SHIRLEY LEITE DOS REIS

SHIRLEY LEITE DOS REIS

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Microstructure and electrical conductivity of sol-gel synthesized and spark plasma sintered doped-lanthanum gallate
2023 - MUCCILLO, E.N.S.; REIS, S.L.; GONIN, C.F.N.; BERTON, M.A.C.; MUCCILLO, R.
Perovskite ceramics consisting of lanthanum gallate with partial substitutions for strontium and magnesium are candidates as solid electrolyte for application in solid oxide fuel cells operating at intermediate temperatures (~550 to ~750ºC). The main concern related to the application of this perovskite solid electrolyte is impurity phases, usually detected even in chemically synthesized powders. The La0.9Sr0.1Ga0.8Mg0.2O3-d (LSGM) composition display relatively low contents of impurity phases. In this work, the LSGM composition was synthesized by the sol-gel method and consolidated by spark plasma sintering to optimize the microstructure and electrical properties of this solid electrolyte. Chemically synthesized powders were consolidated in the 1100 to 1250ºC for 5 min. Sintered specimens with relative densities higher than 98% were obtained. Rietveld analysis of X-ray diffraction data revealed no detectable impurity phases. The microstructure evolution exhibited submicron sized grains with mixed fracture mode. High values of the electrical conductivity were obtained for all specimens.
Thermal and spectroscopic characterization of sol-gel-synthesized doped lanthanum gallate
2021 - REIS, S.L.; GROSSO, R.L.; MUCCILLO, E.N.S.
In this study, a complex oxide solid solution consisting of lanthanum gallate with partial substitutions for strontium and magnesium (La0.9Sr0.1Ga0.8Mg0.2O2.85) was synthesized by the sol–gel route, aiming to obtain a sinter active powder and a final material that could be stoichiometrically controlled. The thermal behavior of the synthesized powder involves several steps of decomposition. The linear shrinkage of green compacts up to 1500 °C was 31%. The bulk conductivity of sintered specimens increases with sintering temperature up to 1400 °C. The stoichiometry was maintained for sintering temperatures up to 1450 °C. Elemental mapping obtained by energy-dispersive spectroscopy evidenced magnesium segregation at the grain boundaries. The overall results evidence the suitability of the synthesis method for preparing doped lanthanum gallate.
Ionic conductivity of doped lanthanum gallate and strontium gallate composites
2017 - REIS, S.L.; MUCCILLO, E.N.S.
Polycrystalline ceramic solid electrolytes based on strontium- and magnesium-doped lanthanum gallate exhibit high ionic conductivity and find potential application in solid oxide fuel cells operating at intermediate temperatures. Sintering of this solid electrolyte is usually carried out at high temperatures, being responsible for loss of Ga and consequent formation of impurity phases. In this work, composites consisting of La0.9Sr0.1Ga0.8Mg0.2O3-δ with additions of strontium gallate were prepared by solid state reaction, and the influence of the additiveon electrical conductivity and phase composition of the composite electrolyte was investigated. The sintered density of composites is higher than 95% of the theoretical value after sintering at 1350°C. The contents of impurity phases decreased with increasing additions of strontium gallate. The overall ionic conductivity of the composites is higher than that of LSGM. This effect is attributed to the decrease in the fraction of free MgO at grain boundaries in the composites.
Influence of yttria-stabilized zirconia on microstructure and electrical properties of doped lanthanum gallate
2019 - FUJIMOTO, TALITA G.; REIS, SHIRLEY L.; MUCCILLO, ELIANA N. dos S.
Lanthanum gallate with partial substitutions for strontium and magnesium shows higher ionic conductivity than traditional ionic conductors such as yttria-stabilized zirconia (8YSZ), and has been considered for application as solid electrolyte and electrode in Solid Oxide Fuel Cells operating at intermediate temperatures (500-700ºC). Parallel to the development of new solid electrolytes, improvement of the already known electrolytes has been one goal. In this work, the effects of adding 8YSZ to La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM), on the microstructure and electrical performance were investigated. Compound electrolytes consisting of up to 20 wt.% 8YSZ were prepared by solid state reactions. Sintered pellets with up to 10 wt.% 8YSZ achieved near full density after sintering at 1450ºC and reduction in the fraction of impurity phases. Addition of 8YSZ promoted grain growth to LSGM. The higher ionic conductivity was obtained for 1 wt.% 8YSZ addition with slight decrease of the apparent activation energy.
