The optimal synthesis condition by sol–gel method and electrical properties of Mn1.5−xCo1.5NixO4 ceramics

A series of Mn1.5−xCo1.5NixO4 (0≤x≤1.2) negative temperature coefficient materials was synthesized by the sol–gel method. The optimal pH of reaction solutions was obtained by theoretical calculation. XRD and X-ray intensity ratios (I220/I440) and (I440/I422) studies showed that as-sintered ceramics are single spinel structures with different cation distributions. The Ni2+ ions not only enter B-site but also occupy A-site. Electrical studies showed that with increasing Ni content, the resistivity decreased from 909.229 Ω cm to 221.873 Ω cm initially in the range of x≤0.5 and then increased to 726.933 Ω cm when x=1.2, whereas the B value decreased from 4427 K to 2134 K. In addition, different sintering temperature ceramics were investigated. A higher sintering temperature results in lower resistivity and B value at same Ni content. The Mn1.0Co1.5Ni0.5O4 material showed much improved electrical stability at 125 °C for 1000 h. The rate of change (ΔR/R) is 0.2%.