High field conduction and prebreakdown phenomena in dielectric liquids

The present paper is mainly devoted to phenomena occurring in point-plane electrode geometry, where breakdown is the result of the initiation and propagation of prebreakdown phenomena called "streamers". In this configuration, an investigative study of the streamer initiation processes, requiring very high electric field strengths (/spl sim/ MV/cm), and of propagation (requiring low electric field, /spl sim/ kV/cm) can easily be carried out for negative streamer development as well as for the positive case. From analysis of experimental results in pure liquids the physical processes connected with streamer initiation and propagation, particularly the electronic ones, are presented and discussed. Estimations of the main parameters of slower subsonic streamers and of the faster filamentary ones (such as field strength at the streamer tip, field inside the channel, charge density, etc.) have been obtained from qualitative considerations and compared to experimental data.     

Positive streamer discharges along insulating surfaces

By measuring the currents associated with the streamer discharge along silicone rubber surfaces, parameters of streamer propagation such as the minimum field of streamer crossing, the field of stable streamer propagation, the mean velocity and the streamer charge distribution have been analyzed and compared to the streamer discharge in air alone. Clear differences were observed in the measured currents for the individual surfaces at low background fields (285 kV/m). For higher fields the streamer crosses the gap almost independently of the surface type. The minimum streamer field was found to be slightly increased compared to air. The field of stable streamer propagation also was higher than in air. It is ~ 570 kV/m, larger than that of the streamer discharge in air (~ 500), under the same conditions. The streamer speed was found slightly increased in the presence of the silicone rubber surface and the distinction between the individual surfaces was modest. A discussion on possible mechanisms for the observed differences in the streamer speed and currents with and without the insulator surfaces is presented. The net positive charge of the streamer along an insulating surface seems to be distributed along the streamer channel rather than localized in the front part of the channel as the case for the streamer in air     

Surface profile effect on streamer propagation and breakdown in air

In a uniform field arrangement, under direct voltage, positive streamer propagation and breakdown are investigated along cylindrical insulators with different profiles, inserted perpendicularly between two parallel plane electrodes. The basic properties of streamer propagation and breakdown, namely the electric field required for a stable propagation together with the associated velocity and the breakdown field together with time to breakdown, are measured as influenced by the pulse voltage amplitude used for the streamer initiation and by the insulator profile. It is shown that a strong relation between streamer propagation and breakdown exists, because the insulator profile exerts a similar influence on the breakdown and propagation fields. The effect of a shed on an insulating surface, forming an `obstruction' to streamer progress, is to increase the stability for propagation and breakdown fields, and to reduce the propagation velocity at all applied fields compared with those in the case of a smooth insulator. Along the surface of a smooth insulator, a streamer system propagates with a `surface' and an `air' component; however, a shed on an insulating surface modifies this system, resulting in only one component reaching the cathode     

The Study of Ferroelectric La-Doped PbTiO3 Thin Films Prepared by RF Magnetron Sputtering

Ferroelectric La-modified lead titanate (PLT) thin film were grown on Pt/Ti/SiO2/Si by sputtering the Pb_0.93La_0.07TiO₃ targets containing an amounts of excess 8% PbO. The effects of sputtering and annealing conditions on the crystalline structures and the surface morphologies of the PLT thin films have been investigated. The remanent polarization (Pr) and the coercive field of the PLT film through rapid thermal annealing (RTA) was 10.2 μC/cm² and 45 kV/cm respectively. The maximum pyroelectric coefficient reached 19 nC/cm².K at 20°C.     

DC conduction behavior of Bi3.15Nd0.85Ti3O12 thin films grown by RF-magnetron sputtering

The Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) thin films were deposited on Pt(111)/Ti/SiO₂/Si substrates by using RF-magnetron sputtering method and studied the ferroelectric and leakage current charateristics. The polarization – electric field (P-E) hysteresis loops of BNT film was well saturated with the remnant polarization (2P _r ) of 29.8 μC/cm² and a coercive field (2E _c ) of 121 kV/cm. The leakage current density – electric field (J-E) characteristics of the Pt/BNT/Pt capacitor reveals the presence of two conduction region, having Ohmic behavior at low electric field (below 50 kV/cm) and Schottky-emission or Poole-Frenkel emission at high electric field (above 60 kV/cm). The barrier height and trapped level of BNT films are estimated to be 1.11 eV and 0.90 eV, respectively.     

