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Ab-Initio and Experimental Techniques for Studying Non-Stoichiometry and Oxygen Transport in Mixed Conducting Oxides
Get PDFThe ability of cerium oxide (CeO2-δ, also called ceria), to vary its oxygen stoichiometry in response to changes in temperature or oxygen activity is key to many of its applications in catalysis and electrochemical energy storage and conversion. This thesis explores ab initio and experimental approaches to study the fundamental thermodynamic and oxygen transport properties of ceria (MxCe1-xOO2-δ), but the methods are applicable to other mixed conducting oxides as well.
In the first part of the thesis, a computational thermodynamics approach that integrates quantum mechanical and statistical ensemble-based simulations is used to calculate the reduction-oxidation thermodynamics of non-stoichiometric ceria entirely from first principles. This procedure is well understood and has been successfully implemented for metallic alloys, but has not been extended to correlated electron systems such as ceria, for which the physics of electronic structure calculations is significantly more complicated. Density functional calculations were used to obtain the ground state energies of ceria with vacancy concentrations ranging from fully stoichiometric up to δ=0.25$. For each δ, numerous vacancy configurations were sampled to capture the interactions between vacancies and other atoms. Using the frozen phonon method, lattice dynamical calculations of phonon density of states were performed for various δ. Based on the ground state energies of nearly 40 structures, a cluster expansion Hamiltonian was used to parametrize the energy as a polynomial in occupation variables. The vibrational energies were used to make the Hamiltoninan temperature dependent. Lattice Monte Carlo (MC) simulations using the cluster expansion Hamiltonian were then used to study, for the first time, the effect of temperature and chemical potential on the vacancy concentration in ceria from first principles. The temperature composition phase diagram constructed from the MC simulations successfully reproduced the experimentally reported miscibility gap. The inclusion of vibrational and electronic contributions to the entropy made the agreement quantitative. Further, the partial molar enthalpy and entropy of reduction as a function of δ were extracted and found to deviate significantly from those of an ideally behaved system. The deviations were quantified by calculating the Warren-Cowley short range order parameters. This was the first demonstration of an ab initio approach being used to accurately model the defect thermodynamics of a correlated electron system without resorting to experimental inputs. Using ceria as benchmark material, this project lays the groundwork for a computational approach to screen new oxides for thermochemical cycling.
The rest of the thesis describes experimental investigations of oxygen transport and non-stoichiometry in doped and undoped ceria. Oxygen transport studies were performed using electrical conductivity relaxation (ECR). In ECR, a small step change in pO2 forces the sample non-stoichiometry δ, and other dependent properties such as electrical conductivity, to equilibrate to a new value. The rate of this re-equilibration is governed by the bulk oxygen diffusivity, DChem, and surface reaction rate constant, kS -- the two principal kinetic properties. By fitting the solution to Fick's second law, with the appropriate boundary conditions, to the conductivity relaxation profile, DChem and kS can be extracted. The instrumental capability for performing electrical conductivity relaxation experiments was set up and a systematic data analysis procedure was developed to reliably and accurately extract DChem and or kS. The experimental and data analytical methodologies were successfully benchmarked with 15 mol% Sm doped ceria, for which approximate values of the two principal transport properties, bulk oxygen diffusivity, DChem, and surface reaction rate constant, kS, can be found in the literature. An unexpectedly high p-type electronic transference number enabled ECR measurements under oxidizing conditions. A systematic data analysis procedure was developed to permit reliable extraction of the kinetic parameters even in the general case of simultaneous bulk and surface limitation. When the surface kinetics were too sluggish compared to bulk diffusion, Pt catalyst nanoparticles were sputtered to catalyze the surface reaction and enable extraction of DChem. The DChem from this study showed excellent qualitative and quantitative agreement with expected values, falling in the range from ~ 2 x 10-5 to 2 x 10-4 cm2/s. The surface reaction constant under H2/H2O mixtures also showed good agreement with literature results. Remarkably, this value increased by a factor of 40 under mixtures of CO/CO2 or O2/Ar. This observation suggests kinetic advantages for production of CO rather than H2 in a two-step solar-driven thermochemical process based on samarium doped ceria.
