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Description
Fuel cells, particularly solid oxide fuel cells (SOFC), are important for the future of greener and more efficient energy sources. Although SOFCs have been in existence for over fifty years, they have not been deployed extensively because they need to be operated at a high temperature (∼1000 °C), are expensive, and have slow response to changes in energy demands. One important need for commercialization of SOFCs is a lowering of their operating temperature, which requires an electrolyte that can operate at lower temperatures. Doped ceria is one such candidate. For this dissertation work I have studied different types of doped ceria to understand the mechanism of oxygen vacancy diffusion through the bulk. Doped ceria is important because they have high ionic conductivities thus making them attractive candidates for the electrolytes of solid oxide fuel cells. In particular, I have studied how the ionic conductivities are improved in these doped materials by studying the oxygen-vacancy formations and migrations. In this dissertation I describe the application of density functional theory (DFT) and Kinetic Lattice Monte Carlo (KLMC) simulations to calculate the vacancy diffusion and ionic conductivities in doped ceria. The dopants used are praseodymium (Pr), gadolinium (Gd), and neodymium (Nd), all belonging to the lanthanide series. The activation energies for vacancy migration between different nearest neighbor (relative to the dopant) positions were calculated using the commercial DFT code VASP (Vienna Ab-initio Simulation Package). These activation energies were then used as inputs to the KLMC code that I co-developed. The KLMC code was run for different temperatures (673 K to 1073 K) and for different dopant concentrations (0 to 40%). These simulations have resulted in the prediction of dopant concentrations for maximum ionic conductivity at a given temperature.
ContributorsAnwar, Shahriar (Author) / Adams, James B (Thesis advisor) / Crozier, Peter (Committee member) / Krause, Stephen (Committee member) / Arizona State University (Publisher)
Created2011
Description
There is an inexorable link between structure and stress, both of which require study in order to truly understand the physics of thin films. To further our knowledge of thin films, the relationship between structure and stress development was examined in three separate systems in vacuum. The first was continued copper thin film growth in ultra-high vacuum after adsorption of a sub-monolayer quantity of oxygen. Results showed an increase in compressive stress generation, and theory was proposed to explain the additional compressive stress within the films. The second system explored was the adsorption of carbon monoxide on the platinum {111} surface in vacuum. The experiments displayed a correlation between known structural developments in the adsorbed carbon monoxide adlayer and the surface stress state of the system. The third system consisted of the growth and annealing stresses of ice thin films at cryogenic temperatures in vacuum. It was shown that the growth stresses are clearly linked to known morphology development from literature, with crystalline ice developing compressive and amorphous ice developing tensile stresses respectively, and that amorphous ice films develop additional tensile stresses upon annealing.
ContributorsKennedy, Jordan (Author) / Friesen, Cody (Thesis advisor) / Sieradzki, Karl (Committee member) / Crozier, Peter (Committee member) / Arizona State University (Publisher)
Created2011
Description
ABSTRACT The behavior of the fission products, as they are released from fission events during nuclear reaction, plays an important role in nuclear fuel performance. Fission product release can occur through grain boundary (GB) at low burnups; therefore, this study simulates the mass transport of fission gases in a 2-D GB network to look into the effects of GB characteristics on this phenomenon, with emphasis on conditions that can lead to percolation. A finite element model was created based on the microstructure of a depleted UO2 sample characterized by Electron Backscattering Diffraction (EBSD). The GBs were categorized into high (D2), low (D1) and bulk diffusivity (Dbulk) based on their misorientation angles and Coincident Site Lattice (CSL) types. The simulation was run using different diffusivity ratios (D2/Dbulk) ranging from 1 to 10^8. The model was set up in three ways: constant temperature case, temperature gradient effects and window methods that mimic the environments in a Light Water Reactor (LWR). In general, the formation of percolation paths was observed at a ratio higher than 10^4 in the measured GB network, which had a 68% fraction of high diffusivity GBs. The presence of temperature gradient created an uneven concentration distribution and decreased the overall mass flux. Finally, radial temperature and fission gas concentration profiles were obtained for a fuel pellet in operation using an approximate 1-D model. The 100 µm long microstructurally explicit model was used to simulate, to the scale of a real UO2 pellet, the mass transport at different radial positions, with boundary conditions obtained from the profiles. Stronger percolation effects were observed at the intermediate and periphery position of the pellet. The results also showed that highest mass flux happens at the edge of a pellet at steady state to accommodate for the sharp concentration drop.
