Shuhei Nakamura

Percolation Threshold of Carbon Black-Polyethylene Composites

In this paper, the percolation threshold of the carbon black-resin composites is discussed based on the experimental results of the changes in resistivity and relative permittivity for loading carbon black, the electric field dependence of current, and the critical exponent of conductivity. It was found that a formation of infinite clusters is interrupted by a tunneling gap in the volume fraction region of carbon black loading where the change in resistivity is extremely large. It was also found that the critical exponent of conductivity for the universal law of conductivity is satisfied if the percolation threshold is estimated at the volume fraction of carbon black where a nonohmic current behavior changes into an ohmic one. It is concluded that the percolation threshold should be defined at this volume fraction of carbon black.

Electrical Conduction of Polypyromellitimide Films at Temperatures of 120-180°C

A measurement of electric current is made on polypyromellitimide films of 25 µm thickness at temperatures of 120-180°C and electric fields 4-500 kV/cm over a time range of 10-3600 s. Since the low field conduction has been confirmed to be ionic, an attempt is made to fit the field-current plot to the theoretical curve for ionic conduction. As a result a good fitting is obtained, giving the jump distance of 80-115 A and the apparent activation energy of 1.1 eV.

Effects of molecular structure on electrical conduction in low-density polyethylene above its melting point

The electrical conduction of various kinds of low-density polyethylene (LDPE) has been studied above the melting point. LDPEs are characterized by the amount and types of branches, double bonds, and oxygen-containing groups. Two components of conduction currents were found: one obeyed Ohms law in the low field range and the other was proportional to the square of the field at lower temperatures in the high field range, and was also inversely proportional to sample thickness for a constant field. The conduction mechanism of the latter component is ascribed to space charge limited current (SCLC). Among the features of the LDPE molecular structure, only the oxygen-containing groups were well correlated with the SCLC. In fact, the oxygen-containing groups reduced the SCLC, suggesting that they act as traps even in the molten state. Branches and double bonds are also known to act as traps in the solid state, but they bore no consistent relation to the electrical conduction in the molten state. >

Electrical Conduction of Nylon 6 at High Temperature

The step-response current i(t) of nylon 6 is described in detail in the time range 5–3600 s at temperatures from 80–120°C with applied fields of about 5×103 to 5×105 V/cm. The log i(t)-log t curve can be expressed by three straight lines and the current consists of an absorption component and an approximately constant one, the latter of which is assumed to be the quasi-steady state conduction current. The absorption current changes in its functional parameters with applied field. The total absorption charge tends to saturate with rising temperature or applied field, and is almost independent of the electrode metal. These results can be interpreted in terms of the immigration of a fixed number of ions existing in the polymer toward the counter electrode. It is also suggested from the ohmic nature of the quasi-steady conduction current that the accumulated ions do not establish a space charge but are neutralized at the electrode.

Effect of Molecular Order on Photoconduction of Polyimide

The photoconduction of commercial polyimide films and polyimide films synthesized in our laboratory was studied, and the photocurrent of the commercial film was found to be smaller than that of the synthesized film. As the film was stretched, the photocurrent increased. X-ray diffraction indicated that as the film is stretched, the molecular order is lowered. It is concluded that the photocurrent in polyimide is closely related to the molecular order.

Improvement of Dielectric Properties of Plasma Polymerized Styrene with Hydrogen Plasma Treatment

Capacitance and dielectric loss tangent of plasma polymerized styrene films are obtained as a function of temperature. The stability of these films is investigated with heating in a vacuum and in air. It was found that the hydrogen plasma treatment is effective in improving and stabilizing the dielectric properties of the plasma polymerized styrene films.

Partial discharge phenomena in artificial cavity in epoxy cast resin insulation system

It is crucial for proper insulation design of cast resin transformer to consider voids and delamination which might exist in cast molding because of several surface boundaries between resin and conductor. Such defects in the insulator lead to reduction of the life of the apparatus. In this report, we investigate the relation between the void size and apparent charge of partial discharge (PD) occurring in a model simulating the insulation system of cast resin transformer. It is also important to determine necessary PD detection sensitivity of PD test in a factory as well as in a field. In this paper, we also discuss the detection sensitivity by considering the relation between charge and void radius using formula derived by L.Niemyer, M.Pedersen and their colleagues in the late 1980?s to apply to the case of the cast resin transformer. Experiments were also performed to obtain PD inception voltage (PDIV) of epoxy resin slab including an artificial void with a given size. Investigation was also made on the effect of X-ray irradiation to the epoxy resin on PDIV characteristics.

Transient Current of Polyimide in the Time Range 10-4-101 Sec at Temperatures 180-280°C

The transient electric current is obtained by applying a step voltage of 167 V on Kapton(R) type H-films of 25 and 76 pm thickness. The current consists of two components: one decays fast originating from a dielectric relaxation having approximately a single relaxation time ¿; and the other is almost flat due to electric conduction. We find a correlation between the conductivity ¿ and the relaxation parameters which is close to ¿=¿0¿¿¿/¿, where ¿¿ is the permittivity of free space and ¿¿¿ is the change in relative permittivity, suggesting that the present dielectric relaxation is closely associated with the conduction process. The change in ¿ with heat treatment time is also discussed.

Thermal Endurance, Electrical Insulating, and Mechanical Properties of Hybrid Made with Poly(dimethylsiloxane) and Tetraethoxysilane

In this paper we present the heat-resisting properties of a hybrid made from poly(dimethylsiloxane) (PDMS) and tetraethoxysilane (TEOS) through a sol–gel reaction. It is found that the hybrid with ratio TEOS/PDMS = 10 shows a weight loss of less than 3% after aging treatment at 200 °C for 480 h in air. The reason for the good thermal endurance property is that low-molecular-weight PDMS chains bond to the surfaces of silica derived from TEOS, leading to the suppression of the thermal decomposition of the chains. After the aging treatment, the mechanical properties of the elastic modulus and breaking stress are improved and elongation at breaking decreases. It is also found that the electrical insulating properties are stable upon aging treatment.

Molecular orientation and photocurrent of alkyl-aromatic polyimide films prepared by vapor deposition polymerization

Vapor deposition polymerization by a coevaporation of pyromellitic dianhydride and decamethylenediamine monomers has been employed for the preparation of polyamic acid as a precursor for polyimide films. By curing at temperatures of 200–250°C the polyamic acid became the polyimide with directional arrangement of the molecular chain in the normal to the substrate observed by wide-angle X-ray diffraction. The sample containing a large amount of oriented crystals showed a small photocurrent. It was concluded that the oriented chain containing a decamethylene moiety hinders the carrier transfer between pyromellitimide moieties, resulting in a decrease of current.