Space Charge Density Research Articles

Numerical modeling of field-assisted Ag+–Na+ ion-exchanged channel waveguides using varied explicit space charge density approach

Numerical modeling of field-assisted Ag+–Na+ ion-exchanged channel waveguides using varied explicit space charge density approach

Three-dimensional nonadiabatic electron- optical gyrotron systems with inhomogeneous current density

The results of a numerical study of the effect of threedimensional perturbations of the space charge density distribution in a nonadiabatic electron-optic systems of gyrotrons are presented. Data of direct numerical simulation using the 3-D program CST Studio Suite are described.

Measuring displacement currents during fabrication of Mg/Si Schottky diodes due to band-bending evolution

The generation of a rectifying metal-semiconductor contact forms a charge depletion layer in the semiconductor surface. The resulting space charge leads to a surface band bending and the formation of a Schottky barrier. The present study introduces an unconventional method to measure and monitor the surface band bending during metal atom deposition by recording the displacement current between the metal and the semiconductor. Magnesium atoms are evaporated at 130 K onto hydrogen-passivated p-Si(001) surfaces. During deposition, the time-dependent reverse current in the diode is detected. A sharp current peak of a few nA can be attributed to the displaced charge when the first monolayers of the Mg film are formed. The currents are proportional to the number of Mg atoms impinging onto the surface. Integrating the observed displacement currents over time yields the total space charge densities at the interface between 8 and 23 nC/cm2. This is in excellent agreement with the calculated value for a Schottky barrier of 0.5 eV and assuming flatband condition for hydrogen-passivated Si(001) surfaces.

Quantifying the large-scale electrification equilibrium effects in dust storms using field observations at Qingtu Lake Observatory

Abstract. Dust and sand electrification, which is a ubiquitous phenomenon in dust events, has a potentially dramatic effect on dust and sand lifting and transport processes. However, the effect of such electrification is still largely unclear, mainly due to its complexity and sparse observations. Here, we conducted an extensive observational analysis involving mild and severe dust storms with minimum visibility, ranging from ∼0.09 to 0.93 km, to assess the electrical properties of airborne dust particles in dust storms. The space charge density has been estimated indirectly based on Gauss's law. Using the wavelet coherence analysis that is a method for evaluating the correlations between two non-stationary time series in the time–frequency domain, we found that the space charge density and dust concentration were significantly correlated over the 10 min timescales (on the order of the typical integral timescale of atmospheric turbulence). We further presented a simple linear regression (SLR) model to quantify such large timescale correlations and found that there was a significant linear relationship between space charge density and dust concentration at given ambient temperature and relative humidity (RH), suggesting that the estimated mean charge-to-mass ratio of dust particles was expected to remain constant (termed as the equilibrium value μ∗). In addition, the influences of ambient temperature and RH on μ∗ were evaluated by a multiple linear regression (MLR) model, showing that the μ∗ is nonlinearly related to environmental factors. The present study provides observational evidence for the environmental-dependent electrification equilibrium effects in dust storms. This finding may reduce challenges in future quantifications of dust electrification, as it is possible to exclude effects, such as the particles' collisional dynamics, on dust electrification.

Design and analysis of electric potential and space charge density in a portable electrostatic precipitator

This paper presents the analysis of electric potential and space charge density distribution for different high DC voltage between discharge electrodes and collecting plates in a Portable Needle-Plate electrostatic precipitator.

Research on Corona Discharge Characteristics Based on Hybrid Numerical Algorithm

Bar-plate configuration has a vital function in the intensive study of corona discharge mechanism. To study the discharge characteristics of the steady current stage in this configuration, this paper proposes a hybrid numerical algorithm composed by the finite-element method and the finite-volume method. The former solves Poisson’s equation and calculates the value of onset field intensity. The latter solves the current continuity equation, in which a time-varying charge item is added. This charge item can increase the stability of the hybrid algorithm. This paper also proposes a new selection strategy for the initial charge and inception voltage on the bar electrode surface. Based on this algorithm, corona current under different electrode radii, gap distances, and applied voltages was calculated. In this way, the distribution characteristics of an electric field intensity and space charge density along the whole gap in the model can be obtained through the experiment. To verify the validity of the algorithm, the related experiment is carried out in the laboratory. The simulation results have a good consistency with the experimental results, verifying the validity of the algorithm proposed in this paper.

