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

Spinel-structured NiMn 2 O 4 ceramics, with different valence Ni sources, were originally prepared using Ni 2 O 3 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 Ni 2 O 3 -based and NiO-based NiMn 2 O 4 ceramics are single cubic spinel structures. SEM/EDS images indicate that the NiMn 2 O 4 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 NiMn 2 O 4 ceramics. Compared with NiO-based NiMn 2 O 4 , the resistivity at 25°C ( ρ 25°C ) of Ni 2 O 3 -based NiMn 2 O 4 increased dramatically from 3109 to 106958 Ω cm, the thermal constant ( B 25 / 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.