Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF5582) is a mixed ionic and electronic conductor which could be used for oxygen separation. The thermochemical behavior of the material is a main factor in the selection of the material for membrane technology. In this work the degradation and the reversibility of degradation is investigated.
It is known from literature that BSCF5582 forms (Ba,Sr)CO₃ if it is annealed in CO₂-containing atmosphere.¹ Hence, annealing experiments in 20% CO₂ in air are conducted. Samples are heated up to 800°C and dwelled for 200h. The microstructure is investigated by SEM+EDX. In Fig. 1A a SEM image of the annealed sample is shown. As can be seen a carbonate layer (3.5μm) is formed on the surface. Below the layer needle-like structures occurred (16μm). The reversibility of the degradation is checked by further annealing. As-annealed samples are then annealed in synthetic air at 800°C for 200h. As can be seen in Fig. 1B the carbonate layer is decomposed and SrO is formed. The needle-like structures are partially destroyed.

Fig. 1: SEM images of BSCF5582 annealed in 20% CO₂ in air (A) and in 20% CO₂ in air, afterwards annealed in synthetic air (B)

The tendency of BSCF5582 to react with CO₂ and the reversibility of the formation of carbonates is investigated by DTA measurements. Therefore, samples are heated up to 1100°C and cooled down in air, heated up to 1100°C and cooled down in 20% CO₂ in air, and heated up to 1100°C and cooled down in air again. Fig. 2 shows the normalized mass change. After heating up and cooling down in 20% CO₂ in air there is an enduring increase in weight. Anyway, after heating up and cooling down in air again the origin is reached. Hence, it could be assumed that the formation of carbonates is reversible. Our results are in good agreement with literature.²

Fig. 2: Normalized mass change of BSCF5582 with and without CO₂

References

1 Arnold et al., J. Membr. Sci. 293 (2007) 44–52

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