Oxygen exchange and transport in (La_0.6Sr_0.4)_0.98FeO_3-d ⇓Ce_0.9Gd_0.1O_1.95 dual-phase composites

The chemical diffusion coefficient and the effective surface exchange coefficient (k_ex) of dual-phase (La_0.6Sr_0.4)_0.98FeO_3-d (LSF) − Ce_0.9Gd_0.1O_1.95 (CGO) composites containing between 30 and 70 vol.% of CGO were determined by electrical conductivity relaxation (ECR) at high oxygen partial pressures (10⁻³ < pO₂ < 1 atm) and at temperatures between 600^◦C and 900^◦C. The surface impurity segregation was detected by TOF-SIMS analysis. A large enhancement of k_ex was observed with increasing CGO fraction in the composite. k_ex was increased from 3.51 × 10−⁵ cm/s for a pure LSF to 1.86 × 10−⁴ cm/s for a 70 vol.% of CGO in the composite at 750^◦C for a pO₂ change from 0.2 to 1.0 atm. The experiments demonstrate that the k_ex is enhanced due to a synergistic effect between the two phases, and suggest a direct involvement of CGO phase in the oxygen surface exchange reaction. Possible mechanisms that could account for the synergy are the oxygen exchange process occurs also on the CGO surface, for example a spillover of absorbed oxygen ions from the LSF surface to the CGO surface or/and scavenging of impurities from one phase to another, thereby improving the oxygen exchange properties of the cleaned phase.