Showing papers on "Electric potential published in 2019"

Implicit self-consistent electrolyte model in plane-wave density-functional theory

Abstract: The ab initio computational treatment of electrochemical systems requires an appropriate treatment of the solid/liquid interfaces. A fully quantum mechanical treatment of the interface is computationally demanding due to the large number of degrees of freedom involved. In this work, we develop a computationally efficient model where the electrode part of the interface is described at the density-functional theory (DFT) level, and the electrolyte part is represented through an implicit solvation model based on the Poisson-Boltzmann equation. We describe the implementation of the linearized Poisson-Boltzmann equation into the Vienna Ab initio Simulation Package, a widely used DFT code, followed by validation and benchmarking of the method. To demonstrate the utility of the implicit electrolyte model, we apply it to study the surface energy of Cu crystal facets in an aqueous electrolyte as a function of applied electric potential. We show that the applied potential enables the control of the shape of nanocrystals from an octahedral to a truncated octahedral morphology with increasing potential.

How transport layer properties affect perovskite solar cell performance: insights from a coupled charge transport/ion migration model

Abstract: The effects of transport layers on perovskite solar cell performance, in particular anomalous hysteresis, are investigated. A model for coupled ion vacancy motion and charge transport is formulated and solved in a three-layer planar perovskite solar cell. Its results are used to demonstrate that the replacement of standard transport layer materials (spiro-OMeTAD and TiO2) by materials with lower permittivity and/or doping leads to a shift in the scan rates at which hysteresis is most pronounced to rates higher than those commonly used in experiment. These results provide a cogent explanation for why organic electron transport layers can yield seemingly “hysteresis-free” devices but which nevertheless exhibit hysteresis at low temperature. In these devices the decrease in ion vacancy mobility with temperature compensates for the increase in hysteresis rate with use of low permittivity/doping organic transport layers. Simulations are used to classify features of the current–voltage curves that distinguish between cells in which charge carrier recombination occurs predominantly at the transport layer interfaces and those where it occurs predominantly within the perovskite. These characteristics are supplemented by videos showing how the electric potential, electronic and ionic charge profiles evolve across a planar perovskite solar cell during a current–voltage scan. Design protocols to mitigate the possible effects of high ion vacancy distributions on cell degradation are discussed. Finally, features of the steady-state potential profile for a device held near the maximum power point are used to suggest ways in which interfacial recombination can be reduced, and performance enhanced, via tuning transport layer properties.

Photo-induced ultrafast active ion transport through graphene oxide membranes

Abstract: Layered graphene oxide membranes (GOM) with densely packed sub-nanometer-wide lamellar channels show exceptional ionic and molecular transport properties. Mass and charge transport in existing materials follows their concentration gradient, whereas attaining anti-gradient transport, also called active transport, remains a great challenge. Here, we demonstrate a coupled photon-electron-ion transport phenomenon through the GOM. Upon asymmetric light illumination, cations are able to move thermodynamically uphill over a broad range of concentrations, at rates much faster than that via simple diffusion. We propose, as a plausible mechanism, that light irradiation reduces the local electric potential on the GOM following a carrier diffusion mechanism. When the illumination is applied to an off-center position, an electric potential difference is built that can drive the transport of ionic species. We further develop photonic ion switches, photonic ion diodes, and photonic ion transistors as the fundamental elements for active ion sieving and artificial photosynthesis on synthetic nanofluidic circuits.

Oscillating Hydrogen Bubbles at Pt Microelectrodes

Abstract: The dynamics of hydrogen bubbles produced via electrolysis in acidic electrolytes is studied in a combination of experiments and numerical simulations. A transition from monotonic to oscillatory bubble growth is observed after $2/3$ of the bubble lifetime, if the electric potential exceeds $\ensuremath{-}3\text{ }\text{ }\mathrm{V}$. This work analyzes characteristic features of the oscillations in terms of bubble geometry, the thickness of the microbubble carpet, and the oscillation frequency. An explanation of the oscillation mechanisms is provided by the competition between buoyancy and electric force, the magnitude of which depends on the carpet thickness. Both the critical carpet thickness at detachment and the oscillation frequencies of the bubble as predicted by the model agree well with the experiment.

