Corona Charging Research Articles

Influence of fluorination time on surface charge accumulation on epoxy resin insulation

In order to suppress surface charge accumulation on the epoxy resin insulation and to investigate the influence of treatment time on the charge accumulation, epoxy samples are surface fluorinated for the different times of 10 min, 30 min and 60 min in a laboratory vessel using an F2/N2 mixture. Attenuated total reflection infrared analyses and the observations of the cross section and the surface of the samples by SEM indicate the increases in degree of fluorination, thickness and compactness of the fluorinated layer, and the decrease in surface roughness, with treatment time increasing. Compared with the deep surface charge traps and stable surface charge of the unfluorinated (original) sample, as indicated by the open-circuit thermally stimulated discharge current measurement, the fluorinated surface cannot store the charge. The corona charges deposited on the sample surfaces fluorinated for 10 min, 30 min or 60 min rapidly decay to zero in about 2 min, 10 min or 15 mi at room temperature respectively, showing a slowed-down release of charge with fluorination time. The measurements of surface conductivity and contact angle and the calculation of surface energy reveal that fluorination gives rise to dramatic increases in surface conductivity, surface wettability and polarity, while they decrease with treatment time. The significant increase in surface conductivity of the fluorinated sample is attributed to a very likely substantial decrease in trap depth and the adsorbed water on the fluorinated surfac. Surface charging current measurements further show that large steady state current flows along the fluorinated surface during corona charging, in comparison with the almost zero steady state current for the original sample. This implies that the fluorinated sample has much lower surface charge accumulation in the period of charging, than the original sample.

Light Induced Curing (LIC) of Passivation Layers Deposited on Native Silicon Oxide

This work presents a novel insight to the aspects of silicon surface passivation and the influence of thin intermediate layers generated by chemically grown silicon oxides. Strong light induced effects on passivation properties are investigated. After exposure to light (0.25 suns) for about 60 s, samples based on a PECVD layer system consisting of SiNx and SiO2 deposited on crystalline silicon with native silicon oxides show an improvement of more than 100% in minority carrier lifetime. These improvements are stable over months and lead to effective surface recombination velocities as low as 10cm/s on chemically polished p-type FZ wafers. With the use of different light sources, corona charging and annealing experiments the effect is investigated in detail. Finally, the effect is proposed to be a photo induced curing process of defects in the Si/SiO2 interface with the incorporation of hydrogen.

Silicon Surface Passivation by Al2O3: Recombination Parameters and Inversion Layer Solar Cells

The interface between p- and n-type FZ-Si and an amorphous aluminum oxide (Al2O3) surface passivation layer deposited by plasma-assisted atomic layer deposition (ALD) was investigated by frequency-dependent conductance measurements. The hole capture cross section in the lower half of the bandgap, σp = (4±3)×10-16cm2, was found to be independent of energy. The electron capture cross section σn in the upper half of the bandgap decreases from σn = (7±4)×10-15cm2 at midgap over two orders of magnitude towards the conduction band edge. Numerical simulations of the effective surface recombination velocity based on these recombination parameters show a good agreement with experimental surface recombination velocities for a wide range of excess carrier and surface charge densities. Carrier transport in the inversion layer formed at the n-Si/Al2O3 interface was investigated yielding a sheet resistance of 15 kΩ/□, which was reduced to 6 kΩ/□ for a surface charge density of –2×1013cm2 obtained by corona charging. The applicability of Al2O3 inversion layers as emitters in n-type inversion layer solar cells was demonstrated by short circuit current densities of up to 25mA/cm2, which show a pronounced dependence on surface charge density.

Surface Passivation by Al2O3 and a-SiNx: H Films Deposited on Wet-Chemically Conditioned Si Surfaces

The surface passivation of p- and n-type silicon by different chemically grown SiO2 films (prepared by HNO3, H2SO4/H2O2 and HCl/H2O2 treatments) was investigated after PECVD of a-SiNx:H and ALD of Al2O3 capping films. The wet chemically grown SiO2 films were compared to thermally grown SiO2 and the surface passivation was benchmarked against single-layer a-SiNx:H and Al2O3 films deposited on HF-last Si surfaces. Directly after PECVD and ALD the surface passivation was found to be similar for all chemically grown SiO2 stacks and higher than for the single-layer films. After annealing at 400°C the level of surface passivation improved but remained similar for the chemically grown SiO2 stacks. Corona charging experiments on the annealed chemically grown SiO2/Al2O3 stacks revealed that the fixed charge density, i.e. the field-effect passivation, was lower compared to single-layer Al2O3 films whereas the chemical passivation was found to be higher. The surface passivation was also investigated as a function of the annealing time. Chemically grown SiO2 stacks achieved the maximum level of surface passivation after annealing for half a minute which is different from the stacks with a-SiNx:H and Al2O3 films deposited on thermally grown SiO2 or HF-last Si surfaces which required considerably longer annealing times.