考虑载流子密度扰动的植物绝缘油中初始流注放电仿真

在现有泊松方程及电流连续性方程的放电模型基础上,通过引入载流子密度波动源建立具有多分支流注形貌的放电改进模型;通过COMSOL软件仿真分析了雷电冲击电压为120kV、160kV和200kV时的放电结果。结果表明:改进模型的流注分支形貌比原模型更接近试验结果;最大场强主要分布在流注头部,且均超过2×108 V/m,其中200kV时由二次流注引起的场强和电荷变化最为明显。此外,3种电压下的z轴流注发展长度和速度还表明流注分支间电荷的相互排斥对流注有抑制作用。而对比电压上升沿时间分别为5ns、10ns、50ns和120ns时的流注形貌还发现:电压上升沿时间越短,流注半径越大,分支越多。该文的研究成果有助于分析放电过程中载流子密度波动对流注分支的影响机理。     

Parallel streamer discharges between wire and plane electrodes in water

Streamer discharges in tap water and distilled water have been generated by applying a voltage pulse from 120 to 175 kV and 500 ns duration to a wire-to-electrode configuration. Electrical and optical diagnostics were used to explore the temporal development of the streamers in tap and distilled water, at various applied voltages and both polarities. With the wire serving as anode, multiple, parallel streamer discharges were generated. The number density of these streamers along the wire decreases with decreasing electric field on the surface of the wire. The dependence of the streamer density on electric field indicates the role of field enhancement at inhomogeneous microstructures along the wire as streamer initiation mechanism. The appearance of the discharge was different for tap and distilled water. However, the measured average streamer propagation velocity from the positive wire to the grounded plane electrode, of 32 mm//spl mu/s, was independent of the water conductivity and the applied voltage. This suggests the existence of a self-sustained electric field at the streamer head. With the wire serving as cathode, only a weak light emission from the area close to the wire was observed, and streamers did not appear for the same voltage amplitude as with the positive polarity. This suggests that an ionic current flowing in the water is not dominant in the streamer propagation process.     

Advances in processing and properties of perovskite thin-films for FRAMs,DRAMs, and decoupling capacitors

Abstract

Highly oriented, dense, and crack-free ferroelectric and paraelectric thin-films on three inch diameter Pt/Ti/Si₃N₄/Si (100) substrates were obtained by polymeric sol-gel processing. Ferroelectric PZT thin-films were fabricated at temperatures as low as 550°C within 15 minutes by rapid thermal annealing. The films heat treated at 700°C for 5 minutes were single grain thick and exhibited P_r, P_sp, and E_c in the ranges of 29–32 μC/cm², 44–53 μC/cm², and 50–60 kV/cm, respectively, and high speed switching times below 5 ns on 30×30 μm² electrodes. A switching time of 2.7 ns was observed on 19×19 μm² area electrodes at a field of 200 kV/cm. Results of low and high field characterization on paraelectric PLT thin-films which were conventionally heat treated indicated that it has an excellent potential for use in ULSI DRAMs and as decoupling capacitors. These films showed a high charge storage density (15 μC/cm²) and a low leakage current (0.5 μA/cm²) at a field of 200 kV/cm. Also, the charging time for a capacitor area of 1 μm² at 200 kV/cm was estimated to be 0.10 ns.     

Effects of transition metal (Ni, Mn, Cu) doping on ferroelectric properties of Bi0.9Nd0.1FeO3 thin films prepared by chemical solution deposition method

