||I am mainly investigating the relationship between degradation and various properties of various polymers. For example, polyethylene, which is a common polymer, polylactic acid, which is known as a biodegradable polymer and is attracting attention as an environmentally friendly material, polyimide with excellent heat resistance, and nanocomposites in which nano-sized inorganic fillers are added to polymers to improve thermal conductivity and electrical properties. In recent years, I have been studying the effects of heat, radiation, and hot steam exposure on electrical and mechanical properties of insulating materials for electrical instrumentation used in nuclear environments.
|Research Field(Keyword & Summary)
- (1) Electrical Conduction and Breakdown Properties of Several Biodegradable Polymers
While the permittivity and conductivity of poly-L-lactic acid (PLLA) and polyethylene terephthalate succinate (PETS) are comparable to low-density polyethylene (LDPE), those of polycaprolactone butylene succinate (PCL-BS) and polybutylene succinate (PBS) are much higher. This is because PLLA and PETS are in the glass state at room temperature, while PCL-BS, PBS, and polybutylene succinate adipate (PBSA) are in the rubber state. All the polymers tested have almost similar impulse breakdown strength at room temperature. As for dc or ac breakdown strength, PLLA and PETS show a relatively higher strength than PCL-BS and PBS.
- (2) Effects of Resin/Filler Adhesion on the Thermal and Electrical Conductivity of Polyimide Nanocomposites
With an aim to develop a good coil winding insulation film, fillers of boehmite alumina in the shape of a roughly rectangular plate were added with ratios of 10 and 20 wt% to polyimide. It was found that samples with fillers treated using the methacrylic silane coupling agent have the strongest adhesion at the filler/polyimide interfaces and the lowest dielectric loss factor at high temperatures. A positive relationship between the filler/polyimide adhesion and the thermal conductivity is also indicated.
- (3) Spatial Resolution between Two Abnormalities in a Cable by Frequency Domain Reflectometry
The spatial resolution of a cable fault location method by frequency domain reflectometry was examined experimentally by giving two abnormalities intentionally along a 31.5-m coaxial cable. As a result, two clear signal peaks appear, which can locate the presence of two hot spots perfectly if they are separated by more than 30 cm at a position about 15 m from the cable end with a measuring device. In addition, a clear signal peak with a small shoulder is recognized if the two hot spots are 1 cm away.
- (4) Degradation of Flame-Retardant Cross-Linked Polyethylene Caused by Heat, Gamma-Rays, and Steam
Various kinds of polymers are used for electrical insulation in nuclear power plants. They are subjected to thermal and radioactive stresses, depending on their locations. They are also subjected to high-temperature steam in the event of an accident. Regarding this, sheets of flame-retardant crosslinked polyethylene (FR-XLPE) were subjected to heat, gamma-rays, and steam exposure, and their infrared absorption spectra, stress-strain relations, indenter moduli, and permittivity spectra were measured. The low dose rate at the high temperature enhances the diffusion of oxygen into the sample. Oxidation and cross-linking proceed at the high dose rate. Such oxidation and cross-linking make FR-XLPE hardened by the formation of three-dimensional structures.
- (5) Influence of Oxygen on Insulation Performance of Safety-related Low-voltage Cables for Nuclear Power Plants under Severe Accident Conditions
Some safety-related low-voltage cables used in nuclear power plants are required to maintain the insulation performance even during severe accidents. It is not clear whether oxygen affects the insulation performance of the cable in the steam exposure during a severe accident. Flame-retardant ethylene propylene diene rubber insulation cables were used as safety-related cables. When oxygen is present in water vapor, the oxygen concentration decreases during the exposure since oxygen is consumed by the oxidation of insulators and other materials. Whether or not the cable was subjected to aging does not alter the insulation resistance of the cable significantly under the assumed severe accident condition.
- Naoshi Hirai, Ryotaro Minami, Toshikatsu Tanaka, Yoshimichi Ohki, Minoru Okashita, and Takashi Maeno, Chemical Group in Crosslinking Byproducts Responsible for Charge Trapping in Polyethylene, IEEE Trans. Dielectrics and Electrical Insulation, Vol. 10, No. 2, pp. 320-330, 2003.
- Yoshimichi Ohki and Naoshi Hirai, Electrical Conduction and Breakdown Properties of Several Biodegradable Polymers, IEEE Trans. Dielectrics and Electrical Insulation, Vol. 14, No. 6, pp. 1559-1566, 2007.
- Yoshimichi Ohki and Naoshi Hirai, Location feasibility of degradation in cable through Fourier transform analysis of broadband impedance spectra, Electrical Engineering in Japan, Vol. 183, No. 1, pp. 1-8, 2013.
- Naoshi Hirai and Yoshimichi Ohki, Location Attempt of a Heated and Irradiated Portion in a Silicone Rubber Insulated Cable by Frequency Domain Reflectmetry and Line Resonance Analysis, IEEJ Transactions on Fundamentals and Materials, Vol. 136, No. 9, pp. 553-560, 2016.
- Naoshi Hirai and Yoshimichi Ohki, Location Attempt of a Coaxial Cable with Multiple Abnormalities by Frequency Domain Reflectmetry, IEEJ Transactions on Fundamentals and Materials, Vol. 139, No. 9, pp. 367-372, 2019.
- Yoshimichi Ohki, Hiroyuki Ishii and Naoshi Hirai, Degradation of Soft Epoxy Resin for Cable Penetrations Induced by Simulated Severe Accidents, Energies, 14, 6932, 2021.
- Yoshimichi Ohki and Naoshi Hirai, A Trial to Estimate the Content and Distribution of Filler in Polymer Nanocomposites by SEM-EDX, Journal of Engineering Science and Technology Review 14 (7) (2022) 8 –15, 2022.
- Yoshimichi Ohki, and Naoshi Hirai、Degradation of Flame-Retardant Cross-Linked Polyethylene Caused by Heat, Gamma-rays, and Steam, IEEE Access, Vol. 10, pp. 62164-62172, 2022
- Yoshimichi Ohki, Naoshi Hirai, Thermal Aging of Soft and Hard Epoxy Resins, High Voltage, pp. 1-9, 2022
- Yoshimichi Ohki, Naoshi Hirai and Sohei Okada, Penetration Routes of Oxygen and Moisture into the Insulation of FR-EPDM Cables for Nuclear Power Plants, Polymers, 14, 5318, 2022.