A solid oxide fuel cell (SOFC) is an energy conversion device that has the potential to efficiently generate electricity in an environmentally-friendly manner. In general, a SOFC operates between 750&deg;C and 1000&deg;C utilizing hydrogen or hydrocarbons as fuel and air as an oxidant. The three major components comprising a fuel cell are the electrolyte, the cathode, and the anode. At present, the state-of-the-art SOFC is made from a dense yttria-stabilized zirconia (YSZ) electrolyte, a porous lanthanum manganite cathode, and a porous nickel/YSZ composite anode. With the advent of the anode-supported SOFC and the increased interest in using a wider range of fuels, such as those containing sulphur, knowledge of the anode properties is becoming more important. <br /> The properties of the current anodes are limited due to the narrow range of nickel loadings imposed by the minimum nickel content for electrical conductivity and the maximum allowable nickel loading to avoid thermal mismatch with the YSZ electrolyte. In addition, there is little research presented in the literature regarding the use of nickel metal as a starting anode material, rather than the traditional nickel oxide powder, and how porosity may affect the anode properties. <br /> The purpose of this investigation is to determine the influence nickel morphology and porosity distribution have on the processing and properties of tape cast Ni/YSZ composites. Specifically, the sintering characteristics, electrical conductivity, and thermal expansion behaviour of tape cast composites created from YSZ, nickel, nickel oxide (NiO), nickel coated graphite (NiGr), and/or graphite (Gr) powders are investigated. In addition to samples made from 100% YSZ, 100% Ni, and 100% NiO powders, five composite types were created for this investigation: NiO/YSZ, NiO&Gr/YSZ, Ni/YSZ, NiGr/YSZ, and Ni&Gr/YSZ each with nickel loadings varying between 4 vol% Ni of total solids and 77 vol% Ni of total solids. Another set of composites with a fixed nickel loading of 27 vol% Ni and 47 vol% Ni of total solids and varying graphite loadings were also created. <br /> During the burnout stage, the composites made from nickel oxide powder shrink slightly while the composites made from nickel metal expand due to nickel oxidation. Graphite additions below 20 vol% of the green volume do not alter the dimensional changes of the composites during burnout, but graphite loadings greater than 25 vol% of the green volume cause significant expansion in the thickness of the composites. <br /> After sintering, the amount of volumetric sintering shrinkage decreases with higher nickel loadings and is greater for the composites made with nickel oxide compared to the composites made from nickel metal. The porosity generated in the composites containing graphite is slightly higher than the volume of graphite added to the composite and is much greater than the porosity contained in the graphite-free composites. <br /> Dimensional changes of the porous Ni/YSZ and NiO/YSZ composites during both burnout and sintering were analysed based on concepts of constrained sintering of composite powder mixtures. In some cases constrained sintering was evident, while in others, a more simple rule of mixtures behaviour for shrinkage as a function of YSZ content was observed. <br /> When nickel oxide is reduced to nickel metal during the reduction stage there is essentially no change in the composite volume for the composites containing YSZ because the YSZ prevents the composites from shrinking. After reduction the additional porosity generated in the composites is equivalent to the change in volume due to the reduction of nickel oxide to nickel metal. <br /> When measuring the electrical conductivity, each composite type demonstrated classic percolation behaviour. The NiGr/YSZ composites had the lowest percolation threshold, followed by the Ni/YSZ and NiO/YSZ composites. When graphite was added with a nickel coating, the added porosity did not disrupt the nickel percolation network and allowed the nickel to occupy a larger effective volume compared to a composite made with similar sized solid nickel particles. When graphite was added to the composites, the electrical conductivity was reduced and the percolation threshold increased. <br /> Generally, the coefficient of thermal expansion (CTE) for Ni/YSZ composites are expected to follow the rule of mixtures prediction since the elastic properties for nickel and YSZ are similar. However when porosity is distributed unevenly between the YSZ and nickel phases, the CTE prediction will deviate from the rule of mixtures. When cornstarch was added to the NiGr/YSZ composites, the CTE increased as the amount of porosity in the YSZ phase increased. The CTE of the NiGr/YSZ composites followed the rule of mixtures indicating that the porosity was evenly distributed between the nickel and YSZ phases. For the other composite types, the measured CTE was higher than the rule of mixtures prediction suggesting that more porosity was contained within the YSZ phase.

