茂金属聚乙烯改善低密度聚乙烯的介电性能

针对高压、超高压直流塑料电缆中存在的空间电荷效应,大多数直流电力电缆常采用改性方法提高其介电性能,如空间电荷特性、体积电阻率和击穿强度等。常用的改性方法主要有添加剂、共混、接枝和二元共聚4种。共混技术较广泛用以改性聚乙烯电缆,提高其介电性能。因此用电声脉冲法(PEA)测量了MPE与LDPE共混试样中的空间电荷分布;用高阻计测量了共混物的体积电阻率,用阶梯电压测量了共混物的交流击穿场强。试验结果表明,1%MPE与LDPE共混能有效降低空间电荷效应,提高交流击穿场强7.9%,略降低体积电阻率。最后讨论了共混物的物质结构、电荷陷阱及介电性能间的关系。     

Vacuum electrical breakdown characteristics and surface condition of Ti electrodes with oxidation conditions

Outgassing from an electrode surface is regarded as a major factor leading to electrical breakdowns in vacuum. Recently oxidation treatment at 200/spl deg/C was reported as an effective means of reducing Ti outgassing. In this paper, we report our measurement and comparison of the electrical breakdown characteristics of Ti electrodes with different oxidation conditions (without oxidation, oxidation at 200/spl deg/C, oxidation at 450/spl deg/C). In addition, we analyzed electrode surfaces before and after breakdown experiments in situ with X-ray photoelectron spectroscopy (XPS). Before oxidation, we machined the electrode's surfaces to the roughness of 0.8 /spl mu/m Rmax with diamond turning. Breakdown experiments demonstrated that the breakdown field is highest at the first application of voltage to electrodes with oxidized at 200/spl deg/C. Before breakdown experiment, surface analysis revealed that all the sample electrodes had a large amount of carbon originating from the hydrocarbons of contaminants, and after the experiments, they revealed that the carbons had disappeared. To obtain breakdown characteristics of electrodes with smoother surfaces, we conducted experiments on electrodes with a surface roughness of 0.05 /spl mu/m Ra. For these electrodes, the breakdown field was higher at first breakdown; the repetitions required to achieve saturated breakdown fields were significantly fewer, and the amount of carbon on electrode surfaces before breakdown was less.     

Off-state breakdown of GaAs PHEMTs: review and new data

This paper reviews the literature dealing with off-state gate-drain breakdown in MESFET and HEMT structures, with particular emphasis on GaAs PHEMTs, in terms of: 1) the physics of the breakdown phenomenon; 2) the breakdown walkout effect; 3) the impact of design and process choices on the breakdown behavior; and 4) the experimental techniques used for breakdown characterization. A thorough temperature-dependent breakdown characterization of commercial PHEMTs is also shown and discussed. It is found that different physical mechanisms may dominate the gate-drain leakage depending on the reverse bias and temperature range considered, and the particular PHEMT technology. The main results shown here tell us the following. 1) The breakdown voltages are decreasing functions of temperature between room temperature and 160/spl deg/C. 2) Between room temperature and 90-100/spl deg/C, thermionic-field emission seems be dominant, with low activation energies below 0.15 eV; as a consequence, the temperature dependence of the breakdown voltage is weak. 3) Between 110/spl deg/C and 160/spl deg/C, higher activation energy mechanisms (possibly trap-assisted tunneling and thermionic emission over a field-dependent barrier) tend to dominate, and the temperature dependence of the breakdown voltages is stronger.     

Comparative laboratory evaluation of TR-XLPE and XLPE cables with Super-smooth conductor shields

This paper provides data on four commercial tree retardant crosslinked polyethylene (TR-XLPE) and one cross-linked polyethylene (XLPE) insulated 15 kV cables supplied by three manufacturers. The cables have "super-smooth" conductor shields and "extra-clean" insulation and insulation shields. AC and impulse voltage breakdown and selected other characterization data are presented for cables that were aged immersed in room temperature water (15-30/spl deg/C) up to 24 months of a planned 48 months aging program. The five cables have high ac voltage breakdown strength, three of the TR-XLPE cables, actually increased in breakdown strength during aging. The one TR-XLPE cable that had the lowest ac voltage breakdown had vented trees at the insulation shield and high dissipation factor, which the other cables did not have. The impulse voltage breakdown strength of all cables decreased during aging; the cable with the lowest ac voltage breakdown also has the lowest impulse voltage breakdown. The dissimilar performance of the TR-XLPE cables and the excellent performance of the XLPE cable indicates evaluations at longer times are required to differentiate between modern TR-XLPE and XLPE insulated cables.     