A phase-field-based lattice Boltzmann modeling of two-phase electro-hydrodynamic flows

Abstract: In this paper, a simple and accurate lattice Boltzmann (LB) model based on phase-field theory is developed to study the two-phase electro-hydrodynamics flows. In this model, three LB equations are utilized to solve the Allen-Cahn equation for the phase field, the Poisson equation for the electric potential, and the Navier-Stokes equation for the flow field. To test the proposed model, the deformation of a single droplet under a uniform electric field is considered. It is found that under a small deformation, the results are in good agreement with the previous work. For a large deformation, however, the theoretical results would give a large deviation, while the present results are close to the available numerical work.

Voltage-induced long-range coherent electron transfer through organic molecules

Abstract: Biological structures rely on kinetically tuned charge transfer reactions for energy conversion, biocatalysis, and signaling as well as for oxidative damage repair. Unlike man-made electrical circuitry, which uses metals and semiconductors to direct current flow, charge transfer in living systems proceeds via biomolecules that are nominally insulating. Long-distance charge transport, which is observed routinely in nucleic acids, peptides, and proteins, is believed to arise from a sequence of thermally activated hopping steps. However, a growing number of experiments find limited temperature dependence for electron transfer over tens of nanometers. To account for these observations, we propose a temperature-independent mechanism based on the electric potential difference that builds up along the molecule as a precursor of electron transfer. Specifically, the voltage changes the nature of the electronic states away from being sharply localized so that efficient resonant tunneling across long distances becomes possible without thermal assistance. This mechanism is general and is expected to be operative in molecules where the electronic states densely fill a wide energy window (on the scale of electronvolts) above or below the gap between the highest-occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). We show that this effect can explain the temperature-independent charge transport through DNA and the strongly voltage-dependent currents that are measured through organic semiconductors and peptides.

Charge-Based EPFL HEMT Model

Abstract: This paper presents a design-oriented charge-based model for dc operation of AlGaAs/GaAs and AlGaN/GaN-based high-mobility field-effect transistors. The intrinsic model is physics-based and does not introduce any empirical parameter. The central concept is based on the linear approximation of the channel charge density with respect to the surface potential, leading to explicit and continuous expressions for charges and current in all the regions of operation, including subthreshold. In addition, an effective circuit design methodology based on the pinchoff surface potential, the pinchoff voltage and the key concept of inversion coefficient (IC) is proposed, likewise for silicon MOSFET circuits.

Mechanism of deep trap sites in epoxy/graphene nanocomposite using quantum chemical calculation

Abstract: Deep trap sites have been proved able to suppress charge injection into nanocomposites even under high electric field. In order to reveal the mechanism of deep trap sites in epoxy/graphene nanocomposite, the trap distribution was obtained through surface potential decay (SPD) method. The energy levels and 3D potential distributions of epoxy and single-layer graphene under vertical or various parallel electric fields were calculated by using the density functional theory (DFT). The experimental and calculation results indicate that doping with graphene sheets introduces deep trap sites in epoxy/graphene composite. Under parallel electric field, the apparent dipoles of single-layer graphene build positive and negative potential wells in pairs, which capture carriers as the trap sites in epoxy/graphene composite. While a vertical electric field seems to have no effect on the trap sites. For better performance in suppressing charge injection, the doped graphene sheets should be exposed partially to a parallel electric field.

Freely Flowing Currents and Electric Field Expulsion in Viscous Electronics.

Abstract: Electronic fluids bring into hydrodynamics a new setting: equipotential flow sources embedded inside the fluid. Here we show that the nonlocal relation between the current and electric field due to momentum-conserving interparticle collisions leads to a total or partial field expulsion from such flows. That results in freely flowing currents in the bulk and a boundary jump in the electric potential at current-injecting electrodes. We derive a new type of boundary conditions, appropriate for the case. We then analyze current distribution in free flows, discuss how the field expulsion depends upon the geometry of the electrode, and link the phenomenon to the breakdown of conformal invariance.

