Surface Potential Decay Research Articles

A Comparative Study of Gas-phase Fluorination and Nano-Al2O3 Doping on Space Charge Behavior and Trap Level in Epoxy Resin

In this work, the effect of gas-phase fluorination concurrent with nano-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> doping on space charge distribution, as well as trap level and density of epoxy resin, is investigated. Epoxy resins with different concentrations of Al2O3 nano-fillers (0, 1, 3, and 5 wt%) are prepared and fluorinated by a gas mixture of F2/N2 (20/80 v/v) for 30 min at 40 °C and 0.05 MPa. Fourier-transform infrared (FTIR) spectroscopy confirms the breakage of molecular-chain during the gas-phase fluorination process. The space charge behavior is observed by the pulsed electroacoustic (PEA) method before and after fluorination. PEA results indicate that 30-minute fluorination is useful against space charge injection into the bulk of epoxy resin/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanocomposites. The trap density and trap energy level distribution are determined by isothermal surface potential decay (ISPD) measurement. The results indicate that fluorination introduces shallow traps on the surface, which can increase the conductivity of surface, and, thus, the trap depth is considerably reduced after fluorination. Moreover, it is found that incorporating 1 wt% nano-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> particles into epoxy resin can level up the energy level of deep traps. However, further increasing the nano-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> concentration, e.g., 3 and 5 wt%, promises the overlap of interaction zones between polymer matrix and nanoparticles, reducing the trap energy level.

Plasma dielectric barrier discharge fluorination modified epoxy resin and its ageing behavior

The modification effect of plasma on the material will be weakened with the storage time, that is, it shows a certain timeliness, which limits further development of plasma modification technology. To explore ageing behavior of plasma dielectric barrier discharge (DBD) fluorinated epoxy resin, the surface fluorination of epoxy resin was realized by plasma dielectric barrier discharge. Scanning electron microscopy (SEM), surface profilometer, X-ray photoelectron spectroscopy (XPS), contact angle tester, high resistance meter, flashover voltage and surface potential testing system were used to characterize the physical morphology, chemical composition and electrical properties of epoxy resin surface before modification as well as being placed in 25 ℃ aging box for 0−30 d after modification. The experimental results show that fluoride grafting on the surface of epoxy resin is realized by DBD fluorination modification, which reduces the surface energy, surface resistivity and trap level of epoxy resin, thus speeds up the surface potential decay rate and increases the flashover voltage along the surface. After storage of 30 d, the fluorine content decreased, the surface energy increased, the attenuation rate of surface potential slowed down slightly, and the flashover voltage also decreased, but it is still higher than that of the untreated sample.

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.

Electrical and thermal performances of epoxy-based micro–nano hybrid composites at different electric fields and temperatures

This paper investigates the electrical and thermal properties of pure epoxy resin (EP) and its micro–nano hybrid composites (20 wt% micro-AlN fillers with 1 wt% and 3 wt% nano-Al2O3 fillers; 50% micro-AlN with 3% nano-Al2O3 fillers) for power electronic packaging applications. Electrical properties such as space charge distribution, electrical conductivity and surface potential decay are measured. The thermal performance of the fabricated samples is estimated using thermal analysis devices. The hybrid composite consisting of 20 wt% micro-AlN and 1 wt% nano-Al2O3 fillers exhibits the least space charge accumulation, higher thermal conductivity and better thermal stability. However, the excessive addition adversely affects space charge and electrical conductivity properties. The micro–nano hybrid composites significantly exhibit higher electrical conductivity than pure EP. The microfiller addition from 20 wt% to 50 wt% significantly improves the thermal conductivity of the EP. The reduced space charge injection and accumulation in the hybrid micro–nano composites are attributed to the enhancement of the injection barrier and reduction of the charge carrier traps in these materials. A theoretical mechanism of the charge dynamics inside the samples under different test conditions is proposed to support the experimental results.

Influence of thermally induced self‐assembly shish‐kebab crystal on charge transport behaviour in polypropylene/elastomer blends

This paper reports on the influence of thermally induced self-assembly shish-kebab crystal on charge transport behaviour in polypropylene (PP) and PP/propylene-based elastomer (PBE) blends. The film samples with the shish-kebab crystal were prepared by adding a β nucleating agent TMB-5 under thermally induced self-assembly. Polarised optical microscope （POM）, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements were performed to understand the crystallisation characteristics of samples. Carrier trap distribution was analysed by isothermal surface potential decay (ISPD) method, and DC breakdown strength measured as well. The decrease of shish-kebab crystal size, the reduction of trap level, the increase of shallow trap density and the decrease of deep trap density were obtained with the TMB-5 content. The hopping distance of charges decreased with the shallow trap density increasing. The DC breakdown strength for PP/PBE/TMB-5 was higher than that for PP/TMB-5. It is suggested that deep traps are formed on the clear shish-kebab crystal boundaries, while shallow traps are induced by the elastomer and the un-crystallisation nucleating agents. Both the carrier trap formation on the crystal boundary and the physical channel orientation of the shish-kebab crystals affect the charge transport behaviour.

