Nano-negative temperature coefficient thermistor with unique electrical properties of high B constant and low resistivity

Abstract

Reports on extraordinary electrical properties of nano-scale (< 200 nm)-negative temperature coefficient (NTC) thermistors are rarely seen. In this study, nano-scale Ni0.54Mn1.26Fe1.2O4 NTC thermistors were prepared using the spark plasma sintering (SPS) approach. After SPS sintered at 750 °C and then heat-treated at 650 °C, highly dense material with an average grain size of about 133 nm was obtained. The sintered samples showed NTC characteristics in their resistance–temperature relationships and exhibited higher material constants (B) and lower room temperature resistivities (ρ25) than their micro-scale counterparts. The B constant and ρ25 of a nano-grained sample were 5630 K and 1.55 MΩ·cm, respectively; these of a micro-meter sample, however, were 4930 k and 2.7 MΩ cm, respectively. The differences can be explained by two factors: smaller grains of the nano-scale sample produce a larger number of grain boundaries, which enhances the energy barriers at grain boundaries, which, in turn, increases the B constant; both the grains and grain boundaries of the nano-scale sample follow the model of hopping conduction, which eases electron migrations and stabilization, thereby leading to lower resistivity.