Mechanical and electrical properties of porous lanthanum strontium manganite at operating temperature

Abstract

Porous lanthanum strontium manganite (LSM) for the cathode in solid oxide fuel cells (SOFCs), was prepared with the fine perovskite oxide powder and organic PMMA particles as the pore-former. After calcination of organic particles, porous LSM with different porosity was synthesized at 1673K for 1 hr by pressureless sintering. Porosity and microstructure of porous LSM were characterized. At room temperature or at 1273K of operating temperature, fracture strength was estimated by three-point bending test and Young's modulus was by the strain under bending test. The fracture toughness was also measured by SEVNB method. Electrical conductivity at operating temperature was confirmed by the four-terminal method, using the specimen similar to that measured mechanical properties. With addition of PMMA particles, the uniformly spherical and isolated pores with approximately 10μm in diameter were observed below 0.25 porosity. Above 0.25, continuous pores were mainly observed. The fracture strength decreased with the increase of the porosity independent of testing temperature. Isolated or continued pores in LSM matrix changed the decreasing rate of strength. Normalized strength, strength for porous LSM divided by that for dense LSM, was agreed at each porosity, though strength at operating temperature was higher than that at room temperature. Apparent Young's modulus for porous LSM also showed similar tendency. Slightly decrease of fracture toughness for porous LSM at operating temperature was observed due to the healed wake zone or enlarged the frontal process zone. Electrical conductivity at operating temperature and thermal conductivity at room temperature decreased similarly with porosity. These properties of porous LSM were discussed using the effective volume of solid phase.