Valence‐induced effects on the electrical properties of NiMn2O4 ceramics with different Ni sources.

Spinel‐structured NiMn2O4 ceramics, with different valence Ni sources, were originally prepared using Ni2O3 and NiO as raw materials, and the effects of different valence Ni sources on their electrical properties were first investigated. XRD patterns show that both Ni2O3‐based and NiO‐based NiMn2O4 ceramics are single cubic spinel structures. SEM/EDS images indicate that the NiMn2O4 ceramics exhibited high density at the experiment‐determined sintering temperatures. XPS results and Raman drifts prove that the Ni valence‐induced changes in Mn ions at B sites played a significant role in the electrical properties and thermal stability of NiMn2O4 ceramics. Compared with NiO‐based NiMn2O4, the resistivity at 25°C (ρ25°C) of Ni2O3‐based NiMn2O4 increased dramatically from 3109 to 106958 Ω cm, the thermal constant (B25/50) increased from 3264 to 4473 K, and the resistance shifts after annealing for 1000 h at 150°C decreased from 0.80% to 0.74%. The investigation of the relationship between the material properties and valence of Ni sources has provided a new and effective way for designing the spinel‐structured negative temperature coefficient (NTC) materials by modulating the valence of ions at A sites in the raw materials. [ABSTRACT FROM AUTHOR]