For researching dielectrics, various spectroscopic analyses must be conducted. Optical absorption spectroscopy is one of such spectroscopic measures. Fourier-transform mid-infrared (FT-MIR) absorption spectroscopy, which uses MIR photons, is very popular and can detect intramolecular vibrations or vibrations of a relatively small chemical group occurring inside a molecule. In contrast, FT-far-IR (FT-FIR) absorption spectroscopy is suitable for the detection of vibrations of an entire molecule or those of a big chemical group. Furthermore, there is another piece of emerging absorption spectroscopy, called THz absorption spectroscopy. The THz spectroscopy is believed to be able to detect intermolecular vibrations. The pros and cons of THz and FT-FIR absorption spectroscopy as a characterization tool for dielectric materials research are discussed in comparison to FT-MIR spectroscopy, by showing several typical examples of experimental results acquired in the authors' laboratory.

Acoustic attenuation of a pulsed electroacoustic (PEA) signal passed through a low- or a high-density polyethylene film is studied using the Kelvin-Voigt model for a viscoelastic material. The transfer functions of the low- and high-density polyethylene films in the PEA system are obtained based on the viscoelastic theory, which allow us to directly calculate acoustic attenuations in the polyethylene films. The calculated acoustic waveforms passed through the polyethylene films show good agreement with the measurements. By using this method, theoretical space charge profiles in the low- and high-density polyethylene films before a charge injection are obtained. This technique is useful for a calibration of the PEA system at the initial condition.

The space charge characteristic of Polyimide (PI) material under different voltage conditions is experimentally studied in this paper. Combined with the PEA method and the IEC Standard, the protection capacitor and resistor are adjusted to 187 pF and 333 kΩ for high-frequency voltage applications. Further experimental results find that, positive charge easily accumulates inside the PI material under rectangular wave voltage and room temperature, and the accumulated charge amount under negative conditions are larger than that under positive ones. Meanwhile, positive charge is more likely to accumulate under positive voltages with low frequencies, while it easily accumulates under negative voltages with high frequencies. When the temperature increases, the space charge injected from the grounded electrode increases obviously under DC voltages. In contrast, it is inferred that under rectangular wave voltages, more negative charge exists inside the sample with temperature. The measured results under different voltage conditions can provide guidance for material research in the corresponding application environment.

This paper presents an experimental study on saturation current for a low-density polyethylene (LDPE) with a semiconduction- anode and aluminum-cathode system under various temperatures. The current density at 30°C saturates and then finally decreases though the applied field E increases step by step from 0.1 to 1.5 MV/cm. The space charge distributions at 30°C are quite different from the theoretical distributions based on the space-charge-limited current model with a constant mobility. A large number of positive charges are injected from the anode and then they form a large packet in the LDPE bulk near the cathode, so that the field distribution is strongly distorted. The saturation current with the charge packet implies that local mobility at the positively charged region is much smaller than that at the other regions. The local mobility calculated with an assumption that only the positive charges are injected into the LDPE bulk exponentially decreases with increasing E at 30°C.

Gas insulated switchgears (GIS) are required to be downsized for the cost reduction, global warming mitigation and energy saving. For the downsizing of GIS, a novel electric field grading technique is highly expected. In order to solve this problem, we have been investigating the electric field control and relaxation technique by “functionally graded materials (FGM)” with spatial distribution of permittivity (ε-FGM) and/or conductivity (σ-FGM). In our previous works, post-type and truncated cone-type ε-FGM spacers were fabricated by “flexible mixture casting method” (FMC method). Compared with the conventional spacers (Uniform spacer) with a uniform permittivity distribution, the electric field grading effects of those ε-FGM spacers were simulated based on Volume-Time theory and verified by the measurement of their breakdown voltages.