Using ECR, the effect of 20% Zr addition on the electrical conductivity and oxygen transport properties of ceria as a function of pO2 and T was investigated. Under oxidizing conditions, both CeO2-δ and Zr0.2Ce0.8O2-δ(ZDC20) showed n type, mixed conduction. The conductivity of ZDC20 was two orders of magnitude higher than that of undoped ceria. Contrary to previous studies, we found that Zr addition does not change the electronic mobility in this pO2 regime. The enhancement in conductivity is a consequence of higher vacancy concentration in ZDC20 under identical conditions compared to ceria. Under reducing conditions, while the n-type conductivity of ceria continued to increase with decreasing pO2, that of ZDC20 reached a broad maximum, eventually decreasing with pO2 (p-type) despite increasing carrier concentration. We show that the electronic mobility becomes strongly concentration dependent at high oxygen non-stoichiometry. This leads to a marked decrease in mobility with increase in δ, causing the conductivity to roll over from n to p type. The chemical diffusion coefficient and surface reaction rate constant of both ceria and ZDC20 showed strong dependence on pO2 under oxidizing conditions, decreasing by nearly an order of magnitude between 10-2 atm and 10-5 atm. The unexpectedly high sensitivity to pO2 was ascribed to the effect of extrinsic vacancies generated by trace quantities of lower valence cation impurities, that dramatically increase both the absolute value of the thermodynamic factor and its sensitivity to pO2 close to stoichiometry. Overall, the addition of Zr lowers the DChem and kS of ceria in the temperature and oxygen partial pressure range of this study, the effect being more pronounced under reducing conditions. Beyond its relevance to ceria, this work demonstrates the potential of ECR to isolate the effect of kinetics from thermodynamics of the real thermochemical cycle, reveal the limiting transport parameters, and ultimately guide microstructure design for maximizing the rate of fuel production.
Lastly, we improve upon an existing formalism to calculate the oxygen non-stoichiometry in thin films of mixed conducting oxides using AC impedance spectroscopy. Cerium oxide was once again chosen as the benchmarking material, since it shows both ideal and non-ideal thermodynamic behavior under different conditions, and has been well studied in its bulk form. In this method, the impedance response of dense, thin films of CeO2-δ deposited on a Y0.84Zr0.16O1.92> (YSZ) substrate was measured using AC impedance spectroscopy. To explore potential grain boundary effects on bulk thermodynamic properties. A physically derived equivalent circuit model was fit to the impedance response to extract a quantity called the 'chemical capacitance', which was subsequently related to the non-stoichiometry. Previous studies employing this method were restricted to systems that can be described using ideal solution thermodynamics, which allows simplifications to the theoretical treatment of their capacitance. Apart from extending this technique to a non-ideally behaved oxide, we report excellent agreement between the non-stoichiometry of single crystal and polycrystalline films and that of bulk ceria. By virtue of using thin films, equilibration times are dramatically decreased, enabling faster measurements compared to bulk techniques like thermogravimetry and coulometric titration. Further, the electrochemical method is ideal for thin films, for which the mass changes are below the detection limits of bulk techniques.</p
Progress Toward the Total Synthesis of Curcusone C and Mechanistic Elucidation of an Unexpected Rearrangement
Get PDFCurcusone C is tricyclic diterpenoid natural product isolated from Jatropha curcas that exhibits potent biological activity and features a 2,3,7,8- tetrahydroazulene-1,4-dione moiety. Herein, we describe a synthetic approach toward ent-curcusone C. Construction of the tricyclic scaffold of ent-curcusone C is achieved from a cyclopentenol boronate and a vinyl bromide, which was synthesized from (S)-perillaldehyde. Suzuki coupling of the two precursors furnished a dieneol, which was converted to a diazoester via transesterification followed by diazo transfer reaction. A divinylcyclopropane was synthesized from the diazoester by intramolecular cyclopropanation and subsequent Kauffmann olefination. The tricyclic core of ent-curcusone C was accomplished by divinylcyclopropane rearrangement, which was initiated by reduction of the lactone moiety.
