Critical Electric Field Strength Research Articles

Analysis of the flow behavior of electrorheological fluids containing polypyrrole nanoparticles or polypyrrole/silica nanocomposite particles

A four-parameter model (Seo-Seo model) was used to analyze the flow behavior of some electrorheological (ER) fluids containing polypyrrole (PPy) nanoparticles, nanocomposite particles of conductive polypyrrole confined in mesoporous silica (MCM-41), and core-shell-structured SiO2/polypyrrole nanoparticles. The static yield stress predictions by the model were compared with the experimental data and dynamic yield stress obtained from the Bingham model and/or Cho-Choi-Jhon (CCJ) model. The static yield stress values were larger than the dynamic yield stress values. It was also found that the static yield stress of the polypyrrole suspension had a quadratic dependence on the electric field strength as predicted by the electric polarization model whereas those of the nanocomposite suspensions showed 1.5 power-law dependency. A master curve describing the yield stress data dependence on the electric field strength was obtained using a single-parameter scaling function to interpret the underlying mechanism of ER activity. A simple method for evaluating the activity mechanism criterion has been proposed and applied to the ER response of those three kinds of suspension. The results show that the critical electric field strength should be checked before the conduction mechanism is asserted, even if the yield stress plot shows 1.5 power-law dependence on the electric field strength.

Fabrication and Characterisation of Silicide/3C-SiC/Si Contacts for Schottky Barrier Diode Application

Silicon carbide is a wide band gap semiconductor which presents excellent physical properties such as a high critical electric field strength which enables the realisation of efficient power devices [1]. The cubic silicon carbide (3C-SiC) polytype has been less investigated than the more popular hexagonal structure 4H-SiC. 3C-SiC has the advantage that it can be epitaxially grown on Si by chemical vapour deposition (CVD) [2] and presents properties suitable for achieving cost effective SiC power devices in the range 650V to 1200V. These 3C-SiC/Si substrates offer a promising alternative to the costly hexagonal SiC (4H-SiC) wafers and can also be used as template for the epitaxy of cubic GaN [3]. However, the presence of the Si substrate does limit the temperature available to anneal out the implantation damage generated by some device processing sequences.This work investigates the properties of metal silicides as a potential Schottky contacts for high blocking voltage 3C-SiC/Si Schottky barrier diodes (SBDs). For this study, Ti and Pt silicide contacts are formed on the (100)3C-SiC surface. The influence of the silicide formation temperature (500oC-650oC) on the contact electrical properties is investigated in detail. Current voltage (I-V) and capacitance voltage characteristics (C-V) are used to evaluate the reverse leakage current and extract the Schottky barrier height as well as the concentration of electrically active dopants in the N type 3C-SiC/Si. The Pt silicide Schottky contact is found to present higher energy barrier height than the Ti silicide Schottky contact and this trend was confirmed by the associated improved reverse current in the Pt Schottky. This is a promising result as it suggests that the reverse leakage is not dominated by the stacking faults inherent to this material. Temperature dependent I-V characteristics are measured and compared to theoretical I-V curves to explore the mechanisms responsible of the reverse leakage. The effort towards high blocking voltage final SBD device requires excellent PN junction edge termination. In this work, to avoid the damage caused by Al ion implantation, the electrical properties of 3C-SiC/Si PN junction diodes fabricated by epitaxy of P type 3C-SiC layers on N type 3C-SiC/Si substrates are also investigated. The reverse current mechanisms in the PN junction and the Schottky are compared and the suitability of the PN for device termination is discussed.[1] Kimoto, T., Cooper, J.A. Fundamentals of Silicon Carbide Technology: Growth, Characterization, Devices and Applications, Wiley, (2014)[2] A. Severino, C. Locke, R. Anzalone, et al., ECS Transactions, 35 (6), 99 (2011)[3] Okumura, H., Ohta, K., Feuillet, G., et al. Journal of Crystal Growth, 178 (1-2), 113, (1997)

Prediction of reduced critical electric field strength of hot dry air in the temperature range 300–5000 K at 0.1 MPa for medium-voltage switchgear

