Effect of CaO-doped in NiMn_2O_4-LaMnO_3 composite ceramics on microstructure and electrical properties

摘要

The composite ceramics (NiMn_2O_4)o.5o(La_(1-x) Ca_xMnO_3)_(0.50) (0 ≤ x ≤ 0.3) consisting of spinel-structured NiMn_2O_4 and perovskite-structured CaO-doped LaMnO_3 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 NiMn_2O_4 compounds with a spinel structure, (La_(1-x) Ca_xMnO_3) 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 urn as the mole fraction of CaO increases from 0 to 0.3. The ρ25 ℃ and B_(25/50) constants of the composite samples are in the range of 0.234-8.61 ft cm and 2,600-2,962 K, respectively. In particular, CaO-doped leads to a decrease in the resistance drift of the (NiMn_2O_4)_(0.50)((La_(1-x) Ca_xMnO_3))o.5o composite NTC (negative temperature coefficient) ceramics after aging test. This indicates that the CaO-doped (NiMn_2O_4)_(0.50) ((La_(1-x)Ca_xMnO_3))_(0.50) NTC ceramics display high electrical stability in comparison with the Ca-free (NiMn_2O_4)_(0.50) (LaMnO_3)_(0.50) ceramics. The X-ray photoelectron spec-troscopy (XPS) analysis verifies that the valence states of the manganese ions have a highly mixed state of Mn~(2+), Mn~(3+) and Mn~(4+) at B site. And the electrical conduction of the composite ceramics can be elaborated by the ions migration mechanism.