A solid oxide fuel cell (SOFC) is an energy conversion device that has the potential to efficiently generate electricity in an environmentally-friendly manner. In general, a SOFC operates between 750&deg;C and 1000&deg;C utilizing hydrogen or hydrocarbons as fuel and air as an oxidant. The three major components comprising a fuel cell are the electrolyte, the cathode, and the anode. At present, the state-of-the-art SOFC is made from a dense yttria-stabilized zirconia (YSZ) electrolyte, a porous lanthanum manganite cathode, and a porous nickel/YSZ composite anode. With the advent of the anode-supported SOFC and the increased interest in using a wider range of fuels, such as those containing sulphur, knowledge of the anode properties is becoming more important. <br /> The properties of the current anodes are limited due to the narrow range of nickel loadings imposed by the minimum nickel content for electrical conductivity and the maximum allowable nickel loading to avoid thermal mismatch with the YSZ electrolyte. In addition, there is little research presented in the literature regarding the use of nickel metal as a starting anode material, rather than the traditional nickel oxide powder, and how porosity may affect the anode properties. <br /> The purpose of this investigation is to determine the influence nickel morphology and porosity distribution have on the processing and properties of tape cast Ni/YSZ composites. Specifically, the sintering characteristics, electrical conductivity, and thermal expansion behaviour of tape cast composites created from YSZ, nickel, nickel oxide (NiO), nickel coated graphite (NiGr), and/or graphite (Gr) powders are investigated. In addition to samples made from 100% YSZ, 100% Ni, and 100% NiO powders, five composite types were created for this investigation: NiO/YSZ, NiO&Gr/YSZ, Ni/YSZ, NiGr/YSZ, and Ni&Gr/YSZ each with nickel loadings varying between 4 vol% Ni of total solids and 77 vol% Ni of total solids. Another set of composites with a fixed nickel loading of 27 vol% Ni and 47 vol% Ni of total solids and varying graphite loadings were also created. <br /> During the burnout stage, the composites made from nickel oxide powder shrink slightly while the composites made from nickel metal expand due to nickel oxidation. Graphite additions below 20 vol% of the green volume do not alter the dimensional changes of the composites during burnout, but graphite loadings greater than 25 vol% of the green volume cause significant expansion in the thickness of the composites. <br /> After sintering, the amount of volumetric sintering shrinkage decreases with higher nickel loadings and is greater for the composites made with nickel oxide compared to the composites made from nickel metal. The porosity generated in the composites containing graphite is slightly higher than the volume of graphite added to the composite and is much greater than the porosity contained in the graphite-free composites. <br /> Dimensional changes of the porous Ni/YSZ and NiO/YSZ composites during both burnout and sintering were analysed based on concepts of constrained sintering of composite powder mixtures. In some cases constrained sintering was evident, while in others, a more simple rule of mixtures behaviour for shrinkage as a function of YSZ content was observed. <br /> When nickel oxide is reduced to nickel metal during the reduction stage there is essentially no change in the composite volume for the composites containing YSZ because the YSZ prevents the composites from shrinking. After reduction the additional porosity generated in the composites is equivalent to the change in volume due to the reduction of nickel oxide to nickel metal. <br /> When measuring the electrical conductivity, each composite type demonstrated classic percolation behaviour. The NiGr/YSZ composites had the lowest percolation threshold, followed by the Ni/YSZ and NiO/YSZ composites. When graphite was added with a nickel coating, the added porosity did not disrupt the nickel percolation network and allowed the nickel to occupy a larger effective volume compared to a composite made with similar sized solid nickel particles. When graphite was added to the composites, the electrical conductivity was reduced and the percolation threshold increased. <br /> Generally, the coefficient of thermal expansion (CTE) for Ni/YSZ composites are expected to follow the rule of mixtures prediction since the elastic properties for nickel and YSZ are similar. However when porosity is distributed unevenly between the YSZ and nickel phases, the CTE prediction will deviate from the rule of mixtures. When cornstarch was added to the NiGr/YSZ composites, the CTE increased as the amount of porosity in the YSZ phase increased. The CTE of the NiGr/YSZ composites followed the rule of mixtures indicating that the porosity was evenly distributed between the nickel and YSZ phases. For the other composite types, the measured CTE was higher than the rule of mixtures prediction suggesting that more porosity was contained within the YSZ phase.

