Dielectric Surface Charge Research Articles

The influence of dielectric surface charge distribution upon the partial discharge behavior in short air gaps

A two-dimensional breakdown model based upon the continuity equations for electrons and ions and the Poisson equation is employed to examine the influence of surface charges upon the partial discharge behavior of a short metallic-dielectric plane-parallel electrode gap of 0.05 cm in an air-like mixture at atmospheric pressure. The form of charge distribution at the dielectric electrode surface is found to significantly affect a number of important discharge parameters, such as the uniformity of the electric field, the charge density and its radial distribution within the gap, as well as the discharge propagation time.

Parasitic charging of dielectric surfaces in capacitive microelectromechanical systems (MEMS)

Silicon dioxide and silicon nitride coatings are preferably used as dielectric layers for short-circuit protection in capacitive silicon microsensors and microactuators. However, their tendency to electrostatic charging can diminish the device reliability. Gas discharges in the air gap of silicon cantilever actuators have been observed, resulting in surface charge accumulation on the electrode passivation of the devices. Charge decay characteristics were recorded for a silicon oxide passivation and a multilayer passivation by silicon oxide and silicon nitride. The charges are found to be highly stable in time. Based on these observations, rules for the application and design of dielectric layers in microdevices are proposed.

Influence of a solid/liquid interface on electric conduction characteristics under nonuniform field in transformer oil

Electric conduction in transformer oil is significantly affected by the presence of a solid/liquid interface. Using a blade-plane electrode geometry, the influence of an insulator inserted along the plane of symmetry of an electrode gap on conduction currents in transformer oil was investigated. Although the stationary relationship between current I and voltage V always showed linear plots of V versus I⅓, conduction currents were prominently decreased by the presence of the insulator. This reduction of current is discussed based on electrohydrodynamic (EHD) motion and the electric field of the blade edge, taking the insulator surface charging into account. © 1998 Scripta Technica, Electr Eng Jpn, 123(4): 1–7, 1998

Influence of a solid/liquid interface on electric conduction characteristics under nonuniform field in transformer oil

Electric conduction in transformer oil is significantly affected by the presence of a solid/liquid interface. Using a blade-plane electrode geometry, the influence of an insulator inserted along the plane of symmetry of an electrode gap on conduction currents in transformer oil was investigated. Although the stationary relationship between current I and voltage V always showed linear plots of V versus I⅓, conduction currents were prominently decreased by the presence of the insulator. This reduction of current is discussed based on electrohydrodynamic (EHD) motion and the electric field of the blade edge, taking the insulator surface charging into account. © 1998 Scripta Technica, Electr Eng Jpn, 123(4): 1–7, 1998

Effect of surface charge on hydrophobicity levels of insulating materials

A correlation between the hydrophobic characteristics and accumulation of static charge on several insulating surfaces (ceramic and nonceramic) is studied. This experiment consists of simultaneous measurements of surface charge decay and contact angle recovery against time on samples previously exposed to corona impingement; a comparison between charge decay and hydrophobicity recovery trends is then made. From the data obtained, a lowering of the original hydrophobicity level for each material as a consequence of surface charge accumulation is identified. The decay of surface charge and the corresponding recovery of the initial hydrophobic characteristics with time are observed. Loss and recovery of hydrophobicity resulting from surface charging and charge decay, respectively, are identified as mechanisms occurring not only on polymeric surfaces, but also on ceramic ones. From a number of laboratory studies, it has been confirmed and extensively reported by several researchers that surface charging of insulators due to electrical activity (partial discharging) can be achieved. Based on the above, as well as on the results obtained from this experimental work, surface charging of insulators is a phenomenon which may occur under realistic operational conditions, and is therefore proposed as a factor responsible for the loss of the initial highly hydrophobic characteristics of polymeric insulators and coatings, in addition to other well established mechanisms.

Comment on "Using the /spl lambda/ function to evaluate probe measurements of charged dielectric surfaces" [with reply

For original paper, see T.O. Rerup et al., ibid., vol. 3, pp. 770-777 (1996). Rerup et al. described the calibration of electrostatic probes used to measure the surface charge on dielectrics. This work showed that the use of the probe response theorem (the /spl lambda/ function), rather than the simple capacitative formula, leads to better experimental values. In this discussion, the commenter wishes to show that a more detailed interpretation can be extracted from the numerical results. In reply, Rerup et al. demonstrate that simple statistics cannot be used to extract, in a conclusive manner, a more detailed interpretation of the /spl sigma//sub /spl lambda//,/spl sigma//sub D/ patterns. Moreover, with the limitations imposed by the axially symmetric /spl lambda/ function, there is little to be gained in pursuing the source of the discrepancies between /spl sigma//sub /spl lambda// and /spl sigma//sub F/. They agree with the commenter that a multi-point scanning procedure would be essential if the functional form of the surface charge distribution were unknown.

