Particle size evolution during the preparation of NTC thermosensitive ceramics from nanopowders and its influence on the structure and properties

Abstract In this study, the evolution of particle size during processing of nano‑sized oxides was systematically investigated during the processing of Mn-Co-Ni-O-based spinel ceramics. And two different approaches - high‑energy milling (HM) versus conventional milling (PM) - were compared in terms of their effects on the microstructure and electrical properties. Particle size after pre‑sintering and re‑milling was tracked by laser diffraction. Phase structure and microstructure were characterized by XRD, SEM, and TEM. The thermistor constant and activation energy were obtained via Arrhenius fitting. The resulting Nyquist plots were analyzed by equivalent circuit fitting to link microstructure with electrical properties. The initial nano‑sized oxide (about 50 nm) was found to coarsen severely to 955.4 nm after pre‑calcination. HM successfully refined the particle size back to the nanoscale (D50: 58-78 nm), while introducing a high density of intragranular dislocations, rough particle surfaces. The increased internal energy drives the powder into a non‑equilibrium, metastable state, which gives a strong, continuous exothermic signal between ⁓ 800 K- 1500 K during thermal analysis. At 1250 °C, phase separation was detected by XRD in the HM samples, which transforms back to the equilibrium spinel phase at 1280 °C. The small‑polaron hopping mechanism, measured on thermistors sintered at 1250 °C, does not depend on the milling process. Based on the analysis of Nyquist plots via equivalent circuit fitting, the phase separation turns out to be merely a kinetic by‑product and does not affect grain boundary conduction. As a result, HM samples show a slightly lower grain boundary resistivity (Rgb) due to higher density. Consequently, the B value is not significantly affected. These findings establish a new paradigm for microstructure engineering of thermosensitive ceramics that goes beyond simple precursor particle size control, and provide solid experimental support for the precise tailoring of such microstructures.