In the last decade new materials appeared that are candidates to be used as an electrolyte in a Solid Oxide Fuel Cell, SOFC. Some materials show high ionic conductivity but lack in important properties, such as mechanical stability or chemical compatibility with other materials in the fuel cell. Scandia Stabilised Zirconia, SSZ, became a possibility when the scandia price dropped with the opening of the Chinese and Russian markets. In the starting system Ce[subscript(x)]Y[subscript(0.2-x)]Sc₀.₆Zr₃.₂O[subscript(8-δ)], (0≤x≤0.2), scandia is introduced to improve conductivity and stabilise the cubic phase; yttria is introduced to fully stabilise the cubic phase and ceria to enhance conductivity lost with the introduction of yttria. The aim of this project is to develop a reliable new method to produce quality ceramics that are not strongly composition dependent, then to prepare a range of compositions and compare intrinsic properties without having to be concerned that poor sintering dominates conduction properties. This project can be divided in two sections, the first section the powder production method, the characteristics of the powders and its final products are in focus. In the second section the relation between electric characteristics and microstructure of the material is reported. In the first section, the effect of different compositions of the system Ce[subscript(x)]Y[subscript(0.2-x)]Sc₀.₆Zr₃.₂O[subscript(8-δ)], (0≤x≤0.2) is studied, in terms of structure, phase and microstructure. The nature, size and shape of the powders are discussed, and their effect on the final product. The sol-gel and combustion method gives the formation of hard agglomerates (shells), during the combustion, a wide range of grain sizes, between less than 1µm and 200 µm, and the formation of grains with non spherical shape. In this project, the sol-gel and combustion process and solid state method are also compared. In the second section of this project, AC Impedance measurements, as a function of temperature, oxygen partial pressure and time are discussed. The Arrhenius plot for all compositions shows two regions (high and low temperature) and the change of region occurs at 580 °C. At low temperatures there is a slight difference between compositions but this difference is less at high temperatures. The obtained ionic conductivity, at 350 °C, varies from 3.84×10⁻⁶ to 5.53×10⁻⁵ S/cm; at 700 °C, ionic conductivity from 0.013 to 0.044 S/cm. At low temperatures, the activation energy associated with bulk process is generally lower than grain boundary process; for example, the composition Ce₀.₁Y₀.₁Sc₀.₆Zr₃.₂O₇.₆₅ has an activation energy, for the bulk process, of 1.05 eV and an activation energy, for the grain boundary process, of 1.17 eV. For compositions with higher ceria content, activation energy, for bulk and grain boundary, have similar values. The AC impedance as function of oxygen partial pressure show that the amount of ceria introduced as an effect on the conductivity at low oxygen partial pressure. For the sample with no ceria in its composition, Y₀.₂Sc₀.₆Zr₃.₂O₇.₆₀, the conductivity does not vary significantly as the oxygen partial pressure is decreased; for oxygen partial of 0.21 atm, conductivity is 0.018 S/cm and when oxygen partial pressure is approximately 10⁻²⁴ atm conductivity is 0.018 S/cm. For the sample with a higher content of ceria, Ce₀.₁₂Y₀.₀₈Sc₀.₆Zr₃.₂O₇.₆₆, there is a decrease in conductivity while oxygen partial pressure decreases; and there is also the appearance of a semi-circle for lower oxygen partial pressures. For oxygen partial pressure approximately 0.21 atm, conductivity is 0.019 S/cm, but when oxygen partial pressure is decreased to 10⁻²⁴ atm conductivity decreases to 0.011 S/cm. AC impedance measurements as a function of annealing time at 600 °C were performed. Total conductivity is fairly stable, for all compositions, until 1800 hours but after this time, conductivity slowly decreases. Some compositions show a second semi-circle in the AC impedance spectra, either from the beginning, time equals 0 hours, or after some working hours. Here, the changes in conduction and conduction processes with time are discussed.

