Search

The electrostatic potential (EP) generated by the protein α-lactoalbumin in the presence of added salt is computed as a thermal average at a given point in space. With this aim, constant pH Monte Carlo (MC) simulations are performed within the primitive model, namely, the solvent is treated as a continuum dielectric. The study of the thermal and spatial fluctuations of the EP reveals that they are remarkably high inside the protein. The calculations indicate that fluctuations inside the protein are mainly due to the asymmetric distribution of the charge groups, while the charge fluctuations of the titratable groups play a minor role. The computed EP matches very well with the one obtained from the Poisson equation for the average charge density in spherical symmetry. The Tanford–Kirkwood multipole expansion reproduces the simulated angular-averaged potential rather accurately. Surprisingly, two of the simplest mean-field models, the linear Poisson–Boltzmann (PB) equation and Donnan potential, provide good estimations of the average EP in the effective protein surface (surface EP). The linear PB equation predicts a linear relationship between charge and surface EP, which is numerically reproduced only if the small ions within the protein are taken into account. On the other hand, the partition coefficients of the small ions inside and outside the protein predicted by Donnan theory reproduce reasonably well the simulation results. [ABSTRACT FROM AUTHOR]

In electromagnetism, materials with a polarization density P → or a magnetization density M → are known to exhibit a bound surface charge density σ b = P → · n ̂ or a surface current density κ → b = M → × n ̂ , respectively, where n ̂ is the unit vector perpendicular to the material boundary surface, directed outward. These expressions can be obtained from volume integrations for the electric potential V, or the magnetic vector potential A → , in which the integrals are restricted to the material volumes delimited by their respective boundaries. In that case, applying the divergence theorem leads to surface integrals on material boundaries and to the above-mentioned surface quantities. In this paper, a simple derivation is presented, which shows that both σb and κ → b are included in the expressions for the volume charge or current densities, provided that the divergence and curl operators are evaluated at the boundary so as to account for discontinuities at interfaces. [ABSTRACT FROM AUTHOR]

As trapping sites for surface charges, the surface trap is an important factor affecting the insulation performance of the epoxy–SF6 interface, which is closely related to the microstructure and its evolution. However, existing studies often ignore the effect of the degradation of the epoxy surface on the surface traps and charge accumulation during the long-term service of the insulation system. In this study, the surface physicochemical evolution and its influence on surface traps of a typical epoxy–SF6 insulation structure were researched under a DC-temperature gradient condition, and an evolution mechanism of surface traps was proposed. The results show that the free radicals introduced during the material degradation process increase the density and depth of both positive and negative charge traps on the epoxy surface. The effect of free radicals formed by the cleavage of epoxy molecules on the electronic orbital and molecular electrostatic potential of epoxy molecules is the intrinsic reason for the evolution of surface trap evolution. This work may help to understand the long-term evolution mechanisms of the physicochemical characteristics of the epoxy–SF6 interface in DC field and may provide a theoretical basis for the long-term failure mechanism revealing and the insulation performance enhancement. [ABSTRACT FROM AUTHOR]

Space plasmas, though tenuous, can lead to the development of large magnitude potentials on spacecraft surfaces relative to the ambient plasma in a process known as spacecraft charging. This effect poses a threat to spacecraft when surfaces charge differentially as it increases the risk of electrical arcing. Spacecraft engineers have primarily focused on the mitigation of charging and not its exploitation. The potentials and current collected by the spacecraft present an opportunity to harvest power from the space environment. Although the current density and floating potential due to incident charged species in the orbit-motion-limited model are identical for all spacecraft surfaces with similar geometries, material property differences between surfaces or charge control devices break the symmetry and allow for net current to flow. Differences in the secondary electron yield, in particular, produce strong differences in floating potential even when the spacecraft is eclipsed or far from the sun. This article presents a theory for computing the power available to harvest from spacecraft charging given the parameters of ambient space plasmas and surface material properties. Given a load connecting two differentially charged surfaces, optimal power is harvested by matching the load resistance to the input plasma impedance. While this holds for any model of the ambient plasma, the orbit-motion-limited theory yields closed-form solutions to the charging equations for spherical and planar surfaces. These solutions provide an upper bound on generated power that scales with the density and temperature of the cold electron population. An environment-specific optimal anode-to-cathode area ratio exists that minimizes the input impedance to the harvesting circuit. Deployable anodes are recommended to satisfy this condition and increase the amount of power harvested. Energy harvesting from spacecraft charging can be used in the Jovian magnetosphere to generate on the order of 10 mW/m2 of power. [ABSTRACT FROM AUTHOR]

