Synthesis and properties of La0.7Sr0.3MnO3 cathode by gel combustion

La0.7Sr0.3MnO3, a good candidate material for a cathode of solid oxide fuel cells, was synthesized by gel combustion method to obtain fine grain size and good sinterability. Powders with the perovskite structure, rather than the amorphous one, were directly obtained after self-propagating combustion of dried gels. The influences of pH and content of citric acid on the stability of the sol system, the combustion process and the phase formation were focused on by this study. The contributions of La3+, Sr2+ and Mn2+ and metal complexes at different pH values were calculated to determine the suitable pH value for the sol system. It was found from experiments that the sol was stable and clear at pH below 3, while white precipitates appeared at pH above 3. The combustion extent was improved with increasing pH and with decreasing the content of citric acid. Almost single phase of perovskite was found at low ratio of citric acid to nitrates (denote this ratio as φ). However a small amount of second phase occurred with increasing φ to above 1. A rational density of the sample prepared via high φ value (1.5) was obtained after sintering at 1300 °C for 2 h. The electrical conductivity of samples was measured by a DC four-probe method. It was demonstrated that conductivity enhanced with increasing φ. The maximum conductivity was 150 S/cm for the above-mentioned sample. It was revealed that the optimal φ was 1.5 when considering both phase formation and sinterability of the powder.