Oxygen tracer diffusion and surface exchange kinetics in Ba0.5Sr0.5Co0.8Fe0.2O3-δ

The oxygen tracer diffusion coefficient, D-b*, and the oxygen tracer surface exchange coefficient, k, were measured in Ba0.5Sr0.5Co0.8Fe0.2O3 - delta (BSCF5582) over the temperature range of 310-800 degrees C and the oxygen partial pressure range of 1.3 x 10(-3)-0.21 bar. Several measurement techniques were used: isotope exchange followed by depth profiling (IEDP) within individual single grains or line scanning (IELS) along the sample cross-section and gas-phase analysis (GPA). Surface exchange kinetics was initially found to be slow and presumably inhibited by the formation of a passivating layer on the sample surface. High temperature pre-anneals (900-950 degrees C) changed the nature of this layer and enhanced surface exchange. Fast bulk oxygen diffusion and surface exchange kinetics were observed after high temperature pre-anneals within the temperature range studied. The activation energies for O-18 tracer diffusion and surface exchange at 0.21 bar were 0.72 +/- 0.05 and 1.10 +/- 0.15 eV, respectively. The tracer diffusion coefficient showed weak dependence upon oxygen partial pressure, whereas the surface exchange coefficient exhibited strong oxygen partial pressure dependence. The microstructure of the samples (the porosity and grain size) had a profound effect on the measured tracer diffusion coefficient. (C) 2014 Elsevier B.V. All rights reserved.