Influence of insulator along the plane of symmetry of a blade-plane gap on I–V characteristics in transformer oil

In the case of a blade-plane electrode geometry in transformer oil, conduction currents under dc stresses were exceedingly decreased by a solid insulator inserted along the plane of symmetry of an electrode gap whatever the polarity of the blade. However, the stationary relationship between current I and voltage V always showed the linear plots of V vs. I sol1 3 . The effect of the insulator on the current is discussed on the basis of the electrohydrodynamic (EHD) motion as well as the electric field of the blade edge taking into account the insulator surface charging.

Study of radiation effects in SiO2/Si by measurement of contact–separation current

Contact and frictional electrification measurements of insulators have been used for a number of years to characterize insulator surface charging. An instrument developed in our laboratory confirms triboelectric series data derived from the conventional field mill technique by measurement of current when a metal plate contacts and then separates from the insulator. In this work we investigated the measurement of this contact–separation current on unbiased gamma (Co-60) irradiated SiO2 thermally grown on single-crystal silicon. To our knowledge, surface charge measurements have not been previously used to characterize irradiated SiO2. Results on wet and dry thermal oxides show that irradiation with total doses of 1 and 2 Mrad (Si) show consistent decrease in contact current. Comparison to flatband voltage shifts indicates that radiation damage of SiO2 grown on Si can be observed with surface charge measurements, although more work is required to determine the potential of this method for quantitative analysis of radiation effects.

Using the λ function to evaluate probe measurements of charged dielectric surfaces

The use of Pedersen's /spl lambda/ function to evaluate electrostatic probe measurements of charged dielectric surfaces is demonstrated. With a knowledge of the probe /spl lambda/ function, the procedure by which this function is employed is developed, and thereafter applied to a set of experimental measurements available in the literature. The values of surface charge density derived are in good agreement with the published data. Through this field-theoretical approach, it is readily shown that areas of charge remote from the probe location can produce a major part of the probe signal. If a circuit-theory approach were adopted to analyze such probe measurements, then as the held features of the probe response cannot be taken into account, a serious misinterpretation of the measurements could arise.

Arcing of negatively biased solar cells in a plasma environment

Experimental and theoretical efforts have been conducted to investigate the arcing of negatively biased solar arrays in a low-Earth orbit plasma environment. Experiments were conducted in an ultrahigh vacuum plasma test chamber, where the environment could be controlled carefully. Outgassing of the adhesive used to bind the protective coverglass to the solar cells was determined to be a key factor hi observed arcing rates. These rates could be reduced by greater than a factor of 100 by eliminating or fully outgassing the excess adhesive remaining at the edge of the solar cells. Optical emission from solar cell arcs was observed to correlate linearly with arc current, both temporally and in total intensity. Solar cell arcing rates were also observed to scale linearly with plasma density. The plasma scaling is in good agreement with a theory based on enhanced field electron emission charging of dielectric surfaces, leading to enhanced electric fields at the conductor/adhesive/plasma triple junction. Apparent thresholds for solar cell arcing are reported.

The effect of insulator charging on breakdown and conditioning

As part of a study on HV design concepts for microwave tubes, a number of different insulator designs have been studied. Analysis of the measured DC current, partial discharge activity and breakdown voltage shows that surface charging of insulators is a key mechanism in the breakdown process and in the conditioning process. Insulator parameters are not only the breakdown voltage, but also the conditioning speed and the sensitivity to gas exposure or charge leakage. In all these respects insulators with a field enhancement at the anode are superior. Field enhancements at the cathode are less harmful if stepped insulator shapes are chosen. Effective conditioning requires at least a limited number of breakdowns. With sufficient conditioning breakdowns, all insulator geometries tested reached a breakdown field exceeding 12 kV/mm. >

Thermally stimulated electron emission from charged dielectric surfaces

Abstract Electron emission from charged surfaces of spontaneously polarized dielectrics has been observed. TGS, a non-linear dielectric-ferroelectric crystal and the linear pyroelectrics lithium sulphate monohydrate and resorcinol have been studied. An electret state is formed at the free surface under the unscreened pyroelectric field. A subsequent thermally activated relaxation process of the electret charge gives rise to the electron emission which can be considered as an electret discharge current into vacuum. It is shown that, for TGS crystals, electron emission in the nonpolar, paraelectric phase occurs due to a space-charge electret effect. A dipole electret polarization formed by reoriented dipolar water molecules in lithium sulphate monohydrate is also sufficient to produce electron emission.

Analysis of electrostatic field in the system: charged dielectric-discharging electrode

Investigations were performed for the configuration: dielectric object-grounded spherical electrode. An axissymetrical system was considered and electric field distribution along the axis of this system was found, in particular at the electrode surface. A model of the electrostatic field in the system under investigation was proposed in order to determine the field intensity. Analysis of this model together with calculation of electric field for given dielectric surface and space charge distribution and for a parametrically variable geometry of the system is the objective of the paper.

