Browsing by Author "Brankovic, Stanko R."
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Item ANALYSIS AND DESIGN OF STRUCTURED MULTI-FUNCTIONAL TRAPPING CATALYSTS FOR CONVERSION OF HYDROCARBONS AND NOX FROM DIESEL AND ADVANCED COMBUSTION ENGINES(2022-08-09) Gupta, Abhay; Harold, Michael P.; Grabow, Lars C.; Rimer, Jeffrey D.; Brankovic, Stanko R.; Louie, Stacey M.Multifunctional automotive catalysts can help abate harmful emissions, such as NOx (x=1, 2), CO, and hydrocarbons (HCs) and provide a potential solution to meet the need of increasingly stringent vehicle fuel economy standards and emissions regulations. The proposed LHCNT (lean hydrocarbon NOx trap) concept involves the combination of NOx trapping with HC trapping and oxidation for conversion of NOx, HC, and CO during sustained, low-temperature exhaust drive cycles and engine cold-start. In the first part of the work, we focus on NOx uptake and release features on Pd-exchanged SSZ-13 (PNA- Passive NOx Adsorber) catalyst to trap NOx in the absence and presence of several diesel exhaust components (CO, C2H4, H2, C7H8, and C12H26). We identified two prospective mechanistic schemes which are consistent with the uptake/release and DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) data. Second, we focused on DOC (Diesel Oxidation Catalyst) which is an essential component of modern vehicle emissions control systems. A new Pd-Cu alloy catalyst ((Pd+Cu/SiO2-Al2O3), prepared in-house, exhibited higher resilience to increased light-off temperature, resultant of mutual inhibition between the various pollutants, in comparison with the commercial DOC (Pt+Pd/Al2O3-CeO2). The benefit of eliminating the inhibition effect of NO on CO oxidation is clearly demonstrated in the juxtaposition of the two catalyst and reactor configurations using a CO and NO containing feed. Next, we investigate the spatiotemporal features of a multifunctional monolith lean hydrocarbon NOx trap (LHCNT) for eliminating NOx and C2H4 in simulated diesel exhaust. Spatially-resolved mass spectrometry (SpaciMS) is used to measure the temporal species concentration profiles spanning the sequentially- positioned PNA (Pd/SSZ-13), HCT (Hydrocarbon Trap; Pd/BEA) and DOC (Pt/Al2O3-CeO2) zones. The working concept of the LHCNT is demonstrated through measured integral trapping efficiency profiles, which show NO and C2H4 trapping followed by delayed NO release along with NO and ethylene oxidation during the simulated warmup. Finally, we utilized the same sequential configuration system to identify the impact of CO on NO trapping. Conditions are identified which lead to CO induced deactivation. We conclude our study with the scope of future work based on our results and observations.Item Application of Gettering Layers for Low Temperature Conversion of Magnetic Oxides into Ferromagnetic Metals in Thin Films, Multilayers, and Nanostructured Arrays(2015-08) Qiu, Wenlan; Litvinov, Dmitri; Ruchhoeft, Paul; Brankovic, Stanko R.; Stein, Gila E.; Chang, LongNanoscale patterning of magnetic metals and their alloys remains a significant challenge due to the lack of reactive-ion etching (RIE) chemistries producing volatile compounds of magnetic elements. Patterning is typically achieved either via a pre-etched substrate deposition, lift-off process, ion milling, or wet etching, all lacking the needed fidelity to achieve the high resolution and high areal density because of fabrication issues such as fencing, shadowing, edge damage, redeposition, or magnetic material residual. Alternatively, electrochemical deposition may circumvent these fabrication issues and produce high aspect ratio structures, but is difficult to make complex structures, i.e., Co/Pd or Co/Pt multilayers, and CoCrPtX alloys, which are widely studied to be the potential candidates for magnetic recording media with areal density beyond 1 Tb/in2. This work demonstrates conversion of nonmagnetic cobalt oxide (CoO) into ferromagnetic cobalt (Co) in thin films, multilayers, and nanostructured arrays by low temperature annealing in the presence of tantalum (Ta) gettering layers. Thin film of CoO sandwiched between Ta seed and capping layers can be effectively reduced to a magnetic Co thin film by annealing at 200◦C, whereas CoO does not exhibit ferromagnetic properties at room temperature and is stable at up to ∼ 400◦C. The CoO reduction is attributed to the thermodynamically driven gettering of oxygen by Ta, similar to the exothermic reduction-oxidation reaction observed in thermite systems. Likewise, annealing at 200◦C of a nonmagnetic CoO/Pd multilayer results in the conversion into a magnetic Co/Pd multilayer with perpendicular anisotropy. A nanopatterning approach is introduced where CoO/Pd multilayer is locally reduced into Co/Pd multilayer to achieve magnetic nanostructured array in the presence of Ta islands. Magnetic properties of thin films, multilayers, and patterned arrays in this work are measured on MicroMag alternating gradient force magnetometer and self-built polar magneto-optical Kerr effect magnetometer. A Physical Electronics 5700 X-ray photoelectron spectroscopy is used to characterize the chemical states and compositions. This technique can potentially be adapted to nanoscale patterning of other systems for which thermodynamically favorable combination of oxide and gettering layers can be identified.Item Applied and Fundamental Studies of LNT-SCR Dual-layer Monolithic Catalysts for Lean NOx Emission Control(2015-12) Zheng, Yang; Harold, Michael P.; Luss, Dan; Epling, William S.; Jacobson, Allan J.; Brankovic, Stanko R.The increasingly stringent both