La0.7Ca0.3-xNaxMnO3 polycrystalline with high magnetoresistance and temperature coefficient of resistance were prepared via the sol–gel method

Doping is a primary method to improve the temperature coefficient of resistance ( TCR ) and magnetoresistance ( MR ) of La 1- x A′ x MnO 3 polycrystalline in a low magnetic field. La 0.7 Ca 0.3- x Na x MnO 3 ( x = 0, 0.02, 0.04, 0.06, 0.08) series of polycrystalline were synthesized by the sol–gel method. X-ray diffraction patterns indicate that the crystal structure changes from orthorhombic to rhombohedral, and the space group changes from Pnma to R 3 - c with increasing x . With Na-doping rise, the increase in grain size reduces grain boundaries, weakening the electron scattering at grain boundaries and decreasing the peak resistivity. The excellent MR (59.51%) is observed in La 0.7 Ca 0.24 Na 0.06 MnO 3 at the 1 T magnetic field. Meanwhile, TCR max reaches 12.36% K −1 in the absence of a magnetic field. The resistivity variation with temperature curve fitting illustrates that grain boundary scattering dominates in the low-temperature stage, and the effect of activation energy dominates in the high-temperature stage.