The dielectric breakdown properties of dry air, which is widely used as the insulation gas in switchgears of electric distribution systems, are determined for dry-air switchgear analysis and design. In this study, a two-term Boltzmann analysis was applied to investigate the dielectric breakdown properties of dry air with increasing temperatures (300–5000 K) at atmospheric pressure (0.1 MPa). According to the changes in the component ratios of dry air, the collision cross sections of high-mole-fraction decomposition species were considered. In order to analyze the contribution of each dry-air decomposition species to dielectric properties, we compared the dielectric properties calculated with and without considering small amounts of the species, such as Ar and CO2, in actual dry air. The results show that reliable calculations can be obtained for dry air at atmospheric pressure and temperatures up to 5000 K only by considering species with mole fractions exceeding 10−2 in the composition. Based on the calculations, it is observed that the reduced critical dielectric strength of dry air does not significantly change for temperatures up to 2000 K. This means that the dielectric properties of dry air show temperature independence over a wider temperature range than those of SF6.

Effect of moderate DC electric field on formation of surfactant-laden drops

The continuous formation of mm-scale surfactant-laden aqueous drops in oil was studied under an applied DC electric field of 0–267 kV/m. The parameters varied included the flow rate of dispersed phase, the electric field strength, the viscosities and densities of dispersed and continuous phase and the surfactant type (anionic or cationic) and concentration. An increase in the electric field strength resulted in a decrease of the drop size due to increasing stresses on the drop interface. The size reduction continued until transition to the irregular regime. The values of critical electric field strength for this transition increased with an increase in both the flow rate and the viscosity ratio between the dispersed and continuous phase. The critical electric field strength and the size of the drops formed in the periodic dripping regime were independent of the electric field polarity. The effect of the electric field increased with a decrease of interfacial tension and density difference between the dispersed and continuous phase.

Investigation of the performance of CF3I/c-C4F8/N2 and CF3I/c-C4F8/CO2 gas mixtures from electron transport parameters

CF3I gas mixtures have attracted considerable attention as potential environmentally-friendly alternatives to SF6 gas, owing to their excellent insulating performance. This paper attempts to study the CF3I ternary gas mixtures with c-C4F8 and buffer gases N2 and CO2 by considering dielectric strength from electron transport parameters based on the Boltzmann method and synergistic effect analysis, compared with SF6 gas mixtures. The results confirm that the critical electric field strength of CF3I/c-C4F8/70 % CO2 is greater than that of 30% SF6/70% CO2 when the CF3I content is greater than 17%. Moreover, a higher content of c-C4F8 decreases the sensitivity of gas mixtures to an electric field, and this phenomenon is more obvious in CF3I/c-C4F8/CO2 gas mixtures. The synergistic effects for CF3I/c-C4F8/70 % N2 were most obvious when the c-C4F8 content was approximately 20%, and for CF3I/c-C4F8/70 % CO2 when the c-C4F8 content was approximately 10%. On the basis of this research, CF3I/c-C4F8/70 % N2 shows better insulation performance when the c-C4F8 content is in the 15%–20% range. For CF3I/c-C4F8/70 % CO2, when the c-C4F8 content is in the 10%–15% range, the gas mixtures have excellent performance. Hence, these gas systems might be used as alternative gas mixtures to SF6 in high-voltage equipment.

Positive synergy of SF6 and HFO1234ze(E)

We measured mixtures of SF 6 and HFO1234ze(E) (C 3 H 2 F 4 ) and determined the density-reduced critical electric field strength (E/N) crit in a pulsed Townsend experiment. HFO1234ze(E) is an environmentally friendly alternative to SF 6 since contrary to SF 6 , its global warming potential is negligible. While SF 6 is unsurpassed in its efficiency of attaching thermal electrons, HFO1234ze(E) appears to moderate electron energies very efficiently. Electron attachment to HFO1234ze(E), on the other hand, is much weaker. We find a positive synergism in SF 6 /HFO1234ze(E) mixtures in the sense that the mixture of both compounds has a higher electric field strength than each gas separately. A maximum (E/N) crit of 425 Td is reached for a mixture of 40% SF 6 and 60% HFO1234ze(E). A mixture of 90% HFO1234ze(E) and 10% SF 6 has a critical electric field strength of 360 Td, similar to pure SF 6 . We attribute this effect to the high efficiency of HFO1234ze(E) in moderating down the electron energies, which enhances electron attachment to SF 6 and reduces electron impact ionization.