In this paper, toward the application of ε-FGM to the actual GIS spacer, i.e. 30% reduction of spacer diameter or 50% reduction of cross section, we fabricated the 1/4-sized cone-type ε-FGM spacer. By grading the mixing ratio of SrTiO₃ (ε_r=332) and SiO₂ (ε_r=4) fillers in epoxy resin, a cone-type spacer with the diameter of 60 mm was fabricated by FMC method. The breakdown voltage of the ε-FGM spacer was measured in a tank filled with SF₆ gas at 0.5 MPa-abs under the negative standard lightning impulse voltage by the step-up method. The maximum breakdown voltage of ε-FGM spacer was verified to be higher by 23% than the Uniform spacer with the same configuration and size of the ε-FGM spacer, which was also consistent with the simulated value.

Electrical penetrations (EPs) in nuclear power plants play an important role in keeping a primary containment vessel airtight and connecting electrical cables from inside to outside of the vessel. Safety-related EPs that are categorized as “severe-accident (SA) equipment” must maintain their intended functions under various SA environmental conditions even after aging under their normal operating conditions throughout their service lifetime. To investigate the insulation performance of EPs in a SA environment, we tested two EPs that were equivalent to those used in pressurized water reactors in Japan. One EP specimen was pre-aged in a concurrent thermal and radiation environment at 100°C and 100 Gy/h for 100 days to simulate the degradation induced during the designated service period. The other EP specimen was used without pre-aging. To simulate the degradation induced by radiation during a SA, the EPs were irradiated with gamma rays to a total dose of 500 kGy at a dose rate of 7 kGy/h. Subsequently, the EP specimens were exposed to saturated steam at 155°C and 0.45 MPaG for 168 h. The electrical insulation resistance between the insulated cores of cables in each EP specimen was monitored during the steam exposure. The minimum insulation resistances measured during the steam exposure for the EP specimens with and without pre-aging were about 10⁹ and 10⁸ Ω, respectively, confirming that they retained sufficiently high insulation performance.

An important element of the power system network is the transformer. The conventional mineral oil (MO), which acts as an insulation and heat transfer agent in a transformer plays a major role in its steadfast operation. The insulation is highly stressed during its operational lifetime. Thus, alternative insulating liquids are being studied to enhance the reliability of the transformer. This paper furnishes a comparative investigation of the breakdown voltages of natural ester-based insulating oil (NEO) and its nanofluids (NEO-NFs) used in power and distribution transformers. The semi-conductive Titanium oxide (TiO₂) nanoparticles (NPs) with a suitable weight percentage were dispersed in the base fluid to formulate the NFs. The assessment of the dielectric behaviour of the insulating liquids was studied for all the varieties of oils-NEO, NEO-NF, aged NEO and aged NEO-NF. The oil samples were aged in an open beaker oxidative ageing (OBOA) apparatus. The AC breakdown voltage (ACBDV) is measured using paired spherical electrodes as per the relevant standard. The hypothesis testing was done to ascertain if the breakdown data for all the oil samples follow a particular probability distribution and a statistical analysis is done. The failure probabilities of 5%, 10% and 50% are considered for the Normal and Weibull distribution functions to analyze the experimental data. It is observed that NEO-NFs performed better than the base fluid in terms of its breakdown value, both before and after ageing process. The goodness of fit analysis is carried out to ascertain which distribution fits the data well.

The cable terminal composed of rubber and epoxy resin is a critical component connecting high voltage direct current (HVDC) cable and gas insulated substations (GIS). The interface is the most vulnerable part of cable terminal insulation because of the interface charge accumulation issues under DC voltage, which causes electric field distortion and even interface discharge or breakdown. In this paper, an interface charge behavior simulation model considering carrier dynamic equilibrium is established, and the interface charge characteristics during polarization and depolarization process under single polarity voltage and polarity reversal voltage are analyzed. The results indicate that the charges mainly accumulate at the solid-gas interface after polarization, which leads to a serious electric field concentration in SF₆. The charge accumulation between the different solid dielectrics first increases and then decreases with time during polarization and depolarization process. The electric field intensity in the solid domain is strengthened obviously under the polarity reversal voltage. The accurate interface charge simulation provides an effective reference for DC cable terminal insulation design.

