Y. Sekiguchi

DC conduction and electrical breakdown of MgO/LDPE nanocomposite

To understand basic electric properties of nano-sized magnesium oxide (MgO) / low-density polyethylene (LDPE) nanocomposite under DC voltage application, the volume resistivity, the space charge distribution and the breakdown strength were investigated. By the addition of nano-sized MgO filler, both the DC breakdown strength and the volume resistivity of LDPE increased. At the average DC electric field of about 85 kV/mm and more, a positive packet space charge was observed in LDPE without MgO nano-filler, whereas a little homogeneous space charge was observed in MgO/LDPE nanocomposite material at the front of electrode. From these results, it is confirmed that the addition of MgO nano-filler leads to the improvement of DC electrical insulating properties of LDPE.

Effects of nano-sized MgO-filler on electrical phenomena under DC voltage application in LDPE

A nanocomposite material, composed of nano-sized MgO-filler added to a low-density polyethylene (LDPE), was subjected to electrical property investigations under DC voltage application. The volume resistivity of LDPE, under 40-80 kV/mm DC field at 90 /spl deg/C, increases by the power of 10 due to the addition of only a few percent of nano-filler. The amount of space charge in LDPE under high electric field is reduced and the ratio of stress enhancement by space charge becomes less due to the addition of nano-sized MgO-filler. The DC breakdown strength of LDPE is increased by adding nano-sized MgO-filler. Based on these results, the mechanism where by the nano-sized MgO filler had improved DC characteristics of LDPE was discussed.

Space charge accumulation and breakdown in LDPE and LDPE/MgO nano-composite under high dc stress at various temperatures

The relationship between space charge accumulation and breakdown at various temperatures is investigated by observing the space charge formation in low density polyethylene (LDPE) and LDPE/MgO nanocomposite material under dc high stress. We have reported, earlier, that, enhancement of electric field in LDPE by packet-like charge under dc stress might be a cause of its electrical breakdown. Furthermore, it has been found that such enhancement does not take place in LDPE/MgO nanocomposite under the same conditions. Therefore, it is believed that the addition of MgO to LDPE results in suppression of charge injection. To understand the effect of temperature, we tried to observe the charge accumulation under high dc stress at various higher temperatures. This is reported in this paper. It is seen that the packet like charge, in LDPE, plays an important role on the electric breakdown at higher temperature. On the other hand, the suppression effect of charge injection by MgO nano filler is still working well even at higher temperatures.

Space charge suppression effect of nano-size fillers added to polymeric materials

Space charge suppression mechanism in nano-composite polymer material is studied using experimental results and numerical simulation. Recently, many kinds of nano-composite polymeric materials have been reported to have improved their characteristics under high electric field. For example, LDPE/MgO nano-composite, which is made up of low density polyethylene (LDPE) and nano size filler of magnesium oxide (MgO), exhibits high volume resistivity and high dielectric strength under dc electric field. Authors have investigated the space charge behaviour in LDPE/MgO nano-composite under high electric field using pulsed electro-acoustic (PEA) method. It has been found that, compared to LDPE, the space charge formation is also suppressed in the nano-composite material. As a reason for the suppression, we have suggested that the induced dipole polarization around MgO filler formed by dc stress application might play a role of carrier trap sites. From the numerical calculation, distortion of electric potential around MgO is seen to be much larger than that around naturally included dipole. It means that the MgO acts as a deep trap site as different from some defect or ions included in LDPE. Using the numerical calculation based on such electric potential distortion, we have tried to simulate the space charge distribution in LDPE/MgO under high dc electric field. The simulation results are in good agreement with the experimental results.

Comparison of Dielectric Properties of Low-density Polyethylene/MgO Composites with Different Size Fillers

Two kinds of low-density polyethylene (LDPE) composites were prepared by adding semi-spherical MgO fillers different in diameter, one being around several ten nm and the other being several mum, and their dielectric properties were compared. At all the measurement temperatures, the conductivity is decreased and the permittivity is increased by the addition of fillers, irrespective of their sizes. However, the conductivity decrease is more significant and the permittivity increase is suppressed more in the case of the nm fillers. In addition, a drastic increase in dielectric loss factor at low frequencies is suppressed more significantly by the addition of the nm fillers. While the formation of packet-like charge observed in LDPE, which is assumed to appear at the front of the high-conductivity region that proceeds from one electrode toward the counter one, is suppressed significantly by both fillers, the effect is more significant in the nanocomposites. This means that the nanofillers suppress the increase in conductivity induced by the voltage application more effectively.

