Space Charge Electric Field Research Articles

Effect of ground-state charge transfer on photoexcited charge transfer in transition metal dichalcogenide heterostructures

We report an experimental investigation on the effect of ground-state charge transfer and its induced electric field on photoexcited charge transfer in van der Waals heterostructures. Two heterostructure samples were fabricated by stacking an undoped WSe2 monolayer with either a Nb-doped or undoped MoSe2 monolayer. While no ground-state charge transfer is expected in the MoSe2/WSe2 heterostructure, the doped holes in the MoSe2:Nb/WSe2 heterostructure can transfer to WSe2, creating a space-charge electric field. By comparing the photoluminescence and time-resolved differential reflectance of the two heterostructures, we find that photoexcited hole transfer from MoSe2 to WSe2 is largely blocked by this field, whereas photoexcited electron transfer from WSe2 to MoSe2 is less affected. These results provide insight into the impact of doping on the charge-transfer performance of van der Waals heterostructures.

Ultrafast oscillation in a field emission-driven miniaturized gaseous diode

We report an ultrafast oscillation (up to ∼104 GHz) in the early stage of field emission-driven microdischarges. Spatiotemporal behaviors of electron density, space charge density, and electric field, exhibiting high-frequency oscillations, are demonstrated based on first-principle particle-in-cell/Monte Carlo collision simulations. Intermittent electron emission fluxes are identified from the electron phase space distributions whereas the ions are rather non-oscillatory in the transient timescale. The mechanisms of oscillation with growing amplitude are found to be related to the rapid modification of the field emission current affected by the space charge electric field, which is also accompanied by the fast response of the electron transport and ionization in a dynamic double-layer sheath. Further, a transport equation for emission current is solved to estimate the oscillation frequency, which agrees well with the simulation results. This study provides a more precise understanding of the formation of the field emission-driven microdischarge, which informs the design and optimization of miniaturized gaseous electronic devices.

Insight into the interface effect and PTC effect on the temperature-adaptive electrical conductivity of epoxy composite

Doping functional fillers into the polymeric matrix is an effective strategy to improve the electrical, thermal, and other performance of insulating materials. It is imperative to understand the influence of fillers on the charge carrier behavior to achieve better regulation effectiveness. In this work, micrometer-sized and nano-sized ceramic particles with positive temperature coefficient (PTC) electrical resistivity are employed to prepare the epoxy composites, whose electrical conductivity under different temperature and electric field, space charge characteristics, permittivity, and electric field distribution are studied. It is found that the doping of a PTC filler shifts the electrical conduction from bulk-controlled to electrode-limited, determining the quantity of charge carriers within epoxy composites. While the interface effect mainly affects the transport process of charge carriers, it would fail to dominate the electrical conduction since the abundant charge carrier introduced by the semiconductive functional filler. Combined with the reinforced interface effect, the electrical conductivity–temperature characteristic of the epoxy nanocomposite is optimized, leading to the reduction in the maximum electric field within electrical equipment insulation by 55%. These findings emphasize the synergistic regulation of charge carrier amount and transport, which contributes to the precision design of polymeric composites doped with functional fillers.

MATOQ: a Monte Carlo simulation of electron transport in environmental-friendly gas mixtures for Resistive Plate Chambers

The increasing interest in environmental-friendly gas mixtures for gaseous particle detectors, especially tetrafluoropropene-based gas mixtures for Resistive Plate Chambers (RPCs), has prompted in recent years the need for simulating electron transport coefficients and reaction rates in these mixtures. MATOQ is a Monte Carlo simulation program that calculates electron transport parameters, specifically designed for studying and optimizing environmental-friendly gas mixtures for RPCs. Unlike other existing codes, MATOQ allows for the simulation of electron avalanches by including the effect of space charge electric field, which can significantly impact the avalanche evolution in gaseous detectors such as RPCs. After the validation of the MATOQ simulation in the temporal and spatial growth configurations, we present the electron transport coefficients and the reaction rates in tetrafluoropropene-based gas mixtures, which may represent a valid alternative to the standard gas mixtures currently used for RPCs.

