The insulation performance and the flashover mechanism of a vacuum gap bridged by an insulator in the shape of a conical frustum have been studied. The cone angle of the insulator is varied from 0 to 45 degrees, and the thickness of the insulator is varied from 5 to 15mm. The gap is subjected to a lightning impulse voltage of ±2/50μs. The insulation performance is investigated by observing the ratio of the flashover voltage of the bridged gap to that of the gap without insulators. The flashover mechanism is investigated by observing the distribution of traces on the cathode due to flashovers and by analyzing the electric field near the insulator-cathode junction. Charging of the insulator due to electron collision on its surface is considered in this analysis. As a result, it is found that the ratio is greater than 90% for a cone angle greater than a critical one, when the top of the frustum is subjected to the positive impulses. It is also found that the trace distribution is closely related to the insulation performance.

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AC flashover voltage in air (ACFOV) decreases, when nylon string is attached on PMMA insulator surface which has a backside electrode. The decrease in ACFOV by the attachment is explained by the generation and the propagation of the negative surface leader along the string. When the contact angle between the H. V. electrode and the insulator surface is in a range from 90° to 120°, some of the negative surface leaders generated proceed along the string to the earthed counter electrode. The magnitude of the negative leader along the string is stronger than that of the leader on the insulator without a string. In the range of the contact angle from 120° to 180°, the negative surface leader which does not appear on the insulator without a string is generated along the string by the attachment. In the both ranges of the contact angle, the negative leader along the string promotes the flashover.

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It has been reported that there exists a significant difference between the AC and impulse partial discharge characteristics of a wedge gap in SF₆ gas. To clarify this, the partial discharge characteristics against polarity-reversing oscillating impulse voltages with relatively high frequency of 5.4 to 154kHz have been studied. At low gas pressures, the partial discharge inception voltage (PDIV) is almost independent of the frequency and attenuation factor of the applied oscillating voltage, and shows fairly good agreement with PDIV of lightning impulses. On the contrary, at high gas pressures, the PDIV becomes lower as the attenuation factor decreases than the value of lightning impulses and the calculated breakdown voltage of SF₆ gas. It is considered that residual charges remained in the wedge gap are responsible for these phenomena.

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The electric charge density accumulated at the interface of a double layer consisting of a liquid and a solid insulator was obtained from the electric field strength in the liquid insulator as a function of time during application of DC stress (24h) and after short-circuiting positive and negative electrodes. The field strength was measured using the high sensitive Kerr electro-optic effect whose minimum measurble electric field intensity was 100V/cm in silicone oil with 6-cm parallel electrodes. The double-layer system consisted of a liquid insulator (Si-oil; pure, water-containing and thermally aged) and a solid insulator (polypropylene (PP), normex and polyester nonwoven fabric). According to simple calculations, the initially applied voltage (3, 000V) was divided in the ratio of 1.5:98.5 (PP, Si-oil, respectively). However, after 24h, the ratio changed to 80:20. This indicates that 80 to 90nC/cm² accumulated at the interface between PP and Si-oil. By observing the time course of the electric field in the Sioil for 24h after short-circuiting the two electrodes, we found two rates of decay indicating two decay mechanisms: the fast rate is related to the charge leakage from the interface and the slower rate is related to the leakage of charges trapped in the PP.

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When the PET film was placed on the anode, the breakdown voltage with a DC-ramp voltage (0.5kV/s) was much lower than that for the oil alone as long as the film was thin and it increased with increasing the film thickness. These results suggested that the complete breakdown was caused by the film reakdown due to negative charge deposition.
The breakdown voltage for the case of the film on cathode was slightly higher than that for oil alone and it was scarcely changed by the change in the film thickness. It was suggested that the discharge in the oil caused the complete breakdown of the composite. However, when the composite whose cathode was screened with the film was subjected to a constant dc voltage for a long period, it was assumed that the complete breakdown was induced by the film breakdown caused by the deposition of positive ions.

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Breakdown time lag obtained from surface discharge propagation on a solid dielectric in insulating oil with point-to-plane electrode configuration was studied using a photo-optical current measuring system and a 1 micro-second rectangular high voltage pulse ganerator. The time lag was directly measured from prebreakdown current. The time lag for negative point is greatly decreased by the presence of solid dielectric surface, compared with that for positive point. The time lag for negative point is longer than that for positive one except at extremely higher electric field condition. At the higher field, the time lag is found to be almost the same for both point polarities. Very little effect of dielectric materials or their thickness on the time lag is observed for both point polarities. On the basis of these results, a new breakdown propagation model involving polarization effect and auger process is proposed.

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Propagation process of an impulse creepage discharge on the surface of a solid insulator in transformer oil is investigated in detail by using a Schlieren method, a dust figure method, a current measuring system by LED, etc. simultaneously when an impulse voltage with various values of duration of wave front (Tf) is applied to a point-back electrode system. The main results may be summarized as follows:
(1) In case of the longer T_f for a constant peak value (V_m=±30kV),
(i) propagation of streamer-like density changes is alwayshalted before the voltage increases up to the crest point.
(ii) appearence of the plural groups of current pulses may be seen sometimes.
(2) As independently of the point polarity, the instantaneous value of the impulse voltage, which corresponds to a time lag to the inception of the discharge current, is larger,
(i) the discharge figure becomes larger in size and more branches appear.
(ii) the charge quantity of the discharge is larger.
(3) For the average propagation velocity υ_sd of streamer-like density changes, , υ⁺ _sd>υ^-_sd, and the remarkable difference of the velocities can not be seen by the T_f for both point polarity.