Effect of pressure on repetitive performance of a corona-stabilized plasma closing switch

This paper reports on research to determine the effect of gas pressure on the repetitive performance of a corona-stabilized plasma closing switch. The influence of gas pressure on the withstand voltage recovery and the after-spark channel decay of a corona-stabilized switch is experimentally and theoretically investigated. The voltage recovery degree vs time curves and the development of discharge plasma channel after breakdown under various pressures were measured to indicate the repetitive performance of switch. A theoretical analysis based on propagation characteristics of corona discharge critical volume is applied to explain the experimental results. The critical breakdown electric field strength and corona critical volume under different pressures were calculated and their effect on the re-breakdown process were discussed. An improved one-dimensional gas thermo-hydrodynamic model was applied to qualitatively investigate the influence of the pressure on neutral gas density evolution over time. The radial distribution of neutral gas density at different recovery time intervals was calculated under various pressures. The results show that a corona-stabilized switch with a higher pressure presents a higher withstand voltage and a better repetitive performance. The repetitve performance of corona-stabilized switches deteriorates with the decrease in pressure.

Reversibility of membrane permeabilization upon pulsed electric field treatment in Lactobacillus plantarum WCFS1

Pulsed electric field (PEF) treatment, or electroporation, can be used to load molecules into cells. The permeabilizing effect of the PEF treatment on the cellular membrane can be either reversible or irreversible depending on the severity of the PEF treatment conditions. The influence of PEF on the reversibility of membrane permeabilization in Lactobacillus plantarum WCFS1 by two different fluorescent staining methods was investigated in this study. Whereas staining with propidium iodide (PI) before and after PEF treatment indicated small reversible permeabilized fractions of maximum 14%, the use of a double staining method with PI and SYTOX Green suggested larger reversible permeabilized fractions up to 40% of the population. This difference shows that the choice for a fluorescent staining method affects the conclusions drawn regarding reversibility of membrane permeabilization. Additionally, the effect of PEF treatment conditions on membrane integrity was compared, indicating a relation between critical electric field strength, cell size and membrane permeabilization. Overall this study showed the possibilities and limitations of fluorescent membrane integrity staining methods for PEF studies.

An improved method for fast evaluating arc quenching performance of a gas based on 1D arc decaying model

The evaluation of an arc quenching medium in circuit breakers usually requires the experimental investigation or the 2D or 3D magnetohydrodynamics simulation of switching arcs, which are expensive and time-consuming. In this work, a fast method is proposed for evaluating the arc quenching performance of gases. In this method, the arc decaying process is divided into three stages based on the results of 1D arc decaying modeling, including the thermal recovery stage, the predielectric recovery stage, and the postdielectric recovery stage. Compared to the previous method, the present method is improved mainly in the three aspects: the thermal recovery stage is featured by the average radial temperature instead of the axial temperature; the criterion of dividing the dielectric recovery stage into the pre- and postdielectric recovery stages is validated by the average electron number density instead of choosing arbitrarily; and the postdielectric recovery stage is characterized by the critical electric field strength Ecr instead of the reduced critical electric field strength (E/N)cr. The case study of SF6, CO2, CF4, and air arcs indicates that the present method yields the same descending order of the thermal recovery rate and the predielectric recovery rate for the four gases as the previous method. Moreover, the present method can avoid negative postdielectric recovery rates, which means that it is more reasonable than the previous method.

