Structure and electrical properties characterization of NiMn2O4 NTC ceramics

Spinel-type NiMn2O4 negative temperature coefficient (NTC) ceramic compound was successfully prepared by the co-precipitation technique. The structure, microstructure, and transport properties are examined for the thermistors application. X-ray diffraction pattern illustrates the formation of spinel structure with cubic crystal symmetry. The scanning electron micrograph shows irregular shape grains with an average size of less than 100 nm. The observed microstructure is mainly due to the low-temperature synthesis and occupancy of Mn3+ at octahedral sites. The dielectric loss was very less in low-temperature regions and increase exponentially at higher temperatures indicating an improvement in electrical conductivity. Similar behavior was observed for relative permittivity and indicates temperature-dependent dielectric polarization along with high-temperature phase transition. The Cole-Cole graph shows a decrease in an area of semicircular with an increase in temperature confirming the improvement of electrical conductivity. The dc-resistivity measurement shows a decrease in resistivity with increasing temperature typically observed in NTC materials.