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In practical insulating systems epoxy resin is used mostly in the form of a composite with fillers. In order to use the epoxy-resin composite insulating systems at higher fields, it is important to investigate electronic properties at the epoxy-filler interface. We have prepared an epoxy-alumina two-layer composite film as a model system of a polymer-filler interface and studied the effect of interface on electronic conduction by measuring photocurrents and thermally stimulated currents (TSC).
At high fields and low temperature, holes are more mobile than electrons in epoxy-alumina composites. Most of the holes in the epoxy layer can pass through the epoxy-alumina interface and move in the alumina bulk, increasing the conduction current in the alumina by about two orders of magnitude. This interfacial phenomenon can not be explained by the conventional MaxwellWagner model. TSC measurements on the omposites showed that some of holes are trapped at the epoxy-alumina interface, in alumina bulk or the epoxy bulk near the interface, and give rise to new TSC peaks. At high fields and room temperature, the photoconduction is governed by the alumina layer and the dark conduction by the epoxy layer. These are well explained by the Maxwell-Wagner model. The electrical properties at the epoxy-filler interface are greatly affected by temperature and carrier species.

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The high-field currents in poly-p-xylylene (PPX) and poly-2-chloro-p-xylylene (PCPX) thin films are due to hole injection from anode metal and collision ionization. PPX thin films showed the negative temperature dependence of high-field currents. Since the introduction of chlorine atoms in PCPX thin films increases the carrier scattering, high-field currents in PCPX thin films are suppressed in the low temperature region.
This paper discussed the high-field currents of PCPX/PPX composite thin films which are PCPX thin films (d _??_ 150nm) deposited on PPX thin films (d _??_ 200nm). They were strongly suppressed at room temperature, but not in the low temperature. Therefore, PCPX/PPX composite thin films showed a remarkable negative temperature dependence of high-field currents. High-field currents were more suppressed in PCPX (60nm)/PPX (230nm) composite thin film than in PCPX (130nm)/PPX (230nm). This can be explained by the suppression of hole injection from anode by homo space charge at the interface between PCPX and PPX.

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Ion implantation involves problems of wafer charging causing the deterioration of thin gate oxides. We investigated how the thin oxide (25nm) of MOS capacitor devices deteriorates during As⁺ ion implantation. The effect of ion beam density, the distribution of the oxide deterioration on a wafer and the effects of photoresist coverage around a gate electrode were quantitatively analyzed using the generated number of interface state at SiO₂/Si inteface of MOS capacitor. The deterioration of gate oxides was found to be affected by the four charge sources: the irradiated ion beam into a gate electrode, the secondary electrons emitted from the gate electrode, the charges accumulated on the photoresist surface around the gate and the secondary electrons emitted from the surface of a wafer holder. The former three charges accelerate the deterioration of the thin oxide and the latter one reduces it.

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Not only in the field of Electrical Engineering, but in the fields of Mechanical Engineering, Metal Engineering, Pharmacology and Medical Science, the Weibull distribution is widely used in discussing the probability distribution of breakdown, failure and lifetime. Therefore, it is very important to estimate the Weibull parameters and percentile points from the actual data.
It has been often used the Weibull probability chart to estimate the parameters of the Weibull distribution. That is, the graphical method is taken. This method has a good advantage that we can estimate the parameters easily. On the contrary, there are some disadvantages such as the inaccuracy in estimation and having no confidence. Thus, we have to estimate the parameters with accuracy by the analytical methods such as maximum likelihood estimation method, and then to calculate the reliability of the estimates.
Here, I mainly describe the calculated results of reliability of the estimates of the parameters and percentile points when all the three parameters are unknown, along with the explanation of the theoretical background. In addition, I describe the confidence limit and confidence region with the method of asymptotic normal theory of maximum likelihood estimation comparing with the method of likelihood ratio test.

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High-density polyethylene films of 50μm thick were treated in a down-stream plasma of rf glow discharge of argon, hydrogen, or oxygen gas. It is found that many transvinylene and unreacted radicals remain after argon or hydrogen plasma treatment, and that carbonyl groups are formed by oxygen plasma treatment.
Conduction current is smaller in the hydrogen-plasma treated samples and the samples exposed by oxygen or ozone after hydrogen-plasma treatment than the untreated samples. As for this reason, thermal-pulse-current measurements indicate that the treated surface layer traps electrons or holes and reduces field strength at the electrode surface, thus suppressing further injection of electrons or holes.

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Relation between morphological change, induced by copolymerized benzene rings or by kneading, and the dielectric strength of ethylene-styrene copolymer was studied. In unkneaded samples, the lamellar thickness decreases and the lattice constant along the a-axis increases with increasing styrene content. However, in kneaded samples, the lamellar thickness and the lattice constant remain constant up to the styrene content of 3.10wt%. Spherulite is observed with a polarizing microscope in unkneaded samples, but it disappears in kneaded samples. Kneaded samples, especially the kneaded sample with the styrene content of 3.10wt%, have higher breakdown strength than unkneaded samples. Styrene content of the sample showing the highest breakdown strength increases as the temperature decreases. At -50°C or -80°C the sample with the styrene content of 15.40wt% has the highest breakdown strength of all the samples. It is considered that benzene rings are moved to the interfacial region between lamellar and amorphous parts by kneading and hinder the acceleration of electrons along the lamellar. This effect of benzene rings becomes pronounced in low temperature region as the breakdown process is electronic.

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