Charge migration of multilayer oil paper on the process of partial discharge under AC voltage

As one of the main reasons for the deterioration of Nomex paper and charge migration, partial discharge (PD) plays a critical role during PD degradation. In this study, the PD characteristics of multilayer oil paper are focused on. In order to explore the characteristics of charge migration, the experiments of the relative permittivity, conductivity, isothermal surface potential decay, scanning electron microscopy and Fourier infrared spectroscopy have been carried out. The results show that the discharge branches of the second layer are two and three times longer than those in the first layer. In addition, the trap level of the first layer for PD degradation samples increases with the degree of PD degradation. The charges captured by deep traps provide seed charges for PD on the surface of the first layer. The charge transportation will accelerate the formation of shallow traps. The charge distribution and shallow traps contribute to the expansion of discharge branches.

Semolina extraction from wheat bran using an electro‐separation process

The present paper is aimed at studying the improvement of the semolina extraction from durum wheat grains through a recycling of wheat bran, as a by-product, issued from traditional milling and sieving. Abrasion of the wheat bran and then electro-statically separating obtained components allowed a recovery of a significant amount of semolina, reaching 50% of the treated mass. Three components were obtained after separation, namely, semolina, fine bran, and relatively big bran. The electrostatic characterization, including surface potential decay and charge measurement, showed a clear conductive character with a slight difference between components in terms of charge conservation. This electric conductive behavior of the wheat bran leads to choose an induction-type electrostatic separator to extract residual semolina from the grinding process. The design of experiments methodology was used to study the influence of the main parameters: the applied high voltage, the conveyor speed, and the speed of the rotating part. The process was optimized taking into consideration both the quantity and quality of the recovered semolina, targeting a high recovery rate with the best possible quality. The physical and chemical analysis showed a good quality of the recovered semolina with 1.1% ash content. Practical applications The wheat milling process in an industrial installation passes through several steps that are optimized to extract maximum semolina from grains. However, still a residual endosperm stays attached to the bran due to the process itself. Literature estimates wheat bran around 15% of the grain, which can be an important quantity taking into account the huge amount of cereal produced and consumed around the world. The suggested complementary process aims at the extraction of residual semolina attached to wheat bran through treating wheat bran by abrasion and later separate obtained components using and an electrostatic field. Obtained results, showed that treating wheat bran using the suggested method allows recovery of around 50% as semolina from the treated quantity. So, this reduces thrown wheat bran quantity to the half, which is important from an industrial point of view since it increases the production from the same wheat quantity.

Correlation of traps inside XLPE and polypropylene with space charge dynamics under polarity reversal condition

Polypropylene (PP) is being considered as a suitable alternative to the cross-linked polyethylene (XLPE) insulation as the latter is prone to the space charge accumulation when subjected to the voltage polarity reversals in line-commutated converter high voltage direct current transmission. This novel contribution correlates the space charge accumulation with the trap distribution inside XLPE and PP under the polarity reversal condition. The pulse electro-acoustic and surface potential decay methods are used to measure the space charge and trap distributions, respectively. These measurements are carried at different poling durations (1 and 6 h at each polarity) and cycles of voltage polarity reversal (one and three cycles) at the electric field of 60 kV/mm. The XLPE shows more homo and hetero charge accumulation than PP. Dominant shallow traps in the XLPE are responsible for the space charge accumulation, whereas deep traps with higher trap density inhibit space charge accumulation within the PP. Furthermore, the positive and negative threshold fields for the space charge accumulation are measured. It is observed that the space charge can be easily accumulated under the negative electric field. Interestingly, the space charge decays rapidly inside the XLPE with the increased number of polarity reversal cycle. A valid theoretical model is developed to explain the space charge accumulation inside the XLPE and PP. The field-assisted ionization plays a key role in the space charge accumulation inside the XLPE.

