脉冲电场下ZnO压敏陶瓷动态击穿过程研究

陶瓷电击穿问题涉及热、光、电多场耦合效应, 一直是非平衡物理学研究的重点和热点。本工作在不同烧结温度下制备了晶粒尺寸大小不同的氧化锌陶瓷, 采用脉冲高压发生装置对陶瓷进行击穿实验, 通过对陶瓷击穿过程的分析和对比, 研究了ZnO陶瓷体击穿的时间步骤。结果显示, 不同晶粒大小的陶瓷击穿过程均在7 μs之内, 典型的压降曲线分为三个阶段。第一个阶段对应于材料中的气孔击穿和击穿通道初步形成; 第二阶段对应于晶界击穿; 第三个阶段是导电通道的完全形成。研究数据显示, 晶粒击穿过程的持续时间最长, 晶界次之, 气孔的击穿时间最短。不同烧结温度下, 样品晶界和晶粒的击穿时间以及气孔的击穿速度均存在差异。     

Electrical breakdown studies in didymium oxide thin films

I–V characteristics of sandwiched Al-didymium oxide-Al structures are studied for different thicknesses. The breakdown voltage increases and the dielectric strength decreases with increase of film thickness. Optical photomicrographs of the breakdown patterns during different stages of dc voltage are taken and the results discussed.     

电子束辐照后聚甲基丙烯酸甲酯的电荷特性

用电子束辐照的方法将电荷引入聚甲基丙烯酸甲酯中,研究了电子束辐照对样品电导率、介电常数的影响,以及电荷脱阱对样品的破坏行为.结果表明,电介质的击穿并不总是发生在电荷的注入过程中,也可以发生在空间电荷的脱阱过程中.同时探讨了注入电荷在样品内的输运情况和空间电荷脱阱对样品的破坏机理.     

On the origin of the dielectric breakdown mechanism in plasma-polymerized hexamethyldisiloxane films

Dielectric breakdown phenomena of plasma-deposited hexamethyldisiloxane are discussed. We show that the deposition conditions and post-treatment can change the dielectric strength. A lowering of the breakdown field was attributed to reactions with oxygen species during or after deposition.     

The electrical breakdown during anodizing of high purity aluminium in borate solutions

High resolution scanning electron microscopy of the surfaces of aluminium specimens supporting barrier-type anodic films and transmission electron microscopy of the stripped films have been employed to gain further insight into the mechanism of the electrical breakdown during anodizing of aluminium in borate solutions. Particular attention has been paid to the following aspects of the phenomenon: (1) the sites at which breakdown initiates, (2) the development of breakdown and (3) the structure and morphology of the films after the breakdown events. The observations indicate that the breakdown is possibly of a thermal nature and is triggered by local heating effects caused by highly localized processes taking place at flaws during anodizing, including the possibility of initial electron avalanching.     

Modeling optical breakdown in dielectrics during ultrafast laser processing

Laser ablation is widely used in micromachining, manufacturing, thin-film formation, and bioengineering applications. During laser ablation the removal of material and quality of the features depend strongly on the optical breakdown region induced by the laser irradiance. The recent advent of amplified ultrafast lasers with pulse durations of less than 1 ps has generated considerable interest because of the ability of the lasers to process virtually all materials with high precision and minimal thermal damage. With ultrashort pulse widths, however, traditional breakdown models no longer accurately capture the laser-material interaction that leads to breakdown. A femtosecond breakdown model for dielectric solids and liquids is presented that characterizes the pulse behavior and predicts the time- and position-dependent breakdown region. The model includes the pulse propagation and small spatial extent of ultrashort laser pulses. Model results are presented and compared with classical breakdown models for 1-ns, 1-ps, and 150-fs pulses. The results show that the revised model is able to model breakdown accurately in the focal region for pulse durations of less than 10 ps. The model can also be of use in estimating the time- and position-resolved electron density in the interaction volume, the breakdown threshold of the material, shielding effects, and temperature distributions during ultrafast processing.     

Kinetic and electrical phenomena in gas–liquid systems

Disperse systems consisting of a liquid and gas bubbles located in it are considered. Two possible versions of evolution of bubbles under the conditions studied are assessed. In simple liquids, contact between two bubbles causes them to merge, as the separating film breaks. In the case of complex organic liquids, amphiphilic film is formed on the surface of bubbles, and the lifetime of bubbles in contact increases with their size. Under an external electric field, chains of bubbles are formed, lined up along the electric field potential lines. The presence of bubbles in liquid greatly lowers the breakdown threshold, as the critical parameters of the breakdown field in liquids are two to three orders of magnitude higher than those in gases at atmospheric pressure. Various breakdown mechanisms in liquids are discussed from the viewpoint of formation of the gas phase during the passage of an electric current through a liquid medium. The character of propagating a streamer in separate bubbles is studied with their random distribution in liquid and in the case of formation of some structures of bubbles; the critical parameters of disperse systems, that can lead to their electrical breakdown, are presented. Along with the general concepts of electrical breakdown in dispersed systems, experimental studies of these processes are considered, and the nature of electrical breakdown in liquid dielectrics, including transformer oil, is discussed.     

