The high-field currents in poly-p-xylylene (PPX) thin films have an anode metal dependence and a negative temperature dependence. They are accompanied by electroluminescence. They are explained in terms of hole injection from anode metal and collision ionization (or electronic avalanche).
In this paper, the authors attempted and discussed the improvement of insulation thin films by chemical or physical modifications. Chlorine atoms in poly2-chloro-p-xylylene (PCPX) thin films suppressed high-field currents and raised the electric strengthes (8.4 MV/cm) in the low tempera ture region. Mean free pathes in PPX and PCPX films are 6.5 nm and 3 nm, respectively. It is clear that the probability of carrier scattering in PCPX is higher than that in PPX, because there are polar groups C-C1 in, PCPX. Plasma treatment introduced some defects and oxidation products in PPX thin films. It strongly suppressed high-field currents in PPX and resulted in a high dielectric strength(-10MV/cm)in all temperature region. Both plasma modification and ozone oxidation also suppressed hole injection from anode. Especially, the latter was more effective to improve the electric strength.

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The high-field currents in poly-p-xylylene (PPX) thin films have an anode metal dependence and a negative temperature dependence. They are accompanied by electroluminescence. They are explained in terms of hole injection from anode metal and collision ionization (or electronic avalanche). The authors attempted and discussed the improvement of insulation thin films by chemical or physical modifications. Especially, ozone oxidation much effectively suppressed highfield current.
In this paper, the authors discussed the mechanisms of high-field current and breakdown of PPX thin films using ozone oxidation layer. High-field currents in PPX-oxPPX thin films were suppressed depending on the oxidation time and the position of an oxidation layer above 4 MV/cm at room temperature and above around 3 MV/cm in the low temperature region. High-field currents inoxPPX thin films were more strongly suppressed than those in PPX-oxPPX thin films. We applied a single-electron collision-ionization theory to the breakdown properties of PPX, PPX-oxPPX and oxPPX thin films. Our experimental results are almost consistent with the theory. It is clear from the theory that the mean free path in PPX thin film is longer than that in PPX-oxPPX thin film and that the mean free path in oxPPX at -160°C is longer than that at room temperature. These experimental results are explained by the suppression of collision ionization due to carrier scattering caused by polar groups (C=0) or lattice vibration.

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Breakdown mechanism was studied for plasma polymer films of ethylene and those of ethylene and trifluoromethane. In order to determine carrier species which causes avalanche, high-field conduction, dielectric breakdown and photoconduction were studied. It was found that all these characteristics depend on the anode metal. When the anode is aluminum, the barrier height for tunneling injection obtained from the Fowler-Nordheim relation is higher than the gold anode. The aluminum anode also has a higher dielectric strength and a longer time-lag-to-breakdown than the gold anode.
From these results, together with the effect of the shape of illuminated electrode on photocurrent, it is considered holes are injected through tunneling from the anode by high electric stress applied and that avalanche is caused by these holes.

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Using newly developed specimen for the measurement of intrinsic electric strengths of polymeric films at cryogenic temperature, the dielectric breakdown characteristics of polar polymeric (Polyvinyl Chloride, PVC) films were studied. The DC electric strengths of PVC film measured with this specimen increased with decrease of temperature, and decreased after showing a peak around 200K. Below 150K they were almost constant. This temperature dependence is quite different from those already reported with recessed specimens. Since the recessed specimens made of the same PVC material gave almost the same temperature dependence as that reported, the present results are inherent to the elongated film specimens. The dependence of electric strengths on the rise rate of applied field and the prestressing showed the electrical breakdown of polar polymers at cryogenic temperature are affected by the space charge effect. Although it has been open question why DC electric strengths of polar polymers show a negative temperature dependence in low-temperature region, it becomes clear from this study that the appearance of this dependence is closely related to the space charge formation.

