Characterization and site distributions of cation in nickel manganite thermistors

Abstract

Cubic spinel Ni0.79Mn2.21O4 ceramics with varying sintering temperatures were prepared using the mixed-oxide method. Field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis were used to determine the structural properties of the nickel manganite specimens, especially the distribution of manganese ions. The specimens sintered at 1200 degrees C showed a uniform and pore-free grain structure and the average grain size was approximately 5.08 mu m. All specimens exhibited the typical XRD patterns of a single-phase cubic spinel structure. Based on the results of XPS investigations, in all specimens, manganese existed in three oxidation states as Mn2+, Mn3+ and Mn4+ ions. The inversion parameter estimated from XRD increased with an increase in sintering temperature due to some of the Mn ions in octahedral sites moving to tetrahedral sites. The Ni0.79Mn2.21O4 specimen sintered at 1150 degrees C showed the lowest activation energy value of 0.329 eV.