Electrical behavior of electric field-assisted pressureless sintered ceria-20 mol% samaria
2019 - REIS, SHIRLEY L.; CARVALHO, SABRINA G.M.; MUCCILLO, ELIANA N.S.; MUCCILLO, REGINALDO
CeO2:20 mol% Sm2O3 green ceramic pellets were sintered conventionally at 1500 C/2 h and flash sintered by applying a 200 V cm􀀀1 electric field at 800 C, 1000 C and 1200 C. The thickness shrinkage of the pellets was followed bythe specimen being positioned inside a dilatometer adapted with platinum electrodes and terminal leads connected to a power supply for application of the electric voltage. The microstructure of the surfaces of the sintered samples were observed in a scanning electron microscope. The electrical properties were evaluated by the impedance spectroscopy technique in the 5 Hz–13 MHz frequency range from 210 C to 280 C. The main results show that (i) the final shrinkage level is nearly independent of the temperature when the electric field is applied and slightly better than that of the 1500 C sintered pellet, and (ii) the bulk conductivity of the sample flash sintered at 1200 C is similar to that of the sample sintered at 1500 C. The availability of a pathway for the electric current pulse derived from the applied electric field is proposed as the reason for the achieved shrinkages. Scavenging of the grain boundaries by Joule heating is proposed as the reason for the improved oxide ion bulk conductivity.
Sintering under AC electric field of samarium doped ceria
2018 - REIS, SHIRLEY L. dos; CARVALHO, SABRINA G. de M.; MUCCILLO, ELIANA N. dos S.; MUCCILLO, REGINALDO
Samarium doped ceria (SDC) is a high oxygen-ion conductor compound with application in solid oxide fuel cells operating at intermediate temperatures (500-700ºC). Polycrystalline SDC exhibits low sinterability, requiring high temperatures to achieve full densification under conventional sintering. Flash sintering is a recently developed method in which ceramic compacts may be sintered at lower temperatures and shorter times than those under conventional sintering. It consists in applying an electric field to a green compact, either during heating or at a fixed temperature up to the occurrence of an electric current pulse. In this study, samarium doped ceria (Ce0.8Sm0.2O1.9) was sintered by applying AC electric field at a fixed frequency of 1.0 kHz to the specimen during heating (dynamic sintering) or during a fixed temperature (isothermal). The influence of sintering on densification, microstructure and ionic conductivity was investigated. Similar density values were obtained for specimens sintered at 800ºC (dynamic) and 1100ºC (isothermal).
Estabilização de fase e microestrutura da zircônia-céria obtida por reação em estado sólido
2019 - FERREIRA, L.A.S.; REIS, S.L.; MUCCILLO, E.N.S.
Materiais cerâmicos à base de zircônia estabilizada na fase tetragonal têm sido extensivamente estudados e propostos para diversas aplicações devido suas destacadas propriedades termomecânicas. A zircônia contendo 12 mol% em céria (12Ce-TZP) possui estrutura tetragonal, alta resistência à fratura e estabilidade térmica. Este trabalho tem como objetivo verificar o efeito da sinterização na estabilização da fase tetragonal e na microestrutura da 12Ce-TZP preparada por reação em estado sólido. As amostras de 12Ce-TZP foram preparadas pelo método de mistura dos óxidos reagentes seguida de sinterização em diversas temperaturas. As fases cristalinas, densificação e microestrutura foram analisadas por difração de raios X, medidas de densidade hidrostática e microscopia eletrônica de varredura, respectivamente. O teor de fase monoclínica foi quantificado utilizando o método dos polimorfos. Amostras com fase única tetragonal foram obtidas para sinterizações a 1400 °C e densidades próximas à teórica (> 99%) para amostras sinterizadas a 1500 °C.
