Made available in DSpace on 2016-06-02T19:10:20Z (GMT). No. of bitstreams: 1 5789.pdf: 9603376 bytes, checksum: a1a36543919125363099dce508d349a5 (MD5) Previous issue date: 2013-12-13 / Financiadora de Estudos e Projetos / Yttrium-doped barium cerate perovskites are promising electrolyte for solid oxide fuel cell (SOFC) due to high proton conductivity at intermediate temperature, 400-600°C. However, the proton conductivity is not an intrinsic characteristic but appears after proton insertion on the crystalline structure during heat treatment in rich hydrogen atmosphere. The relationship between proton insertion efficiency and the type of microstructure is still unclear. Another limiting factors in obtaining these perovskites is the high sintering temperature which modifies the stoichiometry generating secondary phases. In this work, the influence of ZnO as sintering aid on the microstructure development and also the influence of microstructure on the protonation process was systematically investigated. Pure and doped with 10% to 20 at% yttrium BaCeO3, with and without ZnO as sintering aid were prepared through oxide mixture, isostatically pressed and sintered at 1300-1600°C temperature range. The sintered samples were characterized by density measurement using Archimedes principle, X-ray diffraction (XRD), scanning electron microscopy (SEM) equiped with energy-dispersive X-ray detector (EDS) and impedance spectroscopy. The sintered samples microstructures were multiphase and the volumetric relation between the phases was dependent on yttrium content, ZnO addition, and sintering conditions. ZnO acts on the microstructure development through the binary BaO-CeO2 eutectic point but it is only effective as sintering aid in the presence of yttrium. Eutectics microstructures were obtained due the ZnO addition. The protonation efficiency and, in consequence, the electrical conductivity were microstructure dependent. The highest protonic conductivity, 1,44 x 10-2S/cm at 500 °C, was obtaining by 10 at.% yttrium-doped sample with 1.0 wt% of ZnO and sintered at 1600°C-8h. / As perovskitas de cerato de bário dopado com ítrio apresentam características promissoras como eletrólito para célula a combustível de óxido sólido (CaCOS) por apresentarem condutividade protônica elevada em temperatura intermediária 400-600°C. Entretanto, a condução protônica só ocorre após a inserção de prótons na estrutura cristalina durante tratamento térmico em atmosfera rica em hidrogênio. A concentração de prótons acumulados na estrutura e sua relação com as características da microestrutura ainda não está devidamente esclarecida na literatura. Outro fator limitante na obtenção dessas perovskitas é sua elevada temperatura de sinterização o que compromete a estequiometria e favorece a formação de fases secundárias. No presente trabalho, foi investigado de forma sistemática o caminho de atuação do aditivo de sinterização ZnO no desenvolvimento da microestrutura bem como a influência da microestrutura no processo de protonação. Os pós de BaCeO3 puro e dopado com 10% e 20% at de ítrio, com e sem a presença de ZnO, usado como aditivo de sinterização foram preparados por mistura de óxidos e prensados isostaticamente. A sinterização foi realizada entre 1300-1600 °C. Os corpos de prova sinterizados foram caracterizados por medidas de densidade baseadas no princípio de Archimedes, por difração de raios X (DRX), microscopia eletrônica de varredura associada com microanálise (MEV, EDS) e por espectroscopia de impedância. Todas as composições investigadas geraram corpos de prova multifásicos após sinterização sendo que a relação volumétrica entre as fases foi dependente do teor de ítrio, da presença de ZnO e das condições de sinterização. O mecanismo de atuação do ZnO foi estabelecido sendo que ele atua no ponto eutético do binário BaO-CeO2, mas somente na presença do Y2O3 gerando uma microestrutura eutética. A eficiência de protonação, e por consequência, a condutividade elétrica foram dependentes da microestrutura do corpo de prova. O maior valor da condutividade protônica; 1,44 x 10-2 S/cm a 500 °C, foi obtido para a composição com 10% at de ítrio e 1% em peso de ZnO sinterizada 1600 °C ao ar e com patamar de 8 horas.

