Fast oxide-ion and proton conductors are the subject of considerable research due to their technological applications in sensors, ceramic membranes and solid oxide fuel cells (SOFCs). This thesis describes the use of computer modelling techniques to study point defects, dopants and clustering effects in fluorite-and perovskitetype ion conductors with potential SOFC applications. Bi2O3 related phases are being developed with the objective of high oxide-ion conductivities at lower operating temperatures than 1000°C, as in current generation SOFC electrolytes. Doped Bi2O3 phases have shown promise as materials capable of accomplishing this goal. First, the Y-doped phase, Bi3YO6, has been investigated including the ordering of intrinsic vacancies. The defect and dopant characteristics of Bi3YO6 have been examined and show that a highly mobile oxygen sub-lattice exists in this material. A preliminary structural modelling study of a new Re-doped Bi2O3 phase was also undertaken. A comprehensive investigation of the proton-conducting perovskites BaZrO3, BaPrO3 and BaThO3 is then presented. Our results suggest that intrinsic atomic disorder in BaZrO3 and BaThO3 is unlikely, but reduction of Pr4+ in BaPrO3 is favourable. The water incorporation energy is found to be less exothermic for BaZrO3 than for BaPrO3 and BaThO3, but in all cases the results suggest that the proton concentration would decrease with increasing temperature, in accord with experimental data. The high binding energies for all the dopant-OH pair clusters in BaPrO3 and BaThO3 suggest strong proton trapping effects. Finally, a study of multiferroic BiFeO3 is presented, in which the defect, dopant and migration properties of this highly topical phase are investigated. The reduction process involving the formation of oxygen vacancies and Fe2+ is the most favourable redox process. In addition, the results suggest that oxide-ion migration is anisotropic within this system.

Made available in DSpace on 2014-12-17T14:07:10Z (GMT). No. of bitstreams: 1 GislaineBPF_TESE.pdf: 3300091 bytes, checksum: 8adeb890624a95fdc27ae7d6a3f52ad8 (MD5) Previous issue date: 2012-12-19 / Fuel cells are electrochemical devices that convert chemical energy in electrical energy by a reaction directly. The solid oxide fuel cell (SOFC) works in temperature between 900?C up to 1000?C, Nowadays the most material for ceramic electrolytes is yttria stabilized zirconium. However, the high operation temperature can produce problems as instability and incompatibility of materials, thermal degradation and high cost of the surround materials. These problems can be reduced with the development of intermediate temperature solid oxide fuel cell (IT-SOFC) that works at temperature range of 600?C to 800?C. Ceria doped gadolinium is one of the most promising materials for electrolytes IT-SOFC due high ionic conductivity and good compatibility with electrodes. The inhibition of grain growth has been investigated during the sintering to improve properties of electrolytes. Two-step sintering (TSS) is an interesting technical to inhibit this grain growth and consist at submit the sample at two stages of temperature. The first one stage aims to achieve the critical density in the initiating the sintering process, then the sample is submitted at the second stage where the temperature sufficient to continue the sintering without accelerate grain growth until to reach total densification. The goal of this work is to produce electrolytes of ceria doped gadolinium by two-step sintering. In this context were produced samples from micrometric and nanometric powders by two routes of two-step sintering. The samples were obtained with elevate relative density, higher than 90% using low energy that some works at the same area. The average grain size are at the range 0,37 μm up to 0,51 μm. The overall ionic conductivity is 1,8x10-2 S.cm and the activation energy is 0,76 eV. Results shown that is possible to obtain ceria-doped gadolinium samples by two-step sintering technique using modified routes with characteristics and properties necessary to apply as electrolytes of solid oxide fuel cell / As c?lulas a combust?vel s?o dispositivos eletroqu?micos que convertem energia qu?mica em energia el?trica por uma rea??o direta. As c?lulas a combust?veis de ?xido s?lidos (Solid Oxide Fuel Cell - SOFC) operam em temperaturas entre 900 e 1000?C, com eletr?litos de cer?mica. Atualmente o material mais utilizado ? a zirc?nia estabilizada com ?tria, no entanto a alta temperatura de opera??o pode causar problemas de instabilidade e incompatibilidade de materiais, degrada??o t?rmica e alto custo dos materiais perif?ricos. Com a inten??o de minimizar esses problemas, s?o realizadas pesquisas para desenvolver c?lulas a combust?vel de ?xido s?lido de temperatura intermedi?ria (IT-SOFC) que operam na faixa de temperatura de 600 a 800?C, utilizando c?ria dopada com gadol?nia como um dos mais promissores materiais para eletr?litos de IT-SOFC devido ? alta condutividade i?nica e uma boa compatibilidade com os eletrodos. Formas de inibir o crescimento do gr?o durante a sinteriza??o para melhorar as propriedades dos eletr?litos s?o investigadas. Para tal, ? utilizada a t?cnica de sinteriza??o em dois passo (two-step sintering - TSS), que consiste em submeter a amostra a dois est?gios de temperatura. O primeiro est?gio visa atingir a densidade cr?tica para dar in?cio ao processo de sinteriza??o. Em seguida a amostra ? submetida a um segundo est?gio de temperatura capaz de dar continuidade ? sinteriza??o sem que haja acelerado crescimento de gr?o, at? sua total densifica??o. O principal objetivo deste trabalho foi produzir eletr?litos de c?ria dopada com gadol?nia pelo processo de sinteriza??o em dois passos. Neste contexto foram produzidas amostras a partir de p?s microm?tricos e nanom?tricos atrav?s de duas rotas de sinteriza??o em dois passos. Foram obtidas amostras com elevada densidade relativa, superior a 90%. Os tamanhos m?dios de gr?os obtidos est?o na faixa de 0,37 μm a 0,51 μm. Foram obtidas amostras com condutividade i?nica total de 1,8x10-2 S.cm e energia de ativa??o de 0,76 eV. A partir dos resultados obtidos neste trabalho, foi poss?vel obter amostras de c?ria dopada com gadol?nia atrav?s da t?cnica de sinteriza??o em dois passos, utilizando rotas modificadas com caracter?sticas e propriedades necess?rias para serem aplicadas como eletr?litos de c?lulas a combust?vel de ?xido s?lido

Carbon emissions are known to cause decay of the Ozone layer in addition to creating pollutant, poisonous air. This has become a growing concern among scientists and engineers across the globe; if this issue is not addressed, it is likely that the Earth will suffer catastrophic consequences. One of the main culprits of these harmful carbon emissions is fuel combustion. Between vehicles, power plants, airplanes, and ships, the world consumes an extraordinary amount of oil and fuel which all contributes to the emissions problem. Therefore, it is crucial to develop alternative energy sources that minimize the impact on the environment. One such technology that is currently being researched, is the Solid Oxide Fuel Cell (SOFC). This is a relatively simple device that converts chemical energy into electrical energy with no harmful emissions. For these devices to work properly, they require an electrolyte material that has high ionic conductivity with good phase stability at a variety of temperatures. The research presented in this study will concentrate intensively on just one of the many candidates for SOFC electrolytes. 1 mol% CeO2 – 10 mol% Sc2O3 – 89 mol% ZrO2 manufactured by Treibacher Industries was analyzed to better understand its sintering properties, phase stability, and molecular structure. Sintering was performed at temperatures ranging from 900oC to 1600oC and the shrinkage, density and porosity were examined for each temperature. Raman Spectroscopy and X-Ray Powder Diffraction were also conducted for comparison with other known compositions to see if the powder undergoes any phase transitions or instability.