This article presents the derivation of semianalytic solutions to a new 1-D photoelectron sheath model near the lunar surface. The plasma species include the cold solar wind protons, drifting Maxwellian solar wind electrons, and Maxwellian photoelectrons emitted from the surface. The semianalytic model is then numerically solved to obtain profiles of quantities of interest as functions of the vertical distance from the surface. A fully-kinetic 3-D finite-difference (FD) particle-in-cell (PIC) code is then utilized to simulate the 1-D photoelectron sheath and the results agree well with the numerical solution to the semianalytic model. A $\kappa $ -distribution for solar wind electrons is also implemented to the FD-PIC code to compare with the Maxwellian distribution. Results show that photoelectron sheath may reach as high as close to 100 m above the illuminated flat lunar surface near the terminator region and up to about 50 m near the equator region. Our results show that under average solar wind condition, the photoelectron sheath profiles obtained with Maxwellian and $\kappa $ -distribution (with $\kappa = 4.5$) are very close for 1-D numerical results. [ABSTRACT FROM AUTHOR]

In this study, the temporal variation of an electric potential distribution was measured in a surface dielectric barrier discharge experiment using the Pockels effect. A branching pattern and homogeneous distribution were respectively observed when positive and negative voltages were applied. When continuous alternating current was applied to the electrode, the spatial distributions of the electric potential significantly changed between the first and subsequent discharge cycles. The results indicate that the electrostatic repulsive forces caused by the residual charges on the dielectric surface affect the dynamic patterns of the discharge morphology. [ABSTRACT FROM AUTHOR]

Surface charge accumulation on epoxy insulator installed in gas insulated switchgear (GIS) or gas insulated transmission line (GIL) has been recognized as an important factor for electric field distortion that results in reduction of surface flashover voltage. This paper reports on surface charge accumulation behavior on a actual epoxy insulator for 100 kV GIL prototype with multi-arc surface profile. A coaxial electrode system is used to investigate the charging behavior under DC voltages with metal particles positioned at various positions of the surface so as to estimate its influence on the charge accumulation behavior using a Kelvin type probe. The results show that positive charges are accumulated mainly on the nonplanar region around the high voltage electrode in the shape of speckles, and the average charge distribution along the radial direction presented a double-peak feature. Moreover, the charge accumulation is enhanced by metal particles contacting to the grounded electrode. Using a numerical simulation method, it is suggested that the charge accumulation is mainly determined by gaseous ionization which is enhanced by the metal particle induced electric field distortion. [ABSTRACT FROM AUTHOR]

One of the associated factors that controls the performance of a triboelectric generator (TEG) is the mechanical deformation of the dielectric layer. Therefore, a good contact model can be a prominent tool to find a more realistic and efficient way of determining the relationships between the contact and electrical output of the generator. In this study, experiments are conducted on a vertical contact mode triboelectric generator under an MTS machine. The open-circuit voltages are measured at different loads imposed by the MTS by controlling the cyclic displacement of the top tribo layer of the generator. A finite-element-based theoretical model is developed to explain the behavior of the generator during the experiments. The 2D-contact problem of the micro-structured tribo layers is simulated and then the contact results are integrated into 3D to find the actual contact area between the two surfaces. These numerical contact results improve the existing theoretical model by evaluating the correct surface charge density and contact area as a function of the input parameters. The excellent agreement between our experimental and theoretical results illustrates that theoretical modeling could be used as a robust approach to predict the mechanical and electrical performance of TEGs. In addition, some parametric studies of the harvester are presented here for different geometrical parameters of the microstructures. [ABSTRACT FROM AUTHOR]