Effects of collision and ion Mach number on magnetized plasma sheath with two species of positive ions

We study the effect of collision and ion Mach number in a magnetized plasma sheath having two species of positive ions using fluid model. It is assumed that both the ions are singly ionized and in thermal equilibrium, and the electrons obey Boltzmann distribution. Our work signifies that the collision greatly affects the ion densities, which explicitly affect space charge density, net current density and the transverse drift of ions. On increasing the collisional frequency, ions accumulation point and current saturation point shift towards the sheath entrance (away from the wall). The ion Mach number affects potential profile and velocity profiles of both ion species. The magnitude of wall potential increases from 34 V to 45 V with the increase in ion Mach number from 0.3 to 0.9. Furthermore, it is shown that the effect of collisional force and Lorentz force is reduced at the sheath entrance by increasing the ion Mach number, which helps to control the flux of particles, momentum flow and energy flow towards the wall.

1D Mathematical Modelling of Non-Stationary Ion Transfer in the Diffusion Layer Adjacent to an Ion-Exchange Membrane in Galvanostatic Mode.

The use of the Nernst–Planck and Poisson (NPP) equations allows computation of the space charge density near solution/electrode or solution/ion-exchange membrane interface. This is important in modelling ion transfer, especially when taking into account electroconvective transport. The most solutions in literature use the condition setting a potential difference in the system (potentiostatic or potentiodynamic mode). However, very often in practice and experiment (such as chronopotentiometry and voltammetry), the galvanostatic/galvanodynamic mode is applied. In this study, a depleted stagnant diffusion layer adjacent to an ion-exchange membrane is considered. In this article, a new boundary condition is proposed, which sets a total current density, i, via an equation expressing the potential gradient as an explicit function of i. The numerical solution of the problem is compared with an approximate solution, which is obtained by a combination of numerical solution in one part of the diffusion layer (including the electroneutral region and the extended space charge region, zone (I) with an analytical solution in the other part (the quasi-equilibrium electric double layer (EDL), zone (II). It is shown that this approach (called the “zonal” model) allows reducing the computational complexity of the problem tens of times without significant loss of accuracy. An additional simplification is introduced by neglecting the thickness of the quasi-equilibrium EDL in comparison to the diffusion layer thickness (the “simplified” model). For the first time, the distributions of concentrations, space charge density and current density along the distance to an ion-exchange membrane surface are computed as functions of time in galvanostatic mode. The calculation of the transition time, τ, for an ion-exchange membrane agree with an experiment from literature. It is suggested that rapid changes of space charge density, and current density with time and distance, could lead to lateral electroosmotic flows delaying depletion of near-surface solution and increasing τ.

Global and Regional Electricity Components in Undisturbed Midlatitude Lower Atmosphere

The physical mechanisms determining the variability of the vertical profiles of electrical conductivity, space charge density, and electric field in the undisturbed midlatitude lower atmosphere are discussed. The influence of the global and local mesoscale processes on the variability of electrical conductivity and the main component of the atmospheric electric field is estimated. The sunrise effect is studied, estimates are obtained for the charge accumulation rate in the column of the lower atmosphere and the corresponding growth rate of the field strength close to the ground. It is shown that the increase in the average charge density is mainly due to the breakdown of the stable stratification of the atmospheric boundary layer and transformation of the vertical profile of electrical conductivity following the convective mixing of a radon and its daughter products.

