Influence of BaTiO3 additive and electrode material onspace-charge formation in polyethylene. Evidence from thermal stepspace-charge measurements

The effects of 5% wt BaTiO₃ additive and of electrode material on space charge formation and electric field distribution in low density polyethylene (LDPE) were investigated using a thermal step technique. Space charge was formed at an average dc field of ~28 kV/mm and at 50°C. Results indicate that the addition of BaTiO₃ to LDPE has considerably reduced the remanent space charge and electric field and changed their distribution patterns in the doped material when compared with the plain material. It is also shown that the remanent space charge and electric field in plain LDPE are strongly dependent on the type of electrode material     

Weibull statistical analysis of area and volume effects on thebreakdown strength in liquid nitrogen

In order to examine the area and the volume effects on breakdown strength in liquid nitrogen (LN₂), we measured dc and ac breakdown voltages in LN₂ with sphere to plane and coaxial cylindrical electrode configurations. We also carried out statistical analysis of the experimental results using the Weibull distribution and discussed with the statistical stressed electrode area (SSEA) and/or liquid volume (SSLV). The dc and ac breakdown strength in LN₂ decreased to 1/5 with increasing SSEA and SSLV over the wide range from 10⁰to 10⁵ mm² and from 10^-1 to 10⁵ mm³, respectively. The Weibull shape parameter m for the sphere to plane and the coaxial cylindrical electrodes was estimated to be 6 to 8 and 11 to 13, respectively; the breakdown strength in LN₂ saturated with increasing SSEA and SSLV. From these results, it was verified that the area and the volume effects definitely determined the breakdown strength in LN₂. The results of the Weibull statistics were compared with the conventional concept of the stressed electrode area (SEA) and liquid volume (SLV). Consequently, SSEA or SSLV was found to be nearly equal to 80 to 85% of SEA or SLV     

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     

The influence of defects on the short-term breakdowncharacteristics and long-term dc performance of LDPE insulation

Extruded polyethylene is used as the bulk insulation for ac HV cables because of its high electric resistivity and breakdown strength. Although the material at present has limited use in dc power cables, it is used extensively in submarine optical communication cable systems. This paper reports on the study of the short-term characteristics and long-term performance of low-density polyethylene (LDPE) insulation under dc electric stress. The results are presented in which controlled defects as found in practical systems (voids, metallic and non-metallic particles) were introduced into well-characterized polymer material so that their influence on electrical strength and breakdown mechanisms could be determined. Samples were compression-molded under laboratory conditions and subjected to ramp-to-failure and electrical aging tests at various stresses. Weibull statistics are used to analyze the results. Examination of electrically aged samples indicates an oxidation degradation around the defects due to stress enhancement. From the failed samples a value of n in the inverse power model was estimated. The residual life of aged samples was also examined     

Area and volume effects on breakdown strength in liquid nitrogen

We measured dc and ac breakdown voltages in liquid nitrogen (LN ₂) with a sphere-to-plane electrode configuration. Experimental results revealed that the breakdown voltage in LN₂ did not increase monotonously but partially decreased as the sphere diameter increased at a constant gap length. Thus, the existence of the area and the volume effects on the breakdown voltage in LN₂ was verified quantitatively; the breakdown strength decreased when increasing the {SEA}₉₀ (90% stressed electrode area) and the {SLV}₉₀ (90% stressed liquid volume). By changing the experimental conditions, it was verified that both area and volume effects, having a mutual correlation, simultaneously lead to the degradation of the breakdown strength in LN₂. In order to examine the area and the volume effects for the larger {SEA}₉₀ and {SLV}₉₀, we also measured the breakdown voltage with a coaxial cylindrical electrode. It was concluded that the dc and ac breakdown strengths in LN₂ decreased as the {SEA}₉₀ and the {SLV}₉₀ increased varying widely from 10⁰ to 10⁵ mm² and from 10 ^-1 to 10⁵ mm³, respectively     

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。     

Space charge behavior in low-density polyethylene

The space charge distribution in low-density polyethylene (LDPE) was measured with the pulsed electroacoustic (PEA) method. We used three types of LDPE: LDPE-L and LDPE-H were prepared by the high pressure process, and m-LDPE was polymerized with a metallocene catalyst. Space charge in LDPE strongly depended upon the electrode material. Semiconductive electrodes enhanced carrier injection into LDPE and, as a result, space charge. The density, polymerization process, applied field, temperature and so on also affected the space charge behavior. This space charge behavior was compared with the results of dc current measurements     

