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.

The activation energy of a material is an important factor that significantly affects the lifetime and can be used to develop a degradation model. In this study, a thermal analysis was carried out to evaluate and collect quantitative data on the degradation of insulation materials like EPR and CSP used for nuclear power plant cables. The activation energy was determined from the relationship between log β and 1/T based on the Flynn-Wall-Ozawa method, by a TGA test. The activation energy was also derived from the relationship between ln(t) and 1/T based on isothermal analysis, by an OIT test. The activation energy of EPR derived from thermal analysis was used to calculate the accelerated aging time corresponding to the number of years of use, employing the Arrhenius equation, and determine the elongation corresponding to the accelerated aging time.

In this paper, EPR (ethylene propylene rubber) insulation material was accelerated degradation test at 121℃, 136℃, 151℃, and experiment the typical EAB (elongation at break) at mechanical characteristics analysis. It is shown that the failure-time at the point of 50% of the initial value of Elongation rate to obtain the activation energy. The failure-time was shown each 5,219 hr, 3,165 hr, and 668 hr at three temperatures. In order to derive the activation energy, Arrhenius methodology was applied. Also, we got the Arrhenius plot from three accelerated temperatures. The activation energy values got 0.98 eV from EAB test. The experimental data were evaluated for estimating the probability density, and the suitable distribution by using statistical program MINITAB. It is shown that EAB data by the acceleration thermal degradation is most suitable for the Weibull distribution.