Electrical treeing in hexagonal ice crystals under applied impulse voltage

Using two hexagonal samples of ice, polycrystalline and single-crystalline, the electrical treeing has been investigated under an applied impulse voltage. The crystallinity and temperature of the ice play an important role in the events of the treeing. In the single-crystalline ice, the tree in the samples at -25 /spl deg/C always progressed along the basal plane of the ice crystal, but at -196 /spl deg/C, it progressed along the c-axis. These distinctive patterns of the tree progression were attributed to the crystal axis dependence on the electrical conductivity and the relative permittivity at each temperature. The current accompanying the trees were detected by means of an electro-optic coupling with light emitting diodes and photodiodes. After the large current pulses which occur just after voltage application, small intermittent current pulses were visible in the wave tail of the applied voltage. These intermittent pulses appear to generate by the movement of accumulated charges after the formation of tree channels. The electrical breakdown strength of ice at -25 C was large for the electric field parallel to the c-axis of the single-crystalline ice.     

取向度对聚乙烯介电性能的影响

聚合物取向度对电气绝缘性能具有重要的影响。该文研究通过两种方法获得具有不同取向度的聚合物材料。在第一种方法中，通过在聚乙烯中加入5％～10％的乙烯．丙烯共聚物使在相同工艺条件下制备的共混物薄膜比纯聚乙烯薄膜具有更高的取向度。在第二种方法中，采用传统的拉伸方法使聚乙烯薄膜的取向度沿拉伸方向得到提高。研究中，多种方法被用于评价薄膜的取向程度，如热收缩率、偏光显微镜和测量声速法等。文中同时研究了具有不同取向度的聚合物薄膜的电气击穿强度，电流和空间电荷特性。分析了聚合物取向度与其电气性能之间的关系，并对其机理进行了探讨。     

Space charge studies in LDPE using combined isothermal and nonisothermal current measurements

Using a recently developed procedure combining isothermal and nonisothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the nonisothermal measurements appeared a broad peak (40/spl deg/C to 50/spl deg/C) that was possible to decompose into two or three peaks (35, 45 and 65/spl deg/C). At even higher temperature another peak was sometimes present (85/spl deg/C) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of /spl ap/15 /spl mu/m). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the nonisothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.     

A consideration of potentials of low‐density polyethylene using metallocene catalyst as insulator for power cable

We investigated the high‐field conduction, DC and impulse breakdown strength, space charge distribution, and tree inception voltage for three kinds of new low‐density polyethylene (LDPE) prepared using a metallocene catalyst (mLna, mLao, mLldao), linear LDPE prepared using a Ziegler catalyst (LLao), and LDPE prepared by a high‐pressure process (LDna). The dc and impulse breakdown strengths of LDPEs prepared using a metallocene catalyst were higher than those of LLao and LDna. The high‐field currents of LDPEs prepared using a metallocene catalyst were lower than those of LLao and LDna. A homo‐space charge was accumulated near the cathode in mLna. The tree inception voltage of mLna was higher than that of LDna. From these results, it is concluded that LDPE prepared using a metallocene catalyst has electrical insulating properties superior to the conventional LDPE and that the former has potential as a power cable insulator. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 139(4): 17–25, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.1164     

High‐field electrical properties of new LDPE films prepared using metallocene catalyst

Now low‐density polyethylene (M‐LDPE) prepared using metallocene catalyst has narrower composition distribution and molecular weight distributions than linear low‐density polyethylene (LLDPE) using Ziegler catalyst. The authors compared the electrical insulating properties of M‐LDPE films and conventional LLDPE films. The high‐field current of M‐LDPE was found to be lower than that of LLDPE. The difference in current increased with the decrease of temperature. The high‐field current was much reduced in M‐LDPE with a low melting point. The impulse breakdown strength of M‐LDPE increased with the decrease of temperature. The insulating properties of M‐LDPE with a low melting point were improved at 30 °C. These results were explained by the fact that M‐LDPE with a low melting point includes more low‐molecular‐weight components. We also discuss the effects of antioxidant on the electrical properties of M‐LDPE. We compared the electrical conduction and breakdown strength of undoped M‐LDPE and antioxidant‐doped M‐LDPE. Differences in their electrical properties were minor. It was found that the excellent properties of M‐LDPE do not depend on the effects of antioxidant. © 1999 Scripta Technica, Electr Eng Jpn, 130(2): 1–9, 2000     

Breakdown voltages of polymers in the temperature range 23/spl deg/-250/spl deg/C

The direct voltage electric strengths of several polymers are measured using a new type of environmental chamber in which a new specimen of the material can be introduced without the necessity of cooling and reheating after every voltage application. The range of temperatures at which investigations are carried out is 23/spl deg/C-250/spl deg/C. The plots of the dielectric strength on Weibull paper demonstrates departure from the two parameter Weibull distribution, particularly at low field strengths. In each case the method of determining the more favorable distributions are discussed with the view of obtaining consistent results for the dielectric strength. The lower voltage breakdown cannot be ignored if high reliability is desired and the evaluated dielectric strengths on this basis are also presented. The polymers studied are aromatic polyamide, PTFE, aromatic polyimide and polyamide-polyester-polyamide composites. Thickness effects are presented for selected polymers.     

