Solid oxide fuel cells (SOFCs) are the most efficient electrochemical devices to directly convert stored chemical energy to usable electrical energy. The infiltration of ceramic conductors and catalytic metals (e.g. Ni, Pt and Pd) into porous scaffolds that had been pre-sintered onto the electrolyte is regarded as an effective way of promoting the electrode performance via producing nano-scale particles by in-situ sintering at relatively low temperatures. Large-scale fuel cells (5 cm x 5 cm) are prepared with this method and tested to demonstrate its scalability so as to achieve industrial applications. Four configurations are examined in respect of variation in the thickness of cathode, anode and electrolyte to investigate their effect on the infiltration process and electrochemical losses. To further improve infiltration as a method of fabricating high-performance electrodes, much effort is also devoted to optimising and understanding the microstructure of pre-sintered scaffold and its effect on infiltration using image analysis and electrochemical impedance. First, we have prepared the nano-structured electrodes on the 200-μm thick electrolyte-supported planar fuel cell with a 5 x 5 cm dimension. The 8YSZ scaffold is impregnated with La₀.₈Sr₀.₂Cr₀.₅Mn₀.₅O₃ (LSCM) for the anode and La₀.₈Sr₀.₂FeO₃ (LSF) for the cathode. The large planar cell achieved a maximum power density of 116 mWcm⁻² at 700°C and 223 mWcm⁻² at 800°C in humidified hydrogen. Moreover, with the addition of catalyst of 10 wt.% CeO₂ and 1 wt.% Pd, the cell performance reached 209 mWcm⁻² at 700°C and 406 mWcm⁻² at 800°C. Compared to the cell without catalysts, ceria and Pd are efficient in decreasing the electrochemical reaction resistance but making the diffusion resistance more obvious. Second, supported thin electrolytes are prepared by scalable tape casting to reduce the ohmic losses as that in electrolyte-supported cell. The cell with thick LSF-infiltrated support is very efficient in decreasing the ohmic loss thanks to the high solubility of its nitrate precursors in water and fairly high electric conductivity, but the thick cathode causes higher diffusional losses, especially at 800°C. Even though with thinner electrolyte, the ohmic loss from the cell with thick infiltrated anode is comparable to that of 200-μm electrolyte supported cell. The extra ohmic loss can be attributed to the compositional segregation of La₀.₇Sr₀.₃VO₃ (LSV) in the infiltration process in the anode, and lower loading, ca. 25 wt %. A trade-off between the diffusional loss from the cathode and the extra ohmic loss from the thick anode can be achieved by sandwiching the electrolyte between electrodes with identical thickness. A flat large area cell prepared with this method can achieve a high performance of 300 mW cm⁻² and 489 mW cm⁻² at 700°C and 800°C, respectively, if Pd-ceria is added to the anode LSV as catalyst. Third, image analyses and modelling are performed on the constrained sintering of porous thin film on a rigid substrate to study the evolution of pores at different stages. Result shows that both the anisotropy of the pore former/pores in the green body and transport of materials during the sintering process have effect on the orientation of the final microstructure. Specifically, the in-plane orientation of large-scale pores will be intensified during the constrained sintering process, while those small pores whose shape are subjected to materials transport during sintering tend to erect during the constrained sintering process at 1300°C. Fourth, image analyses and semi-quantification are used to predict the correlation between the microstructure and performance of the LSF-infiltrated electrode. Two types of YSZ powders, Unitec 1-μm powder with a broad particle-size distribution having two maxima at ~ 0.1 μm and 0.8 μm, and Unitec 2-μm powder with only one at ~1 μm are selected to fabricate the porous scaffold for infiltration. The porous structure using Unitec 2-μm powder shows finer YSZ grains and a higher boundary length than the 2-μm powder. Ac impedance on symmetrical cells was used to evaluate the performance of the electrode impregnated with 35-wt.% La₀.₈Sr₀.₂FeO₃. At 700°C, the electrode from Unitec 2-μm powder shows a polarization resistance (Rp) of 0.21 Ω cm², and series resistance (Rs) of 8.5 Ω cm², lower than the electrode from Unitec 1-μm powder does. The quantitative study on image indicates that Unitec 2-μm powder is better in producing architecture of high porosity or long triple phase boundary (TPB), which is attributed as the reason for the higher performance of the LSF-impregnated electrode. Finally, oxides of transition metals are doped into the YSZ-infiltrated LSF electrode and the impedances of symmetrical cells are tested to evaluate their effect on the ohmic and polarization resistance. Cobalt oxides are able to reduce the ohmic resistance and polarization resistance only when it is calcined at 700°C, but nickel oxide can reduce both the ohmic and polarization resistance if it is well-mixed and fully reacted with the previously infiltrated LSF. Doping of manganese oxide into LSF-YSZ electrode slightly changes the ohmic resistance but significantly increases the polarization resistance. Detailed analyses of the impact of infiltration process on the impedance data and oxygen reduction process are also presented.

