In this paper, we investigated the effect of Co content on the microstructural and electrical properties of Ni_0.79Mn_2.21-xCo_xO₄ (x=0 to 0.25) specimens. Solid-state reaction was used to prepare the bulk specimens. XRD (X-ray diffraction) patterns showed that all compositions had a cubic spinel phase. As a result of the microstructural properties, FE-SEM(field-emission scanning electron microscopy) analysis showed a dense structure, and the mean grain size increased from 5.24 μm to 7.33 μm with an increase of Co content from x=0 to 0.25. All specimens exhibited the typical NTC thermistor characteristics as the electrical resistance exponentially decreased with increasing temperature. The resistivity and the B-value of Ni_0.79Mn_1.96Co_0.25O₄ were 2959 Ω·cm and 3719, respectively.

In this study, Ni0.79(Mn2.21-χCuχ)O4 (x=0~0.25) specimens were prepared by using a conventional mixed oxide method. All specimens were sintered in air at 1,℃ for 12 h and cooled at a rate of 2℃/min to 800℃, subsequently quenching to room temperature. We investigated the structural and electrical properties of Ni 0.79(Mn2,21-χCuχ)O4 specimens with variation of CuO amount for the application of NTC thermistors. As results of X-ray diffraction patterns, all specimens showed the formation of a complete solid solution with cubic spinel phase. The relationship between ln ρ and the reciprocal of absolute temperature(1/T) for the NTC thermistors was shown linearity, which exhibited the typical NTC thermistor properties. With increasing the amount of CuO, resistivity at room temperature, B-value, and temperature coefficient resistance decreased.