MnFe₂O₄-based NTC thermosensitive ceramics modified by Bi₂O₃/TiO₂ additives

Negative temperature coefficient (NTC) thermistors are the key components in the fields of temperature sensors. In order to significantly reduce the sintering temperature of NTC ceramics and simultaneously obtain a large NTC material constants (B values), herein MnFe₂O₄ based ceramics modified by various contents of Bi₂O₃ or Bi₂O₃ + TiO₂ were fabricated for the high sensitive NTC thermistors. The Bi₂O₃/TiO₂ modified MnFe₂O₄ ceramics can be obtained by sintering at 1060°C, while the sintering temperature of pure MnFe₂O₄ ceramics was 1250°C. All the prepared ceramics show typical NTC characteristics. Both Bi₂O₃ and Bi₂O₃/TiO₂ additives reduce the room temperature resistivity of the MnFe₂O₄ based ceramics that maintain high NTC material constants with the B values higher than 4460 K. The effect of Bi₂O₃/TiO₂ contents on the phase component, elemental valence states, microstructures, electrical properties and aging behavior of the MnFe₂O₄ based ceramics were investigated. The optimized Bi₂O₃/TiO₂ modified MnFe₂O₄ ceramics exhibit high electrical stability with a resistance drift rate of 1.27% after 500 h aging at 125°C in air. The aging characteristics were investigated by analyzing the evolution of the cation valence states before and after aging treatment.