Impedance analysis and conduction mechanism of Ba doped Mn1.75Ni0.7Co0.5−x Cu0.05O4 NTC thermistors

Polycrystalline Mn1.75Ni0.7Co0.5−xCu0.05BaxO4 (x = 0, 0.05 and 0.1) based negative temperature coefficient (NTC) thermistors were synthesized using conventional solid state reaction method. X-ray diffraction (XRD) analysis confirmed the presence of single phase cubic spinel structure for all the compositions and successful substitution of Ba. Scanning electron microscopy (SEM) revealed a dense microstructure along with slight increase in grain size due to Ba doping. Impedance spectroscopy (IS) studies showed that the increase in temperature caused both grain and grain boundary resistance to decrease indicating NTC behavior of the samples. The grain boundary resistance was several magnitudes greater than the resistance of grains which showed that the NTC characteristic of doped samples was mainly dependent on grain boundaries. The time constant for both grain and grain boundary decreased with temperature indicating a hopping conduction mechanism. These results can lead to design the optimum microstructure for various practical applications of NTC thermistors.