Surface Charge Decay Research Articles

Heat-Excitation-Based Triboelectric Charge Promotion Strategy.

The surface charge decay is observed at high temperatures due to thermionic emission, which, however, may not be the only mechanism contributing to the surface charge variation. Here, a triboelectric charge promotion strategy due to the heat-excitation effect of hot electrons near the fermi level is demonstrated, while the final charge is determined by the balance between thermionic emission and the heat-excitation effect. It is demonstrated that metals with lower work function exhibit a better heat excitation capability, and polymers with lower fluorine content in molecule chains further boost the charge output, where metal/Kapton pairs demonstrated a charge promotion of over 2 times at the temperature of 383K with good durability during 90min measurement. The heat-excitation effect and charge durability in sliding freestanding-triboelectric-layer (SFT) mode triboelectric nanogenerator (TENG) is demonstrated as well, where the energy is promoted by over 3 times and the capacitor charging speed is doubled as well, with an energy promotion from 109.34 to 373µJ per cycle to successfully trigger a discharger. This work suggests a promising future of the heat-excitation effect as a new charge promotion strategy for TENG toward different applications in high-temperature environments.

Effect of dielectric material on the uniformity of nanosecond pulsed dielectric barrier discharge

Dielectric barrier discharge (DBD) is considered as a promising technique to produce large volume uniform plasma at atmospheric pressure, and the dielectric barrier layer between the electrodes plays a key role in the DBD processes and enhancing discharge uniformity. In this work, the uniformity and discharge characteristics of the nanosecond (ns) pulsed DBD with dielectric barrier layers made of alumina, quartz glass, polycarbonate (PC), and polypropylene (PP) are investigated via discharge image observation, voltage-current waveform measurement and optical emission spectral diagnosis. Through analyzing discharge image by gray value standard deviation method, the discharge uniformity is quantitatively calculated. The effects of the space electric field intensity, the electron density (N e), and the space reactive species on the uniformity are studied with quantifying the gap voltage U g and the discharge current I g, analyzing the recorded optical emission spectra, and simulating the temporal distribution of N e with a one-dimensional fluid model. It is found that as the relative permittivity of the dielectric materials increases, the space electric field intensity is enhanced, which results in a higher N e and electron temperature (T e). Therefore, an appropriate value of space electric field intensity can promote electron avalanches, resulting in uniform and stable plasma by the merging of electron avalanches. However, an excessive value of space electric field intensity leads to the aggregation of space charges and the distortion of the space electric field, which reduce the discharge uniformity. The surface roughness and the surface charge decay are measured to explain the influences of the surface properties and the second electron emission on the discharge uniformity. The results in this work give a comprehensive understanding of the effect of the dielectric materials on the DBD uniformity, and contribute to the selection of dielectric materials for DBD reactor and the realization of atmospheric pressure uniform, stable, and reactive plasma sources.

Unravelling the Ageing Effects of PDMS‐Based Triboelectric Nanogenerators

AbstractAgeing of elastomeric materials in triboelectric nanogenerators (TENGs) often leads to compromised electrical performance and can greatly affect their real‐world application as next‐generation energy harvesters and self‐powered sensors. Herein, the ageing behavior of PDMS‐based TENGs is investigated by probing the dielectric and mechanical properties of the membrane materials. Over time, ageing of PDMS after 17 months is evinced by a decline by 71%, 68% and 52% in open‐circuit voltage, short‐circuit current and charge transfer, respectively, and an increase by 6.8 times in surface charge decay rate. The reduced electrical performance can be attributed to a decrease in work function, dielectric constant, surface adhesion and heterogeneity in stiffness, as well as an increase in loss tangent. The effect of chemical chain scission on the PDMS surface is confirmed through nearfield infrared nanospectroscopy. This study gives insights into the underlying mechanism behind the ageing of PDMS‐based TENGs, paving the way to future work for ameliorating these ageing‐related issues with the aim to ensure long‐term stability of practical triboelectric devices.