ContributorsLim, Harn Chyi (Author) / Peralta, Pedro (Thesis advisor) / Dey, Sandwip (Committee member) / Sieradzki, Karl (Committee member) / Arizona State University (Publisher)
Created2011
Description
Novel materials for Li-ion batteries is one of the principle thrust areas for current research in energy storage, more so than most, considering its widespread use in portable electronic gadgets and plug-in electric and hybrid cars. One of the major limiting factors in a Li-ion battery's energy density is the low specific capacities of the active materials in the electrodes. In the search for high-performance anode materials for Li-ion batteries, many alternatives to carbonaceous materials have been studied. Both cubic and amorphous silicon can reversibly alloy with lithium and have a theoretical capacity of 3500 mAh/g, making silicon a potential high density anode material. However, a large volume expansion of 300% occurs due to changes in the structure during lithium insertion, often leading to pulverization of the silicon. To this end, a class of silicon based cage compounds called clathrates are studied for electrochemical reactivity with lithium. Silicon-clathrates consist of silicon covalently bonded in cage structures comprised of face sharing Si20, Si24 and/or Si28 clusters with guest ions occupying the interstitial positions in the polyhedra. Prior to this, silicon clathrates have been studied primarily for their superconducting and thermoelectric properties. In this work, the synthesis and electrochemical characterization of two categories of silicon clathrates - Type-I silicon clathrate with aluminum framework substitution and barium guest ions (Ba8AlxSi46-x) and Type-II silicon clathrate with sodium guest ions (Nax Si136), are explored. The Type-I clathrate, Ba8AlxSi46-x consists of an open framework of aluminium and silicon, with barium (guest) atoms occupying the interstitial positions. X-ray diffraction studies have shown that a crystalline phase of clathrate is obtained from synthesis, which is powdered to a fine particle size to be used as the anode material in a Li-ion battery. Electrochemical measurements of these type of clathrates have shown that capacities comparable to graphite can be obtained for up to 10 cycles and lower capacities can be obtained for up to 20 cycles. Unlike bulk silicon, the clathrate structure does not undergo excessive volume change upon lithium intercalation, and therefore, the crystal structure is morphologically stable over many cycles. X-ray diffraction of the clathrate after cycling showed that crystallinity is intact, indicating that the clathrate does not collapse during reversible intercalation with lithium ions. Electrochemical potential spectroscopy obtained from the cycling data showed that there is an absence of formation of lithium-silicide, which is the product of lithium alloying with diamond cubic silicon. Type II silicon clathrate, NaxSi136, consists of silicon making up the framework structure and sodium (guest) atoms occupying the interstitial spaces. These clathrates showed very high capacities during their first intercalation cycle, in the range of 3,500 mAh/g, but then deteriorated during subsequent cycles. X-ray diffraction after one cycle showed the absence of clathrate phase and the presence of lithium-silicide, indicating the disintegration of clathrate structure. This could explain the silicon-like cycling behavior of Type II clathrates.
ContributorsRaghavan, Rahul (Author) / Chan, Candace K. (Thesis advisor) / Crozier, Peter (Committee member) / Petuskey, William T (Committee member) / Arizona State University (Publisher)
Created2013
Description
Dealloying induced stress corrosion cracking is particularly relevant in energy conversion systems (both nuclear and fossil fuel) as many failures in alloys such as austenitic stainless steel and nickel-based systems result directly from dealloying. This study provides evidence of the role of unstable dynamic fracture processes in dealloying induced stress-corrosion cracking of face-centered cubic alloys. Corrosion of such alloys often results in the formation of a brittle nanoporous layer which we hypothesize serves to nucleate a crack that owing to dynamic effects penetrates into the un-dealloyed parent phase alloy. Thus, since there is essentially a purely mechanical component of cracking, stress corrosion crack propagation rates can be significantly larger than that predicted from electrochemical parameters. The main objective of this work is to examine and test this hypothesis under conditions relevant to stress corrosion cracking. Silver-gold alloys serve as a model system for this study since hydrogen effects can be neglected on a thermodynamic basis, which allows us to focus on a single cracking mechanism. In order to study various aspects of this problem, the dynamic fracture properties of monolithic nanoporous gold (NPG) were examined in air and under electrochemical conditions relevant to stress corrosion cracking. The detailed processes associated with the crack injection phenomenon were also examined by forming dealloyed nanoporous layers of prescribed properties on un-dealloyed parent phase structures and measuring crack penetration distances. Dynamic fracture in monolithic NPG and in crack injection experiments was examined using high-speed (106 frames s-1) digital photography. The tunable set of experimental parameters included the NPG length scale (20-40 nm), thickness of the dealloyed layer (10-3000 nm) and the electrochemical potential (0.5-1.5 V). The results of crack injection experiments were characterized using the dual-beam focused ion beam/scanning electron microscopy. Together these tools allow us to very accurately examine the detailed structure and composition of dealloyed grain boundaries and compare crack injection distances to the depth of dealloying. The results of this work should provide a basis for new mathematical modeling of dealloying induced stress corrosion cracking while providing a sound physical basis for the design of new alloys that may not be susceptible to this form of cracking. Additionally, the obtained results should be of broad interest to researchers interested in the fracture properties of nano-structured materials. The findings will open up new avenues of research apart from any implications the study may have for stress corrosion cracking.