Densificação de céria-samária por meio de sinterização sem pressão assistida por campo elétrico AC
2019 - REIS, S.L.; CARVALHO, S.G.M.; MUCCILLO, E.N.S.; MUCCILLO, R.
A solução sólida céria-samária é uma das principais candidatas para aplicação como eletrólito sólido em células a combustível de óxido sólido, devido sua alta condutividade iônica em temperaturas intermediárias de operação (500-750 °C). Um dos problemas ainda não solucionados é sua relativamente baixa sinterabilidade. Altas temperaturas de sinterização, em geral acima de 1500 °C, são necessárias para obter densificação com impermeabilização a gases (> 92% da densidade teórica). Em altas temperaturas pode ocorrer redução de Ce4+ para Ce3+, com consequente aumento na condutividade eletrônica. No entanto, a utilização de métodos de sinterização não convencionais como a sinterização sem pressão assistida por campo elétrico é uma das estratégias para diminuir a temperatura de sinterização e otimizar as propriedades. Este trabalho tem como objetivo avaliar os efeitos do método de sinterização isotérmica assistida por campo elétrico AC durante o patamar em uma temperatura fixa. Foram realizados experimentos variando campo elétrico, tempo de aplicação do campo e limite de corrente elétrica. Os resultados mostraram que é possível densificar a céria-samária em temperaturas inferiores às convencionais por meio de sinterização sem pressão assistida por campo elétrico. O aumento do valor do campo elétrico promoveu diminuição nos valores de resistividade intragranular e intergranular. O aumento na temperatura de aplicação do campo elétrico resultou em amostras com condutividade elétrica maior em relação à amostra sinterizada convencionalmente em 1500 °C/2 h.
Influence of calcium addition on the electrical conductivity of samarium doped ceria
2018 - REIS, S.L.; MUCCILLO, E.N.
Ceria-based ceramics have been proposed to be used as solid electrolyte in solid oxide fuel cells operating at intermediate (500-750°C) temperatures, due to their high ionic conductivity. Samarium ion is recognized as one of the most reliable dopants for cerium oxide because of its lower association energy with oxygen vacancies. One challenge posed to ceria-based solid electrolytes is how to improve sinterability, avoiding high temperatures to attain good densification. Thermal treatments at high temperatures may reduce Ce4+ to Ce3+, favoring electron transport and generation of micropores in the sintered electrolyte. Introduction of a second additive is a usual approach to overcome these constraints. In this work calcium ion was chosen as the second additive. Sm0.2-xCaxCe0.8O1.9-x/2 compositions, with x = 0, 0.025, 0.05, 0.1, 0.15 and 0.2, were prepared by solid-state reaction, and the influence of the additive content on densification and ionic conductivity was investigated. All compositions were found to have cubic fluorite-type structure. The optimal composition was Sm0.175Ca0.025Ce0.8O1.888, which showed a relative density of 97%, single phase and higher ionic conductivity than the Sm0.2Ce0.8O1.9 parent electrolyte.
Influence of gallium-based additives on microstructure and ionic conductivity of doped-lanthanum gallate
2018 - REIS, S.L.; MUCCILLO, E.N.
Sr- and Mg-doped lanthanum gallate is a well known oxide-ion conductor with potential application in Solid Oxide Fuel Cells operating at intermediate temperatures (500-700oC). One of the main concerns on this solid electrolyte is related to impurity phases, frequently observed even in chemically synthesized powders, due to gallium loss during sintering. La0.9Sr0.1Ga0.8Mg0.2O3-d, LSGM, containing small amounts of Ga2O3 and Sr3Ga2O6 were prepared by solid state reaction, and the effects of the additives on microstructure and ionic conductivity were investigated after sintering at 1350oC. Gallium oxide addition promoted grain growth of LSGM and increased the fraction of the gallium-rich impurity phase. In contrast, strontium gallate addition favored reduction of the fraction of impurity phases. The intragrain conductivity of LSGM increases with gallium oxide addition, whereas strontium gallate improved both the intra- and the intergrain conductivities of LSGM.