Sintering temperature and impedance analysis of Mn0.9Co1.2Ni0.27Mg0.15Al0.03Fe0.45O4 NTC ceramic prepared by W/O microemulsion method

•The ceramics are mainly in spinel phase and cubic cobalt oxide phase.•A perfect sintering temperature of 1450°C for the Mn–Co–Ni–Mg–Al–Fe–O was proposed.•The NTC characteristic of the ceramics derive from grain boundary resistance Rgb. The Mn0.9Co1.2Ni0.27Mg0.5Al0.03Fe0.45O4 negative temperature coefficient (NTC) ceramics derived from nano-particles were sintered at 1380°C, 1450°C and 1560°C, respectively. X-ray diffraction (XRD) result showed that the ceramics sintered at 1380°C and 1450°C were mainly in the cubic spinel structure except for a little of tetragonal spinel, and that sintered at 1560°C was consisted of cubic spinel and cubic cobalt oxide phase. Scanning electron microscopy (SEM) image indicated that the grain size of the ceramic increased sharply when the sintering temperature increased from 1380°C to 1450°C, and it changed little when the temperature further increased to 1560°C, while the porosity was enlarged seriously. Thus a perfect sintering temperature of 1450°C was proposed. Impedance analysis revealed that the grain resistance Rg showed positive temperature coefficient thermistor characteristic, while the grain boundary resistance Rgb possessed negative temperature coefficient characteristic. Because the grain boundary resistance Rgb was two orders of magnitude larger than the grain resistance Rg, the material thus showed negative temperature coefficient thermistor characteristic.