This work was on the effect of electric current on the processing of ceramics. The focus was on electromigration/electrochemistry and plasma effects. While there is no solid evidence that there is plasma in Spark Plasma Sintering, (SPS), newer techniques e.g. flash, use different conditions so there is an interest in understanding the conditions under which a plasma forms. The minimum arcing voltage was found from literature to be from 10-15V for materials of interest. This is above that found in SPS (10V). However, due to the many contact points in a powder compact much higher voltages (50V) were required in practical experiments. Optical spectroscopy was used to verify the formation of a plasma, and emission peaks from the powder compact material were visible implying they were vaporised and formed the plasma. Electromigration was exploited to alter the oxidation of zirconium diboride, by passing current through the oxide layer (120μm zirconia base grown at 1200°C) oxygen could be pumped either away or toward the diboride bulk. Small cubes (3mm) of diboride had platinum foil electrodes applied on both sides and oxidation was performed at 1400°C for 5hr. Without a field the oxide grew to 360μm, by applying 10V and 100mA the oxide grew to 150μm under the +ve electrode but 1400μm under the -ve electrode. Electrochemical reduction was believed to have occurred due to the electrical properties of the material changing during oxidation and visible blackening of the oxide. Combining the techniques from both earlier works, a contactless flash sintering setup was developed. This used two plasma arcs as electrodes to heat and pass current through the sample. Various materials, currents and times were used, but the best result was with SiC:B4C which was sintered in 3s with 6A, the microstructure showed sharp grains, no segregation and limited grain growth ( initially 0.7μm SiC and 0.5μm B4C, this grew to 1.1μm and 1.4μm). This was the first recorded case of contactless flash sintering and the technique has the potential to sinter ceramics in a continuous manner.

Planar solid oxide fuel cells (SOFCs) were designed, fabricated and characterized in order to develop a (1) cost-effective method for fabrication of thin electrolyte layers, (2) hermetic sealing and (3) stable interconnects. Electrophoretic deposition (EPD) was discovered to be an excellent method for fabricating dense electrolyte layers of about 5m thick on porous non-conducting substrates. The EPD process was thoroughly studied from proof-of-concept to statistical reproducibility, deposition mechanism, modeling and process optimization. Deposition on non-conducting substrates was found to follow many of the same fundamental trends as that on conductive substrates except for the voltage efficiency and detailed charge transfer mechanism. Eventually, the process was optimized such that an SOFC was fabricated that achieved 1.1W/cm2 at 850C. Further, a novel sealless planar SOFC was designed that incorporates a hermetic interface between the electrolyte and interconnect similar to tubular and honeycomb designs. The hermetic interface successfully acted as a blocking electrode under DC polarization, indicating its potential to act as a sealant. Leakage rates across the interface were 0.027sccm at 750c, similar to polycrystalline mica seals. Through a process of tape casting and lamination, a two-cell stack without sealant was fabricated and achieved a power density of 75mW/cm2 at 750C. Finally, the degradation rate of silver and silver-based interconnects was studied under static and dual-atmosphere conditions. Corrosion of silver grain boundaries along with sublimation losses results in the formation of large pores, resulting in up to 30 of anode oxidation after 8hrs testing at 750c. Further stability studies indicated that silver-based interconnects would be better suited for applications at operating temperatures less than 650C.