ContributorsSun, Shaofeng (Author) / Sieradzki, Karl (Thesis advisor) / Jiang, Hanqing (Committee member) / Peralta, Pedro (Committee member) / Arizona State University (Publisher)
Created2012
Description
The focus of this investigation includes three aspects. First, the development of nonlinear reduced order modeling techniques for the prediction of the response of complex structures exhibiting "large" deformations, i.e. a geometrically nonlinear behavior, and modeled within a commercial finite element code. The present investigation builds on a general methodology, successfully validated in recent years on simpler panel structures, by developing a novel identification strategy of the reduced order model parameters, that enables the consideration of the large number of modes needed for complex structures, and by extending an automatic strategy for the selection of the basis functions used to represent accurately the displacement field. These novel developments are successfully validated on the nonlinear static and dynamic responses of a 9-bay panel structure modeled within Nastran. In addition, a multi-scale approach based on Component Mode Synthesis methods is explored. Second, an assessment of the predictive capabilities of nonlinear reduced order models for the prediction of the large displacement and stress fields of panels that have a geometric discontinuity; a flat panel with a notch was used for this assessment. It is demonstrated that the reduced order models of both virgin and notched panels provide a close match of the displacement field obtained from full finite element analyses of the notched panel for moderately large static and dynamic responses. In regards to stresses, it is found that the notched panel reduced order model leads to a close prediction of the stress distribution obtained on the notched panel as computed by the finite element model. Two enrichment techniques, based on superposition of the notch effects on the virgin panel stress field, are proposed to permit a close prediction of the stress distribution of the notched panel from the reduced order model of the virgin one. A very good prediction of the full finite element results is achieved with both enrichments for static and dynamic responses. Finally, computational challenges associated with the solution of the reduced order model equations are discussed. Two alternatives to reduce the computational time for the solution of these problems are explored.
ContributorsPerez, Ricardo Angel (Author) / Mignolet, Marc (Thesis advisor) / Oswald, Jay (Committee member) / Spottswood, Stephen (Committee member) / Peralta, Pedro (Committee member) / Jiang, Hanqing (Committee member) / Arizona State University (Publisher)
Created2012
Description
There has been much interest in photoelectrochemical conversion of solar energy in recent years due to its potential for low-–cost, sustainable and renewable production of fuels. Despite the huge potential, there are still a number of technical barriers due to the many constraints needed in order to drive photoelectrochemical reactions such as overall water splitting and the identification of efficient and effective semiconductor materials. To this end, the search for novel semiconductors that can act as light absorbers is still needed. The copper hydroxyphosphate mineral libethenite (CHP), which has a chemical formula of Cu2(OH)PO4, has been recently shown to be active for photocatalytic degradation of methylene blue under UV-–irradiation, indicating that photo-excited electrons and holes can effectively be generated and separated in this material. However, CHP has not been well studied and many of its fundamental electrochemical and photoelectrochemical properties are still unknown. In this work, the synthesis of different morphologies of CHP using hydrothermal synthesis and precipitation methods were explored. Additionally, a preliminary investigation of the relevant fundamental characteristics such as the bandgap, flatband potential, band diagram, electrochemical and photoelectrochemical properties for CHP was performed. Better understanding of the properties of this material may lead to the development of improved catalysts and photocatalysts from natural sources.