Characteristics and electrical properties of polyimide films fluorinated for different durations

Surface fluorination of polyimide films was conducted for different durations (30, 60, and 90 min) in a laboratory vessel using an F2/N2 gas mixture containing 12.5 % F2 by volume. The differences in the structural, morphology, wettability, surface free energy, and electrical properties of the fluorinated polyimide films were systematically studied to assess the effects of fluorination and fluorination time, and to investigate the relationship between the surface electrical properties and surface physicochemical properties. The obtained results indicated that fluorination cracked the film surface, with the number of cracks increasing with increasing fluorination time. Fluorination also enhanced the surface wettability and polarity. Surface conductivity and surface potential decay measurements consistently showed that the surface-fluorinated samples had a higher surface conductivity than unfluorinated samples, and that the surface potential rapidly decayed because the surface conductivity increased with enhanced water adsorption on the surface. The increased occurrence of surface cracks, as well as enhanced polarity and surface conductivity of the films, were mainly attributed to the increase in the degree of chain scission that occurred with increasing fluorination time.

Impact of gamma‐irradiation on space charge and charge trap characteristics of Al/epoxy nanocomposites

AbstractEpoxy nano Al composites were prepared using epoxyresin, hardener, and Al nanoparticles with required proportions and irradiated with gamma rays of required dosage. Surface potential measurements and space charge studies were employed to investigate the charge trapping properties of gamma‐irradiated specimens. The surface potential decay with DC voltage was sluggish for the gamma‐aged specimens irrespective of the polarity of the applied voltage. Trap distribution characteristics showed that trapped charge carriers occupy the energy range from 0.7 to 0.9 eV and revealed the existence of deep traps for the gamma‐irradiated samples. The mean lifetime and trap depth have followed the direct correlation. Space charge accumulation increased with the dosage of gamma irradiation but did not vary when space charge accumulation reached saturation. Space charge decay rate as well as saturated space charge density is higher for gamma‐aged specimens vs. unaged counterparts. Deep traps inhibited the space charge decay process. Permittivity and loss tangent measurements indicate that dielectric properties were altered with nanofiller inclusions and gamma irradiation.

Effect of polarity‐reversal voltage on charge accumulation and carrier mobility in silicone rubber/silicon carbide composites

Polarity-reversal voltage in high-voltage direct current transmission system would cause serious electric field distortion, accelerating charge injection. The effect of polarity reversal on charge transport characteristics in silicone rubber (SiR)/silicon carbide (SiC) composites with nonlinear conductivity at different temperatures is studied. The charge transport characteristics of silicone rubber/silicon carbide composites under unipolarity voltage and polarity-reversal voltage at 30°C, 70°C and 90°C are obtained by the isothermal surface potential decay method, and the carrier mobility and trap characteristics of the composites are further calculated. Studies have shown that a large amount of hetero-charge remains in silicone rubber materials under polarity-reversal voltage, which in turn leads to intensive charge neutralisation and severe electric field distortion. The silicon carbide doped composites exhibit excellent performance in suppressing the accumulation of residual charges and weakening the charge neutralisation under polarity-reversal voltage. In addition, high temperature enhances the conductivity of silicone rubber/silicon carbide composites and reduces the accumulation of residual charges, which restrains the distortion of the local electric field under polarity-reversal voltage. This research shows that the application of non-linear conductivity material is an effective measure to solve the problem of cable insulation under polarity-reversal voltage.

Effect of microcapsules doping on DC flashover and trap level of self-healing epoxy resin composites

Minor physical defects can decrease the dielectric efficiency of epoxy resin and severely threaten the electrical device’s dependability. Self-healing may be considered an effective method to preserve the electrical and mechanical properties of the epoxy resin. Besides, self-healing significantly decreases the influence of small physical destruction on the power system. In this research, epoxy resin incorporated with various concentration of microcapsules was prepared. The polyurea formaldehyde (PUF) microcapsules were treated using a silane coupling agent (KH550) to enhance dispersion. Moreover, the self-healing performance of the epoxy resin incorporated with microcapsules was analyzed by FTIR, SEM, and DC Flashover in air and vacuum. In addition, trap energy and trap density were obtained from measurements of surface potential decay (ISPD). The SEM results indicate that in comparison with pure epoxy resin, the microcapsule/epoxy resin composite has an improved self-healing performance. The DC Flashover of epoxy resin in air and vacuum was found relatively higher after incorporation of microcapsules than pure epoxy resin. The distribution of trap energy and trap density measured by surface potential decay measurements confirms that 5 wt% have deeper traps and the highest trap energy levels.