We discovered an unexpected rearrangement during the course of our investigation toward the synthesis of curcusone C. Surprisingly, a silyl enol ether was converted to a complex tetracyclic compound under mild heating conditions. The transformation was elucidated as a unique reaction cascade of [3,3] Cope rearrangement, [1,5] silyl migration, Ireland–Claisen rearrangement, retro Claisen rearrangement, and [1,5] silyl migration by computational and experimental efforts.
Additionally, our work on the development of a bis(phosphine) copper catalyst for the asymmetric alkylation of 3-Bromooxindoles with α-arylated malonate esters is described. Versatile copper sources and chiral bis(phosphine) ligands were investigated.</p
Multifunctional Materials: Bottom-Up and Top-Down
Get PDFThis thesis is thematically separated into two complimentary approaches to advanced materials synthesis: bottom-up and top-down. Part I will discuss the self-assembly of photonic crystals, a unique class of periodic nanostructured materials featuring resonant optical response. Chapter 1 will introduce the concepts of self-assembly, specifically in the context of colloidal crystals and block copolymer nanostructures. Chapter 2 summarizes many years of work towards the goal of utilizing brush block copolymers as paintable photonic crystals. We employed 2D colloidal crystals as resonant light-trapping elements to improve the performance of thin-film solar cells; this work is described in Chapter 3. Part II of the thesis is centered around the concept of functional lithography: the ability to directly pattern materials with tailored physical properties and chemically active interfaces. We will briefly provide a history of photolithography in Chapter 4, and outline some of the limitations of the incumbent lithographic methods. The discovery of latent reactivity in ruthenium vinyl ether complexes, and the subsequent development of PhotoLithographic Olefin Metathesis Polymerization (PLOMP), are discussed in Chapter 5. This discovery has since blossomed into a true platform technology. We will discuss improvements to the functional group tolerance of PLOMP, as well as a few of our efforts to use PLOMP towards specific applications in Chapter 6. In the final Chapter 7 we document our attempts to activate PLOMP resists via multiphoton absorption, towards 3D printing of chemically functional microstructures
Enhanced Thermoelectric Performance at the Superionic Phase Transitions of Mixed Ion-Electron Conducting Materials
Get PDFThe quality of a thermoelectric material is judged by the size of its temperature de- pendent thermoeletric-figure-of-merit (zT ). Superionic materials, particularly Zn4Sb3 and Cu2Se, are of current interest for the high zT and low thermal conductivity of their disordered, superionic phase. In this work it is reported that the super-ionic materials Ag2Se, Cu2Se and Cu1.97Ag0.03Se show enhanced zT in their ordered, normal ion-conducting phases. The zT of Ag2Se is increased by 30% in its ordered phase as compared to its disordered phase, as measured just below and above its first order phase transition. The zT ’s of Cu2Se and Cu1.97Ag0.03Se both increase by more than 100% over a 30 K temperatures range just below their super-ionic phase transitions. The peak zT of Cu2Se is 0.7 at 406 K and of Cu1.97Ag0.03Se is 1.0 at 400 K. In all three materials these enhancements are due to anomalous increases in their Seebeck coefficients, beyond that predicted by carrier concentration measurements and band structure modeling. As the Seebeck coefficient is the entropy transported per carrier, this suggests that there is an additional quantity of entropy co-transported with charge carriers. Such co-transport has been previously observed via co-transport of vibrational entropy in bipolaron conductors and spin-state entropy in NaxCo2O4. The correlation of the temperature profile of the increases in each material with the nature of their phase transitions indicates that the entropy is associated with the thermodynamcis of ion-ordering. This suggests a new mechanism by which high thermoelectric performance may be understood and engineered
Structural and Clumped-Isotope Constraints on the Mechanisms of Displacement Along Low-Angle Detachments
Get PDFDespite years of research on low-angle detachments, much about them remains enigmatic. This thesis addresses some of the uncertainty regarding two particular detachments, the Mormon Peak detachment in Nevada and the Heart Mountain detachment in Wyoming and Montana.