Transition metal (Ni, Mn, Cu) doped Bi_0.9Nd_0.1FeO₃ thin films were prepared on Pt(111)/Ti/SiO₂/Si(100) substrates by using a chemical solution deposition method. Compared to pure BiFeO₃ (BFO) thin film, improved ferroelectric and leakage current properties were observed in the transition metal doped thin films. The values of remnant polarization (2P _r ) and coercive electric field (2E _c ) of the transition metal doped thin films were 59 μC/cm² and 690 kV/cm at 700 kV/cm for the Ni-doped Bi_0.9Nd_0.1FeO₃ thin film, 57 μC/cm² and 523 kV/cm at 670 kV/cm for the Mn-doped thin film, and 85 μC/cm² and 729 kV/cm at 700 kV/cm for the Cu-doped thin film, respectively. The 2P _r values observed in the transition metal doped thin films were much larger than that of the BFO thin film, 21 μC/cm² at 660 kV/cm. Also the 2E _c values of in the transition metal doped thin films were lower than that of the BFO thin film, 749 kV/cm at 660 kV/cm. The reduced leakage current density was observed in the transition metal doped thin films, which is approximately two orders of magnitude lower than the BFO thin film, 2.6?×?10^?3 A/cm² at 100 kV/cm.     

Bird streamer caused flashover in EHV transmission line

The bird-streamer caused failures have become one of the major causes of transmission line outages. It is more common and widespread than generally believed, and greatly threatens the reliability of transmission lines. The laboratory study of such failures mechanism is seldom reported especially for long gaps such as 500 kV insulators. Presented in this paper are the results of long air gaps bird streamer flashover experiments conducted on 500 kV and 220 kV transmission line insulators. The experimental results demonstrated that bird-streamer caused surface flashover can happen in 500 kV insulators, however space flashover, because the bird streamer will split into different jets under high electric field cannot short circuit the long air gaps. The high risk areas of flashover for 220 kV and 500 kV insulators were defined in this paper: an elliptical area for 220 kV I string insulator and a symmetrical area beside the central line for 500 kV V string insulator. The influence of some main factors on flashover processes is also elucidated, such as volume, conductivity and viscosity of the streamer, and the insulator type and configuration. The paper has significant value in understanding the bird caused failure mechanisms and designing remedies.     

Measurement of the space charge field in transformer oil using itsKerr effect

Nonuniform electric fields >80 kV/cm, have been measured via a relative optical intensity measurement by exploiting the Kerr effect in transformer oil. Transformer oil was used as a Kerr medium in these measurements. Neither a knowledge of the Kerr coefficient nor the actual value of maximum transmitted intensity are required to obtain the absolute value of the electric fields. Electric fields were measured with ac and dc excitations. Space charges resulting from charge injection have been found in transformer oil at room temperature under electric stresses >150 kV/cm with ac excitation and 90 kV/cm under dc excitation. The magnitudes of space charges in transformer oil with ac applied voltages varied from 2 to 50 nC/cm³. The magnitudes of space charges with dc excitation varied from 60 pC/cm³ to 10 nC/cm³. The results illustrate the limitations of the electro-optic technique for investigations of the interfacial electric fields (gas-solid) with transformer oil as the Kerr medium     

Streamer propagation along profiled insulator surfaces

Experimental results on the propagation of positive streamers along contoured, axially symmetric, polytetrafluoroethylene (PTFE) insulation surfaces are presented. A plane parallel electrode configuration provides a substantially uniform electric field for streamer propagation and a point electrode at the ground plane initiates the avalanche process. Basic streamer properties of velocity and propagation probability with field strength are measured and compared with the corresponding characteristics of air and cylindrical insulators. Several insulator profiles are investigated and the occurrence of multiple streamer paths is demonstrated, which are generally distinguishable as surface and air components with different propagation velocities. Comparative data of the breakdown fields for the various insulators is included     

5.0 Nanosecond surface discharge development

Contents The nanosecond developing mechanisms of the positive and negative corona streamers on the insulation surface have been studied using the Dust figure technique (=Lichtenberg figure technique). This method has an advantage of measuring the velocity of corona discharge developments, because the distribution of the positive and negative charges produced in the discharge space can be colorfully separated and recorded on the insulation surface. The velocities of the corona streamer development were measured at the voltage of 821 kV and the pressure of 50760 Torr. From the result, the velocities obtained at a voltage of 21 kV and a pressure of 150 Torr were 8.2×10⁸ cm/sec in the positive and 5.8×10⁸ cm/sec in the negative. The observed maximum velocity of negative corona streamer was 1.0×10⁹ cm/sec at the conditions of 18 kV and 50. Torr. The result suggests that the velocity of the negative streamer will be larger than that of the positive at lower pressure and also nanosecond pulse voltage. Moreover, it was found that the field distribution in the corona streamer on the insulation surface could be estimated using Gauss's law.

