Electrical and thermal properties of spinels

The properties of spinel oxides were studied in an effort to identify a potential solid oxide fuel cell (SOFC) interconnect coating which can hinder chromium oxide growth and evaporation as well provide acceptable electrical conductivity. Transition metal spinels based on aluminum, chromium, manganese and iron were examined. The electrical conductivities were measured in air from 500 to 800°C by the standard four-probe DC method. Additionally, the thermal expansion coefficients of some samples were measured in air from room temperature to 1000°C. The stoichiometric aluminates, chromites and manganites have thermal expansion coefficients that lie consistently in the 7-9 ppm/K range. Thermal expansion coefficients of the ferrites are in the 11-13 ppm/K range, and their conductivities are generally 0.1-10 S/cm. Conductivities of the aluminates are all less than 10^-2 S/cm. Among the potential candidates for SOFC interconnect coatings, Coe2MnO4 has a conductivity of 55 S/cm at 800°C and a. thermal expansion coefficient of 9.7 ppm/K and CuFe2O4 has a conductivity of 9.1 S/cm at 800°C and a thermal expansion coefficient of 11.2 ppm/K. The ternary spinel oxides of CuMn2-xCrxO4 (x = 0.2, 0.6, 0.8 and 1.0) and Co2Mn0.5Cr0.5O4 are also acceptable in terms of their electrical conductivities and thermal expansion coefficients.