Submitted by LIECA HASSEGAWA KAVASHIMA null (lie-ca@hotmail.com) on 2016-11-07T22:26:34Z No. of bitstreams: 1 Lieca Hassegawa Kavashima - Dissertação - Mestrado.pdf: 7864947 bytes, checksum: 6b3f02c88be52df0bcf622c6caeaba96 (MD5) / Approved for entry into archive by Felipe Augusto Arakaki (arakaki@reitoria.unesp.br) on 2016-11-10T16:51:24Z (GMT) No. of bitstreams: 1 kavashima_lh_me_bauru.pdf: 7864947 bytes, checksum: 6b3f02c88be52df0bcf622c6caeaba96 (MD5) / Made available in DSpace on 2016-11-10T16:51:24Z (GMT). No. of bitstreams: 1 kavashima_lh_me_bauru.pdf: 7864947 bytes, checksum: 6b3f02c88be52df0bcf622c6caeaba96 (MD5) Previous issue date: 2016-09-23 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A zircônia tetragonal policristalina estabilizada com ítria ou zircônia Y-TZP, atualmente é utilizada como biomaterial na odontologia, para a confecção de próteses por meio da tecnologia CAD/CAM, a partir de blocos comerciais pré-sinterizados. A cerâmica Y-TZP é biocompatível, bioinerte e apresenta estética e propriedades mecânicas adequadas à aplicação. Contudo, as características mecânicas podem variar de acordo com o método de fabricação podendo estar sujeita à falhas por fratura frágil causada por defeitos oriundos das etapas de conformação, pré-sinterização, usinagem da prótese ou sinterização. Dessa forma, o objetivo do presente trabalho foi avaliar o processamento cerâmico e determinar as propriedades mecânicas da zircônia Y-TZP. Os blocos foram conformados por prensagem uniaxial e isostática a 200 MPa, pré-sinterizados com diferentes parâmetros (900°C e 950°C) de temperatura, submetidos a ensaio de microdureza Vickers e usinados por CAD/CAM para confecção de próteses. O material pré-sinterizado foi cortado em barras, sinterizado e submetido aos ensaios mecânicos para a determinação da resistência à flexão, módulo elástico dinâmico e estático. Os resultados indicaram que o processo térmico de pré-sinterização com patamar a 950°C proporcionou material adequado ao manuseio e usinagem CAD/CAM com microdureza similar aos materiais disponíveis comercialmente. Os valores de módulo de elasticidade do material após a sinterização final, determinados pelos métodos dinâmicos e estático, entre 205,7 e 231,9 GPa, estão próximos aos relatados na literatura (entre 200 e 220 GPa). As análises estatísticas pelo método ANOVA (p>0,05) e testes de Tukey-Kramer demonstraram que o processo de pré-sinterização não exerceu influência na resistência mecânica após a sinterização final. Assim, o processo de fabricação estudado neste trabalho apresentou parâmetros que possibilitaram a confecção de próteses odontológicas, por meio da tecnologia CAD/CAM, obtendo materiais com propriedades mecânicas adequadas à confecção de copings odontológicos. / The polycrystalline tetragonal yttria stabilized zirconia or Y-TZP zirconia is currently used as a biomaterial in dentistry for the manufacture of prostheses by means of the CAD / CAM technology with pre-sintered commercial blocks. Y-TZP ceramic is biocompatible, bioinert and presents aesthetic features and appropriate mechanical properties for the application. However, the mechanical characteristics can vary according to the manufacturing method and may be subjected to failure by brittle fracture caused by defects originated from forming, pre-sintering, prosthesis machining or sintering. Thus, the goal of this work was to evaluate the ceramic processing and determine the mechanical properties of the Y-TZP zirconia. Blocks were formed by uniaxial and isostatic compression at 200 MPa, pre-sintered with different temperature parameters (900°C and 950°C), subjected to Vickers microhardness test and machined by CAD / CAM for prostheses manufacture. The pre-sintered material was cut into bars, sintered and submitted to mechanical tests in order to determine flexural strength, static and dynamic elastic modulus. Results showed that the pre-sintering thermal process with 950°C as a baseline provided a suitable material handling and machining CAD / CAM with a microhardness similar to the materials commercially available. The Young’s modulus values of the material after the final sintering, determined by the dynamic and static methods were found between 205.7 and 231.9 GPa are close to those reported in the literature (between 200 and 220 GPa). The statistical analyses via ANOVA (p> 0.05) method and Tukey-Kramer tests showed that the pre-sintering process had no influence on the mechanical resistance after the final sintering. Therefore, the manufacturing process studied in this work presented parameters that enabled the manufacture of dental prostheses by means of the CAD / CAM technology, obtaining materials with suitable mechanical properties to dental copings confection.

