Sol-gel silica electret films were coated on silicon substrates by a spin coating technique. The process of preparing the films is described and the electret behavior of the films is discussed. The films were corona charged at room temperature. Then, isothermal decay of the surface potential at room temperature and open-circuit thermally stimulated discharge (TSD) experiments were carried out. The results showed that the main peak of the TSD current spectra was situated at approximately 250-degrees-C and charge deposited in the film indicated good stability for the negative charged sample. After storage at room temperature for 50 days, the surface potential of the sample still retained at approximately 90% of the initial value. We found activation energies of 1.3 and 0.7 eV for negative and positive corona charged samples. Our experimental results suggest that the sol-gel process could be useful in electret and electronic technique applications.
A Monte Carlo simulation is carried out in SF6 in uniform electric fields with 7 and 14% overvoltages and at two gas densities at each overvoltage. The electron motion and avalanche growth is simulated by tracing individual paths and the effect of space charge is included by solving the Poisson equation. The streamer propagation, electron, positive and negative ion distribution and space charge fields are studied in detail as more time has lapsed after voltage application. The simulated streamer shape explains for the first time the dark space in SF6 streamers observed experimentally. It is found that the mechanism of streamer propagation in an attaching gas is different from that in a non-attaching gas. The maximum field enhancement is just behind the streamer or between two successive streamers in SF6. The anode directed streamer propagates with a velocity of 10(7) to 10(8) cm/s, dependent on the percentage of overvoltage and the gas number density.
HV outdoor insulators are always subjected to pollution of different nature and severity and the performance of a HV insulator under polluted conditions is quite different from that under pollution-free conditions. The present paper is concerned with the field distribution around a post-type insulator for different severities of surface pollution. The capacitive-resistive field distribution around the polluted insulator is calculated by a charge simulation method. The field calculations are carried out for power frequency voltage and also for lightning and switching impulse voltages. The effects of uniform or partial surface pollution and the presence of dry bands on the field distribution around a polluted post insulator are presented and discussed in detail. >
A comprehensive set of data of electron swarm parameters in nitrogen, oxygen and air are calculated by the Monte Carlo technique. It is shown that for mixtures of gases the calculated values differ considerably from those obtained using the simplified procedure of partial addition of the parameters of the constituent gases
The radiation-charged electret ionization-chamber uses an electret formed in situ by applying a bias voltage to the chamber electrodes while the chamber is being irradiated with X-rays. An electrometer permits measurement of the charge on the electret surface before and after the use of the chamber as a dosimeter. The authors present measurements of dosimeter range, sensitivity, and energy dependence as functions of bias potential, electrode separation, guard-ring size, and various materials. Reproducibility permits detection as low as 55 mu Gys with range up to 100's of cGys depending on the design of the chamber, with charge retention properties similar to those of electrets formed from corona discharge.<>
The main activities in the field of vacuum interrupters carried out by the authors' research groups at technical universities in Germany and Austria are discussed. A wide field of dielectric programs has been treated. Work concerning arc modes and motion is reported. Questions of current zero at line frequency and high frequency, as well as chopping characteristics, have been investigated. In many investigations, the influence of contact material has been of great importance
Electrets of amorphous polyethylene-terephthalate were studied, using thermal windows to polarize and thermally stimulated depolarization current (TSDC) dielectric spectroscopy as an analysis technique. It was found that the polar peak ( alpha ) and the free charge peak ( rho ) show a maximal area at polarization temperatures, T/sub p/, called the temperature of optimal polarization, T/sub po/. The T/sub po/ of alpha is approximately=70 degrees C, and for T/sub p/, >T/sub po/ the temperature at which the maximum intensity appears remains constant with T/sub p/. For this range of temperature the intensity of the maximum decreases and the polarization is linear with 1/T/sub p/. The second peak, rho , which has a T/sub po/ approximately=87 degrees C, shows the maximum intensity at temperatures that are linear with T/sub p/, T/sub p/>T/sub po/. For this range of temperatures the intensity of the maximum decreases with T/sub p/ and the polarization is not linear with 1/T/sub p/. These results are compared with those obtained from polyvinylchloride and polymethylmethacrylate. >
A critical study of the performance of several digital filters for rejecting discrete spectral interference (DSI) in partial-discharge (PD) testing is reported. The filters are evaluated with particular reference to distortion introduced on successive PD pulses. A digital filtering method based on a cascaded second-order IIR lattice notch filter is proposed. The study shows that, for similar performance, methods based on linear prediction require longer filter orders when compared to cascaded IIR notch filters. The nonadaptive methods are found to be stable against impulsive disturbances, while adaptive methods are unstable. The method proposed gives the best performance, low distortion to a sequence of PD pulses, good stability, and fast filtering time. The performance at higher sampling rates, even with PD pulses of long widths, is also found to be superior. The method is implemented using fixed-point arithmetic leading to faster processing. Hence, the method is suitable for on-site PD measurements.< >
The effects of electrical, thermal and radiation stresses, singularly or simultaneously, on dielectric films are reviewed. The types of accelerated aging under these stresses and the statistical methods used to evaluate the experimental data for life, including the two-parameter Weibull and the log-normal distributions, are described briefly. The experimental life models currently used for aging are discussed. Recent data on the complex electrical/thermal/radiation aging of dielectric films are described
The author comments on some of the statements made about the aging compensation effect (ACE) correlation by J.P. Crine (ibid., vol.26, pp.811-818, 1991) and on discussions of it elsewhere in the literature
This paper presents a study of the flashover mechanism initiated by metallic particles in SF6 gas. Using a high-speed Imacon photographic camera it was established that a metallic particle near the insulator surface plays an important role in flashover development. The influence of the particle on flashover development depends on the pressure of the SF6 gas. At atmospheric pressure, the flashover was always initiated at a point on the particle farthest from the nearest electrode. At high pressures, it was found that breakdown develops from the electrode of relative positive polarity in the form of a leader. The velocity of the discharge crossing the gap increases exponentially when the distance between the particle and the insulator surface decreases.