Improvement of surface insulating performance for polytetrafluoroethylene film by atmospheric pressure plasma deposition

The surface flashover phenomenon across a vacuum-dielectric interface severely limits the service life and operational reliability of high voltage electrical equipment. Surface modification by atmospheric pressure plasma treatment is a promising method to improve the surface insulating performance of polymers. In order to explore the mechanism of plasma processing on the vacuum flashover characteristics of polymer materials, atmospheric pressure plasma deposition was used to treat polytetrafluoroethylene (PTFE) film. The surface parameters under different processing conditions, such as surface chemical composition, surface resistivity, surface charge decay and trap distribution, were tested and analyzed. The space charge distribution of PTFE and the flashover voltage in vacuum were measured. The results show that Si–O–Si and Si–OH groups are introduced on the surface of PTFE, and the characteristic peaks of PTFE are gradually weakened with the increase of processing time. The surface trap density increases and more traps with lower energy level arise with longer processing time. The plasma deposition changes the space charge distribution in PTFE body, and leads to positive charge accumulation inside the sample. The flashover field strength respectively increases by 15% and 70% in direct current (DC) voltage and microsecond pulse voltage after plasma deposition. The rapid dissipation of surface charge is the main reason for pulse flashover voltage enhancement, while the increase of surface leakage current due to lower surface resistivity and space charge accumulation in PTFE body make the DC flashover voltage reach the saturation point. Therefore the surface insulating and body performance of polymer materials after plasma modification processing should be considered comprehensively based on different applications.

All-organic modification coating prepared with large-scale atmospheric-pressure plasma for mitigating surface charge accumulation

The surface charge accumulation on polymers often leads to surface flashover. Current solutions are mainly based on the introduction of inorganic fillers. The high-cost process and low compatibility remain formidable challenges. Moreover, existing researches on all-organic insulation focus on capturing electrons, contrary to alleviating charge accumulation. Here, an all-organic modification coating was prepared on polystyrene (PS) with the large-scale atmospheric-pressure plasma, which exhibits outperformed function in mitigating surface charge accumulation. The surface charge dissipation rate and surface conductivity are promoted by about 1.37 and 9.45 times, respectively. Simulation and experimental results show that this all-organic modification coating has a smaller electron affinity potential compared with PS. The decrease of electron affinity potential may result in accelerated surface charge decay of PS, which has never been involved in previous works. Moreover, this coating also has good reliability in a repeated surface flashover. This facile and large-scale approach brings up a novel idea for surface charge regulation and the manufacture of advanced dielectric polymers.

Distribution and evolution of surface charges in nanosecond pulsed dielectric barrier discharge under the quiescent air and airflow

The surface charges in nanosecond pulsed dielectric barrier discharge under quiescent air and airflow are detected based on the Pockels effect of electro-optical crystals. In quiescent air, it is found that the surface charge spot propagates and moves in a certain direction due to the combination of the transverse electric field and the thermal accumulation during dozens of consecutive discharge cycles. However, the position of the surface charge spot remains fixed throughout a single discharge cycle (0.83 ms). At the same time, the noticeable decay of surface charges emerges in the above time scales. Furthermore, when the airflow is introduced into the discharge gap, the propagation and movement of surface charges are accelerated. With the increase of airflow velocity, the discharge transforms from a filamentary mode to a diffuse mode, and the distribution of surface charges varies from discrete to uniform. The transition point of the discharge state and charge distribution corresponds to the airflow velocity of 10 m s−1. The airflow accelerates the decay of surface charges, resulting in the shrink and dispersion of surface charges, which is considered to be the fundamental reason for the airflow’s potential to improve discharge uniformity. The inherent mechanism for achieving uniform discharge is revealed in this study.

Correspondence Between Charge Accumulated in Voids by Partial Discharges and Mapping of Surface Charge With PEA Detection

Partial discharge (PD) dynamics in gaseous inclusions are related to the memory effects that originate from surface charge accumulation. In this article, the correspondence between a deposited charge that is inside a void during the PD activity with the surface potential mapping and space charge quantity that are obtained through the pulsed electroacoustic (PEA) method was investigated. The experiments were performed in the configuration of an asymmetric void that is embedded in a polyethylene (PE) material. Since the air gap in a void blocks acoustic wave propagation, the measurements were focused on one dielectric void wall (opposite to the metallic electrode). It was discovered that the polarity of the last period in a PD sequence influences the polarity of a deposited charge. Two types of readout pulsers were investigated and compared with the PEA method. In the surface charge decay process, a faster rate for negative charges was noticed as compared with positive ones. The experimental results showed a good quantitative correspondence between a real charge that was deposited by a PD during the phase-controlled sequence with an accumulated charge value that was obtained from surface mapping as well as charge detection—both being performed by means of the PEA method on the same layer of a void wall. The presented experimental results and analysis help extend the insight into PD mechanisms and quantitative surface charge accumulations.

