SYNTHESIS AND PROPERTIES OF STRONTIUM-DOPED YTTRIUM MANGANITE

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. P owders were prepared using an organometallic precursor; however, achie ving homogeneous compositions was complicated due to the presence of i ntermediate, metastable phases. The desired hexagonal Y1-xSrxMnO3 phas e formed from the precursor at 800 degrees C, while small amounts of a metastable orthorhombic (Y, Sr)MnO3 phase formed in the temperature r ange between 850 degrees and 960 degrees C, and another orthorhombic Y Mn2O5 phase between 840 degrees and 1200 degrees C. The metastable (Y, Sr)MnO3 phase readily transformed into the stable hexagonal phase at about 960 degrees C. The other metastable intermediate phase, YMn2O5, was formed as a decomposition product of a portion of the major hexago nal YMnO3 at 840 degrees C, and subsequently reacted with Y2O3 back to the hexagonal YMnO3 at 1200 degrees C. For the studied compositions, densities higher than 95% theoretical could be obtained by sintering i n air at temperatures above 1400 degrees C for 2 h. The investigated s ystem was comparable in electrical conductivity with the current catho de material La1-xSrxMnO3, and had an average apparent thermal expansio n coefficient between 5 and 7 ppm/degrees C in the temperature range b etween 200 degrees and 1000 degrees C. Unfortunately microcracking was observed in all sintered specimens, possibly caused by a high-tempera ture phase transition between the hexagonal and cubic polymorphs of Y1 -xSrxMnO3. The microcracking presents a major obstacle to the use of t his material as a cathode in solid oxide fuel cells.