The effect of methane gas flow rate on the streamer propagation in an atmospheric-pressure methane-air plasma jet

In this paper, a two-dimensional axisymmetric fluid model is applied to investigate the streamer discharge characteristics in an atmospheric pressure methane-air plasma jet as a function of methane flow velocities (2.5 m/s and 20 m/s, respectively). Although the streamer ignition and propagation in the dielectric tube are not sensitive to the methane gas flow velocity, the concentration field of methane and air in the mixing layer established by a balance between convective methane flow and back-diffusion of air ambient is crucial for streamer propagation in the gap. As the methane flow velocity is 2.5 m/s, the structure of the streamer head transits from ring-shape into solid disk-shape, while the streamer head always maintains a donut-shaped pattern at high flow velocity of 20 m/s until it impinges on the substrate. At lower gas velocity, the back-diffusion of ambient air into the methane jet is even more pronounced, which causes a larger space charge density at the streamer head, and thus the local electric field near streamer head is greater. Therefore, the overall trend in streamer propagation speed versus methane flow velocity is that the larger the flow velocity, the lower plasma bullet speed. Besides, as the gas flow velocity increases from 2.5 to 20 m/s, less oxygen/nitrogen radical species and charged ions are produced in the streamer discharge, while the produced methane-related particles increase slightly. As for different methane flow velocities, the streamer advances within the methane core.

Electrohydrodynamic flows in electrostatic precipitator of five shaped collecting electrodes

The electrohydrodynamic (EHD) flows in wire-plate electrostatic precipitators (ESP) of five shaped collecting electrodes are simulated by employing the CFD code FLUENT with the aid of a user defined function (UDF). The results show that the EHD flow patterns are different in the five configurations. Bending of the collecting plate can easily lead to the peak of the electric field intensity near the collecting surface. The space charge density near the W-type collecting surface is more uniform than those of the other four types. The flow induced by ionic wind in the C-type plate is strongest among the five channels.

Sheath criterion in constant mean free path collisional plasma with two distinct temperature q-nonextensive electrons

A constant mean free path collisional plasma sheath model is investigated in the presence of two-temperature q-nonextensive electrons and fluid ions. By using Sagdeev potential technique, a modified Bohm sheath criterion is derived and also verified. It is shown that the density distribution of positive ions reduces monotonically when the Bohm velocity lies between the derived limits. The effect of collision on the plasma sheath profiles viz. density, potential, net space charge density and positive ion velocity in the sheath in presence of the proposed plasma configuration is investigated. It is also shown that the increasing values of ion-neutral collisionality leads to a decrease of sheath thickness, increase of sheath potential and net space charge density in the sheath region.

Influence of space charge density on electron energy distribution function and on composition of atmospheric pressure He/O2/air plasmas

Atmospheric pressure non-equilibrium plasma represents an efficient source of reactive species for different kinds of applications. Rich chemistry of such plasmas develops on longer time scales and is difficult to handle by kinetic models so global models are often applied to study such processes. The Maxwell-Boltzmann (MB) distribution is often used in global modelling of these non-equilibrium systems, even for the calculation of electron rate coefficients. In order to test the sensitivity of plasma composition on the space charge density and consequently on the assumed electron energy distribution function (EEDF), the zero-dimensional global model is applied to the helium/oxygen mixture (0.5% of O2 with humid air impurities. To test the effect of the distribution function on global models we have included the data calculated based on the non-equilibrium EEDF for the processes where often exponential Arrhenius-like formulae as a function of the effective temperature are used. The initial calculation showed that the change in the form of distribution function mainly affects the processes with thresholds considerably higher than the mean electron energy while it does not change much the rates for the processes with the thresholds and peaks of the cross sections in the region of the mean energy. We have calculated variations of the EEDF with the charge density and the resulting changes in chemical kinetics.

