Air Corona Research Articles

Action of corona discharges on bacteria and spores

We have measured the E.coli bactericidal efficiency of DC and AC point-to-plane coronas in air and in argon in the corona modes positive glow, positive streamer, negative Trichel pulse and negative glow. Negative Trichel pulse and positive streamer coronas are the most efficient for production of bactericidal agents in air. Less than 5 minute exposure to the products from 30 μA of these coronas will sterilize the E.coli covered agar surfaces in a 3l volume to a survival ratio of 10−5. Positive glow coronas are two orders of magnitude less efficient. 50 Hz AC coronas seem to mainly work during the negative half cycles. The bactericidal agent(s) produced by the coronas have not all been identified, but ozone O3 with concentration 35–40 ppm is probably the most important one. We show that charged particles and photons can play only a minor role, so the agent(s) must be neutral particles. The main part of them must have a lifetime in the vessel of more than 3 minutes. The much smaller, and volume/distance dependent, efficiency of coronas in humid argon indicates that metastable neutrals or radicals from H2O can play only a minor role. A preliminary test showed negligible effects of the corona treatment on the spores of Bacillus subtilis.

A generalized model for partial discharge processes based on astochastic process approach

A general framework for the physical description of partial discharge (PD) processes is presented that holds for different types of PD causing defects. A PD process is treated as a stochastic process consisting of short duration discharges (point-like in time) and charge carrier drift/recombination intervals between these discharges. It is determined by few basic physical parameters and, in a stochastic process framework, can be described in a closed form by a master equation. Since usually only the fast discharges can be measured as PD signals, a restricted possibility of observing a PD process results. The link between the stochastic process and observable quantities is derived. A specific type of measurements is reported, the so-called phase-resolved partial discharge (PRPD) patterns. Here the total charge transferred during a discharge and the time or alternating current phase at which the discharge occurs are measured. Thus each discharge event is described by the two quantities, charge and phase angle. The modelling of the observation process is explicitly derived for this case. However, the used method can easily be generalized to other types of PD measurements. The proposed approach yields new possibilities for the interpretation and analysis of PD patterns. Features of PD patterns can be derived analytically from the process parameters. Conversely, quantitative information about the discharge physics can be gained from measured patterns. Some limiting cases of model parameter values leading to typical pattern features are discussed explicitly. Examples are presented that demonstrate the applicability of the model for three different discharge types (internal discharge in a gas-filled void, surface discharge in oil, corona in air).

PHYSICAL AND CHEMICAL STUDIES OF CORONA DISCHARGES IN AIR

This paper is devoted to a physical and chemical approach of DC coronas in air from a few Torr to 150 Torr. In the first part, the physical results obtained using a positive point-to-plane corona discharge are presented and described. In the second part, the ionic chemical species detected by mass spectrometry at the same conditions are introduced and commented. Finally, these results will be analysed in order to point-out correlations between the physical and chemical phenomena.

Surface modification strategies for multicomponent polymer systems. V. Interaction states and mechanical properties of LLDPE/PVC blends with corona-modified rutile pigment

Rutile pigment with preadsorbed monomers of acrylamide (AM) or acrylic acid (AA) has been treated in air corona discharges at various input power levels for times from 30–120 s. Inverse gas chromatographic data showed that the treatments reduced dispersive surface energies and significantly altered the acid/base interaction potential of the surfaces. Inferred is the corona-activated synthesis of oligomeric or polymeric structures anchored to the pigment surface. XPS analyses report modifications in the chemical structure of pigment surfaces, which are consistent with the suggested consequence of corona treatment. When incorporated into LLDPE and PVC host polymers, compounds with the corona-modified rutiles have better mechanical properties than analogues with untreated pigment, notable being improved elastic moduli, yield stresses, and stress/strain relationships at break. AM-modified rutiles were preferable to AA-modified versions in this regard. The addition of treated pigments to immiscible LLDPE/PVC (75/25) blends resulted in similar benefits to mechanical properties, AM pretreatment again being preferred. Stronger acid-base interaction at contacts between corona-modified rutiles and the PVC component is an apparent reason for improved mechanical properties. Speculatively also, AM pretreatments lead to attached chains of sufficient length to entangle with the LLDPE, further strengthening the interphase and relevant bulk properties. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1597–1604, 1998