Two-Dimensional Fourier-Based Modeling of Electric Machines—An Overview

Abstract: An increasing need for fast and reliable models has led to a continuous development of Fourier-based (FB) analytical modeling. This paper presents an overview of the techniques that are currently available in FB modeling for electric machines. By coupling that overview to the most relevant literature related to the subject, an interesting starting point is provided for anyone who wants to use or improve FB models. The following seven aspects of FB models are discussed in detail: 1) the magnetic potential (scalar or vector potential); 2) the coordinate system and the solution of the partial-differential equations for each magnetic potential and for each coordinate system; 3) the way in which time dependence is accounted for; 4) the implementation of the source terms; 5) the possibilities to account for slotted structures; 6) the modeling of eccentricity; and 7) the post-processing computation of physical quantities, such as flux density, electromotive force, torque, losses, and eddy currents in conductive objects. Furthermore, this paper gives the closed form solution of the Laplace, Poisson, and Helmholtz equations in each coordinate system. In addition, this paper tackles other important features of FB models such as computational time reduction and coupling the machine model to an electric circuit.

Peristaltic motion of MHD nanofluid in an asymmetric micro-channel with Joule heating, wall flexibility and different zeta potential

Abstract: A mathematical scrutiny is introduced for the flow of magneto-hydrodynamic nanofluid through an asymmetric microfluidic channel under an applied axial electric field. The impacts of wall flexibility, Joule heating and upper/lower wall zeta potentials are considered. Electric potential expressions can be modeled in terms of an ionic Nernst–Planck equation, Poisson–Boltzmann equation and Debye length approximation. Appropriate boundary conditions have been utilized to get the results of highly nonlinear coupled PDEs numerically. The impact of physical factors on the characteristics of flow, pumping, trapping and heat transfer has been pointed out. The outcomes may well assist in designing the organ-on-a-chip like gadgets.

Systematic derivation of a surface polarisation model for planar perovskite solar cells

Abstract: Increasing evidence suggests that the presence of mobile ions in perovskite solar cells (PSCs) can cause a current–voltage curve hysteresis. Steady state and transient current–voltage characteristics of a planar metal halide CH 3 NH 3 PbI 3 PSC are analysed with a drift-diffusion model that accounts for both charge transport and ion vacancy motion. The high ion vacancy density within the perovskite layer gives rise to narrow Debye layers (typical width ~2 nm), adjacent to the interfaces with the transport layers, over which large drops in the electric potential occur and in which significant charge is stored. Large disparities between (I) the width of the Debye layers and that of the perovskite layer (~600 nm) and (II) the ion vacancy density and the charge carrier densities motivate an asymptotic approach to solving the model, while the stiffness of the equations renders standard solution methods unreliable. We derive a simplified surface polarisation model in which the slow ion dynamics are replaced by interfacial (non-linear) capacitances at the perovskite interfaces. Favourable comparison is made between the results of the asymptotic approach and numerical solutions for a realistic cell over a wide range of operating conditions of practical interest.

Interaction between a helium atmospheric plasma jet and targets and dynamics of the interface

