Effect of CaO-doped in NiMn2O4-LaMnO3 composite ceramics on microstructure and electrical properties

The composite ceramics (NiMn2O4)(0.50)(La1-x Ca (x) MnO3)(0.50) (0 a parts per thousand currency sign x a parts per thousand currency sign 0.3) consisting of spinel-structured NiMn2O4 and perovskite-structured CaO-doped LaMnO3 were prepared by classical solid state reaction. The X-ray diffraction (XRD) patterns have shown that the major phases presented in the sintered samples are NiMn2O4 compounds with a spinel structure, La1-x Ca (x) MnO3 with a perovskite structure. The Scanning Electron Microscope (SEM) pictures have exhibited that the grain size of the composite ceramics decreases from ca. 6.5 to 2.0 mu m as the mole fraction of CaO increases from 0 to 0.3. The rho (25 A degrees C) and B (25/50) constants of the composite samples are in the range of 0.234-8.61 Omega cm and 2,600-2,962 K, respectively. In particular, CaO-doped leads to a decrease in the resistance drift of the (NiMn2O4)(0.50)(La1-x Ca (x) MnO3)(0.50) composite NTC (negative temperature coefficient) ceramics after aging test. This indicates that the CaO-doped (NiMn2O4)(0.50)(La1-x Ca (x) MnO3)(0.50) NTC ceramics display high electrical stability in comparison with the Ca-free (NiMn2O4)(0.50)(LaMnO3)(0.50) ceramics. The X-ray photoelectron spectroscopy (XPS) analysis verifies that the valence states of the manganese ions have a highly mixed state of Mn2+, Mn3+ and Mn4+ at B site. And the electrical conduction of the composite ceramics can be elaborated by the ions migration mechanism.