greenhouse gas (GHG) and tailpipe NOx emission standards have driven the continuous improvement of commercial deNOx technologies, NOx reduction & storage (NSR, also referred to as lean NOx trap (LNT)) and selective catalytic reduction (SCR) technologies. This dissertation conducts applied and fundamental studies of coupled LNT-SCR dual-layer catalysts with the aim of expanding the operating temperature window of a conventional NSR system at lower cost. This is accomplished by a systems approach to identify the influencing factors such as catalyst composition and architecture, types of reducing agents, operating and regeneration strategies, as well as synergistic interactions between the LNT and SCR. We start with performance evaluation of dual-layer catalysts under different regeneration conditions such as H2 alone, CO/H2 mixture and a simulated diesel exhaust containing the CO/H2/C3H6 mixture. Spatial analyses of NH3 yield and NOx conversion along the LNT monolith identify the upstream zone as major NH3 generator and NOx reducer, especially at temperatures exceeding 300 oC. Zoning of either or both the SCR and LNT having a dual-layer structure enables an increase in the low-temperature (200-250 oC) NOx conversion, and minimizes the high temperature (300-400 oC) conversion loss caused by the SCR diffusion resistance and undesired NH3 oxidation by the LNT. The hydrocarbon (HC) reductant leads to an alternative LNT-SCR synergy to classical NH3-pathway; a LNT-assisted HC-SCR pathway. The LNT promotes the formation of partially oxidized HC intermediates during the rich purge which are otherwise difficult to be generated by the Cu-zeolite layer at low temperatures. These activated intermediates can be captured and utilized by the SCR catalyst via HC-SCR during the ensuing lean phase. This pathway plays a major role at low temperatures (<= 225 oC) using the simulated diesel exhaust feed. We investigated the steady-state and transient effects of reductants (CO, H2 and C3H6) on Cu-SSZ-13 catalyzed NH3-SCR as the SCR component in the combined system is periodically exposed to a rich exhaust. The three reductants affect to different extent the NH3-SCR reactions. Propylene is most effective in promoting NO2 reduction to NO by formation of organic intermediates. CO effectively reduces nitrates to nitrites that react with NO2, releasing NO. H2 follows a similar pathway as CO but is less effective. Finally, the effects of the lean/rich cycling frequency on both LNT and combined catalysts are investigated. Rapid C3H6 pulsing into a lean exhaust steam expands the operating temperature window of a conventional NSR system in both low and high-temperature regions. The combination of rapid propylene pulsing and the dual-layer catalyst architecture achieves the highest low-temperature NOx conversion. The working mechanisms of rapid propylene pulsing on both LNT and LNT-SCR catalysts are elucidated. Optimization of top-layer material and catalyst configuration like SCR and PGM zoning can improve system performance at lower cost.Item Benzimidazole-Based Fluorophores and Noncovalent Organic Frameworks(2015-12) Le, Ha Tuyet Mai 1987-; Miljanić, Ognjen Š.; Daugulis, Olafs; May, Jeremy A.; Xu, Shoujun; Brankovic, Stanko R.The previous studies of molecular cruciforms based on distyrylbis(arylethynyl)benzenes, tetrakis(arylethynyl)benzenes, tetrakis(alkynyl)ethenes, and benzobisoxazole cruciforms, and their use as chemical sensors will be introduced and summarized. We construct a simpler "half cruciform" architecture with benzimidazole core, namely L-shaped benzimidazole fluorophores. These fluorophores showed separate frontier molecular orbitals in donor–acceptor systems, similar to benzobisoxazole cruciforms. They display change in their fluorescent emission upon exposure to bases, acids and small anions. The preparation of other benzimidazole-based sensor precursors and initial study is also included. A new class of cruciform based on the benzobisimidazole central motif is introduced together with their synthesis and computational evaluation. These benzobisimidazole cruciforms exhibit strong optical responses to bases, albeit negligible to acids. A representative cruciform was examined and showed pronounced colorimetric response to anions. Noncovalent organic frameworks are porous materials; their porosity results from noncovalent interactions. The series of perfluorinated precursors was synthesized and crystalized to obtain desired noncovalent organic frameworks. The porous framework was analyzed by porosity measurement, single and powder X-ray diffractions.Item Benzobisoxazole Cruciforms and Guanosine Derivatives: Syntheses, Structural Analyses, and Optical Properties(2012-12) Lim, Jaebum 1978-; Miljanić, Ognjen Š.; Lee, T. Randall; Gilbertson, Scott R.; Jacobson, Allan J.; Brankovic, Stanko R.This dissertation presents two studies in supramolecular chemistry; one is the chemistry of benzobisoxazole cruciforms in which two linearly conjugated π systems meet at the unsaturated center, and the other is that of guanosine derivatives. Chapter One. This chapter summarizes the previous work on molecular cruciforms: tetrakis(arylethynyl)benzenes, distyrylbis(arylethynyl)benzenes, tetrakis(aryl-vinyl)benzenes, and tetraethynylethenes. Chapter Two. The synthesis, optical properties, and computational calculation of benzobisoxazole-based molecular cruciforms are presented. The potential use of donor/acceptor substituted cruciforms as sensors is also discussed, and optical response of cruciform compounds to different concentration of trifluoroacetic acid in the liquid state is