Influence of nano layer structure of polyimide film on space charge behavior and trap levels

Measurements of space charge density and trap levels are informative ways to predict failure conditions of insulation materials. The charges are injected from electrodes and penetrate from surface towards deep inside of the insulation material. In order to resist the charge injection, trapping, migration and increase charge recombination, detrapping and extraction rate, layer (nanocomposite-PI-nanocomposite) sandwich structure samples are used. Two techniques Pulse Electro Acoustic (PEA) and Thermal Stimulated Current (TSC) are combined to characterize space charge behavior and trap levels. Lab-view based program is used to detect the charge density distribution and total charge decay, different peaks of current using TSC technique are used to get activation energy and trap level density. The effects of structural interface (single, double, triple layers) and types of nano (Al 2 O 3 , SiO 2 ) fillers are analyzed to influence charge accumulation polarity and trap levels. Trap levels are calculated using TSC data as well as total charge decay data. The trap values calculated from both data match each other. Al 2 O 3 -PI-Al 2 O 3 sandwich structure showed remarkable improvements to resist accumulation of space charge for longer time. Mostly charges are shallow traps with 1.126 eV trap level and recombination rate is higher. In double layer structure in which interface is made up of SiO 2 and Al 2 O 3 , deep trap level of 1.26 eV and higher trap density is observed. The charges decay is faster in three-layer structure than double and single layer structure. It is concluded that the interface formed with different type of nano and structure of insulation, both are an important parameter for manufacturing industry to suppress space charges accumulation and traps.

Quantum Transport by Spin‐Polarized Edge States in Graphene Nanoribbons in the Quantum Spin Hall and Quantum Anomalous Hall Regimes

Using the non‐equilibrium Green's function method and the Keldysh formalism, we study the effects of spin–orbit interactions and time‐reversal symmetry breaking exchange fields on non‐equilibrium quantum transport in graphene armchair nanoribbons. We identify signatures of the quantum spin Hall (QSH) and the quantum anomalous Hall (QAH) phases in non‐equilibrium edge transport by calculating the spin‐resolved real space charge density and local currents at the nanoribbon edges. We find that the QSH phase, which is realized in a system with intrinsic spin–orbit coupling, is characterized by chiral counter‐propagating local spin currents summing up to a net charge flow with opposite spin polarization at the edges. In the QAH phase, emerging in the presence of Rashba spin–orbit coupling and a ferromagnetic exchange field, two chiral edge channels with opposite spins propagate in the same direction at each edge, generating an unpolarized charge current and a quantized Hall conductance G = 2e2/h. Increasing the intrinsic spin–orbit coupling causes a transition from the QAH to the QSH phase, evinced by characteristic changes in the non‐equilibrium edge transport. In contrast, an antiferromagnetic exchange field can coexist with a QSH phase, but can never induce a QAH phase due to a symmetry that combines time‐reversal and sublattice translational symmetry.

Failure Characteristics and Mechanism of Nano-Modified Oil-Impregnated Paper Subjected to Repeated Impulse Voltage.

Nano-modification is a prospective method for improving the electrical properties of transformer oil. In most situations, transformer oil combined with cellulose paper is used to construct an insulation system for power equipment, such as power transformers. However, the influence of nanoparticles on the electrical performance of oil-impregnated paper is still unclear. Therefore, in this paper, we identify the failure characteristics of both fresh and nano-modified oil/paper. Specifically, the accumulative failure characteristics of nano-oil-impregnated paper (NOIP) are experimentally determined. The space charge distribution and trap characteristics of fresh paper and NOIP were measured, and the effect of nanoparticles on the space charge behavior are then analyzed. Finally, we measure the microstructure of fresh paper and NOIP subjected to repeated impulses. The test results indicate that nano-titanium oxide (TiO2) particles have a limited effect on the breakdown voltage of NOIP. However, the particles can dramatically improve the resistant ability of NOIP against repeated impulses. For the NOIP with a nano-concentration of 0.25 g/L, the improvement reaches 62.5% compared with fresh paper. Under repeated applications of impulse voltages, the space charge density of NOIP is much lower than that of fresh paper. The deep trap density of NOIP is much higher than that of fresh OIP, whereas shallow trap density is relatively lower. Micropores are generated in paper insulation subjected to repeated impulses. The amount of the generated micropores in NOIP is lower than that in fresh paper. Nano-TiO2 particles suppress the accumulation of space charge in the oil paper insulation, which weakens the electric field distortion in the dielectric. However, nanoparticles reduce the accumulative damage caused by repeated impulses. The above two points are considered the main reasons to improve the resistant ability against repeated impulses.