A numerical simulation of Trichel-pulse formation in a negative corona

A new quasi-one-dimensional model for a negative corona in air is formulated allowing for the quantitative description of the mechanism of Trichel-pulse formation. Detailed analysis of the processes controlling pulse dynamics is made. Comparison with experiment for a short-gap corona demonstrates a reasonable agreement with the shape of the pulse and in the average characteristics of the negative corona.

The Oscillations of the Positive Glow Corona

The paper treats experiments, computer simulations, and the physical explanation of the oscillatory behaviour of positive glow coronas at or below atmospheric density. Experiments show that most positivc glows have ionization regions that oscillate with frequencies depending on the gas, geometry, and average discharge current. Our circuit simulating program CIRCSIM with the general gas discharge elemcnt GPCSI reproduces quantitatively Beattie's sphere-to-plane coronas in air, and also our cylindrical coronas in air and in argon with additives. The oscillations demand a fast secondary ionization (feedback) mechanism, and are damp or inhibited by slower feedback processes like positive ion or metastable feedback, and detachment from negative ions. Linear damp oscillations are due to ionization region capacitance and equivalent inductance. Nonlinear self-sustained oscillations are formed by periodic quenching by the generated positive ions.

The barrier effect on the onset voltage of negative corona in air

This paper is aimed at calculating the onset voltage of negative corona in rod - plane gaps as influenced by the presence of a dielectric barrier located either parallel or normal to the gap axis. The effect of the barrier thickness, permittivity and location with respect to the stressed rod on the calculated onset voltage is investigated. The method of calculation is based on the criterion of self-recurring single electron avalanches developed in the gap when stressed negatively. The electric field values used for avalanche growth are evaluated using a combined technique based on boundary-element and discrete-charge simulations. In this technique, the surface charge on the rod is simulated by fictitious discrete charges while the surface charge on the faces of the barrier is simulated by surface charged boundary elements. The electric field assumes values higher than those in the absence of the barrier whatever the position of the barrier, parallel or normal to the gap axis. The smaller the distance between the barrier and the rod, the larger the thickness and the higher the relative permittivity of the barrier, the lower is the corona onset voltage.

Air corona removal of phenols

Exposure to the neutral activated species of an a.c. point-to-plane type corona discharge in air of aqueous alkaline solutions of phenol and substituted phenols (pentachlorophenol, triiodo 2,4,6 phenol and picric acid) leads to the oxidative destruction of the solutes up to the CO2 state in the case of phenol. The influence of the discharge parameters, such as the current intensity, is considered. The destruction of phenates follows overall pseudo first-order kinetics, and the kinetic constants are linearly dependent on the current. A scheme for the degradation kinetics is proposed involving a multistep mechanism, and the occurrence of dihydroxybenzenes as reaction intermediates is suggested. Applications of corona treatment to waste waters may be considered as a versatile method for the elimination of toxic chemicals and the purification of industrial waters.

Temperature effect on the performance characteristics of beta rays open air corona streamer counter

Open air three electrode (point-grid-plane) corona streamer counter with a preamplification gap has been utilized for detecting weakly ionizing beta rays. This is based on the fact that when the primary electrons produced by beta rays are multiplied in the external preamplification gap, they form a trigger electron swarm before entering the counter, produce the same effect as the direct incidence of strongly ionizing particle in the corona counter, and allow the counter to operate efficiently. However, operation of this counter in open air as one of its most attractive features requires a complete analysis of the counter response to the changes of ambient conditions. The effect of temperature on the beta particle corona counter performance is studied. A theoretical model is suggested to calculate the counter operating voltage as well as the beta particles triggered corona pulse characteristics at different temperatures (20/spl deg/C-50/spl deg/C). The temperature effects on the counter-energy response is explained. A compensation for the errors due to temperature effects is suggested. All the theoretical findings are confirmed by experiments.