presented. Chapter Three. A method for the identification method of carboxylic acids, organoboronic acids, phenols, amines, ureas, and tetrabutylammonium salts is described using one of donor/acceptor substituted benzobisoxazole-cruciforms as the sensor. Chapter Four. Structural study of benzobisoxazole cruciforms is presented: their solid state structures, obtained by X-ray crystallography, are presented. Chapter Five. The synthesis and structural analysis of 8-arylethynyl substituted guanosine derivatives is presented. The structural effects of this substitution, in comparison to the previous work in guanosine chemistry, is also discussed.Item CARBON MONOXIDE ADSORPTION ON Pt MONOLAYER ON Pd(hkl) SURFACE STUDIED BY SUBTRACTIVELY NORMALIZED INTERFACIAL FOURIER TRANSFORM INFRA-RED SPECTROSCOPY(2012-08) Slavkovic, Milan 1981-; Brankovic, Stanko R.; Akay, Metin; Chen, Ting Y.The progress of our civilization is critically dependent on the discovery of new materials and structures with better properties, novel functionality and applications. The ultra thin metal overlayers represent one class of such structures with increasing application in fuel cells. In our work, the two-dimensional Pt monolayers on Pd(hkl) were used as model system to study CO adsorption. The PtML/Pd(hkl) were obtained using surface limited red-ox replacement reaction of underpotentially deposited Cu monolayers on Pd(hkl) with, and without presence of citrate in the reaction solution. The strength of the CO adsorption on Pt monolayer was studied by subtractevily normalized interfacial fourier transform infra-red spectroscopy (SNIFTIRS). The SNIFTIRS results indicate synergistic effect of both; Pt monolayer and Pd(hkl), yielding a qualitatively new phenomenon where the bond of adsorbed CO on PtML/Pd(hkl) is weaker than the corresponding CO bond on each of the bulk metal surfaces. This result has fundamental importance when catalysts for oxygen reduction and hydrogen oxidation reactions are considered where the surface poisoning by intermediates or impurities is responsible for reduced catalyst activity.Item Characterization and Optimization of Multifunctional Automotive Catalysts(2019-12) Malamis, Sotirios A.; Harold, Michael P.; Epling, William S.; Rimer, Jeffrey D.; Bollini, Praveen; Brankovic, Stanko R.Multifunctional automotive catalysts provide new opportunities for gasoline and diesel engines to meet the constantly tightening emissions and fuel economy standards from various regulatory agencies. Meeting these demands is important not only for securing industrial compliance but also for improving human health and air quality. Combining multiple functions into a single catalyst saves design space and reduces material cost. Here we conduct steady state and transient experiments on multifunctional catalysts that span the operational range of gasoline and diesel engines including cold start and high-temperature operations in order to reduce NOx and hydrocarbon (HC) emissions. We investigate first the Three-Way NOx Storage Catalyst (TWNSC), a concept that combines three-way and NOx storage functionalities for optimal performance during high-temperature vehicle operation. A series of experiments identifies operating conditions that maximize conversion and performance for application with a downstream selective catalytic reduction (SCR) catalyst. Second, we characterize new catalysts that address the cold start issue of modern engines, namely the Lean Hydrocarbon NOx Trap (LHCNT) concept. The LHCNT is a precious group metal (PGM)-zeolite material that combines low temperature NOx and hydrocarbon storage and catalytic conversion of both species into a single unit. By conducting transient uptake and release experiments we obtain useful insight about competitive adsorption, release temperature, conversion activity, and water impact. We find that hydrocarbon concentration and identity, as well as PGM content and zeolite geometry can affect NOx/HC uptake and release performance. Findings from single catalyst function experiments are used to evaluate sequential and dual-layered configurations that improve the overall LHCNT performance. The final part of this work investigates the feasibility of modeling such a catalyst to predict performance and screen new materials. These results provide guidance for improving catalytic systems in the automotive catalysis industry in order to keep up with emission standards.Item CO and Hydrocarbon Oxidation on PD/Ceria-Zirconia/Alumina Three-Way Catalysts(2018-12) Lang, Wendy; Harold, Michael P.; Grabow, Lars C.; Bollini, Praveen; Brankovic, Stanko R.; Jacobson, Allan J.The optimization of combustion engine aftertreatment systems is critical in reducing the negative health and environmental impact of automotive emissions. Over the years, three-way catalysts (TWCs) have been developed and widely implemented to control and reduce tailpipe emissions of carbon monoxide (CO), hydrocarbons (HCs), and nitrogen oxides (NOx) by converting these harmful and undesirable species to CO2, H2O, and N2. The oxidation behavior of CO and model hydrocarbons (propylene, acetylene, and ethylene) and performance of model TWCs is investigated in this dissertation to systematically evaluate trends such as self- and mutual inhibition within reaction and catalytic systems with the intent of improving low-temperature combustion. The use of alumina as a conventional washcoat support material was compared to that of ceria-zirconia (CZO) as an alternate support with oxygen storage capacity in experimental studies. CO, propylene, and acetylene oxidation were enhanced using Pd/CZO compared