Electric State of the Near-Surface Atmosphere according to the Results of Tethered Balloon Observations

The electric state of the near-surface atmosphere up to a height of 400 m is investigated using a tethered balloon with a measuring platform and a ground-based information-measuring complex of the Borok middle-latitude geophysical observatory. For the first time, measurements were taken simultaneously for vertical profiles of the atmospheric electric field, polar electrical conductivities, size distribution of aerosol particles, and the volume activity of radon, which have allowed estimating the average values and variability of the space charge density and conduction current in the atmosphere. The height dependence of the electric potential with respect to the Earth’s surface and electrical resistance of the near-surface atmospheric column under different conditions of the temperature stratification is studied.

Effect of Rare-Earth Metal Oxide Nanoparticles on the Conductivity of Nanocrystalline Titanium Dioxide: An Electrical and Electrochemical Approach

Doping of rare-earth metal oxides into nanocrystalline titanium dioxide (NTD) films is known to improve performance for photovoltaic and photocatalytic applications; however, the reasons for this improvement are not well understood. To explore the enhancement mechanism, an electrical and electrochemical study of rare-earth oxide-doped NTD films was performed. Doped films were found to be 40–50 times more conductive than undoped films, with linear current–voltage characteristics and decreased light sensitivity. Cyclic voltammograms of doped samples show an enhanced scan rate dependence in the deep trap regime due to a slower charge trapping rate. Finally, electrochemical impedance measurements reveal a decrease in space charge density and a shift in the flat-band potential. Taken together, these results suggest that charge transfer from the rare earth oxide neutralizes the deep trap states in the NTD film, decreasing charge scattering, and improving the NTD performance as an electron acceptor and electron ...

The role of low air pressure in the variation of negative corona‐generated space charge in a rod to plane electrode

The corona-generated space charge may vary in different air pressures, and the quantitative law between the corona-generated space charge density and air pressure has never been investigated. This work utilised computational approaches to explore the role of air pressure in the variation of negative corona-generated space charge. A negative fluid model in which the air pressure could be varied was built up to calculate the charged-particle density and electric field under different applied voltages. The continuity equations were solved by flux corrected transport algorithm and the Poisson equation was computed by the successive over relaxation method. The variations of important parameters due to air pressure were analysed. The particles densities of electrons, positive ions and negative ions at standard and low atmosphere pressures were computed, which increased markedly due to the decrease of atmosphere pressure. At fixed applied voltages, the exponential decay law between the maximal values of electron/positive ion densities at the head of the streamer and air pressure could be obtained. The quantitative law between the maximal values of electron/positive ion densities at the head of the streamer and the applied voltage, relative air density was also achieved.

Space charge behavior in XLPE cable insulation under ac stress and its relation to thermo-electrical aging

The space charge features of cable insulation peelings are investigated under ac stress as a function of electrical aging imposed previously on cable sections. The investigated samples are 230-μm-thick peelings of crosslinked polyethylene cable insulation. The space charge profiles are resolved into 32 symmetric phase angles under 50 Hz ac stress. The space charge density in the phase angle of maximum applied field is the largest, as expected, but does not change visibly as a function of aging time. A method that involves averaging space charge in all phases is proposed. The all-phase averaged space charge, which appears in essence as stationary charge, exhibits an obvious increasing trend with stressing time. The aging characteristics of space charge formed under ac stress are investigated considering the all-phase averaged profiles, and the mechanical properties and degree of crosslinking are characterized as complementary methods. The change in the accumulation of space charge under ac stress follows opposite trends unlike the degree of crosslinking, which means that space charge under ac stress has a close relationship with defect concentration in cable insulation. This research contributes to the understanding of space charge features under ac conditions and the understanding of the ac aging principle of cable insulation.