Abstract: This paper investigates the interaction of a helium atmospheric plasma jet impinging onto liquid and metal targets using experimental and numerical techniques. The primary motivation of this study is to experimentally understand the effect of liquid and metal targets on the propagation of a plasma jet, and to numerically characterize chemical and electrical behavior at the plasma-water interface. This study is relevant in the ongoing research of plasma self-organization for the development of medical devices capable of self-adaptation. Experimental measurements are made to obtain ionization wave (IW) propagation, average electron density, and optical emission spectrum of the plasma near the interface for cases with copper and water targets and a freely expanding jet. The induced acidity on the plasma-treated water is measured by pH balance. Results confirm that both liquid and metal targets alter the IW propagation velocity and electric potential. IW average propagation speed is highest with the copper target and lowest with the water target by a factor of two. Corresponding IW head potentials are highest for the metal target and lowest in the presence of the water target. Spatial average of electron density in the plasma column is of the same order of magnitude for all cases, with the case of the water target being the lowest. For the copper target, single IW contact areas on the plasma-target interface are observed to last well into the micro-second range after initial impact within each discharge period. A reflected IW is observed in the case of the metal but not the liquid target. The plasma jet delivers a small acid dose which changes the pH of the liquid. The numerical model uses parameters from plasma-liquid experiments to simulate reactive oxygen and nitrogen, acidic, and charged species in interacting gaseous and aqueous layers by employing a transient advection-diffusion-reaction-solvation equation. The interacting layers are connected by a two-way coupling governed by solvation through Henry's law. Accumulation of charges in the aqueous layer results from varying diffusion and mobility time scales of charged species. The electric potential on the aqueous layer is characterized to be in the order of tens of volts, a small fraction of the kilo-Volt measurement of the IW head potential. Electric fields and Maxwell stresses on the aqueous layer are also simulated and are proposed to affect the movement of the plasma contact spot on the gaseous layer. The mechanism for the plasma contact spot movement on the gaseous layer is implemented using a level-set technique which uses charge-induced electric stresses to calculate local advection velocity. Energy equations are solved in both phases for neutrals, ions and electrons by considering the effect of the Joule heating term. Current density through the gaseous-aqueous interface is calculated to be in the order of micro-Amperes, which creates a small enough Joule heating term so that the temperature of heavy species remains unchanged.

Analysis of Multipactor Effect in a Partially Dielectric-Loaded Rectangular Waveguide

Abstract: This paper presents a study of the multipactor effect in a partially dielectric-loaded rectangular waveguide. To obtain the simulations presented in this paper, a detailed analysis of the dynamics of the electron inside this waveguide has been performed, taking into account the radio frequency electromagnetic fields propagating in the waveguide and the dc electric field that appears because of the charging of the dielectric layer. This electrostatic field is obtained by computing the electric potential produced by an arbitrary charge distribution on the dielectric layer in a dielectric-loaded waveguide. The electron trajectory is then found by numerically solving the equations of motion. The results obtained show that multipactor discharges do turn off by themselves under certain circumstances when they occur in such dielectric-loaded waveguide.

Electroosmotic flow in soft microchannels at high grafting densities

Abstract: This paper reports the results of a theoretical modeling of the fully developed electroosmotic flow in a rectangular microchannel with a high-density polyelectrolyte layer (PEL) attached to the walls. At these conditions, the ions are partitioned between the PEL and the fluid outside the PEL owing to the difference between the permittivities of the two media. It is taken into account that the dynamic viscosity is higher within the PEL because of hydration effects. Solutions are obtained for the electric potential and velocity distributions as well as the mean velocity by making use of a variational approach, applied to the linearized form of the governing equations, which treats the whole area under consideration as a single domain with variable physical properties. The resulting equations are solved using a spectral method. Closed-form analytical expressions are obtained for a slit geometry, representing the case of high aspect ratios. The solutions obtained are validated by comparing to finite-element simulations of the full nonlinear equations. It is shown that the electric potential drop inside the channel increases due to the depletion of the counterions within the PEL caused by the ion partitioning effect. This effect, surprisingly, magnifies the electroosmotic flow rate because of the increase of the space charge outside the PEL. As expected, the hydration effects reduce the flow rate, especially for thick PELs.

Spectroscopic study on hot-electron transport in a quantum Hall edge channel

Abstract: Hot electron transport in a quantum Hall edge channel of an AlGaAs/GaAs heterostructure is studied by investigating the energy distribution function in the channel. Ballistic hot-electron transport, its optical-phonon replicas, weak electron-electron scattering, and electron-hole excitation in the Fermi sea are clearly identified in the energy spectra. The authors find that the electron-electron scattering is significantly suppressed with increasing the hot-electron energy well above the Fermi energy. This can be understood as deriving from a suppressed Coulomb potential at larger distances and higher energies.