Influence of liquid temperature and electrode size on insulated breakdown characteristics in saturated superfluid helium

The dc breakdown strength and pulsed breakdown time lag of saturated superfluid liquid helium (HeII) at 1.4 and 1.85 K are measured over a wide range of electrode sizes and applied field strengths. The results of the measurements are statistically analyzed by using Weibull distribution functions. It is found that the area effect is dominant for dc and pulsed breakdown rather than the volume effect as in the case of liquid helium of 4.2 K (HeI). The shape parameter in the Weibull distribution function for dc breakdown strengths of HeII is smaller than that of HeI. As a result, dc breakdown in superfluid helium can take place in a wider electrode area than in normal liquid helium, while no remarkable differences in breakdown areas are observed between 1.85 and 1.4 K. On the other hand, the statistical time lag in HeII becomes shorter with a decrease in temperature. According to the Fowler‐Nordheim theory, it is suggested that a higher electron mobility in HeII may increase the field‐emitted electron kinetic energy to trigger liquid breakdown with a higher probability. © 1999 Scripta Technica, Electr Eng Jpn, 128(3): 16–23, 1999     

Carrier mobility in LDPE at high temperature and pressure

Current peaks due to transient SCLC (space charge limited current) were observed in LDPE (low-density polyethylene) at 70°C. From the time at which the current peak occurs, carrier mobilities ranging from 5×10^-15 to 7×10^-14 m²/Vs were obtained. The mobility values obtained for LDPE depend on both applied field and hydrostatic pressure and verify hopping mechanism. The presence of space charge has been confirmed by analyzing discharge currents measurements     

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.     

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     

Reliability of Low-Temperature-Processing Hafnium Oxide Gate Dielectrics Prepared by Cost-Effective Nitric Acid Oxidation Technique

In this paper, the physical and electrical characteristics of low-temperature-processing hafnium oxide (HfO₂) films are studied. A simple cost-effective room-temperature process was introduced to prepare high-k HfO₂ dielectrics. A novel technique of direct oxidation of an ultrathin Hf metal by nitric acid, followed by rapid thermal annealing in N₂ is demonstrated. The prepared HfO₂ gate dielectrics show good uniformity, low leakage currents, high breakdown field, and superior reliability under electrical stressing. The long-term ten-year lifetime was also evaluated by a time-dependent-dielectric-breakdown analysis to project the maximum operation voltage of -1.8 V for HfO₂ gate stacks. This low-temperature oxidation technology for preparing high-quality high-k HfO₂ dielectrics is promising for flat-panel-display applications.     

Energy and Spatial Distribution of Traps in SiO2/Al2O3 nMOSFETs

The energy and spatial profiling of the interface and near-interface traps in n-channel MOSFETs with SiO₂/Al₂ O₃ gate dielectrics is investigated by charge-pumping (CP) measurements. By increasing the amplitude as well as lowering the frequency of the gate pulse, an increase of the charge recombined per cycle was observed, and it was explained by the contributions of additional traps located higher in energy and deeper in position at the SiO₂/Al₂O₃ interface. In addition, CP currents, acquired after different constant voltage stress, have been used to investigate the trap generation in this dielectric stack     

应力对诱发空间电荷击穿的作用

电介质在生产和使用过程中通常会受到各种应力的作用,而应力作用与电介质的老化和击穿密切相关.本文利用电子束辐照的方法将空间电荷引入聚甲基丙烯酸甲酯(PMMA)和低密度聚乙烯(LDPE)样品中,在无外加电场下通过外加应力,可能诱发空间电荷脱陷导致树枝化击穿.讨论了应力诱发空间电荷击穿的机理,分析了影响应力诱发的空间电荷击穿的诸多因素,如样品中空间电荷分布情况,机械应力的作用形式(如压缩应力或是拉伸应力),样品中的水份,样品的介电常数等因素在空间电荷树枝化击穿过程的作用.     