to Pd/Al2O3, as observed in light-off performance tests. Differences in reaction mechanisms utilizing the two catalysts were further investigated via steady state kinetic experiments. Ethylene oxidation was studied using Pd/CZO and exhibited behavior deviating from self-inhibition observed during CO, propylene, and acetylene oxidation. Combinations of reactants in binary and ternary mixtures for simultaneous oxidation were studied, revealing competitive mutual inhibition effects and particularly strong inhibition by acetylene. The use of ceria was found to significantly enhance co-oxidation and mitigate inhibition effects. A low-dimensional reactor model was developed and employed to verify experimental results for CO and propylene oxidation on Pd/Al2O3 and predict nonisothermal and heat and mass transfer effects within the monolith catalyst. Refined global and microkinetic models ably captured mechanisms for CO and propylene oxidation and provided key insights into inhibition within three-way catalyst systems.Item Computational Screening of Bifunctional Catalysts for CO and CH4 oxidation(2015-12) Doan, Hieu; Grabow, Lars C.; Harold, Michael P.; Epling, William S.; Brankovic, Stanko R.; Brgoch, JakoahModern advances in density functional theory (DFT) and computing power have allowed us to investigate catalytic reaction at surfaces in great details and with reasonable chemical accuracy. Based on fundamental knowledge of reaction mechanism and key surface properties, new catalysts can be designed and further tuned for optimal performance. In the recent literature, several examples of catalysts that perform multiple site-specific functionalities under steady-state reaction conditions have been reported. The most common systems are bifunctional catalysts where each of the two distinct sites preferentially catalyzes different reaction steps independently. In this dissertation, DFT calculations were used in combination with microkinetic modeling to explore bifunctional catalyst design strategies for CO and CH4 oxidation. Preliminary study suggested that there are theoretical limits for the achievable activity improvement and bifunctional catalysts do not necessarily outperform single-site catalysts. For CO oxidation on bimetallic surfaces, it was found that the optimal activity is not significantly altered when bifunctional mechanism are considered, but equally active bifunctional catalysts may be tailored from less active and cheaper components. In particular, when CO oxidation was probed on the novel RuPt core-edge nanocluster catalyst, a bifunctional mechanism that involves the delivery of two reactants from two different spatial domains to a reacting interface was used to explain the significant activity improvement. In the special case of CO oxidation on Au/TiO2 system, a new mechanism was proposed and water was identified as a co-catalyst in the reaction. To investigate the importance of the material gap in computational catalyst screening, complete CH4 oxidation was evaluated on different representative models of Pd catalysts. It was observed that, although quantitative results may vary significantly, the trend in reactivity is the same across all surface models. Extensive promoter screening was also performed on PdO(101) surfaces, and calculated data for CH4 activation suggested that there exists an additive effect upon promoter substitution at two distinct Pd sites on PdO(101). Our efficient screening strategy has led to predictions of several promising promoters for Pd catalyst for complete CH4 oxidation on the basis of intrinsic activity and resistance toward water inhibition and sulfur poisoning.Item Control of Metallurgical, Magnetic and Electrical Properties of Electrodeposited CoFeNi Thin Films and Cu Nanostructures(2014-05) Dole, Nikhil; Brankovic, Stanko R.; Bao, Jiming; Ruchhoeft, Paul; Meen, James K.; Majkic, Goran; Garfias, Luis F.The ever increasing quality and reliability demand of microsystems indicate that the electrodeposited magnetic alloys used for their fabrication should have minimum possible magnetic losses at the desired frequency range. In order to meet this challenge, the magnetic industry is facing the task of electrodepositing soft high magnetic moment alloys (SHMM) with high permeability (μr>400) and high resistivity (ρ>100μΩ-cm). The ternary ferromagnetic CoFeNi alloys represent the class of SHMM alloys which inherently have a higher resistivity than their binary (NiFe or CoFe) counterparts, with similar magnetic properties. In the first part of this research, the work exploring the development of solution chemistry for the electrodeposition of CoFeNi magnetic thin films is discussed. The experimental results (FIB, EDS, XRD, 4-point probe and VSM) indicate that a high deposition rate CoFeNi film can be deposited from the developed bath chemistry yielding high saturation magnetization (Ms~2.0 T), high relative permeability (μr~700) and high resistivity (ρ~100μΩ-cm). Additionally, the effect of sulfur containing additives like saccharin on the physical, electrical and metallurgical properties of CoFeNi films is studied. The role played by saccharin in stress reduction is exhibited by performing in-situ stress measurements during thin film growth. The second part of this dissertation focuses on a novel approach towards improving the conductivity of Cu interconnects at nanoscale. The externally applied strain on the Cu interconnects during annealing is used to promote the grain growth via grain boundary densification process. The results indicate that this approach yields positive effect on the resistance of Cu