Scintillation in anodic Ta2O5 films

The scintillation of anodic tantalum oxide was investigated by counting the number of breakdown events during anodization at a constant current density. A theory is developed which qualitatively explains the variation in the number of breakdown pulses with time and voltage for different current densities and different electrolyte resistivities. The theory also allows definition of a limiting anodization voltage which increases with the logarithm of the current density. Several experiments are presented which are in agreement with the theory.     

Breakdown of agglomerates in ideal pastes during extrusion

By separating the matrix from the agglomerate it was possible to determine the nature of agglomerate breakdown during extrusion within a known geometry. Previous attempts at observing agglomerate breakdown have been unsuccessful at relating the properties of the paste to the properties of the agglomerate due to the indistinguishability of the paste and the agglomerates. By using well-characterized materials, it was found that the breakdown probabilities of the agglomerates during extrusion were independent of the position of the agglomerate. This indicated that as the shear strain rate was known to be highly position dependent, the agglomerate breakdown was shear strain rate independent. Following breakdown, the distributive mixing was observed to be position dependent.     

Effects on vortex breakdown due to an abrupt change in the rotation of endwall in a disk-cylinder system

Summary The transient flow in a cylindrical enclosure with fixed sidewall is studied numerically. The initial motion due to uniform rotation of the lower endwall is disturbed by setting the top endwall to corotate impulsively with a small angular velocity. The flow parameter values are chosen so as to induce a vortex breakdown in the initial steady state. The unsteady rotationally-symmetric Navier-Stokes equations are solved iteratively using a combination of second-order and fourth order compact difference schemes. At higher values of the Reynolds number, upwind differencing is used in the convective terms. The breakdown bubble in the initial steady state occurs between the stationary top end wall and the mid-plane. The numerical soulution shows that at the early stage of the transient flow the breakdown bubble enlarges in size, and in subsequent time the bubble occurs between the mid-plane and the faster rotating endwall. The role of the azimuthal component of vorticity in the breakdown phenomena is analysed. The torque on the lower endwall is obtained at several values of non-dimensional time during the transient flow.     

Features of development of optical breakdown on an inclined aluminum surface

It is shown that the air breakdown region and the erosion torch during an inclined fall of the laser radiation on the target are separated in time and space??namely, the electron density is higher in the region of the air breakdown, while the concentration of aluminum atoms is higher in the erosion torch. The mechanism of the motion of the laser plasma is determined and the formation time of the erosion torch is evaluated allowing for the film boiling mechanism.     

The influence of operating parameters on pulsed D.C. magnetron sputtering plasma

This paper describes the breakdown voltage characteristics in a pulsed D.C. magnetron sputtering system under varying conditions of frequency, current and pulse-off time. The behaviour of the breakdown voltage with pulsing frequency at different pressures and constant pulse-off time was recorded and revealed that the breakdown voltage decreased consistently as the frequency increased up to 70 kHz. Above this frequency, perturbation in the breakdown voltage was noted, possibly due to the rise in pre-breakdown current during the few microseconds of pulse-on time. This perturbation effect was no longer observed when the operating current was increased. The breakdown voltage was seen to decrease when the pulse-off time was increased while keeping the total period of the pulse constant.     

Spectroscopic analysis of fire suppressants and refrigerants by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy is evaluated as a means of detecting the fire suppressants CF(3)Br, C(3)F(7)H, and CF(4) and the refrigerant C(2)F(4)H(2). The feasibility of employing laser-induced breakdown spectroscopy for time- and space-resolved measurement of these agents during use, storage, and recharge is discussed. Data are presented that demonstrate the conditions necessary for optimal detection of these chemicals.     

Measurement of Electrode Erosion during High-Voltage Vacuum Breakdown

This paper is concerned with the measurement of the mass of electrode material released by the electrodes during a high-vacuum breakdown. The techniques used in the experiment described in this paper for quantitative measurement of material released from the electrodes are neutron activation analysis and gamma-ray spectrometry. The method permits determination of the small masses released during the discharge. The threshold energy density required for material release is 15 J/mm2. An empirical equation is given for the material erosion for copper and a curve is given that relates mass erosion versus stored energy for copper electrodes. The experiment indicates that the mechanism responsible for current transport between the electrodes in a vacuum breakdown is a highly ionized plasma formed from the released electrode material.     

