Domain Reorientations and Its Influence on Mechanical Properties of La0.6Sr0.4Co0.2Fe0.8O3-δ

Mechanical degradations in solid oxide fuel cells (SOFCs) are one of the issues to be solved for their full scale commercialization. In order to suppress the mechanical degradation, the mechanical properties of SOFC components should be correctly understood. Elastic modulus of La_1-xSr_xCo_1-yFe_yO_3-δ (LSCF), which is a typical cathode material, is found to drastically decrease with increasing temperature in the rhombohedral phase [1]. One possible cause of the drastic decrease is domain reorientation. The rhombohedral LSCF can have four different domain states in the rhombohedral phase [2]. When the material is exposed to a mechanical stress, domains start to reorient [2, 3]. Such domain reorientation relaxes the mechanical stress and can cause the enormous decrease in the elastic modulus. In this study, the dynamic elastic modulus of rhombohedral La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF6428) at a range of frequencies was evaluated by a dynamic mechanical analysis (DMA) in a wide frequency range in order to understand the influence of domain reorientation.

The sample were made from powders obtained from AGC Seimi Chemical Co., Ltd.. The samples were sintered at 1573 K for 6 hours and then cut into rectangles and polished with a diamond paste. Relative densities of the rectangles were over 95%. The dynamic Young’s modulus and internal friction of LSCF6428 were evaluated by DMA in 3-point bending geometry. The measurement was performed in 100% N₂ with changing the temperature from room temperature to 773 K and frequency from 0.01 to 100 Hz.

Fig.1 shows the dynamic Young’s modulus and internal friction of LSCF6428 as a function of temperature at a range of frequencies. The dynamic Young’s modulus was almost independent of frequencies at room temperature. It significantly decreased with increasing temperature. The significant decrease was followed by a peak in the internal friction. The temperature at which the dynamic Young’s modulus started to significantly decrease and the peak in the internal friction was observed became higher with increasing frequencies. The above temperature dependence of the dynamic Young’s modulus and the internal friction can be explained by the domain reorientation. The domain reorientation is known to be thermally activated process [4]. At the room temperature, the relaxation time of the domain reorientation is too long. Therefore, the dynamic Young’s modulus was not influenced by the domain reorientation and showed a higher value. With increasing temperature, domains start to reorient and the reorientation causes the apparent elastic softening. Simultaneously, the internal friction shows higher values due to the energy dissipation by the domain reorientation. These results suggest the stress distribution in SOFCs at low temperatures is affected by the domain reorientation in LSCF.

Acknowledgement

This work was supported by the New Energy and Industrial Technology development Organization (NEDO), Japan. This work was supported by JSPS Grant-in-aid for JSPS Fellows Grant Number 25-7156.

References

[1] Y. Kimura, T. Kushi, S. Hashimoto, K. Amezawa and T. Kawada, J. Am. Ceram. Soc., 95, 2608 (2012).

[2] P. E. Vullum, R. Holmestad, H. L. Lein, J. Mastin, M.-A. Einarsrud and T. Grande, Adv. Mater., 19, 4399 (2007).

 [3] W. Araki and J. Malzbender, J. Eur. Ceram. Soc., 33, 805 (2013).

 [4] R. J. Harrison and S. A. T. Redfern, Physics of the Earth and Planetary Interiors, 134, 253 (2002).

Figure caption

Fig. 1. Dynamic Young’s modulus and internal friction of LSCF6428 as a function of temperature at a range of frequencies.