The accumulation of surface charges is a crucial challenge, especially for HVDC components. Accumulated surface charges can change the electric field distribution and generate weak points, especially in case of superimposed transient voltages and, therefore lower the performance e.g. of insulators. Hence, an accurate measurement of the surface potential and so the surface charges is necessary to optimize these components. Commonly, the surface potential is measured with an electrostatic voltmeter. In this study a procedure for determining the spatial impulse response of a compensating electrostatic voltmeter depending on only the surface to probe distance is presented. Based on the performed measurements a mathematical model is developed to describe the capacitive influence and significance of the local surface potential on the measurement result. The actual measurement value is a weighted and averaged value given by the actual position and its surrounding, which is also confirmed by numerical simulations. The performed procedure enables high accuracy of the measurement of surface potentials and so of surface charges using different types of electrostatic voltmeters and shows the limitation of the measurement procedure. This knowledge increases the possibility to design optimized components for HVDC systems. [ABSTRACT FROM AUTHOR]

High-level surface-charging events are frequently observed on spacecrafts in aurora region. Besides the sunlit condition, two major factors causing charging events are the ambient plasma density and the precipitating electron flux. For the first factor, when the ambient plasma density is relatively low due to weak solar UV output during solar minimum, more surface-charging events are reported to occur for polar satellite. Moreover, the increased energetic electron precipitation during high geomagnetic activities in solar cycle decent years, may also contribute to the charging events. To evaluate these two factors comprehensively, we present a statistical survey of over 2000 surface charging events recorded by Defense Meteorological Satellite Program (DMSP) satellites over one solar cycle. The relationship between charging events and Kp index, F10.7 index, and monthly average sunspot number is investigated. It is shown that the occurrence frequency of charging events is larger at descending years of solar cycle. In addition, the conditional probabilities of the charging events increase as Kp increase. There is a threshold for Kp index, under which most of the charging events cannot occur, and this threshold grows as F10.7 increases. Finally, it is demonstrated that the surface charging potential is strongly correlated with the Kp index. Although there is no obvious correlation between the surface charging potential and F10.7 index, we still observe a decreasing trend of the surface potential range as F10.7 increases. [ABSTRACT FROM AUTHOR]

Surface charge accumulation on polymer insulating material has been recognized as an important factor for electric field distortion that results in reduction of surface flashover voltage. This paper reports on the charge accumulation behavior between a pair of finger-shaped electrodes on epoxy/Al2O3 nanocomposites under dc voltage superimposed by repetitive pulses. The charge distribution was measured by means of an electrostatic voltmeter, from which the average charge density could be calculated. Effects of dc voltage, nanofiller content, and pulse voltage on charge accumulation behavior were investigated. Obtained results showed that with the increase in the filler content from 0 wt% to 5 wt%, the average charge density increased initially then appeared to decrease. The superposition of repetitive pulses on dc voltage led to a remarkable growth in charge accumulation as compared with that when only dc voltage was applied. It is suggested that the charges are originated from local gaseous ionization and/or electrode injection, which is determined by the combined voltage. The charge density is dependent upon the surface trap, which is affected by nanofiller content. The superposition of repetitive pulse is considered as a possible reason to encourage local gaseous ionization that results in enhanced accumulation of surface charge. [ABSTRACT FROM AUTHOR]

Laser-ablated template and fluoroalkyl silane modified composite coatings are used to prepare a micro- and nano-structure on silicone rubber surfaces. By testing the surface potential accumulation and dissipation process, we assess the suppression of surface charge on superhydrophobic silicone rubber. The surface physicochemical properties and resistivity are examined. The electron and hole trap distribution are analysed using isothermal current decay theory. Peak density of traps associated with the superhydrophobic surfaces are found to be promoted to lower energy levels relative to unmodified silicone. Surface micro-structure increases trap density and reduce trap depth; nano coatings are shown to further reduce surface trap depth. The improvements shown to suppress surface charge accumulation is beneficial to a range of electrical industries and helps mitigate surface flashover on insulation. [ABSTRACT FROM AUTHOR]