Study on the Partial Surface Discharge Process of Oil-Paper Insulated Transformer Bushing with Defective Condenser Layer

Oil-impregnated paper condenser transformer bushings are an important part of transformer equipment, and partial discharge (PD) occurred when defects exist on the condenser aluminum foil layers. Firstly, to study the PD process of the oil-paper insulated capacitance graded bushing with the defect of broken aluminum foil, a defective oil-paper bushing discharge sample is constructed to study the PD parameters and capacitance, and to discharge carbonization traces at different voltage levels. Then, in order to verify the process of condenser aluminum foil layer discharge and the space charge variation in the oil-paper insulation system of a sample model, the surface flashovers of a needle-plane discharge model based on the bipolar charge transport model and the hydrodynamic model was built. The simulation, by Transport of Diluted Species physics of COMSOL Multiphysics software, points out the discharge process of aluminum foil electrode caused by space charge action and electric field distortion under an electric field at different voltages. The results of simulation and sample bushing experiments showed that the PD process of the defective condenser foil layer is mainly divided into three stages: tip corona discharge, streamer in oil, and surface flashovers. The voltage amplitude is larger the more electrical branches are discharged and the shorter the discharge time is. The findings of the article have important implications for the discharge of the foil layer inside the oil-paper bushing.

Study on high-temperature and high-voltage insulation characteristics of polypropylene blend with highly packed elastomeric domains for power cable applications

In this paper, we report the high-temperature and high-voltage (HV) insulation characteristics of the polypropylene (PP) blend with highly packed elastomeric domains that showed excellent mechanical toughness in our previous study to find its suitability as insulation material for power cable applications. The main focus lies on tensile properties with thermal aging, thermomechanical behavior corresponding to the thermal transition, high-temperature volume resistivity and DC dielectric strength, and space charge accumulation behavior of the PP blend in comparison to the crosslinked polyethylene (XLPE). Our PP blend having low elastic modulus of approximately 266.9 MPa exhibited high tensile strength and elongation at break, and high-temperature stability as well. Using this PP blend, not only good HV insulating performances in volume resistivity and DC breakdown strength (BDS) were obtained similar to XLPE, but also adequate values were obtained at high temperature, even at 110 °C, which was not possible with XLPE. However, there was space charge accumulation and electric field distortion in the PP blend like XLPE, which needs to be suppressed. Therefore, our PP blend is expected to have the potential to replace existing XLPE as insulation materials for future HV power cables.

Interaction of spatial-Gaussian lasers with the magnetized CNTs in the presence of DC electric field and enhanced THz emission

The interaction of spatial-Gaussian lasers with magnetized carbon nanotubes (CNTs) in the presence of a DC electric field is shown to broaden the surface plasmon resonance effectively. The prime feature of the interaction of lasers with magnetized CNTs is that when the electron cylinder of CNT is shifted with respect to the ion cylinder, the space charge electric field is developed in the overlapped region and results in the restoration force of the electrons. The restoration force along with the ponderomotive and magnetic forces assist to increase the nonlinear current which is further enhanced by the drift velocity acquired by the electrons of CNTs due to the applied static electric field. This action is also responsible for the enhanced THz yield at surface plasmon resonance condition where is the polarization coefficient, is the relative permittivity of the dielectric substrate, is the cyclotron frequency of CNT electrons and is the plasma frequency. The study reveals that THz generation from the magnetized CNTs may be a promising source for the detection of dangerous chemicals like royal demolition explosive (RDX), pentaerythritol tetra-nitrate (PETN), trinitrotoluene (TNT), etc.

Investigation of temperature-dependent DC breakdown mechanism of EP/TiO2 nanocomposites

In dielectric science, the electrical breakdown strength of a polymeric material significantly decreases with elevated temperatures, which restricts the development of advanced electrical and electronic applications toward miniaturization. In the present study, to clarify the temperature-dependent DC breakdown mechanisms of epoxy resin (EP)/TiO2 nanocomposites, the effects of nanoparticle incorporation and temperature on charge transport and molecular chain dynamics were studied. The results indicate that space charge accumulation and electric field distortion are reduced by nanoparticle incorporation to enhance the deep trap level, while space charge accumulation, electric field distortion, and molecular displacement are all accelerated as temperature increases. To further investigate the influence of carrier traps and molecular chain dynamics on temperature-dependent breakdown, a DC breakdown simulation model that involves bipolar charge transport, molecular chain dynamics, and breakdown criterion equations is established. The calculated breakdown strengths of EP/TiO2 nanocomposites show great accordance with the experimental results, which indicates that temperature-dependent DC breakdown mechanisms are dominated by the synergetic effects of carrier traps and segment chain dynamics. Through the analysis of the breakdown model, a transition of the dominant mechanism (from segment chain to backbone dynamics) near the glass-transition temperature for DC breakdown of EP/TiO2 nanocomposites is discovered.

