Fabrication of nano-grained negative temperature coefficient thermistors with high electrical stability

Abstract

Dense nano-grained Ni0.7Mn2.3O4 negative temperature coefficient (NTC) thermistors were fabricated by a novel two-step sintering approach that combines rapid sintering and principle of conventional two-step sintering technique. Samples were sintered at 1042 °C for 30 s in the first rapid step and then at 850–950 °C for 20 h in the second soaking step. Crystal phase, microstructure and electrical properties of sintered samples were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), resistance temperature relationship and aging performance. Sintered samples show a single-phase cubic spinel structure and indicate a high relative density ranging from 84% to 91% of the theoretical density. Moreover, average grain sizes of sintered samples under SEM are distributed between 254 and 570 nm. Meanwhile, the resistivity and the aging coefficient significantly decrease when soaking sintering temperature rises. In addition, the obtained material constant (B) ranges from 3931 to 3981 K. Ni0.7Mn2.3O4-3 (soaking at 900 °C) and Ni0.7Mn2.3O4-4 (soaking at 950 °C) present little aging behavior, implying great electrical stability.