Decay Rate Of Surface Charge Research Articles

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.

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.

Understanding the electrical, thermal and mechanical properties of LDPE‐clay nanocomposites

The performance of low-density polyethylene (LDPE) clay nanocomposites was analysed. The inclusion of nano montmorillonite (MMT) clay in LDPE material has significantly increased the contact angle, corona ageing resistance, water droplet initiated corona inception voltage and surface discharge inception voltage of the composites. The surface charge decay rate of the samples significantly reduced on the inclusion of clay indicating modified trap distribution characteristics due to the inclusion of the filler. Dynamical mechanical analysis indicates increased storage modulus and reduced tan (δ) due to nanofillers inclusion. Laser-induced breakdown spectroscopy indicates that on inclusion of nanofillers the plasma temperature increases and crater depth decreases. In particular, increased discharge resistance, improved thermomechanical properties are observed with LDPE–MMT clay composites compared to pure LDPE.

Influence of surface charge decay on cavity PD frequency at DC voltage

In this study, two types of samples, i.e. a disc-shaped cavity embedded within solid dielectrics and the other on an electrode surface, were used to investigate partial discharge (PD) behaviours at DC voltage. It has been found that the discharge frequency of the sample with one metallic surface was much larger under a positive voltage than under a negative one, whereas it was not sensitive to voltage polarities for the sample with two insulating surfaces. In order to explain experimental results, a simulation model based on continuity equations was established to obtain PD sequences at DC voltage, in which surface charge decay and discharge time lag were taken into account. Simulation results showed that positive surface charges dissipated faster than the negative, and the discharge time lag kept almost unchanged at the same voltage amplitude. The discharge time interval consisted of field recovery time and discharge time lag, and the former was determined by surface charge decay rate. In terms of these, the effect of surface charge decay on discharge frequency was discussed. The research is helpful to clarify PD mechanism at DC voltage.

Surface modifications of polystyrene and their stability: A comparison of DBD plasma deposition and direct fluorination

Polystyrene (PS) as an industrialized insulating material is widely used in the pulsed power system, due to the good mechanical and electrical properties. However, the surface flashover of dielectric material limits the development of pulse power technology seriously. It is believed that the surface charge dynamics are related to the surface flashover closely. Therefore, in this paper, several surface modifications including plasma deposition assisted by dielectric barrier discharge (DBD) and direct fluorination (with/without O2) are utilized to enhance the surface electrical properties of PS. The surface charge dynamics characteristics before and after surface modifications are investigated. The experimental results show the decays of surface charges for DBD plasma deposition and oxyfluorination become faster significantly. The surface potential of PS after DBD plasma deposition and oxyfluorination decreases to around 0 V over 100 s. The FTIR spectra indicates that a large amount of polar hydrophilic groups are introduced on the surface of PS samples by DBD plasma deposition and oxyfluorination, which are conducive to the dissipation of surface charges. The decay rate of surface charge after fluorination treatment without O2 is also increased, which is associated with CF and COOH groups in the fluorination layer. Due to the rapid reduction of accumulated charges, the flashover voltage of all modified samples are increased by more than 30%. Furthermore, the stability of surface modification layer is compared. It is found that the stability of fluorination is better than oxyfluorination and DBD plasma deposition.

Investigation of cavity PD physical processes at DC voltage by simulation

In order to reveal partial discharge (PD) physical processes at DC voltage, a model with regard to PD taking place within a cavity embedded in polyethylene is established, in which the charge fluid equations and the Poisson equation are employed to simulate streamer propagation and surface charge accumulation processes after it arrives at the gas–solid interface. Meanwhile, parameters such as discharge current and discharge time during a PD sequence are obtained by considering surface charge decay and discharge time lag variation in the discharge time interval. It is found from the simulation results that the electrons generated by the discharge accumulate on the upper surface of the cavity and the time taken to accumulate is less that 1 ns when a positive voltage is applied to the upper electrode. On the contrary, positive ions accumulate on the lower surface, and the time taken is about 90 ns. Besides, the density of positive ions is slightly higher than that of the electrons. During a PD sequence, the decay of surface charges resulting from previous discharge can be considered to be the key factor contributing to the occurrence of subsequent one, and electric field within the cavity mainly depends on negative surface charges just before the subsequent PD takes place, because of the lower decay rate of the negative charges than the positive ones. The discharge time interval can be divided into two parts: discharge time lag and discharge recovery time, which depend on the surface charge decay rate. As surface charge decay rate increases or the discharge time lag decreases, the discharge time interval decreases. Moreover, the discharge current is affected by discharge time lag and the surface charge decay rate. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.

