are downscaled to micro and nano dimensions. In the research publication “Integration of AlN piezoelectric thin films on ultralow fatigue TiNiCu shape memory alloys.”, I showed the reproducibility of the thermal-induced phase transformation of Ti50Ni35Cu15 is attractive for integration into MEMS devices that require a high cycle lifetime. I showed how the SMA’s phase transformation can be used to tune the resonant of bending cantilever-type sensors like magnetoelectric sensors. I also demonstrated excellent thin-film piezoelectric and shape memory alloy properties for 2 μm AlN/ 5 μm TiNiCu films composites deposited onto silicon substrates. The large work densities and high strength-to-weight ratio offered by SMAs are attractive for the development of micro and nano actuators. The thermal induced phase transformation between martensite and austenite is also used to develop bi-directional micro-actuators with TiNiHf/Si and TiNiHf/SiO2/Si composites. In another research publication, “TiNiHf/SiO2/Si shape memory film composites for bi-directional micro actuation”, I demonstrated the influence of film thickness and substrate on the phase transformation properties of TiNiHf thin-films. Ti40.4Ni48Hf11.6 films with thicknesses as low as 220 nm on SiO2/Si substrates can undergo the phase transformation with high transformation temperatures (As > 100 °C) and a wide thermal hysteresis (ΔT > 50 °C). In this publication, we explain how the wide hysteresis and high transformation temperature obtained in TiNiHf films can be used to develop micro and nano-scale bistable actuators based on PMMA/TiNiHf/Si composites. Even though thin-film NiTi-based SMAs are known to reversibly recover superelastic strains of up to 8%, surprisingly, they have not yet been exploited in the growing field of stretchable electronics. In the technical article “Thin-Film Superelastic Alloys for Stretchable Electronics” I demonstrate the first experimental and numerical studies of freestanding thin-film superelastic TiNiCuCo structured into a serpentine geometry for use as stretchable electrical interconnects. Fabricated electropolished serpentine structures were demonstrated to have low fatigue after cycling external strains between 30% - 50% for 100,000 cycles. The electrical resistivity of the austenite phase of a Ti53.3Ni30.9Cu12.9Co2.9 thin-film at room temperature was measured to be 5.43 × 10-7 Ω m, which is larger than reported measurements for copper thin-films (1.87 × 10-8 Ω m). Expanding upon this work, in the conference proceedings paper “Auxetic Superelastic TiNiCuCo Sputtered Thin-Films for Stretchable Electronics”, I present a new platform for functionalized wearable electronics and implantable medical devices based on superelastic thin-film SMA substrates structured into novel stretchable auxetic geometries. Since thin-film SMAs are conductive, the structured substrate itself could serve as the current collector for such stretchable and flexible devices, or a more conductive electrode can be deposited on top of the stretchable auxetic SMA substrate. Overall, the results discussed in this doctoral thesis look to the future of harnessing the functional properties of thin-film sputtered SMAs for novel uses in next-generation MEMS devices.
RADIATION CHEMISTRY IN PRESSURIZED WATER NUCLEAR REACTORS: H2 GENERATION BY 10B(n,α)7Li, AND THE REACTION OF BORATE WITH •OH
(2023) Guerin, Steven James; Al-Sheikhly, Mohamad I; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Nuclear power plants (NPPs) are complex engineering systems, with malfunctions having enormous potential to lead to widespread and extreme impacts on society and the environment as a whole. Their safe operation depends on a multitude of factors such as intelligent planning, proper design, quality components, high-level safety operations, and economic viability. Due to requiring high temperature and high pressure of an NPP’s cooling fluid, one of the main concerns for further developing safe operating conditions and evaluating component lifetimes is improving our understanding on the issue of corrosion in nuclear systems. In the U.S., all commercially operated Pressurized Water nuclear Reactors (PWRs) are light-water reactors wherein their coolant waters can reach temperatures up to 350 °C. According to a report in 2005 in association with the U.S. Federal Highway Administration, an annual cost of $4.2 billion was directly attributed to corrosion in NPPs in 1998, out of a total $6.9 billion in the electrical utilities industry (Koch, et al., 2005). Boron is added into commercial PWR primary water in the form of boric acid as a soluble chemical neutron “shim” in order to compensate for fuel burnup and allow smooth long-term reactivity control. After a boron nucleus captures a thermal neutron and becomes unstable, the energy of the recoil ions resulting from its fission accounts for up to 33 % of the total dose to the primary water. This event is an important source for H2 and corrosive H2O2, so its product yields must be accurately included in models of the cooling water radiation chemistry. H2 produced in water from the 10B(n,α)7Li fission reaction has been measured up to 300 °C to aid in quantification of the corrosive H2O2 from the same reaction. Thermal energy neutrons from the Rhode Island Nuclear