| La1-xSrxCo1-yFeyO3 perovskite-type composite oxides is a wonderful mixed electrical-ionic conductor, characteristic of good thermal and chemical stability, is a potential candidate for SOFC. In this paper, La1-xSrxCo1-yFeyO3 cathode materials are prepared by solid phase reaction technique, the structure and properties are studied by modern testing technique.The results of XRD.DTA-TG and SEM show that the synthesized samples are single perovskite phase, no other phase is detected. Moreover, the diffraction peak shifts toward high angle as Sr content increases. It is ascribed to the substitution of La3+ by Sr2+ in LSCF increases the average radius of A ions and causes a charge imbalance. In order to maintain electrical neutrality, some trivalent B ions oxidized to form B4+, which resulted in a net decrease in the average radius of the B ions, as a result the volume of the unit cell decreases. The diffraction peak angle doesn't change obviously as Co/Fe ratio, it is attributed to the radius of Co Fe ions are fairly similar, so the influence of substitutions on unit cell is small. TG curves indicate, there were no detectable weight loss below 400 in any composition, the temperature at which weight loss becomes significant decreases, and the magnitude of weight loss increases with increasing Sr content, but the onset temperature of weight loss increases and the amount of this loss decreases with increasing Fe content.The XPS analyses show that there were two different chemical conditions of 0 when Sr doped, corresponding to lattice oxygen and absorption oxygen, respectively. Co ions exists with three chemical conditions, Co2+ Co3+ and Co4+, it is due to Co3+ charge disproportionation and trivalent Co3+ hopping towards Co4+, moreover, disproportionation reaction is available easily as increasing Sr content, while increasing Fe content makes reaction difficult. There are only two chemical conditions for Fe with Fe3+ Fe4+.The electrical conductivity increases to a maximum then decreases as temperature increases. The magnitude of the conductivity maximum increases and shifts to lower temperature with increasing Sr content. In this paper, the electrical conductivity reaches maximum value at x=0.4. Below the temperature corresponding to the maximum value, the electrical conductivity is found to follow the relationship for the small polaron hopping mechanism, charge compensation of oxygen vacancy dominates electrical conduction at high temperature, and oxygen vacancy acts as traps to catch carriers, resulting in thedecrease of carriers concentration and mobility. As a result, the electrical conductivity decreases. In the thermal expansion curves of LSCF, there is a temperature at which thermal expansion curves become steeper, the temperature is found to be coincided with the onset temperature of TG curves, which illustrates that thermal expansion is due to the creation of oxygen vacancy.Ionic conduction is attributed to the charge compensation of oxygen vacancy. At high temperature, oxygen vacancy conducts ions transport as carriers and offers tunnels for oxygen ions transport. Therefore, the ionic conductivity is very high. Electrical conductivity is due to electron hopping between B3+ and B4+, resulting in the creation of B3+-O-B4+ small polaron. Furthermore, charge disproportionation of Co3+; charge compensation of oxygen vacancy; Fe4+ preferential electronic compensation. All these factors contribute to electrical conduction.This work was supported by HuBei Province Natural Science Foundation (Grant No. 2001ABB075)...
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