Effect of Off-Stoichiometry on Electrical Conductivity in Ni-Fe and Mn-Co Spinel Systems

Abstract

This study seeks to clarify the effects of off-stoichiometry on the electrical conductivity of spinels in the (Ni,Fe)3O4 and (Mn,Co)3O4 systems. Spinels with the compositions NixFe3–xO4 (where x = 0.85, 0.875, 0.9, 1.0, 1.05, and 1.15) and MnxCo3–xO4 (where x = 1.0, 1.2, 1.35, 1.5, 1.75, and 2.0) were prepared using conventional solid-state process. Electrical conductivity measurements were taken between 500 °C and 800 °C using the 4-point DC method. The conductivity in Ni-Fe spinels increased with increasing Fe content. Similarly, the conductivity of Mn-Co spinels increased drastically with increasing Co content. The coefficients of thermal expansion (CTEs) of these samples were also evaluated. All the (Ni,Fe)3O4 spinels were found to exhibit excellent CTE matching with prospective stack components. However, thermal expansion characteristics were highly dependent on the composition in the (Mn,Co)3O4 system, and only a narrow range of compositions was well-matched in CTE with prospective stack components. The significant effect of cation off-stoichiometry and sample preparation on the electrical behavior of spinels was discussed as a possible explanation for the large discrepancies in reported conductivity values for both systems.