Enhanced Shrinkage of Lanthanum Strontium Manganite (La_0.90Sr_0.10MnO_3+δ) Resulting from Thermal and Oxygen Partial Pressure Cycling

Abstract

Exposure of (La0.90Sr0.10)0.98MnO3+δ (LSM‐10) to repeated oxygen partial pressure cycles (air/10 ppm O2) resulted in enhanced densification rates, similar to behavior shown previously due to thermal cycling. Shrinkage rates in the temperature range 700°–1000°C were orders of magnitude higher than Makipirtti–Meng model estimations based on stepwise isothermal dilatometry results at a high temperature. A maximum in enhanced shrinkage due to oxygen partial pressure cycling occurred at 900°C. Shrinkage was the greatest when LSM‐10 bars that were first equilibrated in air were exposed to gas flows of lower oxygen fugacity than in the reverse direction. The former creates transient cation and oxygen vacancies well above the equilibrium concentration, resulting in enhanced mobility. These vacancies annihilate as Schottky equilibria are reestablished, whereas the latter condition does not lead to excess vacancy concentrations.

Low-temperature sintering of lanthanum strontium manganite-based contact pastes for SOFCs

“…Thermal processing of steel/contact paste/steel sandwich specimens consisted of repeated and alternating exposure to air (0.21 atm O 2 , 10 min) and to nitrogen (∼10 −5 atm O 2 , 10 min) at 900 • C within a closed-ended tube furnace at 900 • C. These conditions were shown by McCarthy et al [24] to result in a local maximum in the rate of densification of LSM-10. During processing, the coupons were subjected to a uniaxial force of 35 kPa perpendicular to the plane of the contact paste.…”

“…For temperatures of ∼1100 • C and above, rates measured in alternating air-nitrogen and in air were similar. McCarthy et al [24] related this behavior to changes in oxygen non-stoichiometry in LSM-10, which results in the transient co-existence of cation and oxygen vacancies above equilibrium concentrations.…”

“…3 and summarized in Table 1. Thermal processing was performed at 900 • C, shown previously to give the greatest enhancement in sintering of LSM-10 in alternating oxygen and nitrogen exposure [24]. A cycle time of 20 min (10 min in flowing air followed by 10 min in flowing nitrogen) was used, also consistent with conditions that resulted in the highest sintering rates [24].…”

“…Thermal processing was performed at 900 • C, shown previously to give the greatest enhancement in sintering of LSM-10 in alternating oxygen and nitrogen exposure [24]. A cycle time of 20 min (10 min in flowing air followed by 10 min in flowing nitrogen) was used, also consistent with conditions that resulted in the highest sintering rates [24]. Bond strengths did not change significantly for processing times greater than 2 h. Alternating air-nitrogen cycles clearly are critical to the formation of strong bonds with LSM-10: samples processed in air at 900 • C developed negligible bond strengths.…”

“…It is possible to lower the sintering temperature of LSM through partial substitution of cobalt for manganese and through tailoring of the particle size distribution [23]. McCarthy et al [24] recently demonstrated that sintering of (La 0.90 Sr 0.10 ) 0.98 MnO 3 + δ (LSM-10) occurred substantially faster when exposed to alternating exposure to air and to nitrogen (10 ppm O 2 ) than in air for temperatures <∼1000 • C. Enhanced sintering was attributed to the transient co-existence of cation and oxygen vacancies, which increased mobility. In this paper, principles given by McCarthy et al [24] have been applied to the processing of LSM-based contact pastes at low temperature.…”