Abstract：A series of La0.67Ca0.33-xNaxMnO3 ceramics(x= 0.05, 0.10, 0.15, 0.20, 0.25) were prepared by sol-gel technique. The crystal structures, the surface morphology and the temperature dependence of the resistivity (R-T) of the bulk samples were analyzed by means of X-ray diffraction (XRD), scanning electron microscope (SEM), and the standard four-probe method, respectively. The temperature dependence of magnetization (M-T) has been measured at 100~200 K. The results show that with the increase of the content of sodium(Na), the unit cell volume increases and the resistance decreases. Meanwhile, the insulator-metal transition temperature TP shifts to higher temperature and the temperature coefficient of resistivity (TCR) decrease continually. The data of resistivity on low-temperature (T<TP) can be fitted with the relation ρ(T)=ρ0+ρ2.5T2.5. Andthe high-temperature (T>TP) resistivity data can be explained by using small-polaron hopping (SPH) and variable-range hopping (VRH) models. The resistivity data in whole temperature range (100~300K) can be simulated by percolation model.The results indicate that the bond angle of Mn3+-O2--Mn4+ increase with the doping contant of Na (x) increasing, while Polaron activation energy (Ea) decrease. The research results show that the doping Na can enhance the double exchange effect of the materials, which is the reason of the decrese of the resistivity.
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2019, Vol. 26 