Dielectrophoresis (DEP) is a phenomenon with the directional movement of microparticles due to the DEP force under an uneven AC electric field. We utilize polymer surfaces with microstructures such as µm-scale pit arrays for DEP devices to assemble or concentrate nanoparticles. For micro-structuring of the polymer such as PMMA with a thickness of 5-10 µm, we applied proton beam writing (PBW), a direct-write lithographic technique for high-aspect-ratio microstructures of polymeric materials such as PMMA and epoxy resin. Pit arrays such as 5×5 arrays of circular pits with a diameter of 5 µm on the 5-10 µm thick PMMA. We demonstrated the collection and assembly of Ag nanoparticles (diameter of 100 nm) on the PMMA pit arrays from the colloidal suspension of the metallic nanoparticles in water. The DEP experiments were performed with the amplitude (10-20 V) and frequency (1-100 kHz) of applied ac voltages with an additional dc offset voltage of less than 1 V. We also discuss the possible application of this technique to the detection of micro and nanoparticles such as plastics, dielectrics, and metals in environmental water.

Being an important member of polymeric insulation family, low density polyethylene is studied thoroughly. The major issue associated with it, homocharge accumulation (below 50 kV/mm), has got enough attention. Famous theories and simulation models have been put forth in regard to this. At the same time, available models ignored a well demonstrated concentration gradient of charges. According to the fundamental physics behind the concentration gradient of charges, the charges diffuse inside the material and contribute to the diffusion current. Available models/theories have only taken care of drift phenomenon of charges carriers, which is originated due to the electric field while neglecting the diffusion. Considering the gap, an analytical equation is derived after taking care of drift and the diffusion simultaneously. The analytical equation helps in identification and delineation of conduction of charges in bulk, diffusion of charge carriers and injection from the electrodes.

The direct current integrated charge method (Q(t) method) is a useful method to understand the dielectric properties of insulating materials in a simple manner. As total current flowing through an insulating material is integrated by a capacitor, Q(t) data includes various kinds of information about the material in it. To understand Q(t) data, in particular as it relates to electric charge behaviors, a parameter “charge ratio Q(t_m)/Q(0)” which is the ratio of the initial charge amount Q(0) (=CV) and the Q(t) after time t_m is introduced. The relation of the charge ratio, relaxation time and measured time, and analyzing method to distinguish electrode, accumulation and conductive charges are revealed in this paper. Q(t) data of AC- and DC-XLPE are evaluated according to the algorithm.

To evaluate the long-term reliability of polymeric material used for housing material of electric power apparatus, we continuously performed a salt-fog aging test against silicone rubber (SiR) specimens cut out from line post insulators arranged for 10 and 20 years in the coastal area. Then, the surface hydrophobicity and the leakage current were evaluated. The results showed that a required time for recovering hydrophobicity and a discharge ignition time until the leakage current is detectable are good indexes for degradation diagnosis. Especially, the required time for recovering hydrophobicity is useful. The required times for recovering hydrophobicity of SiR used for 10 and 20 years were much longer than that of unused SiR.

The effect of fat globules contained in milk on sterilization using intense pulsed electric field (PEF) is described. PEF with a pulse width of 2 µs and an electric field of 25 kV/cm were applied to four types of bacterial liquids (Listeria innocua, 10⁸ CFU/mL) including carboxymethyl cellulose (CMC) solution, skim milk, homogenized and non-homogenized milk. Temperature history of bacterial solution was precisely controlled so as to be the same under all treatment conditions. The bactericidal effect of PEF was weakest in the non-homogenized milk, whereas it was strongest in the CMC solution. Numerical analysis of the electric field suggests that a fat globule distorts the surrounding electric field and produces the region with the reduced electric field, where bacteria receive less stress. Volume of the region with reduced electric field greatly depends on the diameter of the fat globule. This analysis coincides the experimental results.

We have developed a new stereo-type X-ray camera system that can measure two time-evolutions of 2D soft X-ray emissions with different viewpoints. This system composed of two microchannel plates (MCPs), a bifurcated optical fiber bundle, and a high-speed flaming camera, successfully detected downstream heating of electrons in magnetic reconnection and energy-dependent evolution of the high-energy electron distribution.