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DC and impulse electrical breakdown of polypropylene film was investigated experimentally and discussed theoretically. The breakdown characteristics of DC electric strength suggest an electronic breakdown in low-temperature region and a thermal process due to transient conduction current in high-temperature region. On the other hand, the impulse electric strengths showed almost no temperature dependence from low- to high-temperature region, suggesting the electronic breakdown process.
In order to explain the above breakdown characteristics, a new breakdown model is proposed by introducing an electronic breakdown criterion into thermal breakdown caused by transient conduction current. Numerical analysis was carried out taking account of the transport of both bulkgenerated and electrode-injected carriers in their transient states and the local field distortion. This thermal and electronic composite breakdown model could successfully explain the DC and impulse breakdown characteristics of polypropylene film not only in the low- but also in the hightemperature regions.

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This paper presents experimental results of space charge distribution in cross-linked polyethylene by a pulsed electroacoustic method. The influence of crosslinking agent residues on space charge distribution in XLPE which contained different content of residues is studied to understand the transport and injection of charges under DC voltage. Homocharge distribution is dominant when the content of residues is low whereas heterocharge distribution is dominant when the content of residues is high. A new data display method of a three-dimensional color plot is proposed to display all measurement data on one plot by which the time dependence of space charge distribution can be understood easily and conveniently.

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In the field of HV DC cables, oil impregnated paper insulation is widely used. It has generally been found that polymeric type insulation does not provide sufficient reliability for this purpose. Thispaper relates to the investigation of polymer insulating materials for HV DC cables.
From the results of tests to establish dielectric breakdown strength (E_B) in various types of polymeric materials (using sheet samples), modified HDPE, which was produced by introducing a small amount of polar group into HDPE, was selected as a promising new material. The outstanding DC breakdown characteristics in modified HDPE were assumed to be attributable to the improvement of the materials intrinsic breakdown strength due to the presence of high crystallinity polyethylene, in addition to the control of space charge due to the polar group.
These tests have proved that the excellent Eg of modefied HDPE make it suitable to cable insulation applications, particularly as its DC E_B is 1.5-2.0 times as high as that of a conventional XLPE cable.

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The effect of additives and AC voltage pre-stressing on electrical tree initiation was studied in LDPE using needle to plane electrode system. The tree suppression effect of additive is increased by pre-stressing. It was frequently observed after pre-stressing that tree starts from side portion of the needle electrode instead of the tip. By doping of Methyleneblue as an additive, the migration of additive under high field was proved. The additive migration may be caused by dielectro-phoresis. The voltage hardening effect in additive doped sample was well explained by the term of the concentration of additive at the needle tip.

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We reported that AC breakdown voltage of dry-cured XLPE cables decreases to about half of the initial breakdown voltage by the immersion treatment. In this paper, the causes of this decrease are investigated and are. reported in detail. From some experiments, the decrease is found to be mainly caused by the moisture existing in the insulation. Our results show that the value of the breakdown voltage is not related to the moisture above ten ppm order. However, even little moisture of ten ppm order may reduce the breakdown voltage. To clarify this, we investigated the state of water in the insulation using the FT-IR and the optical microscope. The results show that the moisture is mainly composed of a pure liquid water, and exists in two regions; (1) dissolved in the amorphous region and (2) condensed in the form of microvoids, in amounts exceeding its solubility limit in the insulation. Also, we assume that the moisture in the amorphous region causes the reduction of the AC breakdown voltage.

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The electrical breakdown phenomena in Polymethylmethacrylate (PMMA) for positive polarity with point-to-plane electrode configuration has been investigated using a photo-optical currentmeasuring technique and a 760 ns rectangular high-voltage pulse generator in conjunction with an image convertor camera. The high-speed framing photography revealed the propagation velocity of breakdown channel to be between 30 and 120km/s for positive 50kV point voltage. The waveforms of the prebreakdown current in PMMA is essentially the same as those in liquid dielectrics. The breakdown time lag was precisely measured from the current. A linear relation between formative time lag and gap spacing is observed for gap length greater than a critical value, indicating another constant propagation velocity in this region. The velocity is deduced from the relation to be 2.2km/s for a positive point at 50kV. This value is fairly close to that of longitudinal waves in the solid.

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