interconnects. The drop in the resistance after annealing at 200°C and 250°C is observed for externally imposed compressive strain along the Cu interconnects with critical dimensions (CD) of 50nm and 64nm. The analysis and results suggest that the compressive strain during annealing induces densification of the grain boundaries with the interface vector parallel to the current path. These results are of great significance for the overall improvement of conductivity of the Cu interconnects as well as reliability and life span of microelectronic devices.Item Controlling a Swarm of Robots Using Global Inputs(2018-08) Shahrokhi, Shiva; Becker, Aaron T.; Kavraki, Lydia; Brankovic, Stanko R.; Mayerich, David; Faghih, Rose T.Microrobotics has the potential to revolutionize many applications, including targeted material delivery, assembly, and surgery. The same properties that promise breakthrough solutions---small size and large populations---present unique challenges for controlling motion. When there are more particles than control inputs, the system is underactuated and requires new control techniques. Rather than focusing on a specific microrobotic system, this dissertation designs control laws and algorithms for steering many particles controlled by global fields. First, we identify key parameters for particle manipulation by using a collection of online games where players steer swarms of up to 500 particles to complete manipulation challenges. Inspired by techniques where human operators performed well, we investigate controllers that only use the mean and variance of the swarm. We next derive automatic controllers for these and a hysteresis-based switching control to regulate the first two moments of the particle distribution. Torque control is also necessary for manipulating objects as well as for aligning sensors, emitters, or redirecting an incoming signal. Second, this dissertation proves that swarm torque control is possible, then presents algorithms to automate the task. Torque control enables us to control the position and orientation of an object. Finally, this dissertation investigates particle control with uniform magnetic gradients (the same force is applied everywhere in the workspace). We provide position control algorithms that only require non-slip wall contact in 2D. The walls of in vivo and artificial environments often have surface roughness such that the particles do not move unless actuation pulls them away from the wall. We assume that particles in contact with the boundaries have zero velocity if the shared control input pushes the particle into the wall. All the results are validated with simulations and hardware implementations.Item Cyclobenzoins for Energy Industries(2023-05-01) Robles, Alexandra; Miljanić, Ognjen Š.; Brgoch, Jakoah; Guloy, Arnold M.; Comito, Robert J.; Brankovic, Stanko R.Porous molecular crystals are an emerging class of porous materials with properties reaching and exceeding their polymeric counterparts, metal organic frameworks and covalent organic frameworks. They are composed of discrete molecules containing intrinsic voids or inefficiently packed to create extrinsic pores. Porous molecular crystals have been studied for a variety of applications from more traditional catalysis and gas separations to innovative utilizations in porous liquids and analytical sensing devices. Herein we focus on cyclotetrabenzoins, macrocycles synthesized through benzoin condensation cyclooligomerization of dialdehydes, for applications in energy industries. Chapter 1 introduces porous molecular crystals and discusses their studies in chemical separations, molecular sensing, catalysis, porous liquids, and proton conduction. Chapter 2 focuses on an esterified cyclotetrabenzoin for CO2/CO separation through pressure swing adsorption. Chapter 3 discusses a strategy to systematically expand the extrinsic porosity of cyclotetra(bisarylhydrazone)benzils and the utilization of their virtual pores for iodine capture in the solid state and from solutions. Chapter 4 presents two expanded cyclotetrabenzoins and their oxidation into redox-active cyclotetrabenzils which can be incorporated into organic cathode materials for lithium-ion batteries.Item Design, Synthesis, and Characterization of Quinone Electrode Materials for Sustainable Energy Storage(2017-08) Jing, Yan; Yao, Yan; Brankovic, Stanko R.; Lee, T. Randall; Bao, Jiming; Shan, XiaonanThe energy crisis and environmental issues have instigated the integration of renewable energy sources into the electric grid. A robust, safe, and long-life energy storage system is therefore strongly desired to maximize the benefits of intermittent renewable resources. Organic electrode materials, such as quinones, have recently attracted significant attention due to their high capacity, environmental friendliness, and ability to be derived from biomass. The objective of this dissertation is to design, synthesize, and characterize multiple quinone-based electrode materials in aqueous and non-aqueous electrolytes towards building next-generation energy storage technologies. My aim is to better understand the complex interactions amongst redox molecules/oligomers, ions, electrons, and electrolytes, and find ways to design better materials with improved performance. In this dissertation, I report the design, synthesis, and characterization of quinone-based oligomers and corresponding electrochemical properties in aqueous and non-aqueous electrolytes. I first report the synthesis of two cross-conjugated quinone oligomers and the effects of cross-conjugation and molecular conformations on the electrochemical