Traps energy distribution in Kapton HN polyimide films through surface potential decay model under humidity conditions

In this paper, an experimental study and analysis of the effects of environmental relative humidity (RH %) on the transport mechanisms of electrical charges in 50 μm thick Kapton HN polyimide (PI) films have been presented. The evolution of the surface potential decay with time after negative corona charge deposition have been investigated and analyzed by examining the tdV/dt characteristics to straightly obtain information about trap energy distribution in PI films for different relative humidity levels (35, 50, 70 and 90%). The calculating method of trap energy distribution is based on the double exponential fitting analysis of surface potential decay measurements. Two peaks were observed in the trap charge density distribution curve with trap levels, which is agreed with the presence of the shallow and deep traps. The results indicate that for different charging voltage levels, the surface potential decays faster in humid air (RH = 90%) than in relatively dry ones (RH = 35%). The recorded corona charging current confirms the interpretation of surface potential decay measurements. In addition, the density and depths of both shallow and deep traps change with charging voltage and relative humidity levels. Both shallow and deep trap distribution moves towards lower energy levels, indicating an increase in the film's electrical conductivity.

Effects of Temperature Gradient on Electrical Tree Initiation and Breakdown Phenomenon in XLPE Under Harmonic Superimposed DC Voltage

In this article, the temperature of the needle tip and the ground electrode were changed to obtain different temperature gradients. The harmonic superimposed DC voltage was also simultaneously applied to study the electrical tree initiation, growth and breakdown characteristics in cross-linked polyethylene (XLPE) under the temperature gradient. The tree inception voltage, tree structure, tree length, accumulated damage (AD) and breakdown time were used to analyze the electrical tree degradation process. It was found that the greater the temperature gradient, the easier it is for electrical trees to be initialed which is related to the different charge behaviors affected by the trap distribution indicated by surface potential decay (SPD) results at different temperatures. With the temperature gradient increasing, the electrical tree length increases firstly but then decreases slightly when the temperature of the ground electrode is fixed, but the electrical tree length and AD increase linearly with the temperature gradient increasing when the temperature of the needle tip is fixed. The temperature rise of the ground electrode has a more serious impact on the safety of the insulation compared with the temperature rise of the needle tip. Positive DC bias voltage is more likely to cause damage and breakdown than negative DC bias voltage under the temperature gradient. These results indicate that the temperature gradient and harmonic superimposed DC voltage will promote the electrical growth in cable insulation, so special attention should be paid to the safety of electrical equipment under such special conditions.

Effects of ambient humidity and thermal aging on properties of Nomex insulation in mining dry‐type transformer

Temperature and humidity are the main factors that cause the decomposition of Nomex paper widely used as turn-to-turn insulation material of mining dry-type transformers. In this study, for understanding the effect of ambient humidity and thermal aging on the properties of Nomex paper, specimens with the initial moisture content of 7% were aged in the oven for 168, 504, 672 h at 180°C, respectively. After thermal aging, corona discharge experiments of unaged and aged specimens with different aging degrees were carried out at different relative humidity (RH) levels from 10 to 70%. The characteristic parameters of corona discharge, such as the average discharge amplitude and the number of discharges, were measured. Then, the surface conductivity and the surface potential decay rate were measured. The morphology and attenuated total internal reflectance Fourier transform infrared spectroscopy were also analysed for explaining the difference of properties between unaged and aged specimens. The experiment results indicate that the average discharge amplitude decreases with the increase of RH or the deepening of the aging degree of specimens, but the number of discharges shows the reversal tendency. The changes in the physical structure and chemical composition, which is induced by the hydrolysis reaction of Nomex paper, accelerate the surface potential decay and increase the surface conductivity.

On the crosslinking temperature dependence of charge transport behavior in XLPE film deduced from surface potential decay measurements

