SiO2 and Fe cation synergistically modify the LaMnO3 NTC thermistors with high stability

SiO2 insulator and Fe2O3 dopant was simultaneously used to modify the crystalline phase, morphology and electric properties of LaMnO3 ceramics. Results show that there are several merits after modification. On the one hand, the addition of SiO2 is demonstrated to have multifunction to the LaMn1-xFexO3 (0 ≤ x ≤ 0.5) (LMFO) ceramics. After adding SiO2, the surface of LMFO ceramics tend to be flat, the degree of densification increases, the porosity decreases, and the pores almost disappear. Meanwhile, the resistance drift rate of LaMnO3 with SiO2 is reduced and the aging stability is improved. In addition, the crystalline structure of LMFO ceramics transform from hexagonal perovskite phase (x ≤ 0.2) to cubic perovskite phase (x ≥ 0.25) along with the Fe-doping contents increase. Moreover, the grain size decreases and the grain boundaries increase with the increase of Fe content. X-ray photoelectron spectroscopy (XPS) spectra show that both the Mn4+/Mn3+ ratio and the oxygen vacancy concentration decrease with increasing Fe content. Resistivity ρ25°C raised from 2.84 Ω·cm to 65.38 Ω·cm, and material constant B25/50 increased from 139.36 K to 1751.78 K. Fe doping significantly improves the stability of LMFO ceramics, and the minimum value of resistance drift rate is 0.168 % after aging at 125 °C for 500 h. The above results confirm that the SiO2 modified LMFO ceramic is a high stability negative temperature coefficient (NTC) thermistor.