In this thesis, the focus is on studying BaZrO3-BaCeO3 based proton conductors due to that they represent very promising proton conductors to be used for Intermediate Temperature Solid Oxide Fuel Cells (ITSOFCs). Here, dense BaZr0.5Ce0.3Y0.2O3-δ (BZCY532) ceramics were selected as the major studied materials. These ceramics were prepared by different sintering methods and doping strategies. Based on achieved results, the thesis work can simply be divided into the following parts: 1) An improved synthesis method, which included a water-based milling procedure followed by a freeze-drying post-processing, was presented. A lowered calcination and sintering temperature for a Hf0.7Y0.3O2-δ (YSH) compound was achieved. The value of the relative density in this work was higher than previously reported data. It is also concluded that this improved method can be used for mass-production of ceramics. 2) As the solid-state reactive sintering (SSRS) represent a cost-effective sintering method, the sintering behaviors of proton conductors BaZrxCe0.8-xLn0.2O3-δ (x = 0.8, 0.5, 0.1; Ln = Y, Sm, Gd, Dy) during the SSRS process were investigated. According to the obtained results, it was found that the sintering temperature will decrease, when the Ce content increases from 0 (BZCLn802) to 0.3 (BZCLn532) and 0.7 (BZCLn172). Moreover, the radii of the dopant ions similar to the radii of Zr4+ or Ce4+ ions show a better sinterability. This means that it is possible to obtain dense ceramics at a lower temperature. Moreover, the conductivities of dense BZCLn532 ceramics were determined. The conductivity data indicate that dense BZCY532 ceramics are good candidates as either oxygen ion conductors or proton conductors used for ITSOFCs. 3) The effect of NiO on the sintering behaviors, morphologies and conductivities of BZCY532 based electrolytes were systematically investigated. According to the achieved results, it can be concluded that the dense BZCY532B ceramics (NiO was added during ball-milling before a powder mixture calcination) show an enhanced oxygen and proton conductivity. Also, that BZCY532A (NiO was added after a powder mixture calcination) and BZCY532N (No NiO was added in the whole preparation procedures) showed lower values. In addition, dense BZCY532B and BZCY532N ceramics showed only small electronic conductivities, when the testing temperature was lower than 800 ℃. However, the BZCY532A ceramics revealed an obvious electronic conduction, when they were tested in the range of 600 ℃ to 800 ℃. Therefore, it is preferable to add the NiO powder during the BZCY532 powder preparation, which can lower the sintering temperature and also increase the conductivity. 4) Dense BZCY532 ceramics were successfully prepared by using the Spark Plasma Sintering (SPS) method at a temperature of 1350 ℃ with a holding time of 5 min. It was found that a lower sintering temperature (&lt; 1400 ℃) and a very fast cooling rate (&gt; 200 ℃/min) are two key parameters to prepare dense BZCY532 ceramics. These results confirm that the SPS technique represents a feasible and cost-effective sintering method to prepare dense Ce-containing BaZrO3-BaCeO3 based proton conductors. 5) Finally, a preliminary study for preparation of Ce0.8Sm0.2O2-δ (SDC) and BZCY532 basedcomposite electrolytes was carried out. The novel SDC-BZCY532 based composite electrolytes were prepared by using the powder mixing and co-sintering method. The sintering behaviors, morphologies and ionic conductivities of the composite electrolytes were investigated. The obtained results show that the composite electrolyte with a composition of 60SDC-40BZCY532 has the highest conductivity. In contrast, the composite electrolyte with a composition of 40SDC-60BZCY532 shows the lowest conductivity. In summary, the results show that BaZrO3-BaCeO3 based proton-conducting ceramic materials represent very promising materials for future ITSOFCs electrolyte applications. / <p>QC 20150423</p>