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

The composite ceramics (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50 (0 ≤ x ≤ 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, La_1−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 μm as the mole fraction of CaO increases from 0 to 0.3. The ρ _25 °C and B _25/50 constants of the composite samples are in the range of 0.234–8.61 Ω 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(La_1−x Ca _x MnO₃)_0.50 composite NTC (negative temperature coefficient) ceramics after aging test. This indicates that the CaO-doped (NiMn₂O₄)_0.50(La_1−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.