Synthesis and properties of strontium-doped yttrium manganite

Abstract

The system Y1−xSrxMnO3 (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 Y1−xSrxMnO3 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 La1−xSrxMnO3, 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 Y1−xSrxMnO3. The microcracking presents a major obstacle to the use of this material as a cathode in solid oxide fuel cells.