Abstract
The effects of Sr-doping on Ni-Mn-Co-O spinel-structured NTC ceramics were investigated. And the precursors of Sr-doped NiCoMnO4 NTC ceramics were attained by solid state reactions at room temperature, with no intermediate compounds. Its formation and the structural evolutions were characterized by thermo-gravimetry-differential scanning calorimetry, X-ray diffraction and scanning electron microscopy with energy dispersive spectroscopy. Related electrical analyses were also performed. The values of R-298K, B-298/323 constant and the temperature coefficient of resistance alpha(298K) of the thermistors are in the range of 168-1,738 Omega, 3,257-3,359 K and -3.64 to -3.78 % K-1, respectively. Complex impedance analysis shows, for x = 0.2, single semicircular arc whose shape does not have any change with temperature. The grain boundary properties exhibit thermistor parameters with a negative temperature coefficient characteristic. The conduction for the grain boundary could be explained by oxygen vacancies. The results show that the ceramics exhibits a good electrical property because of narrow grain size distribution, controlled morphology, and high purity. This suggests that the electrical properties can be adjusted to desired values by controlling the Sr cation content. This series of materials could be considered as practical materials for NTC thermistor. Most importantly, this new approach greatly simplifies the preparation processes.
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