Chromophore Poling in Thin Films of Organic Glasses. 2. Two-Electrode Corona Discharge Setup

Chromophore Poling in Thin Films of Organic Glasses. 2. Two-Electrode Corona Discharge Setup In Part 1 of the article we provided description of the corona discharge physics and overview of the methods used for corona poling in thin organic films. Subsequent sections describe comparatively simple technical methods for poling the organic nonlinear optical polymers using a two-electrode (point-to-plate or wire-to-plate) technique. The polarization build-up was studied by the DC positive corona method for poling the nonlinear optical (NLO) polymers. The experimental setup provides the corona discharge current from 0.5 μA up to 3 μA by applying 3 kV - 12 kV voltage to the corona electrode and makes possible selection among the types of corona electrodes (needle, multi-needle, wire, etc.). The results of experimental testing of the poling setup show that at fixed optimal operational parameters of poling - the sample orientation temperature and the discharge current - the corona charging of polymeric materials can successfully be performed applying the two-electrode technique. To study the dynamics of both poling and charge transport processes the three-electrode charging system - a corona triode - should be applied.

Impact of the Deposition and Annealing Temperature on the Silicon Surface Passivation of ALD Al2O3 Films

The effect of the deposition and annealing temperature on the surface passivation of atomic layer deposited Al2O3 films was investigated on n-type Cz silicon wafers. The deposition temperature was varied between 200 and 500̊C and the annealing temperature between 300 and 450̊C, respectively. Films prepared at 200 and 300̊C showed an improvement of surface passivation with increasing anneal temperature. The Al2O3 films grown at 400 and 500̊C did not improve by annealing. By corona charging experiments it was revealed that the improvement in surface passivation with increasing anneal temperature of films grown at 300̊C can be attributed to a significant increase in chemical passivation with a minor increase in field-effect passivation. For Cz and FZ wafers an identical surface passivation was achieved with the chemical passivation being lower for Cz wafers due to the surface morphology and the field-effect passivation being quite similar. Consequently the field-effect passivation was found to be the more important passivation mechanism.

Mechanical and electret properties of polypropylene unwoven fabrics reinforced with POSS for electret filter materials

A novel polypropylene/Polyhedral oligomeric silsesquioxanes (PP/POSS) composite unwoven fabric with permanent electret was prepared through melt-blown process with corona charging. Field-emission scanning electron microscope (FESEM) and wide-angle X-ray diffraction (WAXRD) were employed to investigate the morphology of the composite fibers and the distribution of POSS nanoparticles on the surface of the fibers, respectively. POSS acted as nucleating agent and accelerated the crystallization process during nonisothermal cooling. The utmost stable charge density of PP/POSS melt-blown unwoven fabric was improved by 78.4% compared with the neat PP unwoven fabric. The maximum value of collection efficiency measured by monodisperse polystyrene aerosol (PSL) (particle size: 0.3 μm) collection could reach 97.36% for PP/POSS composite melt-blown unwoven fabric, which improved by 9% compared with neat PP melt-blown unwoven fabric. Moreover, both stress and elongation at break of the PP/POSS melt-blown unwoven fabrics were improved compared with PP unwoven fabric.

Heat transfer at the condensation of a gas-vapor mixture in an electric field

The problems associated with the investigation of heat transfer processes at the condensation of vapor from a vapor-air mixture in the presence of an electric field have been considered. It is established that even very small additions of air impair vapor condensation and, hence, the heat transfer. It is shown that the application of an electric field to a vapor-air mixture can be an effective factor for eliminating the negative effects of the presence of air on the condensation process. It has been found out that a corona discharge specially created by means of notches on the surface of the inner electrode of a cylindrical system and its effects are the main reason for this. The experimental characteristics of the relative heat transfer coefficient at condensation as functions of the air concentration, the rate of the mixture’s delivery into the vapor condenser, and the specific heat flux at different electric field intensities have been obtained. The results are explained by the electric charging of the medium in the corona charge field, by the electric wind, and by the charging of the “vapor-liquid” interface. There has been derived a formula according to which the process of condensation is strongly influenced by the molecule effective diameter (ψ∼d*6) increasing in the electric field due to the solvation of ions by water molecules resulting in a new mechanism for the intensification of the condensation process in an electric field.

