4. Conclusions
The effect of addition of Cu and Fe on the electrical properties of Ni–Mn–Co–O based NTC thermistor compositions is studied. For applications such as inrush current limiters, NTC thermistors with low resistance and moderate sensitivity are required. Addition of Cu in large quantities for reducing the resistivity adversely affects the sensitivity. In this work, we could achieve compositions with very low resistivity without much affecting the sensitivity by the co-doping of very small amounts of Cu along with Co in NiMn2O4. Compositions with high resistivity and good sensitivity were achieved by co-doping Fe with Co in NiMn2O4. Fe was substituted for Mn by keeping the Ni:Co ratio constant. An increase in resistivity with the increasing Fe content was observed. There was an unusual decrease in B-value with Fe content. This may be due to the fact that Fe is substituted for Mn and hence the dilution effect of Fe3+ ions will be predominant in a Mn deficient system. All the compositions exhibited very good reliability.