We investigated the fundamental insulation characteristics of insulating oil with dispersed nanoparticles. The AC breakdown voltage of the insulating oil increased by mixing the dispersed hydrophilic nanoparticles, because the hydrophilic nanoparticles absorbed moisture in the oil, which decreased the number of weak points in the oil for breakdown. In contrast, the hydrophobic nanoparticles did not contribute to the increase of AC breakdown voltage, and decreased the AC breakdown voltage. On the other hand, in the case of overdosing of nanoparticles, the aggregation of nanoparticles caused the decrease in the AC breakdown voltage of the oil, which depended upon the volume concentration of nanoparticles regardless of the diameter of nanoparticles. The aggregation of nanoparticles was led by dielectrophoresis phenomenon, which brought the gap length dependence of the AC breakdown voltage of the insulating oil with the dispersed nanoparticles, because the gradient of electric field strength at the gap was smaller in the case of shorter gap length.

In order to improve the electrical insulation performance of Vacuum Interrupter (VI), it is important to control a surface flashover which occurs via charging on the insulator. In this paper we prepared a model sample of vacuum interrupter with shield rings which mitigates the high electric field around triple junctions. The charge distributions on the inner surface of cylindrical insulator were investigated after applying AC voltage under the various conditions. The charge distributions were measured over whole area of the inner insulator's surface by an electrostatic probe. As a result, we found that surface electrical charges were formed on some local parts of insulator surface between the areas of the shield rings after the initial stage of several cycles of AC voltage application. After AC voltage application during some seconds, the surface charges tend to expand all over the insulator surface. Additionally, depending on times of AC voltage application test, the formation of surface charges becomes more difficult. It was considered as the conditioning effect on the surface electrical charges.

The surface of oil immersed pressboard (PB) influences partial discharge (PD) inception voltage and creepage discharge properties. This paper deals with surface modification of insulating paper by thin solid layer coating such as epoxy resin, Teflon and nano size silica/epoxy resin composites on PB surface to improve electrical insulation properties of oil/pressboard composite insulation system. Five kinds of specimens are prepared with different coating materials to investigate creepage discharge initiation and propagation under positive standard lightning impulse voltage (1.2/50 µs) using a rod-plane electrode arrangement in mineral oil. Creepage discharge properties of conventional PB in oil/PB composite insulation system are compared with that of the solid layer coated PB in terms of discharge final propagation length (l_d) and PD inception voltage (PDIV) as an inception probability (P_i). The results show that PDIV of the solid layer coated PB increases at all applied impulse voltage levels compared with that of conventional PB. Especially, when the filler content of nano silica added in base epoxy was SP-0.2wt%, PDIV characteristic was improved. The effect of solid layer coating of PB surface on the creepage discharge properties is also discussed in terms of the effects of introduced deep electron traps due to the nano composites coating as well as the drying effect to remove water content and the surface roughness of PB.

Recent studies have reported unusual behaviors of geomagnetic diurnal variation (GDV) in the vertical component prior to the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0). To make a better understanding of this phenomenon, time-spatial analysis of GDV has been applied in this study. Geomagnetic data of long term observations at 17 stations in Japan have been analyzed using the same method in Han et al. 2015. Ratios of diurnal variation range between the target station and the reference station KAK have been computed. After removing seasonal variations revealed by wavelet transform analysis, the 15-day mean values of the ratios in the vertical component shows a clear anomaly exceeding the statistical threshold about 2 months before the mega event in both ESA and MIZ stations in the Tohoku Region. Similar results could not be found in other regions of Japan. Spatial distributions of the ratios show a good agreement between the location of the anomalies and the epicenter of Mw 9.0 earthquake. These time-spatial results are consistent with independent results obtained from other observations such as radon density, seismicity, and GPS displacements, which suggest the geomagnetic data might be useful in earthquake monitoring and disaster mitigation.

For analyzing carrier behaviors in organic light-emitting diodes, we have developed a novel microscopic analyzing system using electric-field-induced optical second-harmonic generation measurement and laser beam induced current measurement. Distributions of electric fields, interfacial accumulated charges, EL intensity and photo-current are microscopically probed. Our developed analyzing system showed a strong relationship between EL intensity distribution and photo-current distribution, and thus is found helpful for improvement of organic devices.

CoFe₂O₄(CFO)-doped BaTiO₃(BTO) film is one of composite multiferroic materials. In the materials, magnetoelectric effect occurs from the stress of the interface between ferroelectric and ferromagnetic. Therefore, Interfacial stress between BTO and CFO is important for magnetoelectric effect. However, it has not been clearly yet. It is considered that it can reveal the effect on BTO of CFO to comparing different CFO content samples. Then, we fabricated CFO-doped BTO films with various content and measured lattice constant and temperature dependence of dielectric constant for films. As a result, out-of-plane lattice constant of BTO was reduced with the increase of the CFO content. In addition, phase transition temperature of BTO was increased with the increase of the CFO content. Therefore, it is considered that CFO adds some in-plane tensile stress on BTO.