Chemical Method Synthesis, Structure And Mixed Electronic-ionic Conduction Properties Of La_1-xSr_xCo_1-yFe_yO₃ Complex Oxides

Perovskite-type complex oxides of La1-xSrCo1-yFeyO3 compositions are attracting growing attention because of their superior mixed electronic-ionic conduction properties, which make them promising candidate materials for many important applications, including cathodes for intermediate temperature solid oxide fuel cells, oxygen separation membranes, membrane reactors for syngas production and catalysts for oxidation of hydrocarbons. In this dissertation, chemical methods have been employed to synthesize La0.6Sr0.4Co1-yFeyO3oxides, and the contributing factors to synthesis and preparation, microstructural characteristics and mixed conduction properties of La0.6Sr0.4Co1-yFeyO3 oxides have been investigated. The relationships between the composition, structure and mixed conduction properties have been studied, and the mechanism of mixed electronic-ionic conduction has been discussed.La0.6Sr0.4Co1-yFeyO3 fine powders have been synthesized by a Glycine-Nitrate process(GNP) method and a Citrate method. It was found that the mole ratio of glycine to total metal cation content (abbreviated as G/Mn+), Co/Fe ration, combustion condition and calcining temperature generated significant influence on the crystal structure and morphology of the powders synthesized by the GNP method. The desired synthesis conditions to GNP method were ascertained. The mole ratio of citric acid to total metal cation content (abbreviated as C/Mn+), pH of precursor solution, Co/Fe ration and calcining temperature contributed greatly to the formation of homogeneous sol and morphology of the powder synthesized by Citrate method. The preferred synthesis conditions to prepare fine powder with a pure perovskite-type structure were ascertained.The electrical conductivity and ionic conductivity of La0.6Sr0.4Co1-yFeyO3 ceramics were investigated using a dc four-terminal method and a two-terminal blocking electrode method, respectively. The mixed electronic-ionic conduction properties of the ceramic samples depended closely on sintering temperature. The preferred sintering temperature was ascertained to be 1200 ℃ with respect to the mixed electron-ionic conduction properties. The electrical conductivity of La0.6Sr0.4Co1-yFeyO3 oxides decreased monotonously with the increase of temperature in the range of room temperature to 900 ℃, while the electrical conductivities of other compositions increased with temperature through the maximum values near 600 ℃ and then decreased. The ionic conductivities df La0.6Sr0.4Co1-yFeyO3 oxides enhanced with the measuring temperature. At an identical measuring temperature, La0.6Sr0.4Co1-yFeyO3 oxides with higher Co/Fe ration have higher electrical conductivities and ionic conductivities. The activation energies for small polaron hopping and the activation energies for ionic conduction of La0.6Sr0.4Co1-yFeyO3 ceramics reducedwith the increase of Co/Fe ration.The structure of La0.6Sr0.4Co1-yFeyO3 oxides was investigated by XRD, SEM, XPS, Mossbauer, ESR and iodometric titration. A perovskite-type structure with rhombohedral symmetry was certified for the ceramic samples by XRD. SEM showed that the increase of sintering temperature and Co/Fe ration significantly promoted the grain growth and microstructural densification, but the amount of liquid increased considerably when sintering temperature was excessively elevated above 1 200℃. The relative content of chemically adsorbed oxygen, Co4+ ionic, Fe4+ ionic and the oxygen non-stoichiometry of La0.6Sr0.4Co1-yFeyO3 ceramics increased with Co/Fe ration, whereas the average valence of Co ions and Fe ions decreased. For the ceramic samples with a same composition, the average valence of Co ions and Fe ions decreased with the temperature, whereas the oxygen non-stoichiometry increased. It was found that the microstructural change of La0.6Sr0.4Co1-yFeyO3 ceramics is responsible for the variation of mixed conduction properties with sintering temperature, composition and measuring temperature.The effects of synthesis and preparation methods on mixed conduction properties...