This research characterizes the thermal stress resulting from temperature gradients in hybrid solid oxide fuel cells that are processed using a novel oxide powder slurry technology developed at Georgia Tech. The hybrid solid oxide fuel cell is composed of metallic interconnect and ceramic electrolyte constituents with integral mechanical bonds formed during high temperature processing steps. A combined thermo-mechanical analysis approach must be implemented to evaluate a range of designs for power output and structural integrity. As an alternative to costly CFD analysis, approximate finite difference techniques that are more useful in preliminary design are developed to analyze the temperature distributions resulting from a range of fuel cell geometries and materials. The corresponding thermal stresses are then calculated from the temperature fields using ABAQUS. This model analyzes the manufacturing, start-up, and steady state operating conditions of the hybrid solid oxide fuel cell.

Interfacial polarization resistances limit the performance of many thin film solid-state devices, especially at low temperatures. To improve performance, a fundamental understanding of the electrode kinetics that govern interfacial reaction rates must be developed. The goal of this work is to determine site-specific reaction mechanisms and the relative significance of various reactions in order to quantify optimum structural parameters within the cathode microstructure. Key parameters include the length of triple phase boundary (TPB), the quantity of exposed electrolyte/electrode surface, and the ratio of electrolyte to electrode material. These parameters, when studied in a specific system, can be incorporated into broader models, which will encompass the specific conductivity of each component to develop an optimized three-dimensional network. The emphasis of this work is the systematic control and manipulation of potential cathodic reaction sites in order to develop an understanding of the relative importance of specific reaction sites. Since the physical dimensions of reaction sites are relatively small, an approach has been developed that utilizes micro-fabrication (similar to that used in integrated-circuit fabrication) to produce small and highly controlled microstructures. Investigations were made into the nature and reactivity of Triple Phase Boundaries (hereafter TPB) through the use of patterned platinum electrodes since only the TPBs are active in these electrodes. After the processing details of micro-fabrication were established for the platinum electrodes, patterned Mixed-Ionic/Electronic Conducting (MIEC) electrodes were fabricated and studied using impedance spectroscopy to determine the contributions from the MIEC surface versus the TPB. Systematically changing the geometry of the MIEC electrodes (thickness and line width) allowed for the determination of the effect of ambipolar transport within the MIEC on the activity of MIEC surfaces versus the TPB. This information is critical to rational design of functionally graded electrodes (with optimal particle size, shape, porosity and conductivity). In addition to experimental studies, representative patterned electrode samples were made available for collaborative studies with surface scientists at other institutions to provide additional techniques (such as Raman Spectroscopy) on the carefully designed and controlled cathode surfaces.

In order to facilitate the application of solid oxide fuel cells, in conjunction with reduced research and development costs, there is a need for accurate performance models to aid scientists and engineers in component and process design. To this end, an enhanced transient performance model has been developed. The present thesis enhances transient modeling and simulation via characterization of two important transient phenomena. They are bimodal stimuli (i.e., simultaneous changes in reactant supply and load demand) electrical transients, inclusive of the simulation of electrolysis, and the electrochemical light off phenomenon. One key result of the electrochemical light off simulations was that the realization that electrochemical parameters such as cell potential may be used as dynamic control variables during transitional heating of the cell. Reflective of the state-of-the-art in controls and dynamic simulation development, the modeling efforts are completed in the MATLAB computing environment. There is then a tangible software development that accompanies the modeling and simulation exercises and transient insights resolved.

