Electrical and microstructural characterization of spinel phases as potential coatings for SOFC metallic interconnects

Several spinel samples, i.e., MnxCr3−xO4 (0.5 ≤ x ≤ 2.5), NiCr2O4 and CoCr2O4, were synthesized and studied in terms of phase analysis, density, electrical resistivity and thermal expansion behaviour. The spinel samples were generally single phase; exceptions included MnCr2O4 and Mn0.5Cr2.5O4 with significant amounts of Cr2O3 and NiCr2O4 with trace amounts of NiO.Porosity, in general, decreased with increasing sintering temperature, except for Mn0.5Cr2.5O4, which showed increasing porosity with increasing sintering temperature. NiCr2O4, CoCr2O4 and MnCr2O4, all had similar thermal expansion behaviour, with thermal expansion coefficients (TEC) ranging from 7.2–7.6 × 10−6/°C. The TEC difference between the spinels and ferritic stainless steel was larger than the difference between the steel and chromia, which had a TEC of 9.6 × 10−6/°C. The spinels and chromia exhibited semiconductor-type behaviour, with electrical resistivities decreasing with increasing temperature. Only Mn2CrO4 and NiCr2O4 had resistivities lower than Cr2O3 over the entire temperature range of testing (20–900 °C). For MnxCr3−xO4, resistivity decreased with increasing Mn content.