properties. I further investigate the oligomers in aqueous electrolytes and discover the maximum capacity can be realized when the pH of electrolyte is above the pKa2 of the reduced quinones. Third, I discover that a sufficient swelling of the polyquinone film may be imperative to release full capacity. The combination of electrochemical quartz crystal microbalance and constant current chargedischarge techniques reveal that hydrated cations serve as charge carriers in aqueous electrolytes, and that the hydration numbers dynamically vary with the state of charge and current density. Finally, an oligomer based on pyrene-4,5,9,10-tetraone core capable of 4-electron reduction was synthesized and characterized in various electrolytes (H+, Li+, Na+, and Mg2+ over a pH range of 0-13). A Pourbaix diagram was then derived to understand the competition between H+ and metal-ion coordination. The work described in this dissertation strives to provide an in-depth understanding of the working mechanisms of quinone-based electrodes and guidelines for future organic battery development.Item Electrochemical Characterization of Negative Lead Electrode in Lead-acid Battery(2021-12) Chaudhari, Nikhil Milind; Brankovic, Stanko R.; Robles Hernandez, Francisco C.; Yao, Yan; Bao, Jiming; Stamenkovic, Vojislav R.; Lopes, Pietro P.The energy demands of the growing world are increasing at a fast rate and storage of energy is of extreme importance for many industrial and household applications. Batteries are proving to very useful in energy storage and providing power for applications ranging from portable devices to EVs for transportation. Until recently, Lead-acid batteries were the most used and successful for storing energy, and are used in many applications like UPS system, SLI systems in cars, and for industrial power applications. As the demand for high performance batteries is constantly increasing, Lead-acid batteries are in desperate need of advancements so that maximum efficiency and performance can be achieved since with the lead-acid batteries in use right now, only 30 – 40 % of the theoretical efficiency is achieved. Lead-acid battery has some unique advantages such as about 99.9 % recyclability, low cost, wide operating temperature range and lower risk of explosion. The work included in this dissertation is aimed toward exploring the fundamentals of lead-acid battery electrochemistry using advanced techniques developed in recent past which will help improve their performance. The focus of this work is on understanding and establishing baseline performance of negative lead electrode with variation in temperature. The discharge and kinetic charge acceptance of Pb electrode is explored for a wide range of temperature to understand the performance limits and performance controlling parameters. The cyclic voltammetry electrochemical procedures are established and used for studying discharge and charge processes of Pb electrode. Double layer capacitance measurement for electrochemically polished Pb surface is used as a metric for lead surface area. Quantification of performance of Pb electrode is evaluated using modified Peukert relationship and Kinetic charge acceptance measured from cyclic voltammetry data. Passivation of lead electrode during discharge forms PbSO_4 layer and dissolution of this layer is a limiting process for recharging the electrode. Morphology of PbSO_4 layer is related to its dissolution. Here, a relationship of PbSO_4 thickness and particle size to discharge capacity and charge acceptance is observed. Therefore, morphology and thickness of PbSO_4 layer after discharge at various temperatures is studied using microscopy techniques such as conductive Atomic Force Microscopy and Focused Ion Beam SEM.Item Electrochemical Synthesis of Functional Films and Surfaces(2020-12) Ahmadi, Kamyar; Brankovic, Stanko R.; Robles Hernandez, Francisco C.; Bao, Jiming; Taylor, E. Jennings; Mayerich, David; Dole, NikhilThis dissertation represents experimental work with significant analytical modeling in areas of electrochemical material science and thin films. It produces several fundamental results with great implications for practitioners in the field and various applications in technological enterprises. The dissertation addresses problems in a broad range of electrochemical synthesis methods, including stress control in the electrochemical deposition of functional films, synthesis of functional surfaces, and monolayer deposits. Thin film residual stress developed during the deposition can adversely affect the film's reliability and functionality. This research describes the origin of the crack formation in Cr and Pd films, two industrially significant coatings, using in situ stress measurement during and after the film deposition as the primary experimental approach. Our finding in chromium electrodeposition from Cr(III) electrolytes relates the origin of cracks to the formation and subsequent decomposition of Cr-H. Employing various experimental methods, data analysis, and modeling, the research describes the design of effective PCD (pulse-currents-deposition) that reduces Cr-H accumulation, the stress generating source, at the deposition stage. The designed PCD with t_on/t_off > 5 where t_on > 0.25 seconds showed drastic crack mitigation in the deposits. Additionally, this work investigates stress development during Pd film deposition and shows that introducing Pb2+ ion in the depositing electrolytes can lead to Pb underpotential co-deposition as Pd-Pb intermetallic, which suppresses the generated stress from hydrogen absorption. This dissertation