A program of study has been established for the quantitative study of electrode reactions in solid oxide fuel cells. The initial focus of the program is the mixed conducting cathode material strontium-doped lanthanum manganate (LSM). A formalism was established treating reactions taking place at the gas-exposed surface of mixed conducting electrodes. This formalism was incorporated into a phenomenological model for oxygen reduction in LSM, which treats the phenomenon of sheet resistance. Patterned electrodes were designed that reduce the dimensionality of the appropriate model, and these electrodes were successfully fabricated using DC sputtering and photolithography. A new model for the bulk defect equilibrium in LSM was proposed and shown to be a better fit to nonstoichiometry data at low temperatures. The fitting was carried out with a particle swarm optimizer and a rigorous method for identification. It was shown that a model for the interface structure between LSM and yttria-stabilized zirconia (YSZ) that assumes free oxygen vacancies in YSZ does not accord with experimental observations. Cluster variation method (CVM) was adapted for analysis of the problem, and a new analytical method combining CVM and electrical contributions to the free energy was proposed.

The purpose of this work is to extend the functionality of the Master Sintering Curve (MSC) such that it can be used as a practical tool for predicting sintering schemes that combine both a constant heating rate and an isothermal hold. Rather than just being able to predict a final density for the object of interest, the extension to the MSC will actually be able to model a sintering run from start to finish. Because the Johnson model does not incorporate this capability, the work presented is an extension of what has already been shown in literature to be a valuable resource in many sintering situations. A predicted sintering curve that incorporates a combination of constant heating rate and an isothermal hold is more indicative of what is found in real-life sintering operations. This research offers the possibility of predicting the sintering schedule for a material, thereby having advanced information about the extent of sintering, the time schedule for sintering, and the sintering temperature with a high degree of accuracy and repeatability. The research conducted in this thesis focuses on the development of a working model for predicting the sintering schedules of several stabilized zirconia powders having the compositions YSZ (HSY8), 10Sc1CeSZ, 10Sc1YSZ, and 11ScSZ1A. The compositions of the four powders are first verified using x-ray diffraction (XRD) and the particle size and surface area are verified using a particle size analyzer and BET analysis, respectively. The sintering studies were conducted on powder compacts using a double pushrod dilatometer. Density measurements are obtained both geometrically and using the Archimedes method. Each of the four powders is pressed into 1/4 inch diameter pellets using a manual press with no additives, such as a binder or lubricant. Using a double push-rod dilatometer, shrinkage data for the pellets is obtained over several different heating rates. The shrinkage data is then converted to reflect the change in relative density of the pellets based on the green density and the theoretical density of each of the compositions. The Master Sintering Curve (MSC) model is then utilized to generate data that can be utilized to predict the final density of the respective powder over a range of heating rates. The Elton Master Sintering Curve Extension (EMSCE) is developed to extend the functionality of the MSC tool. The parameters generated from the original MSC are used in tandem with the solution to a specific closed integral (discussed in document) over a set range of temperatures. The EMSCE is used to generate a set of sintering curves having both constant heating rate and isothermal hold portions. The EMSCE extends the usefulness of the MSC by allowing this generation of a complete sintering schedule rather than just being able to predict the final relative density of a given material. The EMSCE is verified by generating a set of curves having both constant heating rate and an isothermal hold for the heat-treatment. The modeled curves are verified experimentally and a comparison of the model and experimental results are given for a selected composition. Porosity within the final product can hinder the product from sintering to full density. It is shown that some of the compositions studied did not sinter to full density because of the presence of large porosity that could not be eliminated in a reasonable amount of time. A statistical analysis of the volume fraction of porosity is completed to show the significance of the presence in the final product. The reason this is relevant to the MSC is that the model does not take into account the presence of porosity and assumes that the samples sinter to full density. When this does not happen, the model actually under-predicts the final density of the material.

