Low-temperature sintered Ni–Zn–Co–Mn–O spinel oxide ceramics for multilayer NTC thermistors

The phase formation, sintering behavior and electrical properties of Ni–Co–Zn–Mn spinel NTC thermistor ceramics of the series Ni 0.5 Zn z Co 0.5 Mn 2− z O 4 with 0 ≤ z ≤ 1 were studied. In contrast to NiMn 2 O 4 , which exhibits limited stability in air below 730 °C and above 970 °C, the Zn–Co-substituted nickel manganite spinels are stable at T 900 °C, with T d increasing with decreasing Zn Content z . The samples were sintered at 900 °C with addition of 3 wt% Bi 2 O 3 as sintering aid and densities of above 92% were achieved. The room temperature resistivity and thermistor B -constants are almost independent of composition at 0 ≤ z ≤ 0.6 and start to increase at higher Zn concentrations. A multilayer NTC thermistor was fabricated using green tapes of a spinel of composition z = 0.75, commercial Ag paste, and cofiring at 900 °C. The firing behavior, microstructure formation and electrical properties of the multilayer thermistor are reported.