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 Ni0.5ZnzCo0.5Mn2-zO4 with 0 <= z <= 1 were studied. In contrast to NiMn2O4, which exhibits limited stability in air below 730 degrees C and above 970 degrees C, the Zn-Co-substituted nickel manganite spinels are stable at T < 730 degrees C and decompose at T-d > 900 degrees C, with T-d increasing with decreasing Zn Content z. The samples were sintered at 900 degrees C with addition of 3 wt% Bi2O3 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 degrees C. The firing behavior, microstructure formation and electrical properties of the multilayer thermistor are reported.