<p dir="ltr">Silicon carbide (SiC) is a ceramic with strength retention at elevated temperatures, oxidation resistance and a high thermal conductivity. These properties make SiC a desirable ceramic for compact, high temperature (> 1000 °C), heat exchangers with improved thermal performance. However, fabricating a SiC heat exchanger is difficult due to the low self-diffusion and high melting temperature of SiC. The aim of this dissertation is to show the viability of using co-extrusion and slip casting as low-cost, scalable processes for creating a compact, high temperature, SiC heat exchanger.</p><p dir="ltr">Co-extrusion is an advantageous fabrication technique as it is capable of producing samples with micron-sized features in two dimensions. To fabricate the heat exchanger body via co-extrusion, a SiC-filled polymer blend and a carbon black (CB)-filled polymer blend (sacrificial) were developed. A 54 vol% SiC-filled polymer blend with the addition of 12 wt% alumina and yttria, sintering aids, in a 2:1 ratio, respectively produced samples with the highest relative densities of 94% while maintaining an extrudable rheology. The SiC-filled polymer blend was co-extruded at 80 °C with a 45 vol% CB-filled polymer blend to produce unit cells that were open and continuous after binder burnout and sintering. The unit cells had an average relative density of 90% with an average strength of 165 MPa.</p><p dir="ltr">The unit cell strengths were lower than expected due to the formation of defects that occurred after removal of the polymers. These defects were categorized into macrodelaminations, defects that occur between two laminated unit cells, and microdelaminations, defects that occur within a single unit cell. The mechanisms causing these defects was studied by investigating the lamination and polymer removal processes. Results showed that poor lamination between extrudates mitigated the macrodelaminations and an oxygen-rich debinding atmosphere caused the formation of microdelaminations. Defect-free unit cells were produced though a partial extrusion step and binder removal in a nitrogen atmosphere.</p><p dir="ltr">An aqueous SiC suspension for slip casting was optimized by investigating the rheological properties, zeta potential, and slip casting behavior. It was determined that a suspension with 40 vol% solids, 1.2 wt% dispersant (polyethyleneimine), and a pH of 7.5 resulted in uniform slip cast parts. This optimized suspension was used to fabricate dense, crack-free SiC headers with an average relative density of 96% and an average strength of 266 MPa.</p><p dir="ltr">This dissertation gives insight into important fabrication parameters that must be considered when fabricating high temperature, SiC heat exchanger components. Additionally, this dissertation showcases the capability of using co-extrusion and slip casting as potential pathways for fabricating a high temperature, SiC heat exchanger.</p>

Next generation aerospace vehicles require thermal protection system (TPS) materials that are capable of withstanding the extreme aerothermal environment during hypersonic flight (>Mach 5 [>1700 m/s]). Ultra-high temperature ceramics (UHTC) such as zirconium diboride (ZrB₂) are candidate TPS materials due to their high-temperature thermal and mechanical properties and are often the basis for advanced composites for enhanced oxidation resistance. However, ZrB₂ matrix impurities in the form of boron trioxide (B₂O₃) and zirconium dioxide (ZrO₂) limit the high-temperature capabilities. Electric based sintering techniques, such as spark plasma sintering (SPS), that use joule heating have become the preferred densification method to process advanced ceramics due to its ability to produce high density parts with reduced densification times and limit grain growth. This study focuses on a combined experimental and thermodynamic assisted processing approach to enhance powder purity through a carbo- and borocarbo-thermal reduction of oxides using carbon (C) and boron carbide (B₄C). The amount of oxides on the powder surface are measured, the amount of additive required to remove oxides is calculated, and processing conditions (temperature, pressure, environment) are controlled to promote favorable thermodynamic reactions both during thermal processing in a tube furnace and SPS. Untreated ZrB₂ contains 0.18 wt%O after SPS. Additions of 0.75 wt%C is found to reduce powder surface oxides to 0.12 wt%O. A preliminary Zr-C-O computational thermodynamic model shows limited efficiency of carbon additions to completely remove oxygen due to the solubility of oxygen in zirconium carbide (ZrC) forming a zirconium oxycarbide (ZrCₓOᵧ). Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) with atomic scale elemental spectroscopy shows reduced oxygen content with amorphous Zr-B oxides and discreet ZrO₂ particle impurities in the microstructure. Processing ZrB₂ with minimal additions of B₄C (0.25 wt%) produces high purity parts after SPS with only 0.06 wt%O. STEM identifies unique “trash collector” oxides composed of manufacturer powder impurities of calcium, silver, and yttrium. A preliminary Zr-B-C-O thermodynamic model is used to show the potential reaction paths using B₄C that promotes oxide removal to produce high-purity ZrB₂ with fine grains (3.3 𝜇m) and superior mechanical properties (flexural strength of 660MPa) than the current state-of-the-art ZrB₂ ceramics. Due to the desirable properties produced using SPS, there is growing interest to advance processing techniques from lab-scale (20 mm discs) to large-scale (>100 mm). The advancement of SPS technologies has been stunted due to the limited power and load delivery of lab-scale furnaces. We use a large scale direct current sintering furnace (DCS) to address the challenges of producing industrially relevant sized parts. However, current-assisted sintering techniques, like SPS and DCS, are highly dependent on tooling resistances and the electrical conductivity of the sample, which influences the part uniformity through localized heating spots that are strongly dependent on the current flow path. We develop a coupled thermal-electrical finite element analysis model to investigate the development and effects of tooling and current density manipulation on an electrical conductor (ZrB₂) and an electrical insulator, silicon nitride (Si₃N₄), at the steady-state where material properties, temperature gradients and current/voltage input are constant. The model is built based on experimentally measured temperature gradients in the tooling for 20 mm discs and validated by producing 30 mm discs with similar temperature gradients and grain size uniformity across the part. The model aids in developing tooling to manipulate localize current density in specific regions to produce uniform 100 mm discs of ZrB₂ and Si₃N₄.

O dissilicato de ítrio (Y2Si2O7) por apresentar propriedades luminescentes consideráveis, é um potencial substituto dos óxidos de terras raras na tecnologia de queimadores de gás para iluminação. Em processos de conformação a partir de suspensões, o controle da estabilidade das partículas em meio líquido consiste em uma importante etapa para a produção de componentes com microestrutura homogênea e estabilidade estrutural. Neste sentido, este trabalho apresenta estudos sobre o comportamento de superfície, de estabilidade e reológico de suspensões aquosas de Y2Si2O7 e &beta;-Y1,95Dy0,05Si2O7 com o objetivo de preparar suspensões compatíveis para o processo de conformação por réplica, usando-se fibras vegetais como matriz para a confecção de componentes reticulados para queimadores de gás. Neste estudo foram avaliados parâmetros determinantes na estabilidade e comportamento de fluxo das suspensões, como também a luminescência estimulada pela temperatura desses materiais na forma de pó e a eficiência radiante do protótipo de queimador de gás. Os resultados obtidos para o &beta;-Y1,95Dy0,05Si2O7 comparados com a ítria (Y2O3) e concentrado de terras raras contento ítria (YTR) mostram-se promissores. Desenvolveu-se por processamento coloidal do &beta;-Y1,95Dy0,05Si2O7 e utilizando-se o processo de conformação por réplica a partir da esponja vegetal Lufa Cylindrica, um protótipo de lâmpada de queimador de gás com boa resistência mecânica ao manejo, emissividade espectral no visível (&lambda;=580nm) e eficiência radiante de 13%, com grande potencial para iluminação de ambientes internos de acordo com as recomendações da Comissão Internacional de Iluminação (CIE). / Yttrium disilicate (Y2Si2O7) shows luminescent proprieties and is a potential material to replace rare earth oxides in porous