Influence of nano-sized cobalt oxide additions on the structural and electrical properties of nickel-manganite-based NTC thermistors

The structural and electrical properties of NiMn2O4 and Ni0.5CoxMn2.5-xO4(where x = 0.5, 0.8 and 1.1) NTC thermistors have been investigated.The samples, prepared by conventional ceramic processing techniques, were calcinated at 900 °C for 2 h and then sintered at 1100 °C and 1200 °C for 5 h.The cubic spinel phase was observed by XRD analysis in the NiMn2O4 and Ni0.5Co0.8Mn1.7O4samples sintered at 1100 °C for 5 h.The sintering at 1200 °C resulted in much denser microstructures with a larger grain size.The room-temperature electrical resistivity (r25) and material constant (B) value of the NiMn2O4 sample sintered at 1100 °C were 7710 W cm and 3930 K, respectively.The electrical resistivity of the samples decreased significantly with the addition of Co3O4.The B25/85 values of the Ni0.5CoxMn2.5-xO4(where x = 0.5, 0.8 and 1.1) samples sintered at 1100 °C were found to be 3820 K, 3525 K and 3270 K, respectively.