Dynamic quantification of the overall effect of dielectric polarization

Due to the significance and universality of dielectric polarization, a quantitative assessment of this phenomenon is crucial for the sake of whether to avoid or exploit it. While triboelectrification and electrostatic induction have a direct relationship with dielectric polarization, there has been no prior work on bridging triboelectrification and dielectric polarization. To investigate the dielectric polarization effect, a dielectric-adjustable nanogenerator is built and the interplay of dielectric constant, electrical output, and surface charge is thoroughly investigated. After confirming the occurrence of self-polarization and polarization reversal during device operation, the surface charge accumulation and decay times are studied to understand the impact of dielectric polarization on prolonging the charge decay time and shortening the charge accumulation time, which are responsible for the device electrical output. Consequently, the feasibility of using the dynamic behavior of surface charge to quantify dielectric polarization is systematically studied.

The surface charge decay: A theoretical and experimental analysis

The ability to retain localized charges at the surface or interface of dielectric materials is a universal property that applies to many different fields such as tribocharging, charge nanopatterning, nanoxerography, etc. Once the surface is charged, its stability and subsequent discharging rate will determine the potential applications of a given system. This decay rate is properly defined by the macroscopic equations which depend on dielectric constants and conductivities of the two media. Here, we derive the equations to model the decay of charge distributions localized at the surface/interface of materials and solve them avoiding additional approximations made so far. Addressing the problem in the Fourier space, we arrive to a compact generic equation which provides a useful tool to determine both the bulk and surface conductivities of a material. Furthermore, we show that Kelvin Probe Force Microscopy (KPFM) is a particularly suited method to exploit this tool. Monitoring the charge decay of previously injected charge patches on silicon dioxide (SiO2), together with an appropriated data analysis, we verify the behavior predicted by our equations. This allows us to characterize the surface and bulk conductivities of SiO2 layers as well as its dependence with the relative humidity.

Generation and decay of surface charge in a background charge environment during surface dielectric barrier discharges

Previously, we reported on the decay mechanism of the positive charge filaments generated by the first microdischarge during surface dielectric barrier discharges using a high-speed camera combined with the Pockels effect of a bismuth silicon oxide crystal. With the existing work, the next milestone is achieved by using this powerful approach to the generation and decay of surface charge in a background charge environment. It is found that the generation locations of new positive charge filaments are influenced by the surface charge memory effect. The background charge environment is an important factor in the decay of surface charge.

Measurements of surface charge dynamics and surface-breakdown characteristics of surface dielectric barrier discharges

The surface charge distribution in a surface dielectric barrier discharge driven by repetitive pulse bias superimposed on AC voltage is measured using the Pockels effect of an electro-optic crystal. The impact of surface charge on surface-breakdown characteristics is investigated by varying the phase of the pulse superimposition. It is demonstrated that the surface charge accumulation varies at different superimposition phases depending on the potential difference between the two electrodes. The accumulated positive/negative surface charge will facilitate the following surface discharge when the AC voltage polarity changes. In addition, different spatiotemporal characteristics of the surface charge distribution are presented when changing the polarity of superimposed pulses. Positive surface discharges are usually easier to develop than negative surface discharges due to their lower breakdown voltage caused by the accumulation of negative surface charges near the edges of exposed electrodes. The decay of positive surface charge is dominated by neutralization of negative surface charge and negatively charged particles (free electrons and negative ions) from the volume above the dielectric. There are two decay modes of positive surface charge: exponential decay and linear decay.

Nonlinear Conductivity and Space Charge Characteristics of SiC/Silicone Rubber Nanocomposites.

To achieve a preferable compatibility between liquid silicone rubber (LSR) and cable main insulation in a cable accessory, we developed SiC/LSR nanocomposites with a significantly higher conductivity nonlinearity than pure LSR, whilst representing a notable improvement in space charge characteristics. Space charge distributions in polarization/depolarization processes and surface potentials of SiC/LSR composites are analyzed to elucidate the percolation conductance and charge trapping mechanisms accounting for nonlinear conductivity and space charge suppression. It is verified that SiC/LSR composites with SiC content higher than 10 wt% represent an evident nonlinearity of electric conductivity as a function of the electric field strength. Space charge accumulations can be inhibited by filling SiC nanoparticles into LSR, as illustrated in both dielectric polarization and depolarization processes. Energy level and density of shallow traps increase significantly with SiC content, which accounts for expediting carrier hopping transport and surface charge decay. Finite-element multiphysics simulations demonstrate that nonlinear conductivity acquired by 20 wt% SiC/LSR nanocomposite could efficiently homogenize an electric field distributed in high-voltage direct current (HVDC) cable joints. Nonlinear conductivities and space charge characteristics of SiC/LSR composites discussed in this paper suggest a feasible modification strategy to improve insulation performances of direct current (DC) cable accessories.

