A sol-gel combustion process was used to synthesize Fe0.48Ni0.3Mn1.32Co0.9O4 (FNMC) nanopowders, which exhibited properties suitable for negative temperature coefficient (NTC) thermistors. The chemical composition of the synthesized FNMC powder after calcination was within the expected values. Thermal analysis of the sol-gel synthesized nanopowders indicated that residual nitrate compounds volatilized and crystallization occurred during calcination. The influence of different calcination temperatures on the crystal structure of FNMC was investigated. A single-phase cubic spinel structure with improved crystallinity was observed after calcination at 1073 K. As expected, an increase in grain size up to 80 nm was observed with increasing calcination temperature. Elements such as Fe, Ni, Mn and Co were homogeneously distributed in both sol-gel synthesized nanopowders and calcined powders. Dense pellets of FNMC nanopowder were sintered at 1473 K in order to evaluate electrical properties. The electrical resistance (ρ20) and B-value (B20/80) were 0.36 kΩ and 3265 K, respectively, which are desirable values for an NTC thermistor. These results suggest that FNMC nanopowders show relatively stable electrical reliability because of the uniform microstructure and single-phase cubic spinel structure, making these nanopowders a suitable candidate for NTC thermistor materials.

Keywords: Fe-Ni-Mn-Co-O, Single cubic spinel structure, Nanopowders, Thermistors, Sol-gel combustion