Investigation on effect of semiconducting screen on space charge behaviour of polypropylene‐based polymers for HVDC cables

High voltage direct current (HVDC) power transmission cable is critical for realising sustainability through renewable energy revolution. Eco-friendly thermoplastic polypropylene (PP)-based polymers/nanocomposites are regarded as promising candidates for replacing current thermoset crosslinked polyethylene (XLPE) cables. As an essential component of the extruded HVDC cable for improving conductor/insulation interface and suppressing charge injection to insulation at high DC electrical stresses, developing semiconducting (SC) screens that are compatible with PP-based insulation is of similar importance but has not been well studied yet. This work aims at designing PP-based semiconducting screens and investigating space charge behaviours of SC/PP/SC sandwich specimen to unfold the effect of semiconducting materials, bonding methods, applied DC electric field, and temperature on charge injection, accumulation, transportation, and dissipation in PP-based insulation. Although conventional thermal, mechanical, and low field electrical characterisations demonstrated that all of the developed semiconducting materials meet the performance criteria of commercial semiconducting materials, their space charge and local electric field distribution varied significantly at high DC fields. Compared with the traditional non-bonded configuration used at lab-scale, charge injection was enhanced in hot-pressed SC/PP/SC samples with tightly bonded interfaces, which better reflects the real situation in extruded cables. High temperature further intensified charge injections. Besides, our results also revealed that high temperature and electric field strongly influence charge mobilities and consequently their distribution and local electric field in PP-based insulations.

Research on electromagnetic environment of HVDC transmission line under sand and dust conditions

To study the impact of dust weather on the electromagnetic environment of the HVDC transmission line, this paper first analyzes the changes of transmission line corona field intensity under different dust conditions, and puts forward the prediction formula of transmission line corona field intensity under dust conditions. According to the Deutsch hypothesis and Kaptzov hypothesis, the calculation formula of ground synthetic electric field under sand and dust conditions is deduced, and the ±500 kV HVDC transmission line is analyzed and calculated. The results show that with the increase of dust particle size and dust concentration, the ground synthetic electric field shows a similar attenuation trend to the Halo electric field, which shows that the dust conditions mainly affect the ground synthetic electric field by affecting the space charge electric field. In the study of dust conditions on the line electromagnetic environment, coarse sand shows a greater impact, and the worse the dust weather intensity, the more obvious the decline of the ground synthetic electric field intensity.

Influence of Polarity Reversal Period and Temperature Gradient on Space Charge Evolution and Electric Field Distribution in HVDC Extruded Cable

During the operation of HVDC extruded cables, voltage polarity reversal (VPR) is considered one of the most severe conditions for cable insulation. In this paper, a bipolar charge transport model developed for cylindrical geometry is improved by introducing ionic carriers from impurity dissociation for the simulation of space charge and electric field in an HVDC extruded cable with thick polymeric insulation under VPR, and the construction of the geometric model is based on a practical 160 kV DC polymeric cable. The influence of polarity reversal period (PRP) and temperature gradient (TG), formed by the load current flowing through the conductor, on the space charge evolution and the electric field distribution are investigated. The mechanisms of the charge dynamics and the field distortion affected by the PRP and the TG are also discussed. The results show that under TG, the maximum transient field appears near the interface between the conductor shield and insulation in the early stage of complete VPR. In addition, the longer the PRP, the more serious the maximum transient field distortion. Moreover, an increase in TG intensifies the maximum field distortion under steady and transient states due to the enhancement of heterocharge accumulation.

Numerical calculation of transient electric field considering space charge in oil-paper insulation of converter transformers

PurposeThe purpose of this paper is to develop a numerical simulation method based on the transient upstream finite element method (FEM) and Schottky emission theory to reveal the distribution characteristics of space charge in oil-paper insulation.Design/methodology/approachThe main insulation medium of the converter transformer in high voltage direct current transmission is oil-paper insulation. However, the influence of space charge is difficult to be fully considered in the insulation design and simulation of converter transformers. To reveal the influence characteristics of the space charge, this paper proposes a numerical simulation method based on Schottky emission theory and the transient upstream FEM. This method considers the influence of factors, such as carrier mobility, carrier recombination coefficient, trap capture coefficient and diffusion coefficient on the basis of multi-physics field coupling calculation of the electric field and fluid field.FindingsA numerical simulation method considering multiple charge states is proposed for the space charge problem in oil-paper insulation. Meanwhile, a space charge measurement platform based on the electrostatic capacitance probe method for oil-paper insulation structure is built, and the effectiveness and accuracy of the numerical simulation method is verified.Originality/valueA variety of models are calculated and analyzed by the numerical simulation method in this paper, and the distribution characteristics of the space charge and total electric field in oil-paper insulation medium with single-layer, polarity reversal of plate voltage and double-layer are obtained. The research results of this paper have the guiding significance for the engineering application of oil-paper insulation and the optimal design of converter transformer insulation.