Science Center 2 MW reactor interacted with boric acid contained in N2O-saturated water in temperature-controlled high-pressure cells made from tubing of either titanium or zirconium alloy. After exposure for a minimum of one hour, the solution samples were extracted and sparged with argon. The H2 entrained by the sparging gas was sampled with a small mass spectrometer. A small amount of sodium was included in the boric acid solution so that after sparging, samples could be collected for 24Na activation measurements in a gamma spectrometer to determine the neutron exposure and thus the total energy deposited in solution. The G-value (µmol/J) for H2 production was obtained for water at a pressure of 25 MPa, over a temperature range from 20 °C to 300 °C. These results have been complemented with Monte Carlo N-Particle® (MCNP®) simulations in collaboration with the National Institute of Standards and Technology, and have been compared with previous experimental results at room temperature and simulated results up to 350 °C. Additionally, boric acid has thus far been accepted as a chemically nondisruptive additive, as it was confirmed long ago to have extremely low reactivity with the two main reactive species produced in reactor primary water by radiolysis, the solvated aqueous electron and the hydroxyl radical (e(aq)- and •OH). However, at the Electric Power Research Institute standard desired pH of 7.3 and the operational temperature of 350 °C, approximately 22% of the boron added in PWR primary water exists in the chemical form of the conjugate base, borate, not boric acid. Although borate was previously confirmed to have no appreciable reactions with e(aq)-, it was not adequately studied for reactions with •OH prior to this work. We have observed a clearly apparent reaction between borate and •OH. Current chemistry models are completely ignorant on both the existence of the resultant species and its reactions. The chemical reaction of [B(OH)4]- (borate) with •OH along with cross-reactions of the product species have been studied up to 200 °C to determine those reactions’ rate constants and the products’ spectra. The University of Notre Dame Radiation Laboratory’s 8 MeV electron linear accelerator (LINAC) was configured to perform pulse radiolysis with pulse widths between 4ns to 20ns providing doses between 5.5 Gy and 62 Gy. High-energy electrons from the LINAC interact with the borated solution which has been N2O-saturated and is continuously flowed through a 316 stainless-steel optical cell. The cell temperature was adjusted by resistive-heating silicon cartridges, and pressure was controlled by two syringe pumps to prevent boiling. The cell had two fused silica windows for transmitting light from a xenon arc lamp through the solution and out to a multichromatic spectrophotometer system. Time-resolved spectral data was obtained over nano- and micro-second timeframes, for wavelengths ranging from the deep UV and into the infrared spectrum (250 nm to 820 nm). The reaction rates and products’ spectra were then obtained by analyzing the data using computational aids, namely IGOR Pro by Wavemetrics and KinTek Explorer by KinTek Corp. The product species of the reaction between borate and •OH is conjectured to be •[BO(OH)3]-, on the basis of ab initio calculations, which likely reacts with boric acid or borate to form a polymer radical.
Development of Low-Cost Autonomous Systems
(2023) Saar, Logan Miles; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
A central challenge of materials discovery for improved technologies arises from the increasing compositional, processing, and structural complexity involved when synthesizing hitherto unexplored material systems. Traditional Edisonian and combinatorial high-throughput methods have not been able to keep up with the exponential growth in potential materials and relevant property metrics. Autonomously operated Self-Driving Labs (SDLs) - guided by the optimal experiment design sub-field of machine learning, known as active learning - have arisen as promising candidates for intelligently searching these high-dimensional search spaces. In the fields of biology, pharmacology, and chemistry, these SDLs have allowed for expedited experimental discovery of new drugs, catalysts, and more. However, in material science, highly specialized workflows and bespoke robotics have limited the impact of SDLs and contributed to their exorbitant costs. In order to equip the next generation workforce of scientists and advanced manufacturers with the skills needed to coexist with, improve, and understand the benefits and limitations of these autonomous systems, a low-cost and modular SDL must be available to them. This thesis describes the development of such a system and its implementation in an undergraduate and graduate machine learning for materials science course. The low-cost SDL system developed is shown to be affordable for primary through graduate level adoption, and provides a hands-on method for simultaneously teaching active learning, robotics, measurement science, programming, and teamwork: all necessary skills for an autonomous compatible workforce. A novel hypothesis generation and validation active learning scheme is also demonstrated in the discovery of simple composition/acidity relationships.