Nesse trabalho foi proposto investigar a estabilidade de fases do sistema zircônia-escândia (ScSZ) por meio do estudo termodinâmico de nanopartículas, na faixa de 0 a 20% em mol de Sc2O3, e a partir da introdução de um segundo aditivo (Dy2O3 e Nb2O5) ao ZrO2 contendo 10% em mol de Sc2O3 (10ScSZ). A estabilidade de fases do ScSZ foi avaliada com base em dados termodinâmicos determinados pelas técnicas de microcalorimetria de adsorção de água e calorimetria de dissolução à alta temperatura. As soluções sólidas foram sintetizadas pelo método de coprecipitação de hidróxidos. Dados termodinâmicos foram determinados para as formas polimórficas encontradas (monoclínica, tetragonal, cúbica, romboédrica β e γ) por difração de raios X no ScSZ. Esse trabalho resultou no diagrama de fases em nanoescala de tamanho de partícula-composição. Os efeitos produzidos pela introdução de aditivos na matriz de 10ScSZ foram investigados visando obter a possível estabilização da estrutura cúbica (c) e a supressão da transformação de fase c-β, característica do sistema binário. As composições foram sintetizadas por coprecipitação de hidróxidos e por reações em estado sólido para fins comparativos. Os materiais foram sinterizados convencionalmente e por sinterização assistida por campo elétrico. A estabilização completa da fase cúbica ocorreu a partir de teores molares de 1% de Dy2O3 e 0,5% de Nb2O5. O menor teor de Nb2O5 necessário para a estabilização da fase foi atribuído à provável formação da fase líquida durante a sinterização e ao menor tamanho do íon Nb5+. Os resultados de difratometria de raios X em alta temperatura e análise térmica mostraram que houve supressão da transição c-β. As amostras contendo 0,5% mol de Nb2O5 apresentaram valores de condutividade iônica similares aos do 10ScSZ sem aditivos em uma ampla faixa de temperatura com elevada estabilidade em um período de 170 h a 600 °C. / In this work, the phase stability of scandia-zirconia (ScSZ) system was investigated by the thermodynamic study of nanoparticles, within the range of 0 to 20 mol% Sc2O3, and by codoping of ZrO2-10 mol% Sc2O3 (10ScSZ) with Dy2O3 and Nb2O5. The phase stability of ScSZ was evaluated based on thermodynamic data collected by water adsorption microcalorimetry and high temperature oxide melt solution. Nanostructured zirconia-scandia solid solutions were synthesized by coprecipitaion method. Thermodynamic data were determined for ScSZ polymorphs (monoclinic, tetragonal, cubic, rhombohedral β and γ) found by X-ray diffraction. This systemic work resulted in an unprecedented phase diagram at the nanoscale of particle size-composition. The effects of additives on 10ScSZ were investigated aiming to stabilize the cubic (c) structure at room temperature and to suppress the characteristic cubic-rhombohedral β phase transformation. Compositions were prepared by coprecipitation and solid state reaction. Materials were sintered by conventional and spark plasma sintering. Full stabilization of the cubic phase was attained by 1 mol% Dy2O3 and 0.5 mol% Nb2O5 additions. The smallest Nb2O5 content required for cubic phase stabilization was attributed to liquid phase formation during sintering and to small ionic radius of Nb5+. Results of high temperature X-ray diffraction and thermal analysis show suppression of the c-β transformation. Samples containing 0.5 mol% Nb2O5 show total ionic conductivity similar to 10ScSZ without additives within a broad temperature range with high stability during 170 h at 600 °C.

Nesse trabalho foi proposto investigar a estabilidade de fases do sistema zircônia-escândia (ScSZ) por meio do estudo termodinâmico de nanopartículas, na faixa de 0 a 20% em mol de Sc2O3, e a partir da introdução de um segundo aditivo (Dy2O3 e Nb2O5) ao ZrO2 contendo 10% em mol de Sc2O3 (10ScSZ). A estabilidade de fases do ScSZ foi avaliada com base em dados termodinâmicos determinados pelas técnicas de microcalorimetria de adsorção de água e calorimetria de dissolução à alta temperatura. As soluções sólidas foram sintetizadas pelo método de coprecipitação de hidróxidos. Dados termodinâmicos foram determinados para as formas polimórficas encontradas (monoclínica, tetragonal, cúbica, romboédrica β e γ) por difração de raios X no ScSZ. Esse trabalho resultou no diagrama de fases em nanoescala de tamanho de partícula-composição. Os efeitos produzidos pela introdução de aditivos na matriz de 10ScSZ foram investigados visando obter a possível estabilização da estrutura cúbica (c) e a supressão da transformação de fase c-β, característica do sistema binário. As composições foram sintetizadas por coprecipitação de hidróxidos e por reações em estado sólido para fins comparativos. Os materiais foram sinterizados convencionalmente e por sinterização assistida por campo elétrico. A estabilização completa da fase cúbica ocorreu a partir de teores molares de 1% de Dy2O3 e 0,5% de Nb2O5. O menor teor de Nb2O5 necessário para a estabilização da fase foi atribuído à provável formação da fase líquida durante a sinterização e ao menor tamanho do íon Nb5+. Os resultados de difratometria de raios X em alta temperatura e análise térmica mostraram que houve supressão da transição c-β. As amostras contendo 0,5% mol de Nb2O5 apresentaram valores de condutividade iônica similares aos do 10ScSZ sem aditivos em uma ampla faixa de temperatura com elevada estabilidade em um período de 170 h a 600 °C. / In this work, the phase stability of scandia-zirconia (ScSZ) system was investigated by the thermodynamic study of nanoparticles, within the range of 0 to 20 mol% Sc2O3, and by codoping of ZrO2-10 mol% Sc2O3 (10ScSZ) with Dy2O3 and Nb2O5. The phase stability of ScSZ was evaluated based on thermodynamic data collected by water adsorption microcalorimetry and high temperature oxide melt solution. Nanostructured zirconia-scandia solid solutions were synthesized by coprecipitaion method. Thermodynamic data were determined for ScSZ polymorphs (monoclinic, tetragonal, cubic, rhombohedral β and γ) found by X-ray diffraction. This systemic work resulted in an unprecedented phase diagram at the nanoscale of particle size-composition. The effects of additives on 10ScSZ were investigated aiming to stabilize the cubic (c) structure at room temperature and to suppress the characteristic cubic-rhombohedral β phase transformation. Compositions were prepared by coprecipitation and solid state reaction. Materials were sintered by conventional and spark plasma sintering. Full stabilization of the cubic phase was attained by 1 mol% Dy2O3 and 0.5 mol% Nb2O5 additions. The smallest Nb2O5 content required for cubic phase stabilization was attributed to liquid phase formation during sintering and to small ionic radius of Nb5+. Results of high temperature X-ray diffraction and thermal analysis show suppression of the c-β transformation. Samples containing 0.5 mol% Nb2O5 show total ionic conductivity similar to 10ScSZ without additives within a broad temperature range with high stability during 170 h at 600 °C.