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The metal oxides of NiCo0.2Mn1.8O4 with spinel structure have the negative temperature coefficient（NTC）characteristics, are one of the main high precision semiconductors in the produce. They have a very wide application in the temperature measurement, control and compensation. The NiCo0.2Mn1.8O4 based NTC ceramics exhibit the spinel-type crystal structure. The electrical conduction is due to the electron jump and the concentration changes between the Mn3+and Mn4+ cations located on the octahedral sites. In this study, the NiCo0.2Mn1.8O4 thermoceramics doped with Cu,Cd, Sm,Ce and Cd-Cu were prepared by the solid state reaction at room temperature, Sol-gel auto combustion method and subsequent sintering. SEM、TG/DTA、XRD、FTIR and XPS techniques characterized the micro-morphologies and struture, phase composition and cations distribution of the precursors and calcined products of the doped NiCo0.2Mn1.8O4 thermoceramics. The electrical properties and aging properties were assessed. Comprehensive analysis results shown as follows:（1） The CuxNi1-xCo0.2Mn1.8O4（0≤x≤0.5） precursors were prepared by solid state reaction at room temperature, then calcined at lower temperature（≥350℃）. A single spinel phase was obtained after doping Cu. As the amount of Cu content in the CuxNi1-xCo0.2Mn1.8O4 samples increased, the grain size decreased, grain distribution tended to uniform and the sintering properties improved greatly. The results showed that Cu had great influence on the electrical properties of the CuxNi1-xCo0.2Mn1.8O4 thermoceramics. When the Cu content was 0≤x（Cu）≤0.5, the values of resistivityρT, thermconstant B, and activation energyΔEa of the thermoceramics were 17-1270?·cm, 1817-3076K and 0.157-0.265eV, respectively. The aging test showed that Cu can decreased the thermal stability of the thermconstant.（2） The precursors and the calcined products of CdxNi1-xCo0.2Mn1.8O4（x=0,0.1,0.2,0.3）and Cd0.2Cu0.1Ni0.7Co0.2Mn1.8O4 were prepared by the solid state reaction at room temperature. The grain of the products refined significntly after doping Cd. The results showed that, Cd was conducive to increase the density and high resistance stability of the CdxNi1-xCo0.2Mn1.8O4 thermoceramics. Compared with the single Cu doped samples, Cd-Cu co-doped can improve the resistivity and the thermal stability of the Cd0.2Cu0.1Ni0.7Co0.2Mn1.8O4 thermoceramics. The values of resistivityρT, thermconstant B, and activation energyΔEa of the thermoceramics were 140-2282·Ωcm, 2647-3852K and 0.245-0.332 eV, respectively.（ 3 ） Nitrate and Citric acid were used as raw material, the precursors CdxNi1-xCo0.2Mn1.8O4 obtained by the Sol-gel method. The ultrafine spinel powder can obtain after the precursors auto combustion, and its product have good sintering properties, achieve the atomic-level doping. The results showed that, the grain further refined with the increasing of Cd doping content, the reasons may be as follows, when Cd content increasing, the distortion energy gradually increased, and the Cd2+ ions not replaced Ni2+ ions, but entered into the lattice forming the second phase, which will hinder the growth of the grain. The resistivityρT, thermconstant B, and activation energyΔEa of the thermoceramics were 1400-2219·Ωcm, 3300-3789 K and 0.228-0.377eV, respectively.（4）The influence of Sm3+ doping on the microstructure and cations distribution of SmxNiCo0.2Mn1.8O4 spinel phases were investigated by XRD and XPS. The results showed that when Sm3+ doped content was low（x≤0.02）, Sm3+ ions partially substituted Mn3+ ions in the octahedral sites of the SmxNiCo0.2Mn1.8O4 spinel phases. It was beneficial to improve the resistance stability of the SmxNiCo0.2Mn1.8O4 thermoceramics; While Sm3+ content was higher than 0.03, SmMnO3 and SmMn2O5 compounds precipitated one after one in the spinel phases matrix, and the resistance stability of the SmxNiCo0.2Mn1.8O4 thermoceramics lower significantly.
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