Sintering Behavior and Phase Characterisation of Composite Perovskite/Fluorite Ceramics for Intermediate Temperature SOF
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0972-AA03-07
Sintering Behavior and Phase Characterisation of Composite Perovskite/Fluorite Ceramics for Intermediate Temperature SOFCs and Oxygen Separation Membranes Natee Tangtrakarn1, Matthew Swanson1, Peter Moran1, Jakob Kuebler2, Jayanta Kapat3, and Nina Orlovskaya3 1 Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI, 49931 2 Empa-Material Science and Technology, Duebendorf, Switzerland 3 Mechanical, Materials & Aerospace Engineering, University of Central Florida, Orlando, FL, 32816-2450
ABSTRACT The sintering behavior and structural changes of fluorite Gd0.2Ce0.8O2-δ (GDC) mixed with one of three perovskites: LaMnO3 (LMO), (La0.7Sr0.3)0.98MnO3 (LSM) and La0.6Sr0.4Fe0.8Co0.2O3 (LSFC) composite ceramics have been studied. Structural changes occur upon sintering depend on the type of perovskite used to make the composite. The LSFC+GDC composite has been found to sinter easily. The two-step sintering mechanism was found to occur in LSM+GDC composite. Of all types of ceramics, only sintered LMO experiences a phase change at sintering temperatures ≥1200oC from rhombohedral R 3 C to orthorhombic Pnma structure with respect to its as received powder phase. The transition is notably suppressed when LMO is part of an LMO+GDC composite. GDC in a fluorite and/perovskite composite, when sintered, undergoes a temperature-dependent expansion in its unit cell that is not observed in pure GDC ceramics. This structural change will impact the function of composite ceramics as fuel cell cathodes or oxygen separation membranes.
INTRODUCTION In recent years, a number of papers have been published related to the development of a composite perovskite/fluorite cathode for intermediate temperature Solid Oxide Fuel Cells [1, 2, 3] and a composite perovskite/fluorite membrane for oxygen separators [4]. Compared to single phase LaxSr1-x MnO3 cathodes, composite (La0.6Sr0.4)1-xFe0.8Co0.2O3+Gd0.1Ce0.9O2-δ cathodes show a significant decrease in polarization resistance [1, 2, 3]. The oxygen ionic conduction in the composite doped-ceria/perovskite oxides is strongly dependent on processing conditions and decreases as interdiffusion of the phases becomes more prevalent. Interdiffusion and the formation of secondary phases during processing can have a negative impact on the oxygen membrane separator [4]. Ionic conduction can be blocked due to the formation of low conductivity layers at the boundary between the GdxCe1-xO2-δ and perovskite grains if lanthanum and strontium cations diffuse into the fluorite phase during elevated temperature processing. High temperature sintering is, however, a critical step for forming a functional composite cathode and an oxygen separation membrane. Sintering temperatures between 900oC-1100oC are required to develop highly porous composite cathodes; the higher sintering temperatures between
1300oC-1500oC are used to produce gas tight oxygen separation membranes. These high sintering temperatures can lead to the interdiffusion of ions, phase changes, and
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