Electrical properties of NiO-based composite ceramics modified by La₂MO₄ (M = Cu or Ni) for NTC thermistors

Negative temperature coefficient (NTC) thermistors are important for temperature sensors and actuators. In this work, La2MO4 (M = Cu or Ni) modified NiO composite ceramics are prepared by conventional solid-state reaction method for NTC thermistors. All prepared composite ceramics have a main phase with a rock-salt structure of NiO and a second phase of La2NiO4 or La2CuO4. The introduction of La2CuO4 or La2NiO4 significantly improved the sintering ability of ceramics and reduced the sintering temperature from 1380 degrees C of pure NiO ceramic to 1250 degrees C composite ceramics. For the existence of highly conductive La2CuO4 or La2NiO4, the room temperature resistivity of ceramics can be effectively reduced, while the ceramics maintain NTC material constant higher than 4000 K. The electrical properties of ceramics were analyzed by combining with XRD, XPS, and complex impedance spectra. Several conduction modes such as band conduction, polaron hopping, percolation conduction, and grain-boundary transition of charge carriers are proposed for the electrical conductivity and NTC characteristics of NiO-based composite ceramics.