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5.0 Nanosekunden—Oberflächen—Entladungsentwicklung

Übersicht Die Nanosekunden Ausbreitungsmechanismen der positiven und negativen Koronaentladung auf den Isolatoroberflächen sind mit der Staubfiguren-Methode (=Lichtenberg-Figuren-Methode) erforscht worden. Diese Methode hat einen Vorteil für die Messung der Ausbreitungsgesch windigkeit der Koronaentladung, weil die Verteilung der positiven und negativen Ladung, die im Entladeraum erzeugt wird, auf den Isolatoroberflächen sorgfältig getrennt und aufgezeichnet werden kann. Die Ausbreitungsgeschwindigkeit der Koronaentladung wurde bei einer Spannung von 5 bis 21 kV und einem Druck von 50 bis 760 Torr gemessen. Aus dem Resultat: Die Geschwindigkeiten, die bei einer Spannung von 21 kV und einem Druck von 150 Torr gemessen wurden, betrugen 8,2×10⁸ cm/sec in der positiven und 5,8×10⁸ cm/sec in der negativen Spannung. Die gemessene Höchstgeschwindigkeit der negativen Koronaentladung lag bei 1,0×10⁸ cm/sec bei einer Spannung von 18 kV und einem Druck von 50 Torr. Das Resultat ergibt, daß die Geschwindigkeit der negativen Koronaentladung größer wird als die der positiven Koronaentladung bei geringerem Druck und bei Nanosekunden-Impulsbreite. Darüber hinaus wurde festgestellt, daß die Feldverteilung in der Koronaentladung auf den Isolatoroberflächen mit Hilfe des Gausschen Gesetzes geschätzt werden konnte.

     

Structure,Ferroelectric, Dielectric and Energy Storage Studies of Ba0.70Ca0.30TiO3, Ba(Zr0.20Ti0.80)O3 Ceramic Capacitors

Ba_0.70Ca_0.30TiO₃-(BCT),Ba(Zr_0.2Ti_0.8)O₃-(BZT) ceramics were fabricated by conventional mixed oxide route to develop inorganic dielectric materials suitable for use as an insulator with high dielectric constant and low energy loss for capacitor applications. The structural phase transition, ferroelectric, dielectric and energy storage properties of BCT, BZT ceramic capacitors were investigated. Room temperature X-ray diffraction (XRD) patterns revealed prominent peaks corresponding to tetragonal perovskite crystal structure for both BCT, BZT solid solutions. Slim ferroelectric hysteresis (P-E) loops were observed for BCT, BZT solid solutions. Temperature dependent dielectric property measurements of BCT, BZT solid solutions have shown a high dielectric constant and low dielectric loss. Room temperature (300K) breakdown field strength and energy densities were obtained from the integral area of P-E loops. For the BCT ceramics, the largest recoverable energy (unreleased energy) density is 1.41 J/cm³ with dielectric breakdown strength as high as 150 kV/cm. For the BZT ceramics, the largest recoverable energy (unreleased energy) density is 0.71 J/cm³ with dielectric breakdown strength as high as 150 kV/cm. Bulk BCT, BZT ceramics have shown interesting energy densities; these might be the strong candidate materials for capacitor applications.     

直流GIL中温度对绝缘子表面电荷积聚时变特性影响研究

柱式绝缘子表面电荷积聚是严重影响直流GIL绝缘水平的重要因素之一。本文在综合考虑GIL内部热交换、绝缘子材料电导特性和绝缘气体中正负离子微观机制的基础上,构建了直流GIL柱式绝缘子表面电荷积聚多物理场耦合时变数学模型,仿真分析了柱式绝缘子的温度分布、空间电荷密度分布和表面电荷密度分布。结果表明:在800 kV直流电压作用下,柱式绝缘子温度由极不均匀分布向均匀分布发展,使得绝缘子电导率不断变化;柱式绝缘子内部主要积聚正电荷,随着温度朝着均匀化发展和时间的延长,绝缘子内部空间电荷密度越来越大,并且柱式绝缘子内部最大空间电荷密度位置由初始状态的中心导体附近变为接地电极附近;温度对表面电荷积聚的影响较大,随着温度朝着均匀分布发展,绝缘子表面电荷密度零点不断右移,表面电荷密度峰值越来越大,切向电场强度也越来越大。     

