Wen-Bin Zhao

Correlation between trapping parameters and surface insulation strength of solid dielectric under pulse voltage in vacuum

As well known, the surface conditions of solid materials strongly affect the flashover phenomena under high electric field. The artificial doping ingredients could change the energy distribution and the density of electron and hole traps. The trapping parameters in the surface layer have some influences on the surface insulation strength. In order to adjust different trapping parameters, the composite materials of epoxy resin intermingled with aluminum hydroxide are prepared. The trapping parameters of the samples with different filler concentrations are investigated through the method of measuring the surface potential based on the isothermal current theory. The surface insulation performance in vacuum of each sample is tested according to the flashover voltage under pulsed excitation. Based on the experiment, the correlation between the trapping parameters and surface insulation strength is discussed and some novel models are presented

Optical investigation of AC preflashover of alumina ceramic in vacuum

Optical phenomena accompanying the preflashover along a planar metal-alumina-metal structure were investigated under stepped AC voltage in vacuum. Two kinds of electrode contacts with and without sputtered gold films were employed. For sputtered alumina, the luminescence became observable at /spl sim/1 kV/sub peak/, and revealed two stages depending on the amplitude of applied voltage. For non-sputtered alumina the optical emission appeared at a much higher voltage and presented irregular and discrete light pulses. The energy band at the metal-alumina interface for the two kinds of electrode contacts is responsible for the relevant optical mechanisms. For non-sputtered contact, the light emission was initiated by field electron emission from the triple junction. While for the sputtered contact, prior to the electron emission, electrons/holes could be injected from electrodes into the surface layer of alumina and electroluminescence phenomena occur due to the radiative electron-hole recombination. Injected electrons form a long-term negative space charge region away from each electrode and hence at a critical applied voltage, the trapped electrons are detrapped resulting in intense light emission. These processes play a significant role in the development of flashover.

Investigation on Surface Insulation Strength of Machinable Ceramic Material under Pulsed Voltage in Vacuum

Ceramic material has been widely used as insulator in vacuum. Their high hardness and brittle property brings some difficulty in the application. A new kind of machinable ceramic was invented recently. The ceramic can be machined easily and accurately after being sintered, which provides the possibility of making the insulator with fine and complicated configuration. However, the insulating strength across the surface of the machinable ceramic is not known clearly. The paper studies the surface insulation performance and flashover phenomena under pulsed excitation in vacuum. The ceramic samples with different crystallization parameters are tested under the vacuum level of 10?4Pa. For the comparison, the Al2O3 ceramic and glass slice is also tested with the same method. The microcosmic patterns of their surface are observed. The surface insulation performance is evaluated primarily. The flashover characteristic and mechanism across the machinable ceramic is discussed.

Surface discharge phenomena of silicon in atmospheric air

Semiconductor devices are being gradually applied in the fields of high-voltage and high-power technology, and thus the high-field characteristics of semiconducting materials have been paid much attention. However, its development meets the limitation of surface discharge/flashover. Based on the measurement of voltage and current waveforms and the optical observation, the surface phenomena across silicon samples were preliminarily investigated under pulsed excitation in atmosphere. Moreover, its discharge behaviors are compared with that of insulating epoxy resin. It is suggested that there was current filament phenomena occurred inside the silicon resulted from the inhomogeneity of injected current density and Joule heating effect, which strongly affect the surface discharge phenomena of semiconductor and liable for its behaviors different from insulating materials.

Optical Investigation of Surface Flashover Plasma Across Silicon Stimulated by Pulsed High Voltage in Vacuum Based on ICCD

Optical investigation of flashover phenomena across silicon is performed in vacuum. A pulse waveform of ~0.4/2.5 mus is employed as stimulation source. The p100-type silicon wafers are the samples. The images of flashover growing are recorded by intensified charge couple device, and the corresponding waveforms of voltage and current are given. The results primarily suggest that the flashover across p-type silicon begins with the distortion of electric field at the edge of anode. Injection of minors may play an important role in the formation process of filament current. It is regarded that the plasma produced by ionized desorbed gas will appear at the final stage of surface flashover.