Interpretation of partial discharges at dc voltages

The aim of this paper is to give an overview on partial discharges at dc voltage. A model is presented which describes the stochastic discharge process. The model is experimentally verified for internal discharges and for corona in air. In addition, the classification of discharge patterns is discussed. The 3-dimensional histogram describing the number of discharges depending on the discharge magnitude and on the time to the successive discharge is suggested to be used as a base for discharge recognition at dc voltage. Experimental proof is presented to show the strength of this approach.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Onset voltage for corona pulses in gaseous Ar under high pressure and in liquid Ar

HV, applied to a point cathode (radius of curvature /spl sim/1 to /spl sim/8 /spl mu/m) in a nonpolar liquid, brings about a regular current pulse regime above a threshold value V/sub s/. This regime is similar to the Trichel pulse regime for corona in air. We present results of an investigation of this threshold voltage V/sub s/ in argon over a wide range of densities. In gaseous argon, the V/sub s/(P) values increase with pressure. In the liquid, V/sub s/ is independent of external pressure. The results show that V/sub s/ is mainly a function of fluid density N and cathode tip radius /spl tau//sub p/. The theoretical analysis of the phenomenon is carried out according to the criterion /spl int//spl alpha/(x)dx constant, where a(x) is the Townsend ionization coefficient. It is shown that if the dependence of V/sub s/ on N for low density gas is extrapolated into the region of liquid densities, it greatly exceeds the experimental values. The dependence of V/sub s/ on the fluid number density /spl sim/ is derived. Its variation is found to be non-monotonic at very high N values.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Physical and chemical aspects of partial discharges and their effects on materials

In the ageing of insulating materials by partial discharges, physical, chemical and electrical factors enter together into play. The paper aims to provide a general overview on the subject, especially for the physical and chemical phenomena, using information obtained from studies carried out with air corona discharges providing both glow and streamer effects on insulating materials.

The multi-avalanche nature of streamer formation in inhomogeneous fields

The streamer formation in a positive point-plane gap both in air and nitrogen has been studied. The primary electrons were created by laser radiation. This gave the opportunity to initiate streamers in a wide range of voltages including the range of a steady corona in air as well as to determine the formative time lags. A streamer starts when the space charge density reaches a critical value in a spatially localized region. In a divergent field the streamer formation is preceded by the accumulation of space charge, i.e. it has a multi-avalanche nature. A model for calculation of formative time lags is presented. For a virgin gap the model describes fairly the accumulation process. For the voltage range of steady corona in air only the qualitative description of the mechanism is given.

Generation and annihilation of traps in metal-oxide-semiconductor devices after negative air corona charging

Surface and bulk traps along with positive oxide charge accumulation have been found to be generated in metal-oxide-semiconductor capacitors, when subjected to negative air corona discharge at slightly reduced pressure (≂10−1 Torr). The effects are neutralized and device quality improved when annealed at 200 °C in air. The bulk traps and a fraction of oxide charges were annealable when kept at room temperature for several months. The results have been analyzed by Nicollian–Goetzberger’s conductance technique and a plausible explanation is given.

Current-density distribution on a plane cathode in DC glow and streamer corona regimes in air

The variation of current densities on a plane cathode in air has been investigated in a positive point-plane gap configurations. This work is concerned with the effects of gap spacing, point diameter and humidity variations on the current density profiles, according to the type of corona regime produced at the point anode during predischarge development. Cases are considered where the point electrode is energized by a positive DC voltage producing either a pure glow or streamers, according to given particular experimental conditions such as point-plane gap spacing, point diameter and ambient absolute humidity. The results obtained show that the current density variation for positive corona in air does not always follow Warburg's law but depends upon the type of corona regime generated at the HV point anode. Comparisons are made with negative point-plane gaps. >

On the role of water in the aging of polymers in air-insulated electrical systems