Electrodes in a solid oxide fuel cell (SOFC) must possess both adequate porosity and electronic conductivity to perform their functions in the cell. They must be porous to permit rapid mass transport of reactant and product gases and sufficiently conductive to permit efficient electron transfer. However, it is nearly impossible to simultaneously control porosity and conductivity using conventional design and fabrication techniques. In this dissertation, computational design and performance optimization of microarchitectured SOFCs is first investigated in order to achieve higher power density and thus higher efficiency than currently attainable in state-of-the-art SOFCs. This involves a coupled multiphysics simulation of mass transport, electrochemical charge transfer reaction, and current balance as a function of SOFC microarchitecture. Next, the fabrication of microarchitectured SOFCs consistent with the computational designs is addressed based on anode-supported SOFC button cells using the laser ablation technique. Finally, the performance of a fabricated SOFC unit cell is characterized and compared against the performance predicted by the computational model. The results show that the performance of microarchitectured SOFCs was improved against the baseline structure and measured experimental data were well matched to simulation results.

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A new reactor and a novel in-situ sampling technique were developed for the study of the synthesis of Ce02 powders produced from dissolved cerium nitrate salts. The reactor minimized particle recirculation and provided a highly symmetric and undisturbed plasma flow suitable for the analysis of the phenomena affecting the formation of Ce02 powders. The sampling probe provided in-situ sampling of in-flight CeCb particles and allowed continuous sampling without cross contamination. The sampled particles were collected using a wet collection system composed of a mist atomizer acting as a scrubber and a custom-made spray chamber. The entire collection system is portable and it was tested in the particle range of 20 nm to 100 jam. This information provided a picture of how Ce02 particles were formed. A numerical simulation of different plasma operating parameters using Fluent was presented. A comprehensive droplet-to-particle formation mechanism was deduced based on calorimetry. thermodynamics of Ce02 formation, numerical simulations and collected particles. The effect of adding water soluble fuels (alanine and glycine) to the original cerium nitrate solutions was investigated. Fuel addition decreased the temperature of CeC>2 formation by acting as a local heat source as a result of fuel auto-ignition. The addition of fuel caused “particle size discrimination’*, and a single mode particle size distribution centered between 50-140 nm was achieved along the centerline of the reactor. [...] / Un nouveau reacteur et une technique innovatrice d’echantillonnage in situ furent developpes pour etudier la synthese de poudres de Ce02 produites a partir de sels de nitrate de cerium dissous. Le reacteur minimise la recirculation des particules et fournit un plasma non perturbe et grandement symetrique approprie pour l’analyse des phenomenes affectant la formation de poudres de Ce02. De plus, une sonde permet un echantillonnage in situ et en vol des particules de Ce02 et ce, en continu et sans contamination croisee. Les particules ainsi captees sont recueillies grace a un systeme de collecte par voie humide qui est compose d’un atomiseur de brume (mist atomizer) qui joue le role d’un recureur (scrubber) et d’une chambre d’atomisation (spray chamber) maison. Le systeme d’echantillonnage est mobile et fut teste sur des particules ayant des tailles de 20 nm a 100 jam. C’est grace a l’analyse des particules ainsi recueillies que nous sommes en mesure de comprendre comment les particules de Ce02 sont formees. Nous presentons aussi une simulation numerique, effectuee avec le logiciel Fluent, qui utilise les differents parametres d’experimentation. Le mecanisme detaille de la formation des particules a partir de gouttelettes fut deduit grace a des etudes calorimetriques, a une etude thermodynamique de la formation du Ce02, a des simulations numeriques et a l’analyse des particules recueillies. Nous avons aussi etudie l’effet d’un ajout de combustibles hydrosolubles (l’alanine et la glycine) a la solution originale de nitrate de cerium. Cette addition de combustibles diminue la temperature de formation du Ce02 en agissant comme une source locale de chaleur resultant de 1’auto-ignition du combustible. Aussi, le combustible cause une « segregation des particules » selon leur taille.