Study On The Structure And Electrical Properties Of The Doping NiCo_0.2Mn_1.8O₄ Spinel Type

The metal oxides of NiCo_0.2Mn_1.8O₄ 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 NiCo_0.2Mn_1.8O₄ 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 NiCo_0.2Mn_1.8O₄ 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 NiCo_0.2Mn_1.8O₄ 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 Cd_xNi_1-xCo_0.2Mn_1.8O₄(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 Cd_xNi_1-xCo_0.2Mn_1.8O₄ 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 Cd_xNi_1-xCo_0.2Mn_1.8O₄ 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 SmxNiCo_0.2Mn_1.8O₄ 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 SmxNiCo_0.2Mn_1.8O₄ spinel phases. It was beneficial to improve the resistance stability of the SmxNiCo_0.2Mn_1.8O₄ 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 SmxNiCo_0.2Mn_1.8O₄ thermoceramics lower significantly.