Influence of alkali concentration, electric waveform, and frequency on the critical electric field strength of droplet–interface partial coalescence

During the process of electro-dehydration, the electric field strength should be set below a critical value (Ecrit) to inhibit droplet–interface partial coalescence. The alkali present in alkaline–surfactant–polymer flooding may affect Ecrit because they change the electrical properties of produced liquids. In view of this, this study investigated the effects of Na2CO3 solution concentration, electric field waveform, and frequency on the root mean square (RMS) values of Ecrit. The results show that elevated Na2CO3 concentrations increase Ecrit, owing to the enhanced solution conductivity which may affect the charge transfer during coalescence. The convection of ions is associated with penetration of fluid into the bulk phase and an increase in Na2CO3 concentration decreases the penetration velocity. Moreover, increasing the AC or pulsed electric field frequency can increase Ecrit, and the optimal waveform (i.e., with the largest Ecrit) varies with frequency. These findings will be useful for optimizing electro-coalescence systems.

Stability conditions of armchair graphene nanoribbon bipolarons.

Graphene nanoribbons are 2D hexagonal lattices with semiconducting band gaps. Below a critical electric field strength, the charge transport in these materials is governed by the quasiparticle mechanism. The quasiparticles involved in the process, known as polarons and bipolarons, are self-interacting states between the system charges and local lattice distortions. To deeply understand the charge transport mechanism in graphene nanoribbons, the study of the stability conditions for quasiparticles in these materials is crucial and may guide new investigations to improve the efficiency for a next generation of graphene-based optoelectronic devices. Here, we use a two-dimensional version of the Su-Schrieffer-Heeger model to investigate the stability of bipolarons in armchair graphene nanoribbons (AGNRs). Our findings show how bipolaron stability is dependent on the strength of the electron-phonon interactions. Moreover, the results show that bipolarons are dynamically stable in AGNRs for electric field strengths lower than 3.0 mV/Å. Remarkably, the system's binding energy for a lattice containing a bipolaron is smaller than the formation energy of two isolated polarons, which suggests that bipolarons can be natural quasiparticle solutions in AGNRs. Graphical Abstract Charge localization of bipolarons in armchair garphene nanoribbons.

Effect of hydrolyzed polyacrylamide used in polymer flooding on droplet–interface electro-coalescence: Variation of critical electric field strength of partial coalescence

Polymer flooding is widely used in petroleum industry to enhance oil recovery. However, presence of polymer molecules in produced liquids can influence the droplet coalescence at oil–water interface in electric dehydration separators. In this study, the influence of hydrolyzed polyacrylamide (HPAM) on droplet–interface coalescence was examined through high-speed recording and observation. Deformation of droplets of aqueous polymer solution under electric fields was experimentally observed. The images showed droplets collapsed to filament without pinch-off due to viscoelasticity. Meanwhile, fine secondary droplets erupted from the summit of filament, which was detrimental to dehydration efficiency. A critical electric field strength (Ecrit) beyond which partial coalescence occurred was identified. The effects of the initial droplet size, HPAM concentration, electric field waveform and frequency were explored. The results showed that Ecrit increased with the decrease in initial droplet radius or the increase in HPAM concentration. Four kinds of AC or pulsatile electric fields (PEFs) were used including sinusoidal AC (sine AC), square, pulsed DC (PUDC) and pulsed AC (PUAC) waveforms. The increase in frequency suppressed the electro-induced stretching of droplets in the vertical direction, inhibiting partial coalescence. For these different electric fields, the increase in Ecrit with elevated frequencies followed different ways. Overall, Ecrit of square waveform was larger than those of other waveforms; however, Ecrit of PUAC and sine AC waveforms may exceed that of square waveform at high frequencies. These findings are useful for the development of electro-dehydration separators for oils containing dissolved polymers in the dispersed aqueous phase.

Multiphysics coupled modelling in HVDC GILs: Critical re-examination of ion mobility selection

Surface charging of spacers is one of the main factors restricting the industrialization of DC gas insulated lines (GILs). The effects of ion mobility, gas pressure and temperature on surface charge behavior are important issues to be clarified. In this paper, a selection method of ion mobility is reviewed and reconsidered. A multiphysics-coupled model considering ion mobility, species of ions, temperature, and gas pressure is investigated. A mathematical model of ion mobility under low and high electric fields is presented and the critical electric field strengths are calculated, based on the current density vs electric field strength. The results show that the two critical electric field strength have different characteristics with gas pressure and temperature. The ion mobility in SF 6 gas has a completely different calculation model in high electric field and low electric field, and these two ion mobilities are equal only at a specific temperature and gas pressure. This paper can provide a reference for the selection of ion mobility in DC GILs under conditions of temperature and pressure.

