Effect of full-range Ni/co ratio regulation on structure and electrical properties of Mn-based NTC thermistors

Most studies on Mn-based NTC thermistors have been limited to fixed or narrow Ni/Co ratios, lacking a systematic investigation of the full compositional range. This gap impedes the rational and precise design of high-performance thermistors. Here, we prepared Co x Ni 1-x Mn 2 O 4 (x = 0, 0.25, 0.5, 0.75, 1) series thermistors via solid-state reaction, achieving single-phase spinel structures across all compositions. The grain size of Co x Ni 1-x Mn 2 O 4 dropped sharply when the Co content reached 0.5, then increased slightly as x was raised further to 1. The Co 0.25 Ni 0.75 Mn 2 O 4 (x = 0.25) thermistor exhibited the lowest resistivity (1.42 kΩ at 25 °C), while Co 0.75 Ni 0.25 Mn 2 O 4 (x = 0.75) delivered optimal comprehensive performance, combining low resistivity (4.16 kΩ) with high sensitivity (B 25-50  = 3.38 × 10 3  K). When the Co content reached the maximum (x = 1), the thermistor exhibited the highest B 25-50 (up to 3.96 × 10 3  K), corresponding to the optimal sensitivity in the temperature measurement range. The resistivity of Mn-Co-Ni based thermistors is mainly determined by the concentration of Mn 3+ /Mn 4+ ion pairs (especially Mn 3+ ) and additionally modulated by the grain size (grain boundary resistance), which establishes a clear structure-performance correlation for the system. This work provides technical references for developing high-performance, precisely designable Mn-based thermistors and lays a foundation for rational Ni/Co ratio regulation in similar spinel NTC systems.