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

摘要

Several spinel samples, i.e., Mn_xCr_(3-x)O_4 (0.5 < = x < = 2.5), NiCr_2O_4 and CoCr_2O_4, 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 MnCr_2O_4 and Mn_(0.5)Cr_(2.5)O_4 with significant amounts of Cr_2O_34 and NiCr_2O_4 with trace amounts of NiO. Porosity, in general, decreased with increasing sintering temperature, except for Mno.5Cr2.5O4, which showed increasing porosity with increasing sintering temperature. NiCr_2O_4, CoCr_2O_4 and MnCr_2O_4, all had similar thermal expansion behaviour, with thermal expansion coefficients (TEC) ranging from 1.2-1.6 X 10~(-6)/ deg C. The TEC difference between the spinels and fenitic stainless steel was larger than the difference between the steel and chrornia, which had a TEC of 9.6 X 10~(-6)/deg C. The spinels and chromia exhibited semiconductor-type behaviour, with electrical resistivities decreasing with increasing temperature. Only Mn_2CrO_4 and NiCr_2O_4 had resistivities lower than Cr_2O_3 over the entire temperature range of testing (20-900 deg C). For Mn_xCr_(3-x)O_4, resistivity decreased with increasing Mn content.