2. Experimental
2.1. Synthesis of NTC compositions
Three sets of compositions based on Ni0.7Mn2.3−xCoxO4 (0≤x≤0.7), Ni0.7Mn1.8−xCo0.5CuxO4 (0≤x≤0.05) and Ni0.7Mn1.8−xCo0.5FexO4 (0≤x≤0.7) were prepared by solid state route. Starting materials such as nickel carbonate basic hydrate, manganese carbonate, Fe2O3, CuO and Co3O4 (Sigma Aldrich, USA) were of high purity (>99%) in order to ensure higher quality and reproducibility of the synthesized thermistor compositions. The raw materials were accurately weighed and ball milled using isopropyl alcohol as solvent and yttria stabilized zirconia balls as grinding media for 24 h to get a uniform mix. The powder mix was then calcined at 1173 K for 3 h. The calcined powder was then ground by ball milling for 48 h to achieve a narrow particle size distribution. The powder after grinding was passed through ASTM 400 mesh size sieve so as to make sure that only particles of uniform size were present in the resulting powder after drying. Crystal structure of the resulting composition was analyzed by X-ray diffraction (XRD) analysis (Bruker AXS D5005, Germany).