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| Author(s): |
Eduardo Caetano Camilo de Souza
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto de Pesquisas Energéticas e Nucleares (IPEN/BT) |
| Defense date: | 2008-12-09 |
| Examining board members: |
Eliana Navarro dos Santos Muccillo;
Vera Regina Leopoldo Constantino;
Fabio Coral Fonseca;
Edson Roberto Leite;
Renata Ayres Rocha
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| Advisor: | Eliana Navarro dos Santos Muccillo |
| Abstract | |
The general proposal of this Doctorate Thesis was to study some aspects of the microstructure of electrical conducting ceramics prepared from nanoparticles and its relationship with the electrical conductivity in the sintered material. For this purpose, samarium doped ceria, a potential candidate for application as solid electrolyte in intermediate temperature solid oxide fuel cells (SOFC) was selected. The main property of interest for this application is its electrical conductivity, which is dependent on the microstructure of the sintered material. Therefore, the main specific objectives of this work were: 1) preparation and physical characterization of ceria nanoparticles containing Sm3+ as additive (Ce1-xSmxO2-x/2 with 0 x 0.3), 2) study of densification and grain growth in these materials, and 3) study of the influence of average grain size on the ionic and electronic components of the overall electrical conductivity. Solution synthesis techniques were used for obtaining powder materials, and the conventional mixing of starting oxides was also used for comparison purposes. Several techniques were used for characterization like X-ray diffraction and Raman spectroscopy for phase characterization and crystallite size calculation; electron microscopy for microstructure observation, and impedance spectroscopy for electrical conductivity determination. The homogeneous precipitation method was found to be suitable for the synthesis of nanoparticles with homogeneous size and shape along with sharp distribution of particle sizes. The use of a mixed solvent (water/alcohol) was quite effective for obtaining nanosized powders with high yield and higher electrical conductivity than those for powders prepared by the conventional method of synthesis. The densification and the electrical conductivity were maximized when water/ethanol mixture was used as solvent. Powder compacts prepared with these powders attained 95% of the theoretical density after sintering at 1200 ºC for 5 h. The electrical conductivity is predominantly ionic in samaria-doped ceria, and the electronic component of the total electrical conductivity becomes significant only at oxygen partial pressures equal or lower than 10-20 atm at 600 ºC. For application of samaria-doped ceria as solid electrolyte in SOFC the maximum operation temperature is ~ 500 ºC. Commercial samaria-doped ceria with high specific surface area may attain good densification at only 800 ºC. The grain boundary conductivity in specimens with ~ 30 nm of average grain size is lower and the space charge potential is higher than those of specimens with higher grain sizes. The electronic conductivity in nanostructured samaria-doped ceria is almost independent on the grain size for temperatures lower than 500 ºC. (AU) | |