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) 3 O 4 and (Mn,Co) 3 O 4 systems. Spinels with the compositions Ni x Fe 3-x O 4 (where x = 0.85, 0.875, 0.9, 1.0, 1.05, and 1.15) and Mn x Co 3-x O 4 (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) 3 O 4 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) 3 O 4 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.

Review—(Mn,Co)3O4-Based Spinels for SOFC Interconnect Coating Application

“…5b as well as Tables I and III, Mn x Co 3−x O 4 spinels with 1.0 < x < 1.5 exhibit a good thermal expansion match to ferritic stainless steel substrates such as Crofer 22 APU (12.3 × 10 −6 K −1 ) 98 and perovskite cathode such as La 0.8 Sr 0.2 MnO 3 (11.8 × 10 −6 K −1 ), 99 with the best CTE match achieved at around x = 1.1-1.3. It should be noted that Mn 1.5 Co 1.5 O 4 spinel experiences a change in expansion rate at around 400-600 °C due to a cubic ↔ tetragonal phase transformation, 35,93 which might affect its thermal compatibility with the adjacent cell components. To optimize the CTE match with the interconnect and cathode (i.e., a targeted CTE value of around 12 × 10 −6 K −1 ), different dopants can be selected, depending on the spinel composition.…”