ContributorsLi, Man (Author) / Chan, Candace K. (Thesis advisor) / O'Connell, Michael (Committee member) / Crozier, Peter (Committee member) / Arizona State University (Publisher)
Created2013
Description
This work focuses on simulation of electrical resistivity and optical behaviors of thin films, where an Ag or Au thin layer is embedded in zinc oxide. Enhanced conductivity and transparency were earlier achieved with multilayer structured transparent conducting oxide (TCO) sandwich layer with metal (TCO/metal/TCO). Sputtering pattern of metal layer is simulated to obtain the morphology, covered area fraction, and the percolation strength. The resistivity as a function of the metal layer thickness fits the modeled trend of covered area fraction beyond the percolation threshold. This result not only presents the robustness of the simulation, but also demonstrates the influence of metal morphology in multilayer structure. Effective medium coefficients are defined from the coverage and percolation strength to obtain simulated optical transmittance which matches experimental observation. The coherence of resistivity and optical transmittance validates the simulation of the sputtered pattern and the incorporation of percolation theory in the model.
ContributorsFang, Chia-Ling (Author) / Alford, Terry L. (Thesis advisor) / Crozier, Peter (Committee member) / Theodore, David (Committee member) / Arizona State University (Publisher)
Created2012
Description
The electrochemical behavior of nanoscale solids has become an important topic to applications, such as catalysis, sensing, and nano–electronic devices. The electrochemical behavior of elemental metal and alloy particles was studied in this work both theoretically and experimentally. A systematic thermodynamic derivation for the size–dependent Pourbaix Diagram for elemental metal particles is presented. The stability of Pt particles was studied by in situ electrochemical scanning tunneling microscopy (ECSTM). It is shown that small Pt particles dissolve at a lower potential than the corresponding bulk material. For the alloy particles, two size ranges of AuAg particles, ∼4 nm and ∼45 nm in diameter, were synthesized by co–reduction of the salts of Au and Ag from an aqueous phase. The alloy particles were dealloyed at a series of potential by chronoamperometry in acid, and the resulting morphology and composition were characterized by electron microscopy, energy dispersive X–ray spectroscopy (EDX). In the case of the smaller particles, only surface dealloying occurred yielding a core–shell structure. A porous structure was observed for the larger particles when the potential was larger than a critical value that was within 50 mV of the thermodynamic prediction.
ContributorsLi, Xiaoqian (Author) / Sieradzki, Karl (Thesis advisor) / Crozier, Peter (Committee member) / Buttry, Daniel (Committee member) / Friesen, Cody (Committee member) / Arizona State University (Publisher)
Created2012
Description
As world energy demands increase, research into more efficient energy production methods has become imperative. Heterogeneous catalysis and nanoscience are used to promote chemical transformations important for energy production. These concepts are important in solid oxide fuel cells (SOFCs) which have attracted attention because of their potential to provide an efficient and environmentally favorable power generation system. The SOFC is also fuel-flexible with the ability to run directly on many fuels other than hydrogen. Internal fuel reforming directly in the anode of the SOFC would greatly reduce the cost and complexity of the device. Methane is the simplest hydrocarbon and a main component in natural gas, making it useful when testing catalysts on the laboratory scale. Nickel (Ni) and gadolinium (Gd) doped ceria (CeO2) catalysts for potential use in the SOFC anode were synthesized with a spray drying method and tested for catalytic performance using partial oxidation of methane and steam reforming. The relationships between catalytic performance and structure were then investigated using X-ray diffraction, transmission electron microscopy, and environmental transmission electron microscopy. The possibility of solid solutions, segregated phases, and surface layers of Ni were explored. Results for a 10 at.% Ni in CeO2 catalyst reveal a poor catalytic behavior while a 20 at.% Ni in CeO2 catalyst is shown to have superior activity. The inclusion of both 10 at.% Gd and 10 at.% Ni in CeO2 enhances the catalytic performance. Analysis of the presence of Ni in all 3 samples reveals Ni heterogeneity and little evidence for extensive solid solution doping. Ni is found in small domains throughout CeO2 particles. In the 20 at.% Ni sample a segregated, catalytically active NiO phase is observed. Overall, it is found that significant interaction between Ni and CeO2 occurs that could affect the synthesis and functionality of the SOFC anode.
ContributorsCavendish, Rio (Author) / Crozier, Peter (Thesis advisor) / Adams, James (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2012