also demonstrates electrochemical techniques for electroless Pb ML deposition and surface decoration as a practical approach for designing chemical sensors and developing new deposition processes like e-less ALD (atomic-layer-deposition). First, it presents a novel chemi-resistive hydrogen-sulfide sensor, designed and synthesized by atomic-layer surface modification. The modification with AuPd alloy improves chemi-resistivity of Au ultra-thin film and results in fast, reproducible, and linear sensing for hydrogen-sulfide from aqueous environments. Further, this research discusses an analytical study on the electroless Pb ML deposition on various substrates. The analysis shows that the surfaces’ electronic nature and catalytic phenomena are the main factors in electroless ML deposition. These findings have significance for electrochemical deposition on nano features and shed light on a pathway to synthesize core-shell nano-catalysts using a new e-less ALD method.Item ELECTROCHEMICAL SYNTHESIS OF MAGNETIC MATERIALS FOR MAGNETIC RECORDING AND MEMS APPLICATIONS(2012-05) George, Jinnie; Brankovic, Stanko R.; Litvinov, Dmitri; Ruchhoeft, Paul; Ignatiev, Alex; Grabow, Lars C.With the increase in the areal density for magnetic recording disks, the bits are becoming smaller so that more bits can be accommodated in a given area of disk. However, there is a certain limit up to which the bits can be made smaller until it reaches the superparamagnetic limit, where bits become thermally unstable. In order for bits to be stable, high coercivity media is needed and the flux coming out from the writer head should be high enough to switch the high coercivity medium bits. Hence Cobalt Iron (Co37Fe63) alloys having the highest magnetic flux density of 2.4 T are used for this purpose. Sulfur containing additives like saccharin are incorporated in the CoFe electrodeposition bath to provide desirable properties like low stress, low coercivity and fine grain size to the deposit. The effect of saccharin incorporation during electrodeposition process on the properties of CoFe films is studied in this research. In-situ stress measurements were performed to determine the reduction in stress with increasing saccharin concentrations in the bath and an analytical model was developed to explain phenomenological dependence of the maximum stress level in CoFe films as a function of saccharin concentration in the bath. However, saccharin incorporation in CoFe bath lowers the corrosion resistance of CoFe films. The corrosion potential dependence on the sulfur incorporation mechanisms in CoFe films was studied via an analytical model based on the mixed potential theory. As bit size decreases, new magnetic sensors that provide high sensitivity and increased magnetoresistance ratio have to be developed. As a part of this research, the novel magnetic field sensors were fabricated based on electrodeposited CoFe nanocontacts and demonstrated magnetoresistance ratio as high as 3000%. The CoFe nanocontacts, ~70 nm in diameter, embedded in insulating Al2O3 layer which separates two plane parallel ferromagnetic layers represent the basic magnetic field sensor design. The magnetoresistance curves of these sensors displayed properties characteristic of both tunneling and ballistic transport of electrons by domain wall scattering. Hence, low temperature measurements were performed to understand the transport mechanisms of electrons in these electrodeposited magnetic nanocontacts.Item Electrochemical Thin Film Growth Assisted by Pb Monolayer(2018-05) Wu, Dongjun; Brankovic, Stanko R.; Bao, Jiming; Yao, Yan; Miljanić, Ognjen Š.; Dole, NikhilThe properties of heteroepitaxial thin films are very much dependent on the method used for their deposition. The electrochemical/electroless deposition represents the ambient temperature growth process for many heteroepitaxial metal overlayers while preserving the integrity of their interface. However, in many substrate-overlayer systems the interfacial free energy of the overlayer/electrolyte is smaller than the sum of the interfacial free energies of the substrate/electrolyte and overlayer/substrate and nucleation and growth of rough 3D films is favored. In epitaxial systems where the thermodynamics predicts Frank-van der Merve or 2D layer-by-layer growth, the difficulties in producing 2D single crystal overlayers by electrodeposition are similar to those encountered during the vacuum deposition at ambient temperature. Owing to strain and kinetic issues, 3D growth occurs often. In recent years, a significant progress has been made in understanding the kinetic aspects controlling the growth modes in vacuum deposition. This development has led to various discoveries in which the growth kinetics during electrochemical growth was manipulated to enhance the evolution of atomically flat epitaxial overlayers. In the first part of this PhD work, electrochemical and electroless Pb UPD phenomena were investigated as a critical step in development of more sophisticated approaches to manipulate growth kinetics during electrochemical/electroless growth of thin films. An electroless atomic layer deposition (eALD) method has been developed using the electroless Pb monolayer as the reducing agent and sacrificial layer in surface limited redox replacement reaction with more noble metal ions (Ptn+). This eALD process is highly selective to metal substrates at which Pb forms the electroless monolayer. Repetition of the eALD cycle leads to formation of a highly compact, smooth and conformal noble metal film. In the second part, electrochemical deposition of 2D Cu thin film