Kinetics of Reaction with Oxygen of LnBa1-XSrxCo2-YFeyO6-δ (Ln - Selected Lanthanides) Oxides as Studied by Relaxation Methods

Kinetics of the reaction with oxygen of the oxides, during changes of the oxygen partial pressure in the surrounding atmosphere, can be studied by various relaxation methods (e.g. mass or electrical conductivity relaxation). These methods allow to determine chemical diffusion coefficient (D) and surface exchange coefficient (K), related to the mentioned reaction [1, 2]. Theoretical basis of relaxation methods relies on the solution combining first and second Fick’s law and flux density at the surface boundaries. Depending on the geometry of the actual sample (3D, disc-like, etc.), the respective solutions are different, however, simultaneous and reliable determination of both coefficients requires additional conduction to be filled: 0.03 ≤ l·K/D ≤ 30 [3]. In real cases, this may limit range of temperatures and oxygen partial pressures, at which such determination is possible. Also, it suggests the importance of the dimensional parameter l.

In this work we present results concerning simultaneous determination of D and K, measured by mass and electrical conductivity relaxation techniques for perovskite-type oxides with LnBa_1-xSr_xCo_2-yFe_yO_6-δ (Ln - selected lanthanides) chemical formula. The measurements were performed at elevated temperatures (600-800 ºC) in a wide range of oxygen partial pressures. The obtained results were analyzed taking into account crystal structure and the oxygen content in the considered compounds at high temperatures. It allowed to determine the correlation between structural (radius of Ln cation, degree of unit cell distortion, metal-oxygen-metal angle, unit cell free volume) and transport properties (oxygen diffusion coefficient, surface exchange coefficient, ionic and electronic component of the electrical conductivity) in the considered LnBa_1-xSr_xCo_2-yFe_yO_6-δ.

Acknowledgements

The project was funded by the National Science Centre Poland (NCN) on the basis of the decision number DEC-2012/05/E/ST5/03772.

References

[1] G. Xiao, Q. Liu, F. Zhao, L. Zhang, C. Xia, F. Chen, J. Electrochem. Soc. 158 (2011) B455.

[2] J.A. Lane, J.A. Kilner, Solid State Ionics, 136-137 (2000) 997.

[3] M.W. den Otter, H.J.M. Bouwmeester, B.A. Boukamp, H. Verweij, J. Electrochem. Soc. 148 (2001) J1.