Space charge formation in low-density polyethylene up to breakdown influenced by addition of MgO nano-filler under DC ramp voltage

To understand the space charge formation of nano-sized magnesium oxide (MgO) / low-density polyethylene (LDPE) nanocomposite materials, the space charge up to the breakdown has been investigated under DC increasing ramp field. A negative charge was dominated in 0 phr sample. A positive charge was dominated in nanocomeposite sample. The field increment (= the maximum field — the average field) of 0 phr increased with increasing average field and then was saturated with a few rises and falls before the breakdown. In case of 1 phr sample, after showing the peak the field increment turned to decrease with further increase of the average field. In case of 2 phr and more, the clear decrease of the field increment was not observed due to a few positive charges. The space charge distributions of 1 phr shows different distribution between 1 st and 2 nd filed applications while that of 0 phr did not. From these results, it was considered that the homo space charge effect at the anode brought by charge trapping effect of MgO nano-filler controlled the space charge formation in nanocomeposite.

Temperature Dependent Space Charge Formation up to the Breakdown under DC Ramp Voltage in MgO/LDPE Nanocomposite

To understand basic electric properties of nano-sized magnesium oxide (MgO) / low-density polyethylene (LDPE) nanocomposite materials, the temperature dependence of breakdown strength and the space charge up to the breakdown has been investigated under DC ramp field. At temperatures at which these investigations were carried out, DC breakdown strength of the sample with the MgO nano-filler (hereafter, called 0 phr sample) exhibited higher DC breakdown strength compared with the sample with the MgO nano-filler (hereafter, called 1 phr sample). DC breakdown strength of 0 phr and 1 phr samples decreased with increasing temperature. In the case of 0 phr sample at 303 K, the negative charge was observed in the bulk and was found to be saturated before the breakdown. The hetero charge was observed at 333 K shortly before the breakdown. In the case of 1 phr sample at 303 K, the positive charge increased with increasing average field until the average field reached a certain value and then decreased with increasing average field until the breakdown occurred. The field increment (maximum field - average field), brought by the space charge, of 0 phr sample at 303 and 333 K increased with increasing average field and found to be saturated before the breakdown. As different from 0 phr sample, the field increment of 1 phr sample turned to decrease after showing a peak under the high field.

Electric characteristics of MgO/LDPE nanocomposite up to breakdown under DC ramp voltage

To understand DC breakdown phenomena of low density polyethylene (LDPE) mixed with nano-sized magnesium oxide (MgO), the electric properties, namely, DC breakdown strength, conduction current, space charge formation and local heat generation up to the breakdown under increasing field with a particular rate of increase was investigated. DC breakdown strength of nanocomposite was found to be high compared to that of LDPE. The increasing rate of the conduction current in nanocomoposite with respect to the average field (under high field) was suppressed by the addition of MgO nano-filler. The space charge formation and the local heat generation were also suppressed under high field. From these results, it is suggested that the suppression effect of conduction current affected by the space charge led to higher DC breakdown strength of nanocomposite material.

Dissipation current and electroluminescence of LDPE/MgO nanocomposite material under trapezoidal waveforms application

Low density polyethylene (LDPE) is widely used as a dielectric insulation of power cable. It has already reported that an incorporation of MgO nanoparticles in LDPE matrix could provide dielectrics with new and improved properties under dc high field. However under ac high field, it has already reported that the effect of the addition of MgO nanoparticles was relatively small compared with that under dc high field. This paper describes the dissipation current waveform and the electroluminescence (EL) of LDPE/MgO nanocomposite material under the high field trapezoidal waveform application by using the simultaneous observation system. From the results of experiments, the EL image and the relation between the dissipation current and the EL under high field trapezoidal waveform application were introduced.

Space charge formation and conduction current of MgO/LDPE nanocomposite under DC ramp field

To understand the space charge formation of nano-sized magnesium oxide (MgO)/low-density polyethylene (LDPE) nanocomposite materials, the space charge and the conduction current up to the breakdown has been investigated under increasing DC ramp field. A negative charge was dominated in the 0 phr sample. A positive charge was dominated in nanocomoposite sample. The field increment (= the maximum field - the average field) of the 0 phr increased with increasing average field and then was saturated with a few rises and falls before the breakdown. In the 1 phr sample, after showing the peak, the field increment turned to decrease with further increase of the average field. In the 2 phr and more, the clear decrease of the field increment was not observed due to a few positive charges. The conduction current of 0 phr sample and 5 phr sample increased monotonically with a increase of the average field. The conduction current of the 1 phr and the 2 phr showed great increase under one average field. From these results, it was considered that the homo space charge effect at the anode brought by charge trapping effect of MgO nano-filler controlled the space charge formation and the space charge influenced to the conduction current under a high field.