The effect of an insulator inserted along the axis of a rod-to-plane gap on an ion flow field in SF6 gas is investigated experimentally in the pressure region where a preceding corona discharge exists. Without the insulator, the calculated electric field on the plane electrode agrees fairly well with the measured one. The high density space charge near the rod electrode produces the corona stabilization effect. With the insulator, the accumulated charges on the insulator affect the current and electric field distribution on the plane electrode which are variable with pressure, while the normalized charge density near the plane electrode is less dependent on pressure. These characteristics may be attributed to the change in the electric field distribution by the accumulated charge, the increase in the ratio of the dielectric strength in gas gap to that along the insulator surface with the gas pressure, and the thermal diffusion of ions near the insulator. On the basis of measured results, the ion flow field in the presence of the insulator is discussed.
Discharges in model capacitors have been studied with a digital analyzer. The dielectric was polypropylene impregnated with an organic liquid. We have investigated the effect of the nature of the liquid (aliphatic or aromatic hydrocarbon) and the influence of parameters such as voltage, pressure and temperature on the main characteristics of the discharges. Discharges were also monitored during aging tests of the models. We conclude that aging of the models we tested at high electric fields (approximately 135 V/mum) is not due to discharges but to the chemical degradation of polypropylene submitted to electrical and thermal stresses.
Summary form only given, as follows. The author reviews the history of thermal and electrical aging relations and analyzes their limitations. The focus is on the variety of electrical aging equations that have been used, and their empirical nature is confirmed. The author argues that the solution to the multifactor aging dilemma lies in the acquirement of an understanding of the basic mechanisms that govern the aging for each environment or stress. Some recent experiments at the Naval Research Laboratory revealed the generation of chemical species in insulation exposed to electrical stresses below corona inception voltages. The quantities of species generated proved to be a function of magnitude of electrical stress and duration of exposure.< >
Summary form only given, as follows. Most polymers used in electret applications have a semi-crystalline morphology. However, due to the limited diffusion of the large and entangled macromolecules, the crystallization is never complete and crystal growth is limited by topological defects, including possible crosslinks, branches or aretic and atactic sequences. This peculiar crystallization process produces tiny lamellar-crystals, which are embedded in an amorphous matrix. These polymeric materials are thus intrinsically composite and their electrical and mechanical properties depend on the degree of crystallinity as well as on the morphology of the mixed phases. The microstructure of a few polymer electrets was examined, and the charge-transport phenomena in the amorphous region were considered in terms of the nature of the carriers and of the mobility of the chains. Ferroelectric polymers exhibit different electrical properties because of the spontaneous polarization of the crystalline phase. The microscopic process of their poling thus involves a reorientation of the ferroelectric axes of the crystallites whose time scale can be very short with respect to characteristic times of carrier-diffusion.<>
This short communication deals with the practical computation of the median rank function, F(i,n). F is the estimator of the cumulative probability of failure for the ith of n identically stressed samples. The exact values of the median rank function are easy to compute, using a reliable and easy-to-code algorithm presented in this paper.< >
A variety of definitions related to partial discharge (PD) phenomena are discussed. There is no general agreement at this time of writing as to the definitions of the various regimes of PD. A certain confusion exists which may be due to the multitude of PD manifestations. An attempt is made to elucidate some aspects of the PD definitions. Related problems needing more work are noted. >
A set of computer programs designed for research and educational objectives in the analysis of low frequency electric and magnetic field phenomena is described. These programs are developed around a standard and easily extensible user interface, run on both PC-compatible and X window platforms, and offer the user a wide range of analysis tools. They are capable of solving Poisson's equation for arbitrarily shaped axisymmetric and two-dimensional geometries
The buildup of the polarization of VDF-TrFE copolymer films, at room temperature and after various thermal treatments, has been studied using the PWP method. This method allows the nondestructive measurement of the homogeneity of the piezoelectric coefficient. In this paper, we have polarized the samples at 100-degrees-C and analyzed the influence of different thermal treatments before polarization, and of the insulator-electrode interfaces. The behavior of the copolymer is compared to that described previously for poling experiments made at room temperature. The effect of electron injection at the cathode by non-blocking electrodes is clearly proved.
Investigations of the luminous events associated with the break-down along the surfaces of large band gap insulators such as polycrystalline alumina and monocrystalline quartz in vacuum were made by employing a phototube and a very sensitive photomultiplier tube (PMT). The onset of luminous signals, in general, was found to start much before the onset of voltage collapse during breakdown. The early onset of luminosity may be due to the presence of various defect or trapping centers within the forbidden gap of the insulator. The spectral nature of the light emitted during breakdown was resolved (in terms of the wavelength) by interfacing a monochromator with the PMT. In the case of the monocrystalline quartz specimen, the breakdown luminosity was observed to be associated with the defects located at 2.76 and 1.91 eV below the conduction band edge. The breakdown luminosity of polycrystalline alumina was found to be associated with the defects corresponding to energy levels at approximately 1.91, 2.25, 2.45 and 2.76 eV. The formation of these defects is discussed in terms of the nonstoichiometric nature of the lattice structure at the surface. Besides, the samples were found to emit light when no breakdown occurs. This no-breakdown light emission was found to be attributed to the deep level defects. The observed results suggest that the surface flashover process is primarily controlled by the defect or trapping centers located within the forbidden gap of the insulators.