聚合物电介质的击穿与空间电荷的关系

空间电荷是聚合物在交流或直流高压作用下发生老化和击穿的主要原因之一,以往的研究基本认为由于空间电荷的注入并集聚使介质内部电场严重畸变,从而使介质老化最终引起击穿,相应的解释模型都是在有外加电场作为前提建立的.但是,根据最新的实验研究发现,外加电场并不是介质击穿的必要条件,介质的击穿可以发生在空间电荷的脱阱过程中而与外加电场无关.本文阐述了空间电荷与绝缘高聚物的老化和击穿的关系,并且结合最新的研究成果,揭示了介质内部空间电荷的存在是击穿的重要条件,而且击穿是发生在空间电荷的脱阱过程中.     

Mechanism of breakdown in the interface region of glass reinforced polyester by artificial weathering

Laboratory studies were conducted to elucidate the mechanism of breakdown in the interface region of glass-fibre reinforced polyester (GRP) composites on outdoor weathering. GRP composites were subjected to the effects of moisture, temperature and radiation. Breakdown in the interface region occurred when the GRP sheets were aged in the presence of water and physically-induced stress (thermally and/or by moisture). The stresses involved are complex, the most predominant being axial shear stresses. Fracture characteristics of breakdown produced during laboratory ageing were very similar to those occurring on outdoor weathering.According to the mechanism proposed, the resin in the interface region is subjected, during environmental ageing, to a stress-fatigue resulting from the differential dimensional changes between glass and matrix induced by moisture and/or temperature cyclic variations. Under the influence of alternating cyclic stresses and in conjunction with the chemical degradation of the matrix, the interface region undergoes cracking, fracture and fibre delamination. This type of breakdown may be referred to as environmental stress cracking.     

Craze development and breakdown in PVC during cyclic loading

The nature of the development and breakdown of crazes during cyclic loading has been examined in poly(vinyl chloride) (PVC). The results show that the relative kinetics of craze development and breakdown are the same, regardless of molecular weight, plasticizer content or stress intensity level. Through optical and fractographic observations, it is found that the full equilibrium length is established early in the cyclic lifetime of the craze. Finally, the fracture path through the craze depends on the amount of cyclically induced damage experienced by the craze prior to the passage of the crack.     

Electrical strength of thin polyaniline films

The electrical strength of in-situ polymerized polyaniline films converted to non-conducting polyaniline base has been compared with the strength of solution-cast polystyrene and poly(methyl methacrylate) films of sub-micrometre thickness. The electrical lifetime of polyaniline film exponentially decreases with the growing electric-field strength. The electric-field strength at breakdown increases with increasing rate of electric field build-up. The breakdown areas in polyaniline films have been characterized by Raman spectroscopy. The possible physical mechanism of the breakdown in thin polymer films is discussed. The proposed concept is based on the steep increase of current density during the transition from the quadratic law at space-limited charge to the regime of complete saturation of traps. As a result, the Joule heat causes the degradation of polyaniline followed by the evaporation of chain fragments in the breakdown area.     

Attenuation of electron gun generated surges in an electron beam evaporation system using ferrite beads

Vacuum breakdown is a very common problem during the operation of any electron gun, more so for e-guns used for vacuum evaporation. This not only spoils the coating or welding job but also causes damage to the electronics of the power system. This paper discusses an approach to attenuate the breakdown current near the gun itself, so that the surge does not propagate to cause any detrimental effect on the power source and thereby on the job. In this paper, ferrite beads on both the feed lines have been used to suppress the high frequency content and reduce the amplitude of the breakdown signal. An induction coil sensor has been designed to monitor the breakdown signal. The high frequency surge in the MHz range could be attenuated to a lower range of KHz by use of ferrite bead. A detailed quantitative analysis of the breakdown signal has been presented in this paper.     

Space charge dynamics in electron beam-irradiated PMMA

In this study, space charge was introduced into polymethyl methacrylate (PMMA) samples by electron beam. The effect of the electron beam on the conductivity and the permittivity of the PMMA sample were investigated, and the breakdown mechanism of the samples during charge detrapping was also studied. The experimental results clearly show that not only space charge injection but also space charge detrapping can lead to the breakdown of the samples. The dynamics of injected charges in the radiated PMMA samples and the mechanism of the breakdown resulting from the rapid discharge are discussed. Translated from Chinese Journal of Materials Research, 2005, 19(4): 431–436 [译自: 材料研究学报]