Surface Segregation Phenomena in La0.6Sr0.4Co0.2Fe0.8O3-δ Subjected to Mechanical Stress

The present study reports on the surface segregation phenomena in La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) subjected to mechanical stress. LSCF samples with mirror-polished surfaces were annealed under uniaxial compression and also four-point bending to investigate the effects of the compressive and tensile stresses on the surface segregation. The annealing was performed at up to 1173 K under up to 100 MPa of mechanical stress. The surface segregaion was examined by using SEM, EDX, and micro-Raman spectroscopy. The Sr-rich phase segregates on the surface, especially where specific surface area is large. The size of the segregated particles increases with increasing annealing time, whereas their number reveals no clear dependence. The number of the segregated particles is fewer, whilst the particle size is larger for the sample annealed under the uniaxial compression, compared to the one annealed without compression. There is a significant difference in the segregation phenomena between the tensile and compressive surfaces annealed under the bending. The effects of the stress distribution as well as the porous structure on the segregation are also demonstrated. In addition to the segregaion, high-temperature creep deformation due to Sr diffusion and grain boundary sliding is observed. Based on the above results, the mechanism for the Sr surface segregation in LSCF under mechanical stress is discussed, considering Sr diffusion under microscopic and macroscopic stress fields. The surface reactivity of LSCF under mechanical stress is briefly discussed.