Nano-cluster-enhanced high-performance perfluoro-polymer electrets for energy harvesting

Development of high-performance electret materials is required to obtain a large power output of the electrostatic vibration-driven energy harvesters. In this study, by introducing aminosilane derivatives into CYTOP-based perfluorinated polymer film, we have successfully formed nano-clusters containing the organic siloxanes in the polymer electrets. Using small-angle x-ray scattering, tapping mode AFM and SEM analysis, the existence of such nano-clusters has been directly observed in the CYTOP film. It is suggested that the observed nano-clusters serve as the charge trap site and enhance the surface charge density and the thermal stability of the trapped charges. As a polymer electret, an extremely high surface charge potential of −1.6 kV with the 15 µm thick film has been obtained under the optimum condition of corona charging.

HOLOGRAPHIC RECORDING IN CORONA CHARGED ACRYLAMIDE-BASED MFI-ZEOLITE PHOTOPOLYMER

The influence of corona charging on holographic recording in acrylamide-based photopolymer nanocomposite containing MFI zeolite nanoparticles has been studied. The holographic recording was carried out in two different geometries — transmission grating recording and total internal reflection grating recording. During the recording process, the layers were charged in a corona field. It was observed that independently of the corona polarity, in the case of transmission geometry of recording, the corona charging led to a decrease in the diffraction efficiency (DE) of the grating. In the case of the total internal reflection grating, the DE increased in the corona field presence.

Aerosol filtration properties of PA6/PE islands-in-the-sea bicomponent spunbond web fibrillated by high-pressure water jets

The bicomponent spunbonding technology offers high productivity and provides great flexibility in the selection of polymer compositions and fiber configurations. This process was utilized to develop islands-in-the-sea (INS) fabrics consisting of 75% nylon-6 islands and 25% PE sea polymer. The number of islands was varied from 1 (sheath-core) to 108. These webs were hydroentangled at high pressure to cause fracture and fibrillation of the sea polymer and to “release” the islands to form hybrid micro and nanofiber structures. Physical properties as well as aerosol filtration characteristics of these webs were investigated to determine their feasibility for aerosol filtration. The submicron-sized dioctyl phthalate (DOP) particles were used as a challenging aerosol at face velocities in range of 3.3–11.7 cm/s. The filtration efficiency of the webs followed typical behavior for fibrous filtration media in which the most penetration particle size was in range of 0.2–0.3 micron. Surprisingly, the 1 INS web exhibited a higher quality factor compared with other INS webs due to its lower solidity. Also, it was found that corona charging improved the filtration efficiency of 1 INS web 4.3 times compared to discharged webs for 0.2 micron of DOP particles at the face velocity of 5.3 cm/s.

Numerical modelling of needle-grid electrodes for negative surface corona charging system

Surface potential decay measurement is a simple and low cost tool to examine electrical properties of insulation materials. During the corona charging stage, a needle-grid electrodes system is often used to achieve uniform charge distribution on the surface of the sample. In this paper, a model using COMSOL Multiphysics has been developed to simulate the gas discharge. A well-known hydrodynamic drift-diffusion model was used. The model consists of a set of continuity equations accounting for the movement, generation and loss of charge carriers (electrons, positive and negative ions) coupled with Poisson's equation to take into account the effect of space and surface charges on the electric field. Four models with the grid electrode in different positions and several mesh sizes are compared with a model that only has the needle electrode. The results for impulse current and surface charge density on the sample clearly show the effect of the extra grid electrode with various positions.

Influence of corona charging in cellular polyethylene film

Cellular polymers have recently attracted attention for their property of exhibiting a piezoelectric constant when they are electrically charged. The electrostatic charge generated in the voids by the internal discharges creates and internal macrodipole which is responsible for the piezoelectric effect. Charging by corona discharge is the most used method for cellular polymers. Many works has been published on polypropylene and polyethylene films mainly focused on the required expansion process or on the results obtained for raw cellular materials electrically activated. Our work is based on commercial polyethylene cellular films which have been physically characterized and electrically activated. The effect of thermal treatment, physical uniaxial or biaxial stretching and corona charging was investigated. The new method of corona charging improved the piezoelectric constant under other activation conditions.