Constraints on the geometry and kinematics of emplacement of the Mormon Peak detachment are provided by detailed geologic mapping of the Meadow Valley Mountains, along with an analysis of structural data within the allochthon in the Mormon Mountains. Identifiable structures well suited to constrain the kinematics of the detachment include a newly mapped, Sevier-age monoclinal flexure in the hanging wall of the detachment. This flexure, including the syncline at its base and the anticline at its top, can be readily matched to the base and top of the frontal Sevier thrust ramp, which is exposed in the footwall of the detachment to the east in the Mormon Mountains and Tule Springs Hills. The ~12 km of offset of these structural markers precludes the radial sliding hypothesis for emplacement of the allochthon.
The role of fluids in the slip along faults is a widely investigated topic, but the use of carbonate clumped-isotope thermometry to investigate these fluids is new. Faults rocks from within ~1 m of the Mormon Peak detachment, including veins, breccias, gouges, and host rocks, were analyzed for carbon, oxygen, and clumped-isotope measurements. The data indicate that much of the carbonate breccia and gouge material along the detachment is comminuted host rock, as expected. Measurements in vein material indicate that the fluid system is dominated by meteoric water, whose temperature indicates circulation to substantial depths (c. 4 km) in the upper crust near the fault zone.
Slip along the subhorizontal Heart Mountain detachment is particularly enigmatic, and many different mechanisms for failure have been proposed, predominantly involving catastrophic failure. Textural evidence of multiple slip events is abundant, and include multiple brecciation events and cross-cutting clastic dikes. Footwall deformation is observed in numerous exposures of the detachment. Stylolitic surfaces and alteration textures within and around “banded grains” previously interpreted to be an indicator of high-temperature fluidization along the fault suggest their formation instead via low-temperature dissolution and alteration processes. There is abundant textural evidence of the significant role of fluids along the detachment via pressure solution. The process of pressure solution creep may be responsible for enabling multiple slip events on the low-angle detachment, via a local rotation of the stress field.
Clumped-isotope thermometry of fault rocks associated with the Heart Mountain detachment indicates that despite its location on the flanks of a volcano that was active during slip, the majority of carbonate along the Heart Mountain detachment does not record significant heating above ambient temperatures (c. 40-70°C). Instead, cold meteoric fluids infiltrated the detachment breccia, and carbonate precipitated under ambient temperatures controlled by structural depth. Locally, fault gouge does preserve hot temperatures (>200°C), as is observed in both the Mormon Peak detachment and Heart Mountain detachment areas. Samples with very hot temperatures attributable to frictional shear heating are present but rare. They appear to be best preserved in hanging wall structures related to the detachment, rather than along the main detachment.
Evidence is presented for the prevalence of relatively cold, meteoric fluids along both shallow crustal detachments studied, and for protracted histories of slip along both detachments. Frictional heating is evident from both areas, but is a minor component of the preserved fault rock record. Pressure solution is evident, and might play a role in initiating slip on the Heart Mountain fault, and possibly other low-angle detachments.</p
Reconstructing Past Depositional and Diagenetic Processes through Quantitative Stratigraphic Analysis of the Martian Sedimentary Rock Record
Get PDFHigh-resolution orbital and in situ observations acquired of the Martian surface during the past two decades provide the opportunity to study the rock record of Mars at an unprecedented level of detail. This dissertation consists of four studies whose common goal is to establish new standards for the quantitative analysis of visible and near-infrared data from the surface of Mars. Through the compilation of global image inventories, application of stratigraphic and sedimentologic statistical methods, and use of laboratory analogs, this dissertation provides insight into the history of past depositional and diagenetic processes on Mars. The first study presents a global inventory of stratified deposits observed in images from the High Resolution Image Science Experiment (HiRISE) camera on-board the Mars Reconnaissance Orbiter. This work uses the widespread coverage of high-resolution