Synthesis and properties of strontium-doped yttrium manganite

The system Y_1−xSr_xMnO₃ (x = 0.000, 0.005, 0.010, 0.050, and 0.100) was studied as a potential cathode material for solid oxide fuel cells. Powders were prepared using an organometallic precursor; however, achieving homogeneous compositions was complicated due to the presence of intermediate, metastable phases. The desired hexagonal Y_1−xSr_xMnO₃ phase formed from the precursor at 800 °C, while small amounts of a metastable orthorhombic (Y,Sr)MnO₃ phase formed in the temperature range between 850°and 960 °C, and another orthorhombic YMn₂O₅ phase between 840°and 1200 °C. The metastable (Y, Sr)MnO₃ phase readily transformed into the stable hexagonal phase at about 960 °C. The other metastable intermediate phase, YMn₂O₅, was formed as a decomposition product of a portion of the major hexagonal YMnO₃ at 840 °C, and subsequently reacted with Y₂O₃ back to the hexagonal YMnO₃ at 1200 °C. For the studied compositions, densities higher than 95% theoretical could be obtained by sintering in air at temperatures above 1400 °C for 2 h. The investigated system was comparable in electrical conductivity with the current cathode material La_1−xSr_xMnO₃, and had an average apparent thermal expansion coefficient between 5 and 7 ppm/°C in the temperature range between 200°and 1000 °C. Unfortunately microcracking was observed in all sintered specimens, possibly caused by a high-temperature phase transition between the hexagonal and cubic polymorphs of Y_1−xSr_xMnO₃. The microcracking presents a major obstacle to the use of this material as a cathode in solid oxide fuel cells.