Structure of rat skin after application of electret characterized by DSC

Polypropylene (PP) electrets with surface potential of −500V, −1000V and −2000V were prepared by constant voltage corona charging. The electrets were applied to excised rat skin for 2 hours respectively and then the skin samples were analyzed with the differential scanning calorimetry (DSC) technique to study the alteration of lipid organization of the skin. There were three peaks at 63°C, 82.7°C and 115.1°C in the DSC spectra for rat skin untreated, which have been assigned essentially to lipid, lipid-protein and protein alterations. For −500V electret treated-skin sample, only a single peak appeared at 79.1°C. With the increase of electret surface potential from −500V to −2000V, the transition temperature and peak areas at moderate decreased first and then increased. The negative electret could result in the transition of stratum corneum (SC) lipid from gel to liquid crystal and protein transition from α helix structure to β folding structure. The regulation action of electret to skin mircostructure presented an effect of "potential window".

Polarization and piezoelectricity in polymer films with artificial void structure

Laminated polymer-film systems with well-defined void structures were prepared from fluoroethylenepropylene (FEP) and polytetrafluoroethylene (PTFE) layers. First the PTFE films were patterned and then fusion-bonded with the FEP films. The laminates were subjected to either corona or contact charging in order to obtain the desired piezoelectricity. The build-up of the “macro-dipoles” in the laminated films was studied by recording the electric hysteresis loops. The resulting electro-mechanical properties were investigated by means of dielectric resonance spectroscopy (DRS) and direct measurements of the stress-strain relationship. Moreover, the thermal stability of the piezoelectric d33 coefficient was investigated at elevated temperatures and via thermally stimulated discharge (TSD) current measurements in short circuit. For 150 μm thick laminated films, consisting of one 25 μm thick PTFE layer, two 12.5 μm thick FEP layers, and a void of 100 μm height, the critical voltage necessary for the build-up of the “macro-dipoles” in the inner voids was approximately 1400 V, which agrees with the value calculated from the Paschen Law. A quasi-static piezoelectric d33 coefficient up to 300 pC/N was observed after corona charging. The mechanical properties of the film systems are highly anisotropic. At room temperature, the Young’s moduli of the laminated film system are around 0.37 MPa in the thickness direction and 274 MPa in the lateral direction, respectively. Using these values, the theoretical shape anisotropy ratio of the void was calculated, which agrees well with experimental observation. Compared with films that do not exhibit structural regularity, the laminates showed improved thermal stability of the d33 coefficients. The thermal stability of d33 can be further improved by pre-aging. E.g., the reduction of the d33 value in the sample pre-aged at 150°C for 5 h was less than 5% after annealing for 30 h at a temperature of 90°C.

Surface Potential Versus Electric Field Measurements Used to Characterize the Charging State of Nonwoven Fabrics

Surface potential and electric field measurement techniques are widely used for the investigation of the corona charging of dielectric surfaces in a wide range of industry applications. The aim of this paper is to estimate which of these techniques are the most appropriate for characterizing the charging state of nonwoven fibrous dielectrics and point out the “noise factors” that might distort the results of the measurements. The experiments were performed on samples of nonwoven polypropylene sheets, in contact with or at a well-defined distance from a grounded plane electrode. The effect of the variability of the position of the probes with respect to the samples was also investigated. Several recommendations have been formulated regarding the use of these techniques for monitoring the charging state of nonwoven fabrics in industry applications.

Improvement of the performance of the PECVD SiO2/Si3N4double-layer electrets

In this paper, multiple approaches were attempted to improve the performance of PECVD SiO2/Si3N4 double-layer electrets on glass, including different electrode structures, as well as various corona charging conditions. Both chargeability and charge stability of the PECVD SiO2/Si3N4 double-layer electret at high temperature and high humidity were investigated. It is proven that samples with both lower and back electrodes has simpler process but exhibits similar chargeability and charge stability to those having lower electrode contact with ground directly by welding lead. High charging temperature can increase chargeability but has no effect on charge stability melioration. Long-term observation implies that if the electrets have to work outside properly, heat treatment will be necessary, although hermetic packaging is the best option. PECVD SiO2/Si3N4 double-layer electrets can endure high temperature lower than 250°C and shows good charge stability under high humidity.

