Sintering temperature dependence on microstructure and electrical properties of Co2.38Mn1.62Fe1.6Zn0.4O8 ceramics for NTC thermistors

Co–Mn–Fe–Zn–O-based ceramics are important negative temperature coefficient (NTC) materials. To clarify the processing-property relationship, Co₂.₃₈Mn₁.₆₂Fe₁.₆Zn₀.₄O₈ ceramics were prepared via the solid-state method, and the effects of sintering temperature (1100–1300°C) on their microstructure, electrical properties, and aging behavior were systematically investigated. XRD and SEM analyses revealed that higher temperatures promoted grain growth and densification, with the average grain size increasing from 1.6 μm to 5.3 μm. The room-temperature resistivity (ρ₂₅) exhibited a non-monotonic trend, initially increasing from 3327 Ω·cm at 1100°C to a maximum of 4287 Ω·cm at 1250°C, then decreasing to 3020 Ω·cm at 1300°C. XPS analysis indicated that the Mn³⁺/Mn⁴⁺ ratio decreased monotonically from 99.5% to 65.4% with increasing sintering temperature. This suggests that the resistivity evolution is governed by a transition in the dominant mechanism: small polaron hopping between Mn³⁺ and Mn⁴⁺ ions at lower temperatures (≤1250°C), and grain boundary scattering due to significant grain growth at higher temperatures (≥1250°C). Furthermore, all samples demonstrated excellent aging stability with a resistance drift (ΔR/R) below 0.20% over 800 hours, with the sample sintered at 1250°C achieving the optimal combination of high resistivity (4287 Ω·cm) and minimal drift (0.16%), attributable to its dense microstructure and balanced cation valence. This work provides valuable insights for optimizing the sintering process of NTC ceramics to achieve superior electrical performance and long-term stability.