脉冲激光清洗典型污秽瓷式绝缘子温度特性分析

脉冲激光清洗瓷式绝缘子表面污秽会产生强烈的热效应,分析清洗过程中的温度特性至关重要。文中以沾污的瓷式绝缘子为研究对象,通过仿真和实验相结合的方法,研究不同典型污秽、不同污秽含水量、不同激光能量密度及波长等工况下温度场的变化规律。结果表明:温度的高低与污秽含水量、激光能量密度成正比,与激光波长成反比,其中激光湿式清洗比干式清洗效率高;在保证绝缘子瓷材料的抗热稳定性的前提下,激光能量密度处于2.52～3.81 J/cm~2,扫描速度1 000 mm/s时,不会损伤绝缘子基底材料,可达到最佳清洗。     

Measurement of space-charge field in doped LiNbO3 single crystals

In order to measure the space-charge field in MgO-doped LiNbO₃, we examined the variation of second-harmonic intensity by an external DC electric field and a single beam pump. Intensities of illuminated beam were 3.9 and 7.8 W/cm². An external DC field was applied in the direction of spontaneous polarization (i.e. c-axis). Generated second-harmonic intensity as a function of DC electric field represented by the function of (cosine)². Space-charge field formed as a function of the exposure time of a laser beam had a exponential function. In 4 mol% MgO-doped LiNbO₃ of 6.8 kV/cm at 3.9 W/cm², the saturated space-charge field had a minimum value and the relaxation time had a maximum value.     

Electrical properties of paraelectric PLT(28) thin films deposited by dc magnetron sputtering

Abstract

Paraelectric [Pb, La]TiO₃ (PLT, La = 28 mol%) thin films were prepared by dc magnetron sputtering using a multi-element metal target. In order to crystallize the as-deposited PLT thin films into the cubic perovskite structure, a heat treatment was applied at annealing temperatures ranging between 450 and 750°C. The electrical measurements such as dielectric properties, polarization-electric field (P-E), and current-voltage (I–V) were investigated with the change of annealing temperature. The dielectric constant and dissipation factor of paraelectric PLT film annealed at 750°C were 1216 and 0.018, respectively. The charge storage density was approximately 12.5 μC/cm². The leakage current density in PLT film annealed at 650°C was around 0.1 μA/cm² at the electric field of 0.25 MV/cm.     

Ferroelectric properties of plzt thin films prepared by mocvd

Abstract

Lanthanum-modified lead zirconate titanate (PLZT) thin films have been grown on Pt/SiO₂/Si substrate at 650[ddot]C by metalorganic chemical vapor deposition. The relative dielectric constant increased as the La content was increased up to about 5 atomic percent (at%). The remanent polarization and coercive field decreased from 30 to 20 μC/cm² and from 53 to 30 kV/cm, respectively, with increasing La content in the range of 0–13 at%. The leakage current of PLZT film was 3 × 10^?9 A/cm² at an applied voltage of 3 V. The degradation of switched charge density of PLZT film was not observed even at 2 × 10¹¹ cycles.     

Effect of vanadium doping on ferroelectric and electrical properties of Bi3.25La0.75Ti3O12 thin film

Bi_3.25La_0.75Ti₃O₁₂ (BLT) and V-doped BLT (BLTV) thin films were prepared on Pt/Ti/SiO₂/Si substrates by a pulsed laser deposition method. The effects of V doping on ferroelectric and electrical properties were investigated by polarization-electric field hysteresis loops and leakage current-voltage measurements. BLTV single phases were confirmed by X-ray diffraction. Remnant polarization was increased and the leakage current density was decreased by V doping. The leakage current density of BLT thin films suddenly increased at 100 kV/cm while that of BLTV thin films increased at the higher electric field of 160 kV/cm. The power law relationship J α Eⁿ of current density vs. applied electric field is estimated to be J αE^2.0 for BLT and J αE^1.0 for BLTV thin films. The leakage current of the BLT/Pt junction can be explained by space-charge-limited current. However, that of the BLTV/Pt junction was characterized by the Schottky emission behavior.