Dielectric-elastomer actuators deliver clean energy

Last year marked the 10th anniversary of the Stanford Research Institute’s landmark Science paper on deformable capacitors based on elastomeric insulators with highly compliant electrodes.1 It showed that attractive electrostatic forces between positive and negative charges on compliant electrodes lead to high-speed, giant-strain, electrically controlled actuators, which have now exceeded the performance of natural muscles in terms of strain, stress, and elastic-energy density. The perspectives of the field are discussed in a recent Science article,2 showing that such dielectric-elastomer actuators (or electroactive-polymer artificial muscle) have become areas of both intense academic research and industrial development. Electrically controlled optical lenses and phase gratings are already on the market,3 while a first mass-market application has been announced for mobile phones, providing users with vibrotactile feedback.4 Key factors promoting the success of the technology are the simple physics behind the actuation (attractive electrostatic forces acting on soft, deformable dielectrics) and its cost-effective implementation with inexpensive materials. The idea that a solid material deforms when stimulated by electricity originated in the late 18th Century, as documented in a letter by Alessandro Volta discussing observations of Felice Fontana on volume changes in electrified Leyden jars, the first electrical capacitors.2 In the late 19th Century, Wilhelm Conrad Rontgen designed an experiment, which is seen today as a beautiful description of the dielectric-elastomer-actuation principle.2 A rubber strip, when electrified by a corona discharge (which sprays opposite electrical charges on either side of the strip), shrinks in thickness and increases in length. Figure 1 shows a sketch of the experiment and a demonstration using simple household materials. We replicated Rontgen’s experiment with modern materials,5 showing that such charge-controlled actuation overcomes the electromechanical pull-in effect, i.e., the mechanical collapse of Figure 1. (top) Dielectric-elastomer-actuation principle anticipated by Rontgen in 1880 and (bottom) demonstration of the elongation of a stretched rubber strip after corona charging. HV: High voltage.

The study of thermal silicon dioxide electrets formed by corona discharge and rapid-thermal annealing

A silicon dioxide (SiO2) electret passivates the surface of crystalline silicon (Si) in two ways: (i) when annealed and hydrogenated, the SiO2–Si interface has a low density of interface states, offering few energy levels through which electrons and holes can recombine; and (ii) the electret’s quasipermanent charge repels carriers of the same polarity, preventing most from reaching the SiO2–Si interface and thereby limiting interface recombination. In this work, we engineer a charged thermal SiO2 electret on Si by depositing corona charge onto the surface of an oxide-coated Si wafer and subjecting the wafer to a rapid thermal anneal (RTA). We show that the surface-located corona charge is redistributed deeper into the oxide by the RTA. With 80 s of charging, and an RTA at 380 °C for 60 s, we measure an electret charge density of 5 × 1012 cm–2, above which no further benefit to surface passivation is attained. The procedure leads to a surface recombination velocity of less than 20 cm/s on 1 Ω-cm n-type Si, which is commensurate with the best passivation schemes employed on high-efficiency Si solar cells. In this paper, we introduce the method of SiO2 electret formation, analyze the relationship between charge density and interface recombination, and assess the redistribution of charge by the RTA.

Mechanical and electrical properties of laminated polytetrafluoroethylene films

Laminated polytetrafluoroethylene (PTFE) films, which are made of compact and porous PTFE layers, are prepared through the process of sintering. The corona charging technique is utilized to make the laminated PTFE films piezoelectric, thus transforming them into piezoelectrets. The crystallinity, Young's modulus in compression mode, stress-strain behavior, charge dynamics, and thermal stability of the fabricated films are investigated by particular techniques, such as differential scanning calorimetry (DSC), dielectric resonance spectra, dynamic mechanical analysis (DMA), thermally stimulated discharge (TSD) spectra, and isothermal annealing, respectively. The results reveal that the crystallinity of the fabricated PTFE films with three and five-layer systems are 79.5 and 59.8 %, respectively. The compressive and tensile moduli at room temperature are 7.4 and 167 MPa for the three-layer system samples. The resulting temperature peak increases by 20°C as the heating rate increases from 2 to 4°C in TSD measurement. Two charge drift mechanisms exist in the films when the samples are thermally stimulated and discharged. With the increase of corona charging voltage from -10 to -25 kV, more and more detrapped charges from the deeper traps in the laminated PTFE films are released, corresponding to the current peaks identified in the temperature range from 130 to 140°C, which prefer to drift through the solid PTFE layers. However, charges also escaped from the relevant shallow traps, corresponding to the current peaks identified in the temperature that range from 80 to 95°C. The charge drift along the surface of the PTFE fibers is always a dominant mechanism, showing resistance of the corona charging voltage under the experimental-study conditions. The sample shows a stable piezoelectric d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> coefficient of 50 pC/N at 120°C after one day annealing at the same temperature.