Atypical secondary electron emission yield curves of very thin SiO2 layers: Experiments and modeling

The secondary electron emission phenomenon often refers to the emission of electrons as a result of the interaction of impinging energetic electrons with the surface of a material. Although it is fairly well described for metals, with a typical shape of the total electron emission yield (TEEY) first increasing to reach a maximum and then decreasing along with the energy increase in the primary electrons, there is still a lack of data and detailed analysis for dielectrics, in particular thin layers. The present work proposes a new insight into the electron emission phenomenon from very thin dielectric layers. It reports on the TEEY from very thin SiO2 layers, less than 100 nm. It is found that a departure from the typical shape of the TEEY curve occurs for primary electrons with energy of around 1 keV. The TEEY curve presents a dip, a local minimum that might be as deep as below 1. This atypical shape depends substantially on the layer thickness. The measured TEEY is compared to an electron emission 1D-model in which we consider the combined effect of the space-charge electric field induced by trapped charges in the dielectric layer and of the processes of field-dependent conductivity and radiation-induced conductivity on the fate of secondary electrons. Those mechanisms govern the charge transport in the dielectric and, consequently, the electron emission. The effects of the SiO2 layer thickness, an incidence angle of the primary electrons, and an applied external electric field on the TEEY curves are reported.

Parameter regulation for periodic wave pair and photovoltaic soliton pair excitations in the bias photovoltaic-photorefractive crystal

In the bias photovoltaic-photorefractive crystal, the two-component nonlinear Schrödinger equation can portray the dynamics of periodic wave pair and photovoltaic soliton pair. These excitations are analytically revealed including the Jacobian sn-cn type and sn-dn type periodic wave pair and their related dark-bright soliton pair, the mixed elliptic function pair excitation and their related bi-peaked soliton and bright soliton pair. In view of these results, the formation conditions of these periodic wave and soliton pairs are related to parameters of the photovoltaic field, the space charge electric field, irradiance and nonlinearity. Moreover, the excitation regulation of soliton pairs including amplitudes, widths and the change of propagation constants based on the parameter related to the photovoltaic field and space charge electric field is investigated in the bias photovoltaic-photorefractive LiNbO3 crystal.

Temperature Gradient Affecting Electrical Tree in Silicone Rubber under Impulse Superimposed on DC Voltage

In this paper, the electrical tree growth characteristics of silicone rubber (SiR) are studied under impulse superimposed on DC voltage and a temperature gradient (TG) field. The temperature difference between high voltage (HV) and ground (GD) electrodes varies from -90 to 90 °C resulting in a TG field in the samples. The electrical tree morphology, length and accumulated damage are discussed. Experimental results show that with the increase of absolute value of TG, the electrical tree length and accumulated damage continue to increase, and the tree morphology turns from branch-like tree to bush-branch hybrid tree. The positive TG has a more obvious acceleration effect on the tree growth than does the negative TG. Under the TG, the temperature dependent carrier transport results in space charge accumulation and electric field distortion at low temperature side, which is the main reason for the different growth characteristics of electrical tree. Besides, the accumulated damage of DC superimposed with heteropolar impulse voltage is larger than that under DC superimposed with homopolar impulse voltage. It is concluded that the temperature gradient and impulse superimposed on DC voltage promote the electrical tree growth of HVDC cable accessories insulation.

Simulations and experiments of the generation apparatus of the DC electric field with the space charge

To make accurate measurements, field mills must be calibrated in a known DC electric field space charge in which the quantities can be control1ed. A simulation model of the generation apparatus of the electric field with a space charge has been built. Based on the combination of the electric field and the transfer of a dilute substances model, the influence of openings in the metal screen was calculated. Simulation results showed that the opening size of the high-voltage electrode has a great influence on the distribution of the electric field and ion density at the calibration zone. Next, a generation apparatus was designed and developed and ion mobility was researched. The experimental results showed that ion mobility decreased with an increase in electric field strength. However, ion mobility tends to be saturated when the electric field reaches a certain critical value. Two fitting formulas of ion mobility of positive and negative charge are proposed that can be used to predicate ion mobility in a high electric field.

Simulation of charge transport in polypropylene-based nano-composites

Polypropylene (PP)-based polymers are environmentally friendly and degradable materials that make them attractive alternatives to polyethylene for the electrical insulation systems of power cables in the future. To enhance the insulation performance of PP and to impede leakage currents through the material, different admixtures are utilized. In this paper, the effect of nano-fillers on charge transport characteristics in PP composites under DC electric fields is investigated by using a bipolar charge transport model accounting for mobile and trapped electrons and holes as well as ionic species. The model was validated by comparing the numerical results with the experimental data. The dynamic distributions of space charge densities and electric field in PP with and without nano-fillers are analyzed and compared. The numerical results confirmed that 0.5 phr ZnO nano-fillers can significantly improve the electric field distribution in PP nanocomposites and suppress the bulk current by reducing the charge generation rate and mobilities of charge carriers, thus resulting in a decrease in the volume conductivity. The obtained results provide a theoretical background for tailoring PP-based composites for specific applications by controlling nano-fillers.