Stresses produced due to moving load in a prestressed piezoelectric substrate

Abstract: The present paper aims to compute the normal and shear stresses, dielectric, and electric potential in an irregular initially stressed piezoelectric substrate under moving load. A mathematical form...

Hölder Stably Determining the Time-Dependent Electromagnetic Potential of the Schrödinger Equation

Abstract: We consider the inverse problem of determining the time and space dependent electromagnetic potential of the Schrodinger equation in a bounded domain of R n , n 2, by boundary observation of the solution over the entire time span. Assuming that the divergence of the magnetic potential is fixed, we prove that the electric potential and the magnetic potential can be Holder stably retrieved from these data, whereas stability estimates for inverse time-dependent coefficients problems of evolution partial differential equations are usually of logarithmic type.

Describing Molecular Polarizability by a Bond Capacity Model.

Abstract: We propose a bond capacity model for describing molecular polarization in force field energy functions at the charge-only level. Atomic charges are calculated by allowing charge to flow between atom pairs according to a bond capacity and a difference in electrostatic potential. The bond capacity is closely related to the bond order and decays to zero as the bond distance is increased. The electrostatic potential is composed of an intrinsic potential, identified as the electronegativity, and a screened Coulomb potential from all other charges. The bond capacity model leads to integer fragment charges upon bond dissociation and displays linear scaling of the polarizability with system size. Bond capacity parameters can be derived from reference molecular polarizabilities, while electronegativity parameters can be derived from reference atomic charges or a reference molecular electrostatic potential. Out-of-plane polarization for planar systems is modeled by off-nuclei charge sites. The model is shown to be able to reproduce anisotropic reference molecular polarizabilities with an accuracy of ∼10% using a limited set of bond capacity parameters and can describe both inter- and intramolecular polarization.

Lattice Boltzmann Electrokinetics simulation of nanocapacitors

Abstract: We propose a method to model metallic surfaces in Lattice Boltzmann Electrokinetics (LBE) simulations, a lattice-based algorithm rooted in kinetic theory which captures the coupled solvent and ion dynamics in electrolyte solutions. This is achieved by a simple rule to impose electrostatic boundary conditions in a consistent way with the location of the hydrodynamic interface for stick boundary conditions. The proposed method also provides the local charge induced on the electrode by the instantaneous distribution of ions under voltage. We validate it in the low voltage regime by comparison with analytical results in two model nanocapacitors: parallel plates and coaxial electrodes. We examine the steady-state ionic concentrations and electric potential profiles (and corresponding capacitance), the time-dependent response of the charge on the electrodes, and the steady-state electro-osmotic profiles in the presence of an additional, tangential electric field. The LBE method further provides the time-dependence of these quantities, as illustrated on the electro-osmotic response. While we do not consider this case in the present work, which focuses on the validation of the method, the latter readily applies to large voltages between the electrodes, as well as to time-dependent voltages. This work opens the way to the LBE simulation of more complex systems involving electrodes and metallic surfaces, such as sensing devices based on nanofluidic channels and nanotubes, or porous electrodes.

Unified Non-equilibrium Modelling of Tungsten-Inert Gas Microarcs in Atmospheric Pressure Argon

Abstract: Analysis of the plasma parameters of a tungsten-inert gas microarc with a length of 0.4 mm is carried out by means of a unified one-dimensional model. The model solves the fluid equations for the particle and energy conservation of the electrons and the heavy species in the plasma, and the heat conduction in the thermionic cathode. The particle transport of the electrons and the ions is coupled with the Poison’s equation. The spatial distributions of the densities of the charged particles, the electric potential and field, the components of the electric current density, the heating mechanisms and the resulting temperatures of the electrons and heavy particles are discussed in detail for an electric current density of 106 A/m2. The discharge voltage, estimates of the Debye length, the near-electrode voltage drop, and the thickness of the regions of space charge adjacent to the electrodes are obtained for current densities in the range from 5.3 × 103 up to 2.3 × 106 A/m2.