Influence of Nitrogen Annealing on Phase Evolution, Microstructure, and Electrical Properties of Mn0.76Fe0.87Co1.07Zn0.3O4 NTC Ceramic

Abstract

This study elucidated the mechanism underlying the differences in the stability of Mn_0.76Fe_0.87Co_1.07Zn_0.3O₄ spinel-type NTC thermistors induced by nitrogen annealing. Prolonged nitrogen annealing led to the precipitation of a secondary CoO phase and significant changes in the microstructure. Additionally, the annealing process reduced the concentration of high-valent cations (Mn⁴⁺, Co³⁺, and Fe³⁺). These structural and compositional changes resulted in substantial alterations in electrical properties, with resistivity (ρ₂₅) increasing from 5430 Ω·cm to 18304 Ω·cm, and the material constant (B_25/50) rising from 4023 K to 4316 K after 6 hours of annealing. AC impedance analysis revealed that these shifts were primarily due to the increased grain boundary resistance (R_gb), which became more pronounced with extended annealing time. These findings provided both theoretical and experimental insights into the mechanisms governing the electrical stability and performance fluctuations of spinel-type NTC thermistors in high-temperature nitrogen environments, offering practical guidance for optimizing thermal stability in manufacturing and application processes.