Pyrolysis of 3D Printed Photopolymers: Characterization and Process Development
(2023) Tyler, Joshua Bixler; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
3D printing has shown to be instrumental in the development of complex structures that have been previously unobtainable through traditional manufacturing processes. Photopolymers have been used in lithography-based 3D printing techniques for decades and have shown to be easily printed from the micro to macro scales. The thermal decomposition, or pyrolysis, of patterned photopolymers of microscale and mesoscale has been shown to create carbon devices such as carbon micro electromechanical systems (MEMS) and electrodes. In this dissertation, I present the characterization of pyrolyzed photopolymers 3D printed via stereolithography (SLA) and two-photon polymerization (2PP). Furthermore, processes in which to bolster the material properties of the pyrolyzed materials was examined.First, I study the effects of increasing the pyrolysis temperature on 2PP photopolymers and how this changes the electrical conductivity and microstructure of the material. From this it was shown the ability to vary the conductivity of 3D printed and pyrolyzed glassy carbon parts by up to 500X through only the temperature of pyrolysis, including reaching conductivities an order of magnitude higher than previously reported work. By extending the characterization of pyrolyzed photopolymers to SLA photopolymers I am able to further develop a generalized understanding of the electrical and microstructural properties of pyrolyzed 3D printed photopolymers. Further, demonstrate a metric in which to understand the deformation of the material during pyrolysis and perform an electrical and microstructural study of the material. Secondly, I investigate increasing the electrical and mechanical properties of pyrolyzed photopolymers through metals deposition via electroplating. In doing so I introduce a novel technique on which to electrodeposit on the surface of pyrolyzed SLA and 2PP 3D printed parts. Metallizing these pyrolyzed samples showed to increase both the electrical conductivity and ultimate strength of both pyrolyzed photopolymers. Lastly, I looked at increasing the stiffness of the pyrolyzed photopolymers through the addition of hBN filler into the precursor photopolymer. In doing so I examine the manufacturing of the composite hBN containing photopolymers for 3D printing with SLA and 2PP systems. Following 3D printing and pyrolysis of the hBN/photopolymer composite compositional and microstructural analysis is performed. Mechanical testing of the pyrolyzed composites shows that a slight increase in the stiffness of the material is observed. I have shown the ability to control the electrical conductivity and microstructure of pyrolyzed 3D printed photopolymers through pyrolysis temperature. Through the addition of metals via electroplating I demonstrate a process by which to increase the electrical conductivity and ultimate strength of pyrolyzed photopolymers and through the addition of hBN into the precursor photopolymer I have shown a way to increase the stiffness of the pyrolyzed materials. These processes have already demonstrated the ability to 3D printed electrical devices and have laid out a groundwork for future development of 3D printed electronics, energy-storage devices, and shielding.
Title of Dissertation: Structural and Electrochemical Variances in Doped Lithiated Cathodes and Ionically Conducting Solid State Materials: Relationships in Solid State Electrolytes, Cathodes, and the Interfaces
(2023) Limpert, Matthew A.; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Lithium-ion conducting Li7La3Zr2O12 (LLZO) garnets are being explored as a replacement for the flammable organic electrolytes used in batteries. However, LLZO garnets require high temperature sintering to densify the structure, but that microstructure and electrochemical properties can vary with lithium content as the lithium volatizes during sintering. The effects of sintering the LLZO garnet requires a detailed examination and study to determine how lithium content can affect physical properties, phase purity and density, as well as performance through ionic conductivity. Studying these parameters produced ionic conductivities above 10-4 S cm-1 in samples that had increased density by enabling liquid phase sintering through the eutectic between Al2O3 and Li2O. Despite this high conductivity, the movement of Li+ through a solid electrolyte encounters even slower kinetics through the rigid electrolyte-cathode interface to the active cathode material. A cathode for LLZO garnets requires a new design with both ionic conduction and electronic conduction pathways while reducing interfacial resistance when co-sintered. Excess lithium within LLZO garnet reduced formation of nonconductive LaCoO3 when co-sintered with the active material, LiCoO2 (LCO), which enables a new completely solid-state cathode for lithium metal batteries to be designed and interfacial resistance to be minimized. LCO, however, is limited to 4.2 V to ensure long life cycle without lattice deformation. Unlocking the potential 5 V cycling with of LLZO garnet necessitated the development of a higher voltage cathode. Chlorinating the oxygen site of lithium spinel, LiMn2O4, using a citric acid method stabilizes the 2 V plateau, which increases the capacity to 180 mAhr g-1, and triple doping with Co, Fe, and Ni enables customization of the properties while shifting the voltage to 5 V. The high voltage spinel and LLZO garnet enables high voltage cycling with increased safety potential enabling a pathway to a safe 400 Wh kg-1 cell, 150 Wh kg-1 higher than the current state of the art.

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