Phase transformation and oxygen equilibration kinetics of pure and Zr-doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite oxide probed by electrical conductivity relaxation

Abstract

Electrical conductivity relaxation is used for monitoring the cubic-to-hexagonal phase transition in the perovskite-type oxide Ba0.5Sr0.5Co0.8Fe0.2O3−δ. After in situ annealing at 1173 K, at which temperature the cubic polymorph is stable, electrical conductivity relaxation experiments are conducted during long-term annealing at 1073 K. The decreases in the surface exchange and chemical diffusion coefficients with time due to the sluggish phase transition are found to be correlated. It is demonstrated that B-site doping with Zr to an amount as low as 3 mol % stabilizes the high-temperature perovskite cubic phase of Ba0.5Sr0.5Co0.8Fe0.2O3−δ, with preservation of its high oxygen transport and exchange properties.

In-situ X-ray diffraction study of carbonate formation and decomposition in perovskite-type BCFZ

“…Poor stability of Co-containing perovskite caused by flexible redox behavior of Co-cations was already discussed in some details in ours previous reports [25][26][27]. Furthermore, these results are in good agreement with reports of Yakovlev et al [17] as well as Yi et al [28] suggesting the stabilizing effect of higher valent B-site cation like Zr 4 þ or Nb 5 þ on the Co-, Fe-containing perovskite materials. The remaining perovskite phase was found to be Fe-enriched.…”

“…Note, the co-existence of two cubic perovskite phases after doping of 5-10% Zr was also reported for similar perovskite systems like Ba 0.5 Sr 0.5 Co 0. 8 [17,18]. The high-temperature XRD pattern taken from the BCFZ powder annealed at 1173 K for 30 min in the flowing synthetic air evidences no structural changes.…”

Permeation model and experimental investigation of mixed conducting membranes

“…The big discrepancies between diffusion coefficients may be related to the phase transformation from cubic to hexagonal at intermediate temperatures. 40 Regarding the different experimental methods listed in Table 2, it should be pointed out that conductivity relaxation and tracer diffusion experiments were performed in thermal equilibrium, whereas the oxygen permeation was studied under permeation steady state. Therefore, the kinetic parameters obtained under permeation steady states are more useful for practical applications in the field of MIEC membranes and SOFCs.…”

High temperature X-ray diffraction and thermo-gravimetrical analysis of the cubic perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ under different atmospheres

“…The creep rate of BSCF ceramics is exceptionally high, illustrating the challenge of keeping the cation diffusion low while optimizing the oxygen vacancy mobility 20 . The low creep resistance questioning the actual use of BSCF in high temperature gas separation membranes and several attempts to stabilize BSCF by chemical substitution have appeared, both to avoid the formation of the hexagonal polymorph and to increase the creep resistance [22][23] . Chemical expansion [24][25] , particularly for ceria and perovskite materials with transition metals (Co, Fe) on the Bsite is another concern with respect to the mechanical stability of oxygen permeation membranes [26][27][28][29][30][31][32] .…”