Experimental results are presented on the direct interaction between corona and insulating surfaces, including the generation of water by corona oxidation of the polymer. Measurements of indirect corona-polymer interaction by corona exposure of distilled water with submerged polymer samples are also reported. It is shown that corona discharges in ambient air will form a water layer on any exposed polymer surface, both by bringing water to the surface made hydrophilic by the corona, and by producing water from the polymer itself, by oxidation. Water exposed to air corona becomes acid and oxidizing, and causes marked aging effects on polymer surfaces. Thus, any model of the action of air corona discharges on polymer surfaces must include the presence and effects of an intermediary aqueous electrolyte surface layer. >

Interactions with aqueous solutions of the air corona products

The interaction with aqueous solutions of an air plasma provided by a D.C. point-to-plane corona device at atmospheric pressure has been investigated with focusing on the chemical properties of the neutral species after separation from the ion flux. Evidence is given of acid effects induced on the target solution. The increase in acidity due to the treatment by the chargeless activated species — mainly singlet oxygen in air and oxygen plasmas — is measured and related to the quantity of electricity involved in the discharge. In addition, acid-base indicators trapped in a gel are used to develop the acid effect and allow to measure the treated area, in order to get an analytical tool to predict the corona treatment. This area depends on the electric parameters voltage and current intensity, on the distance from the point electrode, on the electrode gap and on the initial acidity of the target.

Effect of corona discharge treatment on cellulose prepared from beech wood

AbstractCellulose prepared from beech wood (Fagus crenata Blume) was treated in a corona discharge. The chemical changes produced by the treatment were investigated by use of infrared spectrum, Schiff's reaction, adsorption of methylene blue and Orange II, and intrinsic viscosity. Infrared absorption bands at 1382 and 1715 cm−1 were formed. The color reaction with Schiff's reagent indicated that the 1715 cm−1 absorption was due to the aldehyde group and not to the carboxyl group. The intrinsic viscosity was decreased by the air corona treatment, but was not affected by nitrogen corona treatment. The adsorption of methylene blue was not increased by corona treatment, indicating that no acidic groups were formed. Adsorption of Orange II indicated that some basic groups were formed by the nitrogen corona after a long treatment time. Low molecular weight fractions showing the 1382 cm−1 infrared absorption band were formed by corona treatment. These fractions were removed from cellulose by washing with water, and they increased with an increase in the treatment time. The absorption band at 1382 cm−1 was assigned to the symmetric carbon‐hydrogen bending of methyl group.

The positive corona in air: A simplified analytic approach

The stable positive corona in air is analyzed; the coaxial cylindrical geometry is considered in detail. The analysis separates the corona into two physical regions, corresponding to the absence or presence of significant electron impact ionization. Continuity of electric field and carrier flux across the boundary between these regions gives rise to the criterion for the stable discharge. Electron attachment and detachment are included in the analysis, in an approximate manner. Three secondary-electron-generation processes are considered: secondary emission at the cathode wall due to positive ion impact, photoemission of electrons from the cathode wall, and photoionization in the volume of the air. Comparison of the analysis with experimental data suggests that secondary processes associated with the cathode wall play a minor role in the atmospheric positive corona discharge in cylindrical geometry. Evidence is shown that photoionization of the air is a viable candidate to be the leading secondary-electron-generation process in the discharge.

Electret formation by electrical discharge in air

Electret charging methods on the basis of gas discharge in air offer many advantages, including a very simple arrangement, no direct contact to the electret surface, and no restrictions on charging temperature. For discharge in air two arrangements are in use: (1)discharge in a parallel air gap and (2) corona discharge. A comparison of both methods, showing significant similarities, is given. Starting with investigations of charging in a parallel air gap, the practical knowledge is applied to the more complex corona charging. The characteristics of equilibrum electret voltage and its dependence on applied voltage are measured and interpreted theoretically. The influence of deviations in gap spacing on electret voltage is discussed. Electrical breakdowns of the electret foil affect the results. Therefore the role of breakdown is investigated in more detail.