Surface charge accumulated on insulators plays an important role in flashover process in vacuum, which has not yet been fully understood. In this paper, charging behaviors on insulator surface were experimentally examined under negative DC and impulse voltage. For the case of DC voltage, two charging stages were observed, i.e., the area near cathode was at first negatively charged under lower voltage (stage I) and then followed by a positively charged cathode-anode surface (stage II). Surface charging at stage I is considered to be caused by Schottky injection of homo-charges from electrode into sample surface layer. While at stage II secondary electron emission (SEE) process is responsible to the positively charged anode-cathode surface. However, charging at stage I is quite sensitive to pre-stress process, as well as the electrode-insulator interface conditions. The ongoing injection of electrons from cathode can lead to gradually decreasing in surface charge accumulation as the voltage duration increases. For the case of impulse voltage, Schottky injection of homo-charges has little influence on surface charging as the time is too short for charge injection and migration in surface layer. The SEE process will govern surface charging due to its extremely fast establishment process. A good linear relationship is obtained between saturated surface charge density and applied voltage, which is generally consistent with the numerical calculation result based on secondary electron emission avalanche theory. At last, it is confirmed that the ionization of desorbed gas from insulator surface and subsequent neutralization of electrons with positive surface charge is the main cause to eliminate surface charge in flashover. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The development of electric fields in gas-solid insulation systems under dc voltage stress goes hand in hand with the accumulation and decay of charges on dielectric interfaces. The charge transport through the solid insulator and the gas is determined by the electric properties of the involved insulating materials. Therefore, the nonlinearly field-dependent volume conductivity of a gas, resulting from a constant charge generation rate by natural ionization has to be considered. The transient field distribution on insulators under dc voltage is investigated theoretically, using a gas model, which takes into account the physical characteristics of insulating gases like air and SF6 as well as the effects resulting from electron emission from the electrodes. The influence of the ion pair generation rate in the gas, the electric field strength, and the volume and surface conductivity of the solid insulating material on the development of the electric field is investigated on cylindrical and conical insulators which are typical for gas insulated systems. Results of calculations using the gas model are compared to simulations assuming constant gas conductivity. The simulation results are verified by measurements of the surface potential on cylindrical epoxy resin insulators in air. [ABSTRACT FROM AUTHOR]

We theoretically investigate electrostatic properties between two charged surfaces with a grafted polyelectrolyte layer in an aqueous electrolyte solution by using the Poisson-Boltzmann approach accounting for ion partitioning. In order to consider the ion partitioning effect, we focus on changes of electrostatic properties due to the difference in dielectric permittivity of the polyelectrolyte layer and the aqueous electrolyte solution. We find that ion partitioning enhances electrostatic potential in the region between two charged soft surfaces and hence increases the electrostatic interaction between two charged soft surfaces. Ion partitioning effect on osmotic pressure is enhanced not only by an increase in the thickness of polyelectrolyte layer and Debye length but also by a decrease in ion radius. [Display omitted] • Ion partitioning effect involves an increase of electrostatic potential and osmotic pressure. • An increase in surface charge density amplifies the ion partitioning. • A higher thickness of polyelectrolyte layer results in a stronger ion partitioning. • A small radius of ions enhances ion partitioning. [ABSTRACT FROM AUTHOR]

We report an exact result for the electrostatic potential energy stored in a hemispherical surface with uniform surface charge density. This system lacks the spherical symmetry of a uniformly charged spherical surface or its solid sphere counterpart. Consequently, a hemispherical surface with all its charge uniformly spread on its surface represents an interesting case study in electrostatics. In this work we combine the fact that the body has axial symmetry with clever mathematical tools to obtain an outcome that is remarkably simple. The final exact expression obtained for the electrostatic potential energy stored in a hemispherical surface with uniform surface charge density answers a long-standing question. Though the bulk of the treatment is analytical, the accuracy of the findings can also be ascertained by using symbolic computation software or numerical recipes. Hence, the present analytical results can be compared to ones obtained with various powerful numerical methods which nowadays are routinely used to check the correctness of analytical and semi-analytical calculations. • Hemispherical surface with uniform surface charge density. • Exact analytic result for the value of the electrostatic self-energy. • Method can be generalized to other similar systems with reduced symmetry. [ABSTRACT FROM AUTHOR]