burner technology for lighting. In conformation process from suspensions the control of stability of particles in aqueous medium is an important step to produce ceramic components that show homogeneous microstructure and structural stability. This work presents studies about surface behavior, stability and rheological behavior of aqueous suspensions of Y2Si2O7 and &beta;-Y1,95Dy0,05Si2O7 and aims to prepare suitable suspensions for replica method, and vegetable fibers were used as template to produce porous components for gas burners. In this study key parameters on stability and rheological behavior of suspensions were evaluated, as well as the stimulated luminescence of powders by increasing temperature and radiant efficiency of gas burner prototype. The results as compared with yttria and rare earth concentrate containing yttria are very promising. By colloidal processing of &beta;-Y1,95Dy0,05Si2O7 and using replica method from Luffa Cylindrica vegetable sponge a prototype of gas burner lamp showing good handle strength, visible spectral emissivity (&lambda;=580nm), radiant efficiency of 13%, and great potential to be used for lighting internal spaces according to the International Commission on Illumination (CIE).

O dissilicato de ítrio (Y2Si2O7) por apresentar propriedades luminescentes consideráveis, é um potencial substituto dos óxidos de terras raras na tecnologia de queimadores de gás para iluminação. Em processos de conformação a partir de suspensões, o controle da estabilidade das partículas em meio líquido consiste em uma importante etapa para a produção de componentes com microestrutura homogênea e estabilidade estrutural. Neste sentido, este trabalho apresenta estudos sobre o comportamento de superfície, de estabilidade e reológico de suspensões aquosas de Y2Si2O7 e &beta;-Y1,95Dy0,05Si2O7 com o objetivo de preparar suspensões compatíveis para o processo de conformação por réplica, usando-se fibras vegetais como matriz para a confecção de componentes reticulados para queimadores de gás. Neste estudo foram avaliados parâmetros determinantes na estabilidade e comportamento de fluxo das suspensões, como também a luminescência estimulada pela temperatura desses materiais na forma de pó e a eficiência radiante do protótipo de queimador de gás. Os resultados obtidos para o &beta;-Y1,95Dy0,05Si2O7 comparados com a ítria (Y2O3) e concentrado de terras raras contento ítria (YTR) mostram-se promissores. Desenvolveu-se por processamento coloidal do &beta;-Y1,95Dy0,05Si2O7 e utilizando-se o processo de conformação por réplica a partir da esponja vegetal Lufa Cylindrica, um protótipo de lâmpada de queimador de gás com boa resistência mecânica ao manejo, emissividade espectral no visível (&lambda;=580nm) e eficiência radiante de 13%, com grande potencial para iluminação de ambientes internos de acordo com as recomendações da Comissão Internacional de Iluminação (CIE). / Yttrium disilicate (Y2Si2O7) shows luminescent proprieties and is a potential material to replace rare earth oxides in porous burner technology for lighting. In conformation process from suspensions the control of stability of particles in aqueous medium is an important step to produce ceramic components that show homogeneous microstructure and structural stability. This work presents studies about surface behavior, stability and rheological behavior of aqueous suspensions of Y2Si2O7 and &beta;-Y1,95Dy0,05Si2O7 and aims to prepare suitable suspensions for replica method, and vegetable fibers were used as template to produce porous components for gas burners. In this study key parameters on stability and rheological behavior of suspensions were evaluated, as well as the stimulated luminescence of powders by increasing temperature and radiant efficiency of gas burner prototype. The results as compared with yttria and rare earth concentrate containing yttria are very promising. By colloidal processing of &beta;-Y1,95Dy0,05Si2O7 and using replica method from Luffa Cylindrica vegetable sponge a prototype of gas burner lamp showing good handle strength, visible spectral emissivity (&lambda;=580nm), radiant efficiency of 13%, and great potential to be used for lighting internal spaces according to the International Commission on Illumination (CIE).

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