orbital images to make global-scale observations about the processes controlling sediment transport and deposition on Mars. The next chapter presents a study of bed thickness distributions in Martian sedimentary deposits, showing how statistical methods can be used to establish quantitative criteria for evaluating the depositional history of stratified deposits observed in orbital images. The third study tests the ability of spectral mixing models to obtain quantitative mineral abundances from near-infrared reflectance spectra of clay and sulfate mixtures in the laboratory for application to the analysis of orbital spectra of sedimentary deposits on Mars. The final study employs a statistical analysis of the size, shape, and distribution of nodules observed by the Mars Science Laboratory Curiosity rover team in the Sheepbed mudstone at Yellowknife Bay in Gale crater. This analysis is used to evaluate hypotheses for nodule formation and to gain insight into the diagenetic history of an ancient habitable environment on Mars.</p
Direct Simulation of Proton-Coupled Electron Transfer Reaction Dynamics and Mechanisms
Get PDFProton-coupled electron transfer (PCET) reactions are ubiquitous throughout chemistry and biology. However, challenges arise in both the the experimental and theoretical investigation of PCET reactions; the rare-event nature of the reactions and the coupling between quantum mechanical electron- and proton-transfer with the slower classical dynamics of the surrounding environment necessitates the development of robust simulation methodology. In the following dissertation, novel path-integral based methods are developed and employed for the direct simulation of the reaction dynamics and mechanisms of condensed-phase PCET
Neural and Computational Representations of Decision Variables
Get PDFThese studies explore how, where, and when representations of variables critical to decision-making are represented in the brain. In order to produce a decision, humans must first determine the relevant stimuli, actions, and possible outcomes before applying an algorithm that will select an action from those available. When choosing amongst alternative stimuli, the framework of value-based decision-making proposes that values are assigned to the stimuli and that these values are then compared in an abstract “value space” in order to produce a decision. Despite much progress, in particular regarding the pinpointing of ventromedial prefrontal cortex (vmPFC) as a region that encodes the value, many basic questions remain. In Chapter 2, I show that distributed BOLD signaling in vmPFC represents the value of stimuli under consideration in a manner that is independent of the type of stimulus it is. Thus the open question of whether value is represented in abstraction, a key tenet of value-based decision-making, is confirmed. However, I also show that stimulus-dependent value representations are also present in the brain during decision-making and suggest a potential neural pathway for stimulus-to-value transformations that integrates these two results.
More broadly speaking, there is both neural and behavioral evidence that two distinct control systems are at work during action selection. These two systems compose the “goal-directed system”, which selects actions based on an internal model of the environment, and the “habitual” system, which generates responses based on antecedent stimuli only. Computational characterizations of these two systems imply that they have different informational requirements in terms of input stimuli, actions, and possible outcomes. Associative learning theory predicts that the habitual system should utilize stimulus and action information only, while goal-directed behavior requires that outcomes as well as stimuli and actions be processed. In Chapter 3, I test whether areas of the brain hypothesized to be involved in habitual versus goal-directed control represent the corresponding theorized variables.
The question of whether one or both of these neural systems drives Pavlovian conditioning is less well-studied. Chapter 4 describes an experiment in which subjects were scanned while engaged in a Pavlovian task with a simple non-trivial structure. After comparing a variety of model-based and model-free learning algorithms (thought to underpin goal-directed and habitual decision-making, respectively), it was found that subjects’ reaction times were better explained by a model-based system. In addition, neural signaling of precision, a variable based on a representation of a world model, was found in the amygdala. These data indicate that the influence of model-based representations of the environment can extend even to the most basic learning processes.
