Preparation And Electrical Property Of Rare Earths Modified CaMnO₃ Powders

In order to enhance the electrical conductibility of CaMnO₃, which is considered as the candidate for conductive and thermoelectric devices, Sol-gel auto combustion method was used to fabricate the CaMnO₃ powders modified by Sm, Pr, and Er. The influences of doping ratio and calcination temperature on the constitution, structure, and electrical properties of modified CaMnO₃ powders were investigated.The analyses of TG-DTA, FT-IR, XRD, and electrical resistivity detection at room temperature illustrate that the completed perovskite structure of Sm doped CaMnO₃ was obtained by calcining the powders at 900℃and the lowest resistivity of samples, Ca_0.8Sm_0.2MnO₃, was 0.5345Ω·m. Based on the researches above, the effects of the calcination temperatures on the structure and resistivity of rare earths doped CaMnO₃ are discussed. The results indicate that the higher calcination temperature caused the stronger intensity of characteristic peaks in the XRD pattern which means the better formation of the gain phases of CaMnO₃ powders doped with Sm, Pr, and Er. The grain size of the modified CaMnO₃ powders was increased with the calcination temperature, which demonstrated that high calcination temperature is in favor of the secondary growth of grain. Simultaneously, the calcination temperature also plays a crucial impact on the resistivity of modified CaMnO₃ powders. When calcined the CaMnO₃ powders doped with Sm, Pr, and Er at 1000℃, the resistivity was lower than those calcined at 900℃and 1100℃.The study of modified CaMnO₃ with different type of rare earths at various of doping ratio depicts that Sm, Pr, and Er can decrease the resistivity of CaMnO₃ powders, however, Sm is of the most significance. The variation of resistivity against doping ratio of Sm, Pr, and Er showed the V type tendency of firstly decrease than increase which is the typical characteristic of electron doping.The resistivity of Sm doped CaMnO₃ powders was lower than that of Pr and Er doped ones, however, the resistivity of all doped powders was on the same order of magnitude of 10-1?·m and with the fine stability of frequency. When the CaMnO₃ powders were doped by 20at% Sm and calcined at 1000℃, the resistivity reached the lowest point of 0.2920Ω·m. XRD analysis indicates that with the increase of doping ratio, the 2θof characteristic peaks of Sm, Pr, and Er doped CaMnO₃ powders were moved, which suggests that the doping ratio significantly affects the crystal structure of CaMnO₃ powders. The doping ratio affects the electrical property in terms of the crystal distortion. SEM detection showed that the doping of Sm, Pr, and Er caused the refinement of grains of CaMnO₃ powders, and with the increase of doping ratio, the grain size was decreased firstly then was increased slightly.The average valance and oxygen content of Mn in Pr doped CaMnO₃ powders were determined by indirect iodometry, and ratio of Mn³⁺ against Mn⁴⁺ is also obtained. The results suggest that the higher ratio decreased the resistivity of Pr doped CaMnO₃ powders. It is believed that doping caused the formation of Mn³⁺ and also generated numbers of electron carrier, and the interpretation of variation in resistivity of modified CaMnO₃ powders was given based on that behavior.