Introduction
Solid oxide fuel cells (SOFCs) are an electrochemical conversion system with high efficiency and low-emission of pollution. In order to develop a commercial SOFC system with high performance and long-term stability, it is important to understand temperature distribution in the cell and stack [1]. For this purpose, thermal properties of SOFC components should be evaluated particularly under SOFC operating conditions, e.g. at elevated temperatures and under various oxygen partial pressures. Although there exist several reports on the thermal properties of the SOFC components, most of the data were measured at room temperature or in some specific atmospheric condition. [2-3] Thus, we aimed to investigate thermal properties of SOFC components as functions of temperature and oxygen partial pressure.
Perovskite-type oxides La0.6Sr0.4Co1-xFexO3-δ(LSCF) are selected for this work. They are known to exhibit oxygen nonstoichiometry changes depending on temperature and oxygen potential. [4-7] The variation in oxygen vacancy concentration is known to influence electrochemical properties and mechanical properties. However, little is known about the effect on the thermal properties although. There exist several reports on the thermal properties of the LSCF. In this study, thermal diffusivities, heat capacities and thermal conductivities have been investigated by using the laser flash method and TG-DSC at high temperatures up to 1273 K under controlled oxygen atmospheres. Dependencies of the thermal properties of LSCF on temperature, oxygen partial pressure and the material composition will be discussed in terms of oxygen nonsotichiometry and crystal structures.
Experimental
Powders of La0.6Sr0.4Co1-xFexO3-δ (LSCF) were prepared by a conventional Pechini method. Obtained powders were hydrostatically pressed at 150 MPa into compacts, and then sintered in air at 1573K-1473K for 6h. For the thermal diffusivity measurement, the sintered compacts were cut into rectangles (c.a. 10 x 10 x 1 mm). For the TG-DSC measurement, the sintered compacts were cut smaller than Pt crucible. The phase of sintered samples was characterized by XRD to investigate any phase change after sintering. Thermal diffusivities and heat capacities of the LSCF were measured by using the laser flash method (LFA 457 Micro Flash, NETZSCH) and TG-DSC (STA 447, NETZSCH) in the oxygen partial pressure range from 10-4 to 0.2 bar and in the temperature range from R.T. to 1173 K. In order to control the atmospheres, a gas mixing system and an oxygen sensor are additionally attached to the laser flash system. The thermal conductivities (λ) of LSCF were calculated by using Eq(1). α : thermal diffusivity of LSCF with respect to temperature, ρ : the density of sintered LSCF sample and Cp: heat capacity of the LSCF as a function of temperature.
λ = α·ρ·Cp (1)
Results and Discussion
In this study, the thermal properties of perovskite-type oxides La0.6Sr0.4Co1-xFexO3-δ (LSCF) were investigated. The thermal diffusivities of LSCF were dependence of temperature as well as oxygen partial pressure and Co contents ratio in intermediate temperatures (873K to 1173K).[Fig.1] The thermal diffusivity of LSCF decreased gradually as p(O2) decreased at all investigated temperatures, and decreased as temperature increased in all investigated p(O2) range. The thermal diffusivity depended on oxygen partial pressure more significant in lower oxygen partial pressure and at higher temperature. The thermal diffusivity increases with Co contents ratio increases and each composition increases with temperature through a maximum, then decreases. Heat capacity and thermal conductivity of LSCF (x=0, 0.8, 1.0) were almost constant from R.T to 873K, however increasing with increasing temperature over 873K. However, measured heat capacity compare with the Neumann-Kopp rule-specific heat of LSCF (x=0, 0.8, 1.0) were not consistent. LSCF is known to show the oxygen nonstoichiometry changes at relatively higher temperatures. Therefore, we suggest these dependenciese could be interpreted by the change in the oxygen nonstoichiometry, meaning that the thermal properties of LSCF were significantly affected by the oxygen nonstoichiometry.
References
[1] S.C. Singhal, K. Kendall, 1-85617-387-9 Elsevier, Kidlington Oxford(2003)
[2] I. Yasuda and M. Hishinuma, Solid State Ionics, 80 (1995) 141-150
[3] R. Gawel, K. Przybylski, M. Viviani, Materials Chemistry and Physics147 (2014) 804e814
[4] M.V. Krishnaiah, P. S. Murti and C.K. Mathews, Thermochimica Acru, 140 (1989) 103-107
[5] H. Iwahara, Solid State Ionics,77 (1995) 289.
[6] H. Iwahara, Solid State Ionics, 86-88(1996) 9.
[7] S. Hashimoto, H. Nishino, Y. Liu, K. Asano, M. Mori, Y. Funahashi, Y. Fujishiro, J. Electrochem. Soc. 155–6 (2008) B587