Multiphysics coupled modelling in HVDC GILs: Critical re-examination of ion mobility selection

Surface charging of spacers is one of the main factors restricting the industrialization of DC gas insulated lines (GILs). The effects of ion mobility, gas pressure and temperature on surface charge behavior are important issues to be clarified. In this paper, a selection method of ion mobility is reviewed and reconsidered. A multiphysics-coupled model considering ion mobility, species of ions, temperature, and gas pressure is investigated. A mathematical model of ion mobility under low and high electric fields is presented and the critical electric field strengths are calculated, based on the current density vs electric field strength. The results show that the two critical electric field strength have different characteristics with gas pressure and temperature. The ion mobility in SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> gas has a completely different calculation model in high electric field and low electric field, and these two ion mobilities are equal only at a specific temperature and gas pressure. This paper can provide a reference for the selection of ion mobility in DC GILs under conditions of temperature and pressure.

Molecular Dynamics Simulations of the Electrocoalescence Behaviors of Two Unequally Sized Conducting Droplets.

The electrocoalescence of droplets plays a crucial role in various fields. However, studies on the effects of droplet radius on the electrocoalescence behaviors of droplets have not been conducted until now. In this work, the electrocoalescence behaviors of two unequally sized conducting droplets are investigated via molecular dynamics (MD) simulations. The influences of electric field strength and droplet radius on the electrocoalescence behaviors of two unequally sized droplets are investigated. When the electric field strength increases, the contact cone angle between the droplets increases, and the two droplets are more likely to partially coalesce and bounce. When the radius of the smaller droplet between the two droplets increases at the same electric field strength, the contact cone angle, daughter droplet size, and ions in the daughter droplet increase, whereas the critical electric field strength ( Ed) for generating the daughter droplet decreases. Furthermore, the daughter droplet is ejected from the smaller droplet when the two droplets have different radii.

Identification of the discharge kinetics in the perfluoro-nitrile C4F7N with swarm and breakdown experiments

We follow up on our previous publication on C4F7N and investigate the ion kinetics influencing C4F7N discharges with a pulsed Townsend experiment. The measured current signals at pressures between 0.06 and 6.9 kPa, in the range of density-reduced electric fields E/N from 500 to 1050 Td, show evidence of three anions: one long-lived anion with a detachment rate coefficient in the order of , a presumably non-detaching anion, and a short-lived anion with a detachment rate coefficient in the order of . We obtain the corresponding rate coefficients for ionization, attachment and detachment in C4F7N. Using these results, we calculate a corrected value of the effective ionization rate coefficient , taking into account detachment from negative ions. It results in a corrected value of the density-reduced critical electric field strength Td, much lower than the previously obtained value of Td, which is valid only when electron detachment is negligible, i.e. for low pressure and small geometric distance. To find which value is most relevant for electrical insulation, we perform breakdown voltage measurements in homogeneous electric fields in C4F7N at 5–65 kPa. We compare the measured density-reduced breakdown field strength to that calculated using the streamer criterion with the presently obtained rate coefficients, including electron detachment. We find excellent agreement between the measured and calculated .

Partial coalescence of droplets at oil–water interface subjected to different electric waveforms: Effects of non-ionic surfactant on critical electric field strength

Electro-coalescence technology is commonly used for separation of water from oil. However, the coalescence may not be complete at high field strengths, forming secondary droplets. This process is, hence, undesirable for separation due to the challenging removal of fine droplets. In this study, the effects of non-ionic surfactant (Tween 80) on the critical electric field strength (Ecrit) of partial coalescence of droplets present at oil–water interface were investigated. Different waveforms and frequencies were employed to gain better understanding of the phenomena. The results showed that the increase in initial droplet radius (R) or surfactant concentration may reduce Ecrit. Also, Ecrit was found to vary linearly with R−0.5 and the slope Ecrit/R−0.5 was proportional to the interfacial tension. Compared to DC electric fields, pulsatile electric field (PEF) can affect Ecrit by periodically deforming the droplet. The deformation of droplets was found to be suppressed with elevated frequencies, resulting in an increase in Ecrit. Generally, Ecrit of DC and bipolar square waveforms were the highest, indicating they were more effective in suppressing formation of secondary droplets. However, Ecrit of pulse AC may exceed that of sinusoidal AC and DC at high frequencies. These findings are useful for optimizing electro-coalescence systems.

