| Summary: | 博士 === 國立清華大學 === 化學工程學系 === 89 === Doped ceria is difficult to sinter, at temperature required for SOFC (Solid Oxide Fuel Cell) fabrication, to near-full density ceramics. The development of doped ceria powders capable of sintering to full density below 1400℃ is important to facilitate SOFC fabrication.
20% samarium-doped ceria powders were prepared by the sol-gel method with different processes. The characteristic of the samples was investigated by particle size distribution, X-ray diffraction, crystallite size, and density result. A modified process with sol-gel method we first proposed involving high-carbon alcohol (long chain, high boiling point) distillation of molecular water yields soft-agglomerated nanocrystalline powders are easily sintered in air to yield near-fully relative density at 1300℃ for five hours (the lowest temperature as we know to synthesize doped ceria with high density), which is significantly lower than that for 1400-1500℃ required by the sol-gel method and much lower than that for 1600-1700℃ required by the solid state techniques.
Conductivity, σ, of the samaria-doped ceria electrolyte is studied as a function of temperature and dopant concentration, x, which was from 5 to 30 mole%. It is shown that a maximum in σ versus x corresponds to a minimum in activation energy. It is found that the conductivity is completely due to oxygen vacancy conduction. The conductivity increases with increasing samaria doping and reaches a maximum for (CeO2)0.8(SmO1.5)0.2, which has a conductivity of 5.6×10-1 S/cm at 800℃. A curvature at T = Tc, the critical temperature, has been observed in the Arrhenius plot. This phenomenon may be explained by a model which proposed that, below Tc, nucleation of mobile oxygen vacancies into ordered clusters occurs, and, above Tc, all oxygen vacancies appear to be mobile without interaction with dopant cation. In addition, the composition dependences of both the critical temperature and the trapping energy consist with that of the activation energy.
The overall resistivity of polycrystalline Sm0.2Ce0.8O1.9 obtained by DC four-probe method was found to increase linearly with the reciprocal of the average grain size ( ) at sintering temperature of 1500℃, and the grain resistivity can be got by extended the fitting line to . In addition, we found that by lowering the sintering temperature to 1100-1200℃, the overall resistivity decreases obviously and nearly equal to the grain resistivity obtained at 1500℃ which enable Sm0.2Ce0.8O1.9 working as SOFCs'' electrolyte at temperature lower than 700℃comparable to 800℃ traditionally sintering at 1500℃ or above.
AC impedance spectroscopy has been used to separate grain and apparent grain boundary resistivity in a series of measurements on Sm0.2Ce0.8O1.9 ceramic electrolytes with a range of different grain size distributions. The "brick layer" microstructural model has been used to provide an estimate of apparent grain boundary resistivity and to relate the electrical properties of the ceramic to microstructural parameters. For samples sintering at 1500℃, a clear relationship between apparent grain boundary resistivity and the grain size corresponding to the result from the DC measurement. For samples sintering at 1100-1200℃, true grain boundary resistivity was nearly two order lower than that sintering at 1500℃ resulted from lower charge density of Sm''Ce in space charge layer around the grain boundary which will make the oxygen vacancy pass across with less activation energy.
According to space charge theory, the grain boundary interface would carry an electrical potential resulted from the presence of excess ion of one sign, this will be compensated by a space-charge potential with the opposite sign adjacent to the grain boundary. Owing to the highly disordered structure of the grain boundary interface, the oxygen vacancy transport resistance at interface is negligible to that at space charge layer. Therefore, the intrinsic grain boundary resistivity of Samarium doping ceria is resulted from oxygen vacancy trapping with Sm''Ce in the space charge layer and a simple space charge density model was developed. In this model, we predicted that the Sm''Ce charge density increases with increasing sintering temperature and this will result in the higher probability to form complex for transporting oxygen vacancy. This is confirmed with the higher grain boundary resistivity resulted from higher activation energy.
In addition, by lowering the sintering temperature, the grain resistivity decreases obviously compared to those sintering at high temperature, especially for ceria doped with small amount of samarium. The reason is that insufficient energy to derive the components within the grain diffuse completely therefore resulted in not well-distributed composition. As oxygen vacancy passes through, it chooses the environment with less resistance, and this will lowering the grain resistivity.
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