Influence of different perovskite interlayers on the electrical conductivity between La_0.65Sr_0.3MnO₃ and Fe/Cr-based steels

The electrical conductivity of the three-layer system {La_0.65Sr_0.3MnO₃ (LSM)}/{Perovskite Contact Layer (PCL)}/Steel was studied by a DC four-point contact method for 3200 h in air at 800 °C. The four commercial steels Crofer22APU, ZMG232, X2CrTiNb18 (DIN 1.4509), and X18CrN28 (DIN 1.4749) were investigated. The three perovskite compositions La_0.8Sr_0.2Mn_0.5Co_0.5O₃ (LSMC), LaMn_0.4Co_0.6O₃ (LMC) and Y_0.3Ca_0.7MnO₃ (YCM) were used as contact layer material. Moreover, the electrical conductivity of the systems without a contact layer, i.e. of the {LSM/LSM/Steel} systems, was investigated for comparison. Electrical and thermal cycling was additionally carried out. The LSM/Crofer22APU combination showed the lowest change of resistance over time and can be used in a SOFC without the contact layers considered in this paper. The systems with the steels Crofer22APU, ZMG232, or DIN 1.4509 coated by LSMC and the ZMG232/YCM system showed a low contact resistance and a low rate for the increase of the resistance. This rate was high for all systems with the steel DIN 1.4749 independent of the perovskite used as the contact layer. Microstructure and composition of the oxide layer formed at the different {PCL/steel} interfaces were investigated using high-resolution SEM/EDX analysis. The results indicate that the strong degradation observed with the steel DIN 1.4749 is caused by the formation of Si-containing phases with low electrical conductivity and a low thermal expansion coefficient. The thermal expansion coefficient of the perovskites and the steels was determined.