Core–shell NTC materials with low thermal constant and high resistivity for wide‐temperature thermistor ceramics

Abstract

AbstractCore–shell structures have been proposed to improve the electrical properties of negative‐temperature coefficient (NTC) thermistor ceramics. In this work, Al2O3‐modified Co1.5Mn1.2Ni0.3O4 NTC thermistor ceramics with adjustable electrical properties were prepared through citrate‐chelation followed by conventional sintering. Co1.5Mn1.2Ni0.3O4 powder was coated with a thin Al2O3 shell layer to form a core–shell structure. Resistivity (ρ) increased rapidly with increasing thickness of the Al2O3 layer, and the thermal constant (B) varied moderately between 3706 and 3846 K. In particular, Co1.5Mn1.2Ni0.3O4@Al2O3 ceramic with 0.08 wt% Al2O3 showed the increase of ρ double, and the change in its B was less than 140 K. The Co1.5Mn1.2Ni0.3O4@Al2O3 NTC ceramics showed high stability, and their grain size was relatively uniform due to the protection offered by the shell. The aging coefficient of the ceramic was less than 0.2% after aging for 500 hours at 125°C. Taken together, the results indicate that as‐prepared Co1.5Mn1.2Ni0.3O4@Al2O3 NTC ceramics with a core–shell structure may be promising candidates for application as wide‐temperature NTC thermistor ceramics.