Circuit model of surface arcing

An electrical breakdown on a highly charged dielectric surface can result in a discharge along the surface, i.e., a flashover arc. We construct a simple circuit model for such an arc: the discharge of a capacitor C (related to the initial charged area) through a series inductor L and resistor R (related to the arc considered as a plasma). The arc current assumes a very simple form over most of its dynamic range, and such measured arc quantities as total charge transport, pulse width, peak current, and rise time are easily calculated. Moreover, straightforward a priori estimates of C, L, and R values give calculated arc quantities in good agreement with observation, for both typical magnitudes and areal scaling. We also analyze the effect on areal scaling of allowing the arc resistance R to ‘‘switch’’ during the evolution of the arc, from a small value characteristic of the arc plasma to a large value characteristic of the dielectric surface. Finally, we consider some aspects of the electromagnetic radiation generated by the arc.

Dc surface flashover mechanism along solids in vacuum based on a collision-ionization model

Selected experimental results are presented from an extensive experimental investigation of dc prebreakdown and breakdown processes in vacuum along solid insulators made of a variety of alumina ceramics and polymers. The investigations included measurements of predischarge current, x-ray intensity, insulator surface charge density, breakdown behavior as influenced by insulator surface characteristics and insulator profiles, and the analysis of the spectral characteristics associated with insulator surface luminescence. The prebreakdown and breakdown phenomena were found to be strongly influenced by surface microstructure and the chemical state of the insulator surface. The experimentally observed phenomena in our studies strongly point to a new breakdown model based on collision-ionization by electrons of defect sites and/or traps within the dielectric subsurface at the vacuum interface.

Detection of radio-frequency signals emitted by an arc discharge

We analyze the nature of radiation signals detected from electrical arcing events originating on highly charged dielectric surfaces. Starting from a simple model of the arc current pulse, we calculate the electromagnetic radiation signal incident on a distant radio-frequency receiver. The receiver is tuned to some central frequency and operates in a given bandwidth. Because the receiver responds to the arc current derivative rather than the arc current itself, the detected radiation pulse profile can differ substantially from the current pulse profile, even when the receiver bandwidth is large compared to the reciprocal pulse width. This signal distortion significantly affects the interpretation of arc radiation data. To illustrate the point, we show that at high frequencies the detected pulse width carries very little information on the arc itself, and that even a scan of pulse width versus frequency may provide a misleading picture of possible arc dynamics. Finally, we argue that arc radiation data from radio-frequency measurements are most useful at low frequencies, when the receiver frequency and bandwidth are both comparable to the reciprocal arc duration.

Corona Discharge Studies in SF6 with Insulators under Positive Switching Impulses

Corona discharge currents and corona inception time delays have been measured in SF6 with an insulating support under positive switching impulses. The insulator surface charging is found to have a significant influence on the corona discharge phenomena. Cumulative probability distribution function of the number of corona pulses is found to depend upon the insulator material and gas pressure. Based on the experimental results, the calculated corona inception time delay distribution with an insulator is found to fit into a log-normal distribution. The influence of insulator on electron initiating mechanism is analyzed using Von Laue diagrams for the corona time delay. The effect of gas pressure and the applied voltage on the inception time delay and pulse current with an insulator in the gap is discussed.

Effect of Laser Produced Pinholes upon the Charging Characteristics of Spacecraft Thermal Control Surfaces

Differential charging of dielectric surfaces on spacecraft has received much attention in recent years because of the possible damage to spacecraft systems resulting from electrical breakdowns. Teflon presents a long term hazard because it can retain its charge for long periods after initial charging. In an attempt to reduce the charging of Teflon, a CO2 laser has been used to drill 100 μm diameter pinholes in metal backed Teflon. A radiation induced conduction layer from the front surface through the pinhole to the metal backing may provide a path for increased leakage currents and hence lower equilibrium surface potentials. Measurements of drilled Teflon samples irradiated with a monoenergetic electron beam indicate that an increased conduction to the metallized back of the samples and a reduction of secondary electron emission from the sample surface are accompanied by a reduction in equilibrium surface potential. The most interesting and perhaps most important effect noted is that drilled Teflon emits electrons after irradiation ceases. This emission combined with charge leakage to the metallized backing leads to a rapid reduction in surface potential. Drilled Teflon will therefore not maintain a charge for as long as will undrilled Teflon.

Investigation of Long Drift Chambers for a Nucleon Decay Detector

We have built a successful prototype of a long drift chamber to be used for a nucleon decay experiment. In order to minimize the attenuation of drifting electrons, it was necessary to produce the drift electric field with a continuous electrode, realized with a film of resistive ink, which prevented charging of dielectric surfaces andeliminated outside effects. A focussing electric field shape was used to reduce loss of electrons by diffusion into the walls of the chamber.

Corona Current Control by Mesh Grid

Corona charging is widely used for charging dielectric surfaces. It is desirable to attain precise and uniform surface voltage in its application. A corona charging method using a mesh grid is described. It is found that the corona current control characteristics by mesh grid can be expressed by a nondimensional parameter?the corona current transmission rate. This parameter is obtained experimentally. The method of parameter application in dielectric surface charging is expressed by a differential equation. This technique is applied to charging amorphous selenium xerographic plates and to neutralizing the charges on a dielectric surface. It is found that the surface voltage obtained by charging with a mesh grid is far more uniform than that obtained by charging without a mesh grid.