Flashover development process across silicon and surface microcosmic phenomena induced by pulsed voltage in air

We report the flashover development phenomenon across a polished silicon wafer and its damage effect on the silicon surface under pulsed excitation of ~0.4/2.5 µs with a peak value of >6 kV in atmospheric air. A visible light channel connecting two electrodes is observed before the complete flashover occurs. The filament and pit–jut damages induced by the flashover are found on the silicon surface. A comprehensive flashover development hypothesis is proposed according to which first the filamentary current process occurs in the surface layer of a semiconductor and then the ionization process of air ambient above the surface occurs. The observed surface damages are considered to be closely correlated with the two processes.

Surface Electroluminescence Phenomena from Polymer under AC Voltage

Electroluminescence (EL) phenomena from polymers has been investigated under 50Hz ac electrical stress. Because the light emission is very week, so we employ a sensitive photon counting module (PCM) and put the sample into vacuum chamber to avoid the influence of gases or impurities absorbed on the surface of the sample. We use three kinds of materials such as PTFE, LDPE, and PP in this experiment. Their surface trap parameters are measured according to the theory of isothermal relaxation current (IRC). By that method we acquire the trap energy level and trap density of both electrons and holes distributing in the surface layer of each insulating polymer. We find that there is some relationship between the surface trap parameters of solid polymer insulating materials and their molecule structure, and surface trap has influence on light emission from polymer insulating material

Surface electroluminescence phenomena of insulating polymers

Electroluminescence (EL) phenomena reveal some important information about the electrical ageing of solid insulating materials in discharge-free condition. The EL emission from the surface of several insulating polymers with a planar electrode system was investigated under ac electric field in vacuum. It was observed at a voltage as low as /spl sim/1 kV. Its time-resolved distribution under stepped voltages, the intensity as a function of ac, positive and negative half-rectified ac voltages, the phase-resolved and spectral distributions are studied in detail. Based on the band theory of solid, a new model has been proposed to describe the EL phenomena observed, resulting from the radiative recombination of electrons and holes injected in the surface states. The experimental results and theoretical analysis also support a new approach to understanding the surface and interfacial phenomena in the field of electrical insulation.

Analysis of insulator surface charging due to charge injection and secondary electron emission in vacuum

During the developing process of flashover across an insulator in vacuum, the insulator surface is usually charged. It is of great importance to analyze the charging phenomena for better understanding the flashover characteristics in vacuum. It is considered that there are two kinds of mechanisms closely related to the surface charging phenomena of insulating materials, i.e., the classical secondary electron emission occurring over an insulator, and charge injection and accumulation occurring inside an insulator. Based on the rigorous analysis of the kinetic processes of both primary and secondary electrons, the related surface charges were analyzed. In this process, the influence of image force of mirror charge was taken into account. Involving the detrapping of charges trapped and the recombination of charges injected, the charging process due to charge injection and accumulation was also deduced theoretically. Some formulas were given to express the density of surface charges.

Surface microcosmic phenomena induced by pulsed flashovers

The surface conditions of solids strongly affect the related surface and interfacial phenomena, which implied the difference between its surface and bulk. In this paper, under the pulsed excitation, the pattern and ingredient on surface of two kinds of semi-conductive material, silicon (Si) and gallium arsenide (GaAs) were investigated based on the surface microcosmic analysis techniques, digital metallographic microscope (MM) and scanning electron microscope (SEM) prior to and after the flashover shots. At the same time the waveforms of voltage, current and light of the flashover were recorded for synthetically analyzing microcosmic phenomena. The planar electrode configuration was employed. Some surface microcosmic phenomena about injection and flashover process were displayed and analyzed.