The effect of crosslinking temperature on charge transport behavior in crosslinked polyethylene (XLPE) film has been investigated by means of surface potential decay measurements. 200 μm-thick XLPE specimens were prepared by hot-molding low density polyethylene (LDPE) by adding dicumyl peroxide (DCP) as crosslinking agent and phenolic antioxidant with temperatures ranging from 150 to 180 °C. Surface potentials on corona-charged samples were monitored by using a Kelvin type probe equipped with an electrostatic voltmeter, based on which trap distribution, effective charge mobility and field dependent conductivity were estimated. Fourier Transform Infrared (FTIR) spectrum and Differential Scanning Calorimeter (DSC) analysis were employed to examine the change in chemical structure and crystal morphology of test samples. Results obtained show that the charge transport dynamics is cross-linking temperature and polarity dependent. With the rise of the crosslinking temperature, the mobility of positive charge increases monotonously from 1.05 × 10−16 m2V−1s−1 to 5.42 × 10−15 m2V−1s−1, whereas the mobility of negative charge shows a fall-and-rise tendency that a minimum value of 1 × 10−16 m2V−1s−1 appears at 160 °C. A single trap center for electron is observed at any test sample. However, as regards to hole, a double-trap-center feature tends to appear as the temperature exceeds 160 °C. Moreover, with the increase of the crosslinking temperature, the conductivity of sample becomes field dependent. It is suggested that the variation of charge transport behavior should be ascribed to the presence of by-products as well as the crystallinity change caused by the temperature.

DC Breakdown and Surface Potential Behavior of Epoxy/Al2O3 Nanocomposites at Cryogenic Temperature

For high temperature superconducting (HTS) dc cables, one key issue is the necessity of using polymer dielectrics with higher dielectric strength at cryogenic temperature in liquid nitrogen. Nanocomposites have played an important role in improving the dielectric properties of epoxy resin (EP). This article investigates on the dc breakdown strength and the surface potential behavior of the epoxy/Al2O3 nanocomposites (0, 1, 3, 5 wt%) at the room and cryogenic temperatures. Moreover, the trap distributions are analyzed from the surface potential decay (SPD) measurement. The possible effect of the trap distributions on the dc breakdown strength is discussed with the valid results. The measurements show that the dc breakdown strength and the initial surface potential continually increased with the nanoparticle addition up to 3 wt% at both room and cryogenic temperatures, and decreased thereafter. The enhancement of the breakdown strength and the initial potential is also found at cryogenic temperature. Besides, the addition of the nanoparticles decreased the rate of SPD compared with the pure epoxy. It is proposed that the higher trap density and trap level enhances the dc breakdown strength. Interestingly, under the cryogenic environment, the nanoparticles addition significantly affects the trap distributions, which further influences the dielectric strength. The enhanced dc breakdown strength is attributed to the reduced space charge formation, the charge carrier mobility and the energy of the charge carriers within the sample bulk.

Enhancement of DC Flashover of Liquid Silicone Rubber by Direct Fluorination

Liquid silicone rubber (LSR) sheet samples of a commercial formulation and surface fluorinated 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 at various temperatures are studied in this work. The intrinsic DC flashover of the surface fluorinated samples is evaluated using a sheet sample/finger electrode configuration and under a stepwise increasing voltage. The tests show an increase up to 64% in the intrinsic DC flashover voltage of the LSR by the fluorination and an influence on the flashover on the fluorination temperature. ATR-IR and XPS analyses indicate that there are Si-F bonds as well as C-F bonds in the fluorinated layers. SEM shows the thickness of the fluorinated surface layers with nanostructured surfaces. Surface conductivity and potential decay measurements reveal that the fluorination caused a large increase in surface conduction, in association with fluorination temperature. The improvement in the intrinsic DC flashover is mainly attributed to the suppression of surface charge accumulation on the LSR by the increased surface conduction.

Study on water droplet initiated discharges and charge trap characteristics of laser textured silicone rubber nanocomposites

AbstractSilicone rubber micro‐nanocomposites were fabricated and laser texturing was performed on their surface to investigate super hydrophobic properties. Irrespective of type of texture (square, hexagon or diamond), the contact angle is almost the same but a marginal increase in contact angle is observed with increase in wt% of nano fillers in silicone rubber. Water droplet initiated corona inception voltage (CIV) is measured by fluorescence technique and the CIV tends to increase with texturing and is high under—DC followed with +DC and AC voltages. Fast Fourier Transform (FFT) analysis of the fluorescence signal due to corona discharge indicates that the energy content of the fluorescent signal lies in the range DC to 20 MHz. Surface temperature of the sample after water droplet initiated discharge is found to be high under DC rather than under AC voltages. Textured surface has a relatively lower temperature decay rate under AC and DC voltages profiles, compared to nontextured surface. The formation of shallow charge traps is predominant with the textured specimens. In general, the diamond and hexagonal micro textured surface specimen exhibit better performance in terms of water droplet initiated CIV, hydrophobicity, and surface potential decay characteristics.

Charge transport behavior in gamma‐ray irradiated poly(ethylene terephthalate) estimated by surface potential decay

Charge transport behavior in gamma‐ray irradiated poly(ethylene terephthalate) estimated by surface potential decay