Rate coefficient and collision frequency for electron collision process in hot SF6 gas under electric field: electrical breakdown property at 300–3000 K

Hot SF6 gas of several thousand kelvins is produced during the current interruption process in a gas circuit breaker. If the hot gas is subjected to an electric field, electrical breakdown of the hot gas can occur as a consequence of the ionization and attachment processes by electron impact. The probability of occurrence of the process is stated in terms of a rate coefficient. First, this paper describes the rate coefficients of the electron impact processes in the hot SF6 gas subjected to an electric field. The rate coefficients in SF6 gas in the range 300–3000 K at 0.6 MPa are evaluated for 21 processes such as SF4 + e → SF4+ + 2e and F2 + e → (F2−)* → F− + F. Second, using these rate coefficients we obtain the collision frequencies of the 21 processes and find out the predominant processes in the hot SF6 gas. Finally, 26 continuity equations with the rate coefficients are expressed in terms of the particle number densities. Using numerical solutions of the continuity equations, we derive the critical electric field of SF6 gas for different temperatures. Furthermore, comparative discussions are presented on the critical electric field strength and the collision cross‐sections shown in other published papers. The present study is considered to provide the rate coefficients and the critical electric field strengths with higher accuracy. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.

Perfluoro-1,3-dioxolane and perfluoro-oxetane: promising gases for electrical insulation

Perfluorinated cyclic ethers represent an interesting class of molecules to use as dielectric gases for electrical insulation. Measurements of the two compounds 4,4,5,5-tetrachloro-2,2-difluoro-1,3-dioxolane (, CAS 87075-01-2) and octafluoro-1,4-dioxane (, CAS 32981-22-9) in a pulsed Townsend experiment are presented, their ionization and attachment rate coefficients are obtained and their electron attachment cross sections are estimated. Both gases have a high critical electric field strength of about and times the one of SF, respectively, which is presumably due to very high electron attachment cross sections in the range from 2–4 eV. In particular, for the values appear to be very close to the S-wave scattering limit around 3 eV. Based on these results, we recommend the two analogous compounds perfluoro-1,3-dioxolane (, CAS 21297-65-4) and perfluoro-oxetane (, CAS 425-82-1) for further research for the use as insulation and refrigerant gases, since they are chemically comparable, presumably environmentally friendly, and have high vapor pressures.

Coalescence characteristics of droplets at oil–water interface subjected to different electric waveforms

Droplet–interface electrocoalescence may be partial, which is undesirable for separating water from oil. This paper focused on the coalescence characteristics of water droplets at oil–water interface, and the influences of droplet diameter, electric field strength (E), waveform and frequency were experimentally evaluated. The results showed that the normalized volume of secondary droplets (Vnor) increased as E rose, and it was larger for large initial droplet. Increasing the frequency of pulsatile electric fields (PEF) reduced Vnor. The droplet was periodically stretched under PEF according to the waveform’s periodic characteristic. It was proved that the increase of frequency suppressed the deformation of the droplet in vertical direction, which was conducive to droplet’s vertical collapse, and therefore inhibited the formation of secondary droplets. Critical electric field strength (Ecrit) for the occurrence of partial coalescence also increased with the elevated frequency, and the increase became relatively slow at high frequencies. Ecrit of square waveform was the largest in experiments, indicating it was most effective in suppressing partial coalescence; moreover, Ecrit of PUAC could exceed that of sine AC and DC waveforms at high frequencies. As the initial droplet radius (R) reduced, Ecrit increased and showed a linear correlation with R−0.5.