Effects of Trapping Characteristics on Space Charge and Electric Field Distributions in HVDC Cable under Electrothermal Stress

Space charge behavior has a strong impact on the long-term operation reliability of high voltage–direct current (HVDC) cables. This study intended to reveal the effect of trap density and depth on the space charge and electric field evolution behavior in HVDC cable insulation under different load currents and voltages by combined numerical bipolar charge transport (BCT) and thermal field simulation. The results show that when the load current is 1800 A (normal value), the temperature difference between the inside and the outside of the insulation is 20 °C, space charge accumulation and electric field distortion become more serious with the increase in the trap depth (Et) from 0.80 to 1.20 eV for the trap densities (Nt) of 10 × 1019 and 80 × 1019 m−3, and become more serious with the increase in Nt from 10 × 1019 to 1000 × 1019 m−3 for Et = 0.94 eV. Simultaneously decreasing trap depth and trap density (such as Et = 0.80 eV, Nt = 10 × 1019 m−3) or increasing trap depth and trap density (such as Et = 1.20 eV, Nt = 1000 × 1019 m−3), space charge accumulation can be effectively suppressed along with capacitive electric field distribution for different load currents (1800 A, 2100 A and 2600 A) and voltages (320 kV and 592 kV). Furthermore, we can draw the conclusion that increasing bulk conduction current by simultaneously decreasing the trap depth and density or decreasing injection current from conductor by regulating the interface electric field via simultaneously increasing the trap depth and density can both effectively suppress space charge accumulations in HVDC cables. Thus, space charge and electric field can be readily regulated by the trap characteristics.

Bessel beam approach for photovoltaic trapping of micro- and nanoparticles on Fe-doped lithium niobate crystal

The non-diffracting Bessel beam approach was used for the first time for the elaboration of photovoltaic tweezers. We report the trapping of dielectric microparticles of chalk (CaCO3), as well as silver nanoparticles in glycerin suspension via dielectrophoretic forces on the surface of Fe doped LiNbO3 (LN:Fe) crystal with recorded volume holographic grating by the non-diffracting Bessel beam technique. The photorefractive Y-cut LN:Fe crystal was illuminated by 20–40 mW power Bessel beam at 532 nm wavelength which generates refractive Bessel lattice with ~40 μm periodicity via induction of photovoltaic space-charge fields. The Bessel beam approach provides the induction of high contrast 2D quasi-periodic distribution of space-charge electric field in the crystal volume and proximity of the crystal surface and high quality 2D patterning of the particles. Experiments showed that CaCO3 microparticles are trapped by attractive forces on the crystal surface in the areas of refractive index maxima of the Bessel lattice, while the trapping of Ag nanoparticles dispersed in glycerin takes places at the borderlines of lattice rings due to the repulsive forces. A physical model was developed to explain the experimental results. The photovoltaic approach of trapping and manipulation of micro- and nanoparticles is promising for applications in photonics, integrated optics and biotechnology.

High Temperature Insulation Materials for DC Cable Insulation — Part I: Space Charge and Conduction

Fundamental studies of potential candidates for DC electric power transmission in high temperature environment, including ETFE, FEP, PTFE, PI and PEEK, are carried out and presented in form of the series papers containing space charge and conduction as part I, partial discharge as part II, and the degradation and surface breakdown as part III. In this part, the space charge at 20kV/mm was measured at 25 °C and with a thermal gradient at 50 °C. The electrical conductivity was measured at electric fields ranging from 10kV/mm to 30kV/mm in temperatures ranging from 25 °C to 200 °C. The experimental results showed that considering the effect of thermal condition and electric field, FEP has the lowest total amount of space charge accumulation and electric field distortion among these materials at the measured conditions. PI and PEEK have the lowest amount of trap-controlled mobility at 25 °C due to deeper average trap level because of the aromatic rings in the structure. PTFE and PI have the lowest amount of thermal activation energy and temperature-dependent electrical conductivity due to the more uniform morphological phase comparing to ETFE, FEP, and PEEK. The outcomes of this paper serve as a benchmark for the fundamental researches over high temperature materials for DC applications and lay a basis for Part II and Part III.