Influence of cobalt substitution on the structural and electrical properties of NiMn2O4- based NTC thermistors

Nickel Manganese Oxide (NiMn 2 O 4 ), a spinel-structured material widely studied for Negative Temperature Coefficient (NTC) thermistors, serves as an accurate temperature sensor in industrial and domestic applications like circuit compensation, temperature control, and measurement. Adding divalent or trivalent cations to NiMn 2 O 4 can significantly modify its electrical properties. Substituting manganese with cobalt could potentially alter the resistance characteristics of the material. Hence, studying the effect of cobalt on its structure and properties is valuable both scientifically and for practical applications. This study aims to synthesize NTC thermistors based on Co-substituted Ni-Mn oxides with the composition NiCo x Mn 2-x O 4 (0≤x≤1) using solid-state reaction method and to investigate the structural and electrical properties. X-ray diffraction studies confirmed a cubic spinel structure for all samples, with average crystallite size less than 30 nm and the lattice parameter decreasing at higher doping concentrations. The Fourier-Transform Infrared spectroscopy also confirmed the cubic structure. Surface morphology and structure was also evaluated using SEM and TEM analysis which further confirmed the structural features of samples. Electrical studies demonstrated an NTC behaviour for all samples and revealed a definite effect of Co on the resistance of the thermistor. Dielectric studies showed a decrease in dielectric constant with frequency and an increase with temperature, with the highest value observed for the sample with Co concentration, x=0.4. These results emphasize the notable influence of cobalt substitution on the structural and electrical properties of NiMn 2 O 4 , underscoring the potential for further exploration at higher cobalt concentrations. • Ni-Mn-Co-O spinel based NTC thermistors were prepared by solid-state reaction method. • Co substitution for Mn helped to tailor thermistor parameters in the desired range. • Thermistor parameters obtained were found to be suitable for general purpose applications. • Moderate Co concentrations led to higher value of dielectric constant.