on Ru(0001) surface was investigated. In the first case, Cu was co-deposited with periodically deposited and stripped Pb mediator, the Pb mediator serves to increase the density of 2D Cu islands during deposition and promote a layer by layer thin film growth mode. In the second case, a full UPD monolayer of Pb was used as a surfactant during Cu deposition and 2D growth of Cu was obtained on the Ru(0001) surface. In the third part of this work, an electroless approach was investigated for Co deposition on the Cu(h, k, l) polycrystalline substrate. The effect of substrate pre-treatment on the final morphology and structure of deposited Co film was studied; the results suggest that proper annealing of the Cu substrate in the forming gas (75%H2, 24%N2 and 1%CO) is beneficial for the ultimate quality of the deposited Co thin film. Our study shows that with the presence of the electroless Pb monolayer surfactant, the morphology and quality of the deposited Co film is improved in terms of uniformity, continuity and smoothness of the film. The experimental result indicates that the electroless Pb monolayer has efficiently reduced the Schwoebel barrier and enhanced the surface diffusion of atoms during deposition.Item Electrodeposition of Chromium Film Using Cr3+ Solution: Fundamental Study of Crack Formation(2017) Ahmadi, Kamyar; Brankovic, Stanko R.What is the purpose of this study? Understand issues with electrodeposition (ELD) of chromium (Cr) film from Cr3+ solution and then suggest an approach to address them. What is ELD? In ELD process by applying current metallic ions in solution reduce and form a metallic coating on the surface of an electrode. Why Cr? Cr has attractive appearance -> decorative application. Cr is hard and anti-corrosion -> engineering application. General approaches for ELD of Cr? Cr(IV) ions -> + high quality films, - highly carcinogenic. Cr(III) ions -> + environmentally friendly, - issues with quality due to crack formation. What have we done so far? We fundamentally study the origin of cracks in the films. We propose a promising approach to reduce the cracking.Item Electrodeposition of CoNiFeX magnetic Films with Low Magnetic Losses for Power Applications(2023-05-08) Solanki, Dhaivat; Brankovic, Stanko R.; Robles Hernandez, Francisco C.; Ruchhoeft, Paul; Bao, Jiming; Chang, LongElectromagnetic induction is essential for functioning of the analog circuits used in cell phones, MEMS devices and other power related electronics. In order to meet the demand of higher efficiency and minimum magnetic losses in IC devices, we have developed solution chemistry for electrodeposition of CoNiFeX (X=S,P,B,V) thin films with high magnetic moment (HMM) and permeability with higher than 100μΩ-cm resistivity; higher than 1.7 T magnetic moment. CoNiFeX alloys are fully compatible with existing manufacturing concepts in semiconductor industry. Direct current (DC) and pulse current (PC) deposition methods to produce CoNiFeX alloys is employed to demonstrate control over BCC crystal structure by tuning transport conditions during electrodeposition. The BCC structure of CoNiFeX alloys is characterized with average grain size of 10-20 nm which consequently yields coercivity values of CoNiFeX alloys between 8-15 Oe at 500 nm film thickness. CoNiFe alloys produced by DC and PC deposition have magnetic moment higher than 1.7 T, which is not significantly diminished with addition of X (X=S,P,B,V) as resistivity controlling phase. More than 100% increase in the resistivity of deposited CoNiFeX alloys is demonstrated with controlled concentration of X burring molecules and Fe3+ ions as hydroxide precipitating agent without deterioration in magnetic properties. All samples exhibit shiny surface finish and low stress. Electrodeposition process is scaled up with customized setup to build thin film inductor devices on 4 in wafer, fabricated at Nanofabrication Facility at the University of Houston. Thin film inductor devices with CoNiFeX cores are tested at Intel Corporation facility in wide frequency range (1 MHz to 1 GHz). More than 70% improvement in induction efficiency compared to air core devices is observed when CoNiFeX magnetic cores of various geometry are introduced in device design. Direct measurement of permeability and losses for DC deposited CoNiFeX alloys is performed by resonant cavity setup, which illustrate reduction in losses at 100 MHz frequency due to resistivity improvement by incorporation of X phase in CoNiFeX alloys.Item Electroless Atomic Layer Deposition(2017-12) Solanki, Dhaivat J.; Brankovic, Stanko R.; Robles Hernandez, Francisco C.; Bao, JimingThin film growth is studied by many researchers because of broad applications and significant impact on technological development in modern era. However, thin film nucleation and growth is challenging task for many materials with high surface energy and melting temperature. Such systems tend to grow 3D leading to non-uniform coverage and rough surface. This poses an issue when very thin film deposition is required with 2D morphology and full coverage of deposit is desired. In this thesis, we propose a protocol for aqueous solution based thin film deposition where deposit thickness is precisely controlled down to a monolayer. We demonstrate combination of electroless monolayer formation and Surface Limited Redox Replacement to deposit thin film selectively on Cu nanowires and polycrystalline Cu wafer irrespective of another metal and oxide surrounding. Such protocol expands a new horizon in the field of precise ultrathin film deposition.
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