Cold-sintered NiMn2O4 thermistors: An electrical aging and recovery study

Abstract

Cold sintering using eutectic molten salts transient chemistries presents a promising low-temperature route for fabricating NiMn₂O₄ based NTC thermistors without the need for conventional high-temperature sintering. In this work, we explore the use of a LiCl–LiI flux to densify NiMn₂O₄ at just 400 °C, achieving > 95 % relative density and preserving the spinel with minor impurities that are largely erased after a brief post anneal at 850 °C. While initial electrical properties of the cold-sintered parts are influenced by a flux-mediated reduction and cation inversion segregation, we show that a mild anneal at 850 °C is sufficient to restore a more stable thermistor. Here we argue that the transient phase of LiCl–LiI drives a cation-Frenkel–dominated reduction during the densification: transient dissolution/precipitation and halide redox promote formation of metal interstitials (Ni_i,Mn_i) paired with cation vacancies on the normal sublattices, with a valence shifts (Mn⁴⁺→Mn³⁺). We also report that, in the absence of annealing, these samples undergo a negative aging with a decrease in resistance over time, further supporting the role of the flux modifying the oxidation state and site occupancy. These results demonstrate that cold sintering, when paired with strategic annealing, can produce high thermistor performance with a thermally stable NTC ceramics at a fraction of the processing energy and temperature, with tunable electrical properties driven by the molten salt flux chemistry.