Surface charge behavior of silicone rubber/SiC composites with field-dependent conductivity

High temperature vulcanized (HTV) silicone rubber (SiR) is widely used as HVDC cable accessory insulation considering its outstanding electrical properties along with elasticity and thermal resistance. However surface charge caused by corona would accumulate on SiR surface and distort the local field which in consequence led to accessory premature discharge and insulation failure. This paper focused on the behavior of surface charge of SiR composites and managed to modify it by incorporating silicon carbide (SiC) particles into SiR matrix. The content of SiC varied from 0 to 100 % and its effects on the field-dependent conductivity and surface charge behaviors were discussed. The outcomes of experiments showed that the SiR/SiC composites performed a nonlinear conductivity as a function of electric field when the content of SiC exceeded 30 %. It was also noticed that this field-dependent conductivity became more obvious when the content of SiC increased. Meanwhile, the decay rate of surface charge was observed to rise with the increase of SiC content and voltage. It was confirmed that the SiR/SiC composites could accelerate the decay of surface charge and increase the dc conductivity under high voltage, resulting from the modification of the trap level distribution.

Surface potential decay of functional dielectrics after polarization

High voltage polarization domains in functional dielectrics and the ability to orient them with an external applied electric field lead to the development of a variety of applications of energy conversion from energy harvesting to information storage and reading. Voids are often generated at the bond interface during the production process of polymer films, which can break down easily in the process of polarization with high voltage, but the voids inside dielectric materials play an important role in charge storage and maintenance if the materials are oriented to the application of electrostatic generator, sensor, actuator and transducer etc. Meanwhile, a polymer matrix doped with a small amount of nanoparticles can suppress charge formation and increase surface charge decay rate. Results obtained in this paper show that polymer materials with microvoids have better maintenance properties of space charge compared with nanocomposite polymer materials. The research result is of important reference value and practical significance in the field of insulation and applications of functional dielectric materials.

Creeping Discharge Characteristics of Nanofluid-Impregnated Pressboards Under AC Stress

This paper presents experimental research on creeping discharge characteristics of TiO2 nanofluid-impregnated pressboards (NPs) under sustained ac voltage. It was found that NPs possess higher resistance to creeping discharge than that of pure oil-impregnated pressboard. The results of permittivity and surface charge accumulation and decay characteristics reveal that the modification of nanoparticles can not only make the electric field more uniformly distributed at the interface of nanofluid/pressboard due to a better matching in permittivity but also significantly increase the surface charge decay rate via a trapping and detrapping process in shallower traps. Thus, the creeping discharge characteristics of NPs are greatly improved by the modification of TiO2 nanoparticles.

Effects of AC and pulse voltage combination on surface charge accumulation and decay of epoxy resin

Epoxy resin is widely used in disc-type spacers of gas insulated switchgear (GIS). Surface flashover related to the existence of surface charge often occurs on the spacer, which is a great threat to the application of GIS and the safety of entire grid. Moreover, the influences on surface charge under pulse voltage produced by switch overvoltage, accompanying with the AC voltage, are still unclear. In order to research the comprehensive effect of AC and pulse voltage on surface charge, in this paper, epoxy resin sample was charged by a corona discharge system and the charge distribution was measured by an electrostatic voltmeter. The results are obtained from the research of different pulse amplitudes, pulse numbers and pulse polarities. The surface potential is a negative value after AC voltage applied for a period of time. When AC and negative pulse voltage are applied, smaller pulse voltage would push the original negative charges away. As a result, initial negative surface potential decreases. But bigger pulse voltage could increase the charge accumulation. When the pulse polarity becomes positive, surface potential at the sample center will be positive value at the beginning and then it gradually changes from positive to negative potential. In addition, the influence on surface charge decay rate will be enhanced with the increase of pulse number under the same pulse voltage. Trap energy distributions were used to analyze charge decay process in the last section.

Surface charge and dc flashover characteristics of direct-fluorinated SiR/SiO&lt;sub&gt;2&lt;/sub&gt; nanocomposites

Silicone rubber (SiR), as a basic insulating material, is widely used in insulators for HVDC transmission lines. Incorporation of nanoparticles could not only improve the insulating properties, but also alter surface charge behavior as a consequence. In HVDC transmission lines, corona discharge could even occur on well-designed insulators, which can inject charge into the insulator surface and influence dc flashover voltage. Direct fluorination is a method to change the surface chemical component of polymers, which could induce the corresponding changes in electrical properties of the surface layer, thus influencing surface charge behavior and dc flashover voltage. This paper tries to study the effects of fluorination time and mass fraction of nanoparticles on surface charge behavior and dc flashover voltage of SiR/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanocomposites. Samples were prepared by dispersing nano-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> into SiR matrix with the fraction of 0, 1, 3 and 5 wt% respectively. Then the direct fluorination of the samples were conducted in a stainless reaction vessel at 25 °C using a F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mixture gas with 12% F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> by volume at 0.05 MPa for 5, 10, 15 and 20 minutes. After corona discharge tests performed at room temperature with a relative humidity of ~ 40%, the decay curves of the surface potential were measured using an electrostatic voltmeter, and flashover tests were carried out under dc stress, of which the polarity was identical with the corona discharge. Obtained results show that the dc flashover voltage is affected by the surface charge, which varies with the fluorination time and mass fraction of nanoparticles. It is indicated that the direct fluorination can markedly improve the decay rate of surface charge, and raise the dc flashover voltage of SiR/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanocomposites.