New negative temperature coefficient ceramics in Ca-Ce-Nb-M-O (M=Mo or W) system

Abstract

A series of quaternary powellites with varying cerium contents in a Ca-Ce-Nb-M-O (where M=Mo or W) system has been synthesized through the solid-state route. Powder X-ray diffraction analysis shows that these compounds crystallize in a tetragonal powellite structure with the I4(1)/a space group. The structure was further confirmed by Raman spectroscopic analysis. The electrical conductivity studies reveal that they exhibit negative temperature coefficient (NTC) thermistor characteristics. The characteristic parameter of the thermistor (beta) is found to be in the range of 5000-7000 K, and these values can be tuned with the cerium concentration. The electrical conductivity in these compounds is due to the presence of Ce3+, which remains in the reduced state without being oxidized to Ce4+ due to the structural stabilization. The X-ray photoelectron spectroscopy analysis corroborates the presence of Ce in the 3+ state.