Thermal Expansion And Electrical Conducting Properties Of Ln_0.6Sr_0.4Co_1-yFe_yO₃(y=0.2,0.8) Complex Oxides

Perovskite complex oxides of Ln1-xSrxCO1-yFeyO3(LnSCF, Ln=La, Pr, Nd, Sm) compositions have received increasing attention in recent years due to their superior mixed electronic-ionic conducting 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 the present thesis, the glycine-nitrate process (GNP) was adopted to synthesize Ln0.6Sr0.4Co1-yFeyO3 (y=0.2, 0.8) oxides. The synthesis and preparation, microstructural characteristics, thermal expansion and mixed conducting properties of Ln0.6Sr0.4Co1-yFeyO3 (y=0.2, 0.8) oxides have been investigated. The thermal expansion and mixed electronic-ionic conducting properties have been interpreted with respect to composition and structure.Ln0.6Sr0.4Co1-yFeyO3 (y=0.2, 0.8) fine powders have been synthesized by the GNP method. DSC-TG measurement was employed to analyze the formation process of perovskite structure as a function of temperature. The crystal structure and morphology of synthesized powders have been examined by XRD and SEM, respectively. The preferred synthesis conditions, including the mole ratio of glycine to total metal cation content (abbreviated as G/Mn+), pH of precursor solution, combustion condition and calcining temperature, were ascertained.The thermal expansion properties of Ln0.6Sr0.4Co1-yFeyO3 (y=0.2, 0.8) were measured using a thermal dilatometer method. The thermal expansion curves are almost linear up to 600 ℃, and the slops increase at higher temperatures, which can be attributed to the loss of lattice oxygen and the formation of oxygen vacancies. The lattice energies of Ln0.6Sr0.4Co1-yFeyO3 (y=0.2, 0.8) were calculated from thermodynamic data in order to understand the variation in thermal expansion coefficient (TEC) with the change in the radius of Ln ions. The result infers to a decline of TEC with decrease of Ln ionic radius. The values of average TEC from room temperature to 600℃ are generally consistent with the inference. The anomalies contrast to the inference can be attributed to microstructural reason. The compositions with higher Co/Fe ratio exhibit higher TEC values.The electronic and ionic conductivities of Ln0.6Sr0.4Co1-yFeyO3 (y=0.2, 0.8) ceramics were evaluated by a dc four-terminal method and ac impedance spectroscopy using a two-terminal blocking electrode method, respectively. The electronic conductivities of the complex oxides increase with temperature up to the maximum values around 600 ℃ and then decrease. At an identical measuring temperature, the electronic conductivities of the oxides degrade with the decrease in radius of Ln ions due to increased lattice distortion, and enhance with higher Co/Fe ratio. The activation energy for small polaron hopping exhibits the same trend to that of theelectronic conductivity with compositional change. The ionic conductivities of Ln0.6Sr0.4Co1-yFeyO3 (y=0.2, 0.8) ceramics are not sensitive to the nature of Ln ions, with the compositions with higher Co/Fe ratio providing higher ionic conducting properties.The crystalline structure, microstructure and chemical situation of Fe ion of Ln0.6Sr0.4Co1-yFeyO3 (y=0.2, 0.8) ceramics were characterized by XRD, SEM and Mossbauer spectroscopy. A transition from rhombohedral symmetry to orthorhombic symmetry was found with the decrease in the radius of Ln ions. SEM observation revealed well-grown grains and dense microstructure for the compositions with higher Co/Fe ratio. Large amount liquid was detected in the specimens containing Sm. It was found that the microstructural features of the compositions have an important effect on their physical properties. MOssbauer spectroscopy revealed an increase in Fe-O bond strength and lattice distortion with the decrease in the radius of Ln ions.