Iwao Ishino

High-Field Conduction and Electrical Breakdown of Polyethylene at High Temperatures

The mechanism of the breakdown occurring when a dc ramp voltage is applied to low-density polyethylene (LDPE) and silane cross-linked PE at high temperatures is discussed, using experimental results on the Youngs modulus Y, high-field conduction and pre-breakdown currents. The electric strength FB of PE was found to be a function of Y, irrespective of cross-linking. It was also found that, over a wide temperature range, a sharp pre-breakdown current increase could not be detected till a time before breakdown of 100 ns, the resolution limit of the measuring system. The results showed that, under the present experimental conditions, thermal breakdown is not applicable to PE over a wide temperature range and that breakdown is initiated by an electronic process which has a very short time lag to breakdown of the order of some tens of ns or less.

Effects of Oxidation on Electrical Conduction and Breakdown of Low-Density Polyethylene Films with Different Densities

In this study, the effect of blending on physical and electrical properties of low-density polyethylene (LDPE) was investigated. Two kinds of LDPEs whose densities are evaluated to be 0.9179 g/cm3 and 0.9192 g/cm3, respectively, were used and blended according to different blend ratios. The LDPE with a blend ratio of 50 wt% had the lowest impulse breakdown strength, FBImp, at 30°C. However, the LDPE with a blend ratio of 50 wt% also had the highest FBImp at 90°C among all specimens. The DC breakdown strength, FBDC, decreased with the increase of the blend ratio at 30°C but increased at 60°C and 90°C. However, the FBDC did not depend on the blend. The current densities for all specimens were almost the same at 30°C, but decreased with a blend ratio up to 75 wt% at 90°C. By analyzing X-ray diffraction (XRD) patterns, we found that the crystal size in the (020) plane increased with a blend ratio up to 50 wt%, and the LDPE with a blend ratio of 50 wt% had the largest crystal size in the (020) plane among all specimens. It was found that the FBImp was strongly related to the crystal size in the (020) plane.

Electrical Breakdown of Ethylene Copolymers

co was found that the electric strength FB of ethylene co-polymers containing optimum content of halogen compouns copigher than that of LDPE over a wide temperature range from -196 to 90°C, and that their conduction current was suppressed at the higher fields. It is thought that the introductioned comonomers with halogen groups into polyethylene suppresses the electron acceleration due to an increase of the scattering of conduction electrons, leading to an increase of FB which is determined by the electron avalanche breakdown. However, the reason for a fall of FB with halogen content at high content level is not yet clear. A tentative explanation is presented: Existence of high halogen content in the amorphous part of the ethylene chain gives rise to some kind of defects which arise, for example, from locally brittle structures or stereo hindrance due to strong dipole-dipole interaction, leading to a decrease of FB.

Effects of oxidation on electrical conduction and breakdown of LDPE films with different densities

The effects of oxidation on electrical conduction and breakdown strength were studied in two kinds of LDPE films with different densities. Oxidation enhanced more the conduction current in the LDPE film with density of 0.9172 g/cm/sup 3/ (l-LDPE) than that with density of 0.9255 g/cm/sup 3/ (h-LDPE). This result was attributed to the easier carrier transport in unoxidized region of l-LDPE. Both oxidized specimens had almost the same impulse breakdown strength (F/sub Bimp/) except when the absorbance of C=O groups (A) is higher than 0.6. At 30/spl deg/C, it was considered that the increase in the number of injected electrons which initiate avalanche decreased F/sub Bimp/ for A<0.1, but the carrier scattering due to C=O groups increased F/sub Bimp/ for A>0.1. However, thermal or electromechanical breakdown mechanism would take part in the breakdown process at higher temperatures. The dc breakdown strength (F/sub Bdc/) decreased with oxidation and h-LDPE showed a higher one. These results suggested a contribution of thermal mechanism to the breakdown.

Electrical properties of ethylene/silane copolymers

A method of manufacturing silane-crosslinked polyethylene was developed using random copolymerization of ethylene with silane compounds. In examining the crosslinking properties and electrical properties of these ethylene/silane copolymers, it was found that the crosslinking rate was closely related with the reactivity of the silane, whereas the electric strength did not differ greatly with the type of silane radical. Study of the mechanism of electric breakdown under application of DC voltage in a temperature range of 60 degrees C approximately 160 degrees C revealed that the electric strength can become greater than that for application of impulses near the ordinary temperatures due to the influence of space charges. In the higher temperature range, on the other hand, the electric breakdown appeared to be due to a certain complex process concerned with Maxwellian forces.<<ETX>>

Molecular Structure and Electric Breakdown of Ethylene/Silane Copolymers

Crosslinking characteristics and electric breakdown (F}B) of various kinds of ethylene/silane random copolymers were investigated. The crosslinking rate was found to increase with the reactivity of the silane species, whereas the FB did not significantly differ among the samples with different silane radicals. The FB was measured in a temperature range of 30-160°C using dc and impulse voltages. The results revealed that the dc FB was larger than the impulse one below the critical temperature Tc for the crosslinked silane/ethylene copolymers with two different gel contents. The difference was interpreted in terms of space charge effect. Tc moved to higher temperature with increasing the degree of crosslinking. X-ray induced TSC measurements allowed observation of new TSC peaks at high temperatures due to the crosslinking giving rise to carrier traps, consistent with the breakdown result.

High field conduction and carrier traps in polyethylene copolymerized with various monomers

Polyethylene (PE) has been widely used in the fields of wire and cable insulation because of its excellent electrical properties (low dielectric loss and high electric strength). Recently, the development of 500 kV class PE cable is in progress and the improvement of insulating materials is required to endure under a higher electric stress. Chemical modification (copolymerization, additives, etc.) and physical modification (morphology, crystallinity, etc.) of PE have been tried to increase its electric strength [1,2]. We found that electric strength of PE was improved by the copolymerization of monomers containing halogen atoms [3]. In order to clarify the role of halogen comonomers, in this paper, we have investigated high field conduction and carrier traps in PE copolymerized with various halogen comonomers.

Electrical Breakdown of Linear Low-Density Polyethylene

Disruption electrique de onze sortes de polyethylenes basse densite lineaire avec differentes structures moleculaires de comonomeres et avec diverses teneurs