The Microstructure and Conductivity Evolution of Plasma-Sprayed (Mn, Co)3O4 Spinel Coatings during Conductivity Measurements at Elevated Temperature

Abstract

(Mn, Co)3O4 spinel is widely used to protect the metallic interconnect of solid oxide fuel cells while it suffers deoxidization during the preparation by plasma spray. This work was proposed to study the effect of spray parameters on the microstructure and conductivity of spinel coatings. In this work, spinel coatings were prepared by the atmospheric plasma spray. The prepared coatings were heated up to 700 °C and held on for 15 h to allow the conductivity evolution. The microstructure and composition of coatings were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectrum (XPS). The results show that all coatings were evidently densified in two hours of heating while the measured conductivities were continuously evolved. The phase composition was found contributed more to the conductivity evolutions than the densification. The conversion of CoO to MnCo2O4 was observed and thus endowed the coatings a conductivity of 40 S/cm. A high fraction of Co3+ diffraction peaks, a high amount of Mn2+ and a low content of Co2+ jointly showed that more Co3+ occupied the B site of AB2O4 phase and more Mn2+ stood at the A site, indicating a stoichiometric composition of MnCo2O4. Annealing twins were detected by TEM and EBSD for the heated coatings but only a limited contribution to the evolution of the conductivity was considered. Finally, we suggest a high flowrate of plasma gas and a high input energy to prepare spinel coatings with designed conductivity.