Knowledge of the state of hidden variables in an environment is required for optimal inference regarding the abstract decision structure of a given environment and therefore can be crucial to decision-making in a wide range of situations. Inferring the state of an abstract variable requires the generation and manipulation of an internal representation of beliefs over the values of the hidden variable. In Chapter 5, I describe behavioral and neural results regarding the learning strategies employed by human subjects in a hierarchical state-estimation task. In particular, a comprehensive model fit and comparison process pointed to the use of "belief thresholding". This implies that subjects tended to eliminate low-probability hypotheses regarding the state of the environment from their internal model and ceased to update the corresponding variables. Thus, in concert with incremental Bayesian learning, humans explicitly manipulate their internal model of the generative process during hierarchical inference consistent with a serial hypothesis testing strategy.</p
Strength, Deformation and Fracture in Metallic Nanostructures
Get PDFAn understanding of the mechanics of nanoscale metals and semiconductors is necessary for the safe and prolonged operation of nanostructured devices from transistors to nanowire- based solar cells to miniaturized electrodes. This is a fascinating but challenging pursuit because mechanical properties that are size-invariant in conventional materials, such as strength, ductility and fracture behavior, can depend critically on sample size when materials are reduced to sub- micron dimensions. In this thesis, the effect of nanoscale sample size, microstructure and structural geometry on mechanical strength, deformation and fracture are explored for several classes of solid materials. Nanocrystalline platinum nano-cylinders with diameters of 60 nm to 1 μm and 12 nm sized grains are fabricated and tested in compression. We find that nano-sized metals containing few grains weaken as sample diameter is reduced relative to grain size due to a change from deformation governed by internal grains to surface grain governed deformation. Fracture at the nanoscale is explored by performing in-situ SEM tension tests on nanocrystalline platinum and amorphous, metallic glass nano-cylinders containing purposely introduced structural flaws. It is found that failure location, mechanism and strength are determined by the stress concentration with the highest local stress whether this is at the structural flaw or a microstructural feature. Principles of nano-mechanics are used to design and test mechanically robust hierarchical nanostructures with structural and electrochemical applications. 2-photon lithography and electroplating are used to fabricate 3D solid Cu octet meso-lattices with micron- scale features that exhibit strength higher than that of bulk Cu. An in-situ SEM lithiation stage is developed and used to simultaneously examine morphological and electrochemical changes in Si-coated Cu meso-lattices that are of interest as high energy capacity electrodes for Li-ion batteries
Applications of a Concise and General Strategy for the Syntheses of Transtaganolide and Basiliolide Natural Products
Get PDFHerein are described the total syntheses of all members of the transtaganolide and basiliolide natural product family. Utilitzation of an Ireland–Claisen rearrangement/Diels–Alder cycloaddition cascade (ICR/DA) allowed for rapid assembly of the transtaganolide and basiliolide oxabicyclo[2.2.2]octane core. This methodology is general and was applicable to all members of the natural product family.
A brief introduction outlines all the synthetic progress previously disclosed by Lee, Dudley, and Johansson. This also includes the initial syntheses of transtaganolides C and D, as well as basiliolide B and epi-basiliolide B accomplished by Stoltz in 2011. Lastly, we discuss our racemic synthesis of basililide C and epi-basiliolide C, which utilized an ICR/DA cascade to constuct the oxabicyclo[2.2.2]octane core and formal [5+2] annulation to form the ketene-acetal containing 7-membered C-ring.
Next, we describe a strategy for an asymmetric ICR/DA cascade, by incorporation of a chiral silane directing group. This allowed for enantioselective construction of the C8 all-carbon quaternary center formed in the Ireland–Claisen rearrangement. Furthermore, a single hydride reduction and subsequent translactonization of a C4 methylester bearing oxabicyclo[2.2.2]octane core demonstrated a viable strategy for the desired skeletal rearrangement to obtain pentacyclic transtaganolides A and B. Application of the asymmetric strategy culminated in the total syntheses of (–)-transtaganolide A, (+)-transtaganolide B, (+)-transtaganolide C, and (–)-transtaganolide D. Comparison of the optical rotation data of the synthetically derived transtaganolides to that from the isolated counterparts has overarching biosynthetic implications which are discussed.
Lastly, improvement to the formal [5+2] annulation strategy is described. Negishi cross-coupling of methoxyethynyl zinc chloride using a palladium Xantphos catalyst is optimized for iodo-cyclohexene. Application of this technology to an iodo-pyrone geranyl ester allowed for formation and isolation of the eneyne product. Hydration of the enenye product forms natural metabolite basiliopyrone. Furthermore, the eneyne product can undergo an ICR/DA cascade and form transtaganolides C and D in a single step from an achiral monocyclic precursor.</p