空间电荷对低密度聚乙烯电气击穿特性的影响

为解决聚乙烯用作电线电缆绝缘材料时所受空间电荷问题的困扰,采用在低密度聚乙烯(low density po-lyethylene,LDPE)试品上施加直流预电压使其中积聚一定量的空间电荷,然后测量试品击穿强度的方法,研究了空间电荷对LDPE击穿特性的影响。结果表明,与未经过预电压处理的LDPE的击穿强度相比,在经过较低场强(50 kV/mm)预电压处理后,预电压与击穿电压极性相同时击穿强度提高了约9%,极性相异时击穿强度降低约14%;而经过较高场强(150 kV/mm)预电压处理后,预电压时LDPE中出现空间电荷包现象,预电压后同极性击穿强度提高约19%,而异极性击穿强度反而上升约16%。分析认为空间电荷包在LDPE中的运动导致了部分空间电荷的中和,使得空间电荷积聚量减少,同时LDPE中可能的缺陷得到了一定程度的老炼而使介质得到了均匀化,从而使LDPE的击穿强度得到了提高。     

Electrical characterization of benzocyclobutene polymers for electric micromachines

An approach using interdigitated capacitors for electrical characterization of CYCLOTENE, a spin-on low-k benzocyclobutene (BCB)-based polymer is introduced and the effect of moisture uptake is investigated. The dielectric constant of CYCLOTENE is extracted from capacitance measurements with a systematic error less than 0.1%, giving an average value of 2.49 with a standard deviation of 1.5%. The dielectric constant increases by 1.2% after a humidity stress of 85% RH at 85/spl deg/C. The I-V characteristics of CYCLOTENE show a dependency of breakdown strength and leakage current on the geometrical dimensions of the device under test. A breakdown strength of 225V//spl mu/m and 320 V//spl mu/m for 2-/spl mu/m and 3-/spl mu/m finger spacing, respectively, and a leakage current of a few to tens of pA are measured. The I-V characteristics degrade drastically after the humidity stress, showing a breakdown strength of 100 V//spl mu/m and 180 V//spl mu/m for 2-/spl mu/m and 3-/spl mu/m finger spacing, respectively, and a maximum increase in the leakage current as large as one order of magnitude. The maximum performance and long-term reliability of an electric micromachine are adversely affected by the degradation of the breakdown voltage and the leakage current after moisture absorption. It is expected, however, that the electrical efficiency is improved using BCB-based polymers with negligible dependency on moisture absorption.     

High‐field electrical properties under AC voltage application in acetophenone‐coated LDPE film

This paper deals with the effects of acetophenone coating on dielectric properties such as tan δ and capacitance and on space charge formation up to electrical breakdown under AC voltage application in low‐density polyethylene (LDPE) film. The existence of acetophenone at the electrical interface enhances tan δ. tan δ in acetophenone‐coated specimens increased with electric field and decreased with the frequency in the high‐field and high‐temperature region. AC breakdown test revealed that the electric strength in noncoated specimens was higher than in acetophenone‐coated specimens, and that the electric strength also decreased with temperature. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 142(3): 1–7, 2003; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/eej.10086     

茂金属聚乙烯改性低密度聚乙烯介电性能与机械强度

为提高聚乙烯电缆料的击穿强度和机械性能,将少量茂金属聚乙烯(MPE)与普通低密度聚乙烯(LDPE)共混,用电声脉冲法(PEA)测量直流预压短路后的残余空间电荷量;详细研究了共混物的体积电阻率、直流击穿强度、直流预压反极性击穿电压和机械强度.实验结果表明:当MPE含量为1wt%时,能有效提高共混物的击穿强度和机械强度,但同时降低了体积电阻率和材料韧性,而且聚乙烯的介电性能和机械强度与其结晶形态密切相关.     