Influence of shape effect on dynamic surface charge transport mechanism of cellular electret after corona discharge

Surface charge accumulation and transport on cellular polypropylene play an important role in nanogenerators, which could have a potential impact on energy harvesting and wearable devices for zero carbon energy systems and the internet of things. Different shapes have different charge accumulation and decay characteristics of the polymer. Therefore, we studied the influence of the sample’s shape on the surface charge decay by experiment and modeling. The surface potential of square and circular cellular polypropylene was measured by a two-dimensional surface potential measurement system with electrostatic capacitive probe. The experimental result shows that the surface potential distribution of the square sample dissipates non-uniformly from the bell shape to a one-sided collapsed shape, while that of the circular sample dissipates uniformly from the bell shape to the crater-like shape. Moreover, the simulated results of the initial surface potential distributions of the square and circular cellular polypropylene are consistent with the experimental results. The investigation demonstrates that the charge transport process is correlated with the shape of the sample, which provides significant reference for designing electret material used for highly efficient nanogenerators.

Synergetic optimization of charge transport and breakdown strength of epoxy nanocomposites: Realizing sandwich topological structure through constructing a SiC@SiO2/EP surface layer and m-BNNS/EP insert layer

Polymeric materials doped with wide-bandgap semiconductors exhibit intriguing nonlinear conductivity and rapid charge dissipation characteristics. However, excessive doping to accelerate charge dissipation occurs at the expense of breakdown strength. In this work, a sandwich topological structure is proposed to realize synergetic optimization of charge transport and breakdown strength. SiC nanoparticles decorated with insulation shell SiO2 were prepared, which served as fillers in the two surface layers of sandwich structured epoxy composites with exfoliated and functionalized m-BNNS doped into epoxy as the insert layer. The significantly improved breakdown strength approaches 65.8 kV/mm (44% higher than pristine epoxy resin) for sandwich structured composites due to local electric field redistribution while maintaining excellent charge dissipation ability owing to surface charge dissipation and charge decay along the bulk. Moreover, the dielectric loss still retains a low value of 0.02. These novel sandwich structured composites present promising potential in electrical and electronic insulation systems.

Decay of electrostatic charge on surfaces due solely to gas phase interactions

The electrostatic charge on surfaces can decay over time by bulk or surface conduction to electrical ground, or by charge transfer with ionic species in the surrounding gas phase. In general, these processes occur simultaneously, and it is not possible to disentangle their effects. Here, we study the time dependence of surface charge decay resulting only from interactions with the surrounding gas. Our methodology is based on the magnetic levitation of an electrostatically-charged sample surface to avoid any conductive path to ground. By measuring the surface charge over time, we find that the charge decays more slowly than predicted by previous models based on collisions with gaseous ions, and exhibits a different functional form for the time dependence. The slower decay is attributed to a depletion of ions in the vicinity of the charged surface, and the experimentally-observed charge decay is explained by a new model that considers a spatially-dependent ion velocity.

Distribution and evolution of surface charge in surface dielectric barrier discharge driven by AC and pulse dual power supply

The evolution of surface charge in surface dielectric barrier discharge (SDBD) is observed by using Pockels effect. SDBD is driven by sine AC and pulse dual-power supply voltage. The filamentary discharge and glow-like discharge are enhanced by superimposing positive pulse on sine trough and negative pulse on sine crest, respectively. The interval of enhanced discharge is adjusted by pulse repetition frequency (PRF). The formation and decay of surface charges are analyzed at low PRF, and the accumulation effect is analyzed at high PRF. The results showed that the decay rates of charges decrease with increasing distance from the exposed electrode. When a positive pulse is superimposed on sine trough, the traces of positive charges are filaments with long extending lengths, which are the footprints of discharge channels. The lifetime of positive charges is hundreds of AC cycles (tens of milliseconds). Under certain conditions, subsequent glow-like discharge evolves as ‘flying’ above the dielectric surface (three-dimensional (3D) propagation). Most of the negative charges are neutralized in subsequent filamentary discharge. Some negative charges accumulate downstream and exist longer than positive charges. In the case of negative pulses superimposed on sine crest, the enhanced glow-like discharge appears 3D propagation. The propagation distance is much smaller than that of positive pulse. Most of the negative charges are uniformly distributed near the exposed electrodes with a short lifetime (a few hundred microseconds) and are quickly neutralized in subsequent discharges. The occurrence of 3D propagation requires certain conditions and the mechanism needs further research.