电声脉冲法测量聚合物绝缘表面陷阱能级分布

对紫外光老化不同时间的低密度聚乙烯(low-density polyethylene,LDPE)试样及3%含量的纳米氧化锌(ZnO)粒子与LDPE复合材料试样进行了交流击穿实验,结果表明纯LDPE试样的击穿强度随光老化时间的增加逐渐降低,3%含量的纳米ZnO粒子与LDPE复合材料的击穿强度光老化后基本保持不变。通过对老化前后试样的老化面分别进行电晕放电注入负电荷,并采用电声脉冲法测量短路状态下表面注入电荷随时间的衰减特性,通过公式推导计算得出试样老化面的陷阱能级分布。结果表明,纳米ZnO粒子不仅能提高聚乙烯的光稳定性,还通过成核和界面作用减少聚乙烯体内深陷阱密度,增加浅陷阱密度。     

Decomposition of benzene using alumina-hybrid and catalyst-hybridplasma reactors

In order to investigate the effects of alumina and metal ions in plasma discharge, plasma reactors packed with a mixture of BaTiO₃ pellets and porous Al₂O₃ pellets (alumina-hybrid reactor), and with a-mixture of BaTiO₃ pellets and metal-supported Al₂O₃ pellets (catalyst-hybrid reactor) were examined for oxidation of dilute benzene in air. It was found that the oxidative decomposition of benzene was enhanced by concentrating benzene on the Al₂O₃ pellets and the catalyst pellets. Furthermore, the selectivities to CO ₂ in the alumina-hybrid reactor and the catalyst-hybrid reactors were higher than those in the plasma reactor packed with BaTiO ₃ pellets alone. In particular, the selectivities to CO₂ in the catalyst-hybrid reactors using Ag, Co, Cu and Ni/Al₂ O₃ were higher than those from the alumina-hybrid reactor. In addition, the presence of the alumina and catalysts suppressed the formation of N₂O     

DC breakdown strength of polypropylene films: area dependence and statistical behavior

The DC breakdown strengths of 4-15 mum thick capacitor grade polypropylene (PP) films were determined. The measurements were performed with an automatic measurement system using five electrodes with areas between 0.045 and 9.3 cm². Some samples were studied with larger electrodes. Samples were not aged but were measured as received. The measurements were performed in air at room temperature and typically 40-80 breakdowns per sample per electrode area were collected. The Weibull distribution parameters alpha and beta were fitted to the data. Depending on the electrode area and the film grade, the alpha-parameter values obtained were between 450 and 850 V/mum. The breakdown strength values showed decreasing area dependence with decreasing electrode area and the Weibull scaling law was not valid for the smallest electrode areas. The alpha-values for the large sample areas were extrapolated from the small area data by area and Weibull extrapolation. The area extrapolation overestimated the breakdown strength at 4 m² by 40-50% whereas the Weibull extrapolation gave an accuracy of plusmn15%. The extrapolated breakdown strengths for the full-scale power capacitors deviated from the capacitor manufacturers test data. This indicated that different defect distributions were responsible for the breakdown strengths in the oil impregnated power capacitors than in the small dry film samples     

Breakdown mechanism of liquid nitrogen viewed from area and volumeeffects

We investigated the area and volume effects on the breakdown strength in liquid nitrogen (LN₂) to discuss the breakdown mechanism in cryogenic liquids for superconducting power apparatus. We measured breakdown voltages in LN₂ with and without thermal bubbles over a very wide range of the electrode size. Experimental results revealed that the breakdown mechanism changed from an area dominant to volume effective region at larger electrode configurations in LN₂. Moreover, we discussed the contribution rate of area and volume effects to the breakdown strength in LN₂. It was suggested that a mutual contribution of area and volume effects appeared in breakdown characteristics in LN₂ under thermal bubble conditions, as a phenomenon peculiar to cryogenic liquids. Consequently, we pointed out that it is very important to consider both thermal bubbles and electrode surface condition for HV insulation of superconducting power apparatus     

The influence of ions on the thermally stimulated discharge currentspectra of water-treed additive-free low-density polyethylene

The effects of water treeing on steady state dc currents and thermally stimulated discharge current spectra (TSDC) of additive-free low density polyethylene (LDPE) were studied using heavily water-treed material in solutions of LiCl, LiCl plus HCl, MnCl₂ and FeCl ₃. Significant changes in the dc conductivity of treed samples are attributed to the effects of ions. For such samples, graphs of the logarithm of current versus inverse temperature show three distinct linear regions. The transition temperatures demarcating these regions appear to correlate with those reported for thermal relaxations. TSDC spectra of water-treed samples show a dependence on both the ionic solutions used and on the poling conditions. For LiCl treed samples, twin peaks in the TSDC spectra were produced only under specific poling conditions. These peaks may be correlated with thermal relaxations observed using differential scanning calorimetry (DSC). The melting point temperature and the melting peak area derived from DSC spectra were reduced by treeing     

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。