Electric charge transport and trapping in dielectrics deduced from isothermal and nonisothermal measurements

By sequential use of the isothermal charging, the isothermal discharging, the final thermally stimulated discharge current and the final isothermal discharging current techniques, the charge dynamics in highly insulating materials may be investigated. The method is demonstrated for polyethylene terephthalate. The injected charge for a field of 20 MV m/sup -/1 and polarization temperatures up to 110/spl deg/C is almost totally trapped in the material and is released during the heating of the sample at 180/spl deg/C for a sufficiently long time. A significant current at high temperatures, about 90/spl deg/C above the poling temperature, was observed proving that it originates from charge detrapping. The final thermally stimulated discharge current peaks shift to higher temperature when the polarization temperature increases, and are characterized by activation energies in the range from 1.03 to 1.56 eV. They allowed the identification of the glass transition around 114/spl deg/C. The relaxation time of the trapped charge, at 180/spl deg/C, was determined to be about 3780 s, explaining the very good stability of trapped charge.     

Space charge dynamics in LDPE films immediately before breakdown

Space charge dynamics in low-density polyethylene (LDPE) films were observed by the pulsed electroacoustic (PEA) method during breakdown tests at various temperatures. In order to investigate the intrinsic electrical breakdown, the applied electric field was increased to 300 kV/mm in 150 ms, and kept constant until breakdown of the specimens, or 300 ms at the longest. Space charge profiles were measured at 1 ms intervals. A significant positive charge propagating into the sample was observed only immediately before the breakdown at 90°C. These measurements revealed that the positive charge behavior is strongly related to the breakdown phenomena     

Charge storage in corona-charged polypropylene films analyzed by LIPP and TSC methods

Negatively corona-charged 50-/spl mu/m-thick polypropylene (PP) film is measured using laser-induced pressure pulse (LIPP) and thermally stimulated current (TSC) in order to study the charge storage mechanism in the PP film. The LIPP can reveal the space-charge distribution in the depth direction of the PP films and the TSC can be used to measure the energetic depth of the charge trap. The LIPP shows that negative charge is deposited on the charged surface of the sample. Almost all surface charges are removed by soaking the sample in ethanol. However, about 5% of the surface charge is injected into the sample up to a depth of about 7 /spl mu/m from the surface. The injected charge is not removed by the dip-in-ethanol method because the ethanol does not penetrate into the sample. The injection of the surface charge increases with corona-charging temperature. Besides the negative charge injection, the injection of positive charge from the opposite surface is also observed when the sample is charged at higher than 60/spl deg/C. In addition, negative bulk charge is formed when the sample is charged at higher than 80/spl deg/C. The LIPP profile is compared with the TSC spectrum. It is shown that the space charge observed using LIPP disappears when the temperature of the sample exceeds 80/spl deg/C. However, TSC is observed at even higher than 80/spl deg/C. This indicates that the TSC is observed even after the disappearance of the space charge measured using LIPP.     

Ultra high vacuum properties of some engineering polymers

Engineering Polymers are very good candidates for applications requiring mechanical properties comparable with metals, chemical inertia, high insulation capability, high temperature operation and ultra high vacuum (UHV) compatibility. The results of a systematic test series, aimed at qualifying the engineering resins VESPEL/spl reg/ SP1, PEEK and CELAZOLE/spl reg/ PBI as UHV seals, are reported. The study of the materials behavior has been carried out over a wide temperature interval, ranging from 20 to 400/spl deg/C. In addition to the tightness and permeation tests, thermal desorption and gas chromatographic-mass spectrometer (GC/MS) analysis have also been performed. The results obtained indicate that CELAZOLE/spl reg/PBI provides the best performance, since it can be operated safely up to 375/spl deg/C, without giving any sign of leak or other drawbacks. PEEK, on the contrary, does not stand temperatures higher than 275/spl deg/C but, below this limit, it remains a very cost effective and reliable alternative. VESPEL/spl reg/ SP1, in its turn, can be operated safely up to 325/spl deg/C but above this temperature its properties start to degrade even if not in an abrupt manner as is the case for the other two resins. The possible applications of some of these polymers in the field of nuclear fusion research are also briefly described.     

The breakdown mechanism of poly-p-xylylene film. Prestress effectson the breakdown strength

Impulse and dc breakdown strengths of 4 μm thick poly-p-xylylene (PPX) films were 5.9 and 4.1 MV/cm, respectively. They were independent of temperature in the temperature range from -60 to 60°C. dc prestressing for a long time (t_p=60 s) reduced impulse breakdown strength for both the same and the opposite polarity. But dc prestressing for a short time (t_p<1 s) increased impulse breakdown strength for the same polarity. These results were explained by positive space charge in PPX film. It was also concluded that positive charge carrier injected from the anode, spread in a 4 μm thick PPX film during a short time     

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.