Synthesis and properties of strontium-doped yttrium manganite
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Mladen F. Trubelja and Vladimir S. Stubican Center for Advanced Materials, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 14 February 1994; accepted 17 June 1994)
The system Yi_xSr^MnO3 (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^Sr^MnOa 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 YMn 2 O 5 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, YMn 2 O 5 , was formed as a decomposition product of a portion of the major hexagonal YMnO 3 at 840 °C, and subsequently reacted with Y 2 O 3 back to the hexagonal YMnO 3 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 Lai_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 Yi_xSrxMnO3. The microcracking presents a major obstacle to the use of this material as a cathode in solid oxide fuel cells.
I. INTRODUCTION Due to their ability to convert fossil fuel energy cleanly, directly, and efficiently into electrical energy, solid oxide fuel cells (SOFC's) have been attracting increasing attention from both the industrial and scientific communities.1 In one of the most efficient fuel cell designs, the monolithic design, a dense Y 2 O 3 -stabilized ZrC>2 (YSZ) electrolyte is sandwiched between a porous cathode and a porous anode. In order to increase the overall cell output voltage, alternate cells are electrically connected in series via a dense interconnect. This laminated structure is coflred at temperatures above 1550 °C and operated at 1000 °C. The high corking temperature, as well as the temperature and oxygen partial pressure of operation, impose severe chemical, mechanical, electrical, and materials-compatibility requirements on all components of the SOFC. Successful commercialization of SOFC's calls for improvements in both the materials' selection and properties, and in the cell design.1'2 The ideal cathode material would exhibit a high mix of electronic and oxygen ion conductivity, a high catalytic activity for oxygen molecule reduction, chemical stability in a wide range of oxygen partial pressures, a matching thermal expanJ. Mater. Res., Vol. 9, N
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