Modeling and prediction of loading characteristics of electret filter media for PM2.5

Electret filter media are widely used to improve indoor air quality. However, their filtration performance may degrade severely as particles deposit, resulting in the increasing risk of human/instrument exposure to PM2.5 pollution. Therefore, it is of great significance to predict the performance evolution of electret filter media during particle loading to better meet practical applications. Herein, a novel model was developed to estimate and predict the loading characteristics of electret media, considering the charge screening effect due to particle deposition, the surface charge decay caused by conductance, and the particle deposition morphology affected by RH and particle hygroscopicity. The deposited particles were innovatively divided into homogeneous- and dendritic-deposition particles (HPs/DPs), and the two can be transformed into each other through the collapse of DPs and the growth of HPs. The humidity factor related to relative humidity (RH) and particle hygroscopicity was proposed to quantitatively describe the mutual conversion between DPs and HPs based on experimental results. In addition, a new calculation method of electrostatic efficiency considering deposition distance was proposed to describe screen effect. Furthermore, the dynamic charge decay of electret media caused by the deposition of conductive particles was modelled according to Kirchhoff's current law. The developed simple and efficient model had no adjustable parameter. It showed satisfactory predictive ability under various particle loading conditions in comparison with the results of experiments and references.

Charge Dissipation Behavior on the Insulating Polyimide Backsheet in Photovoltaic System

Deterioration of electric properties will be generated by the accumulation of surface charge on photovoltaic backsheet in the insulating system of photovoltaic (PV) system. The phenomena of potential induced degradation (PID) could be verified in the insulating Polyimide (PI) backsheet of PV module. In the field of photovoltaic system, PI has been found a special insulating backsheet that work in a mild condition. If the dynamic behaviors of the charge in the insulating backsheet could be understand, unexpected flashover events will be avoided. Thus, the mechanism of surface charge decay of PI should be explored under steady condition. The effect of accumulation of surface charge was explored on PI dielectric under constant ambient temperatures in this paper. The present investigation will contribute greatly toward the early potential induced degradation phenomenon of PV system.

Effect of Surface Charges on Flashover Voltage - An Examination Considering Charge Decay Rates

The effect of the position and amount of surface charges on cylindrical polytetrafluoroethylene (PTFE) insulators with different surface conductivities is studied. The surface potential decay property with time is investigated subjected to different electric fields. A simulation model for charge transport is established. The results show that the surface potential decays exponentially after surface treatment. The decay rate of surface charges increases with the surface conductivity. At t = 180 seconds, the decay rates of surface potentials for different surface conductivity are 2%, 42%, 60% and 84%, respectively. For samples with the same surface conductivity, the decay rates of surface potentials under different initial values are about 57%, and the decay rates at different accumulated locations are about 62%. It is demonstrated that the surface charge decay rate has little difference when initial charges with different amount and positions are introduced. However, the dissipation of surface charges can be promoted by applying a voltage of the same polarity. When the surface conductivity is increased to 5.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-16</sup> S, the potential decay rate under the applied voltage increases to 76%. Furthermore, the flashover voltage decreases with the increase of surface conductivity. As a result, the effect of homopolar surface charges on the flashover voltage is reduced.

Studies of the effect of temperature on charge accumulation in PVDF-PMMA double-layered thin films based on depolarization current measurements

In this paper, we have reported the interpretation of air gap (Thermally Stimulated Depolarization Current) of surface charges in PVDF-PMMA (Polyvinylidene fluoride (PVDF)–Polymethyl methacrylate (PMMA)) double-layered polymer thin films whose decay could not be observed in metalized electrets with short-circuit TSDC. Since short-circuit TSDC is caused by the relaxations of homo- and hetero-charge flow in one direction and thus makes it difficult to identify, separate, and characterize its components, therefore, an air gap was introduced to the short-circuit TSDC technique to carry out air-gap TSDC. This technique enables one to observe the orientation of dipoles, excess charge decay by ohmic conduction, and decay of surface charge. When a dielectric is charged by an application of an external field, two charges with opposite polarity and different nature can be found. Air-gap TSDC’s of double-layer samples revealed the presence of homo charge; charges trapped at the surface are due to dissipation of space charges thus, the depolarization current is observed to have the same polarity as that of the polarization current. Hetero-charge persists at high temperatures due to the bulk polarization formed because of the electric field created by the homo-charge. Hence, the depolarization current observed in the present study was observed to be of opposite polarity as that of polarization current. The above discussed polymeric layered structure was found to be a source of charge trapping which was identified and confirmed by various calculated parameters.