Showing papers on "Electric potential published in 2000"

Influence of Electrical Potential Distribution, Charge Transport, and Recombination on the Photopotential and Photocurrent Conversion Efficiency of Dye-Sensitized Nanocrystalline TiO2 Solar Cells: A Study by Electrical Impedance and Optical Modulation Techniques

Abstract: The role of electrical potential, charge transport, and recombination in determining the photopotential and photocurrent conversion efficiency (IPCE) of dye-sensitized nanocrystalline solar cells was studied. Electrostatic arguments and electrical impedance spectroscopy (EIS) are used to obtain information on the electrical and electrochemical potential distribution in the cell. It is shown that on the macroscopic level, no significant electrical potential drop exists within the porous TiO2 when it contacts the electrolyte and that the electrical potential drop at the transparent conducting oxide substrate (TCO)/TiO2 interface occurs over a narrow region, one or two layers of TiO2. Analyses of EIS and other data indicate that both the photopotential of the cell and the decrease of the electrical potential drop across the TCO/TiO2 interface are caused by the buildup of photoinjected electrons in the TiO2 film. The time constants for the recombination and collection of the photoinjected electrons are measur...

Measurement of the interaction potential of microspheres in the sheath of a rf discharge

Abstract: The interaction potential of two microspheres that are levitated in the sheath region of a radio frequency (rf) argon discharge is studied experimentally by analyzing their trajectories during head-on collisions. It is shown that the interaction parallel to the sheath boundary can be described by a screened Coulomb potential. Thus, values for an effective charge and a screening length can be obtained. The horizontal part of the interaction potential has been determined for several plasma conditions. There is no evidence for an attractive part in the potential within the accuracy of the present measurements and the given plasma conditions.

Pumping of water with ac electric fields applied to asymmetric pairs of microelectrodes.

Abstract: Bulk fluid flow induced by an ac electric potential with a peak voltage below the ionization potential of water is described. The potential is applied to an ionic solution with a planar array of electrodes arranged in pairs so that one edge of a large electrode is close to an opposing narrow electrode. During half the cycle, the double layer on the surface of the electrodes charges as current flows between the electrodes. The electrodes charge in a nonuniform manner producing a gradient in potential parallel to the surface of the electrodes. This gradient drives the ions in the double layer across the surface of the electrode and this in turn drags the fluid across the electrode surface. The anisotropic nature of the pairs of electrodes is used to produce a net flow of fluid. The flow produced is approximately uniform at a distance from the electrodes that is greater than the periodicity of the electrode array. The potential and frequency dependence of this flow is reported and compared to a simple model. This method of producing fluid flow differs from electrical and thermal traveling-wave techniques as only a low voltage is required and the electrode construction is much simpler.

Conditions for Similitude between the Fluid Velocity and Electric Field in Electroosmotic Flow

Abstract: Electroosmotic flow is fluid motion driven by an electric field acting on the net fluid charge produced by charge separation at a fluid−solid interface. Under many conditions of practical interest, the resulting fluid velocity is proportional to the local electric field, and the constant of proportionality is everywhere the same. Here we show that the main conditions necessary for this similitude are a steady electric field, uniform fluid and electric properties, an electric Debye layer that is thin compared to any physical dimension, and fluid velocities on all inlet and outlet boundaries that satisfy the Helmholtz−Smoluchowski relation normally applicable to fluid−solid boundaries. Under these conditions, the velocity field can be determined directly from the Laplace equation governing the electric potential, without solving either the continuity or momentum equations. Three important consequences of these conditions are that the fluid motion is everywhere irrotational, that fluid velocities in two-dime...

Electrowetting : a model for contact-angle saturation

Abstract: Electrowetting (EW) involves the application of an electric potential across a solid–liquid (SL) interface, which modifies the wetting properties of that interface by reducing the SL surface energy and induces a contact-angle change without altering either the bulk liquid or solid properties. Reversible contact angles are achieved at low potentials, while the application of high potentials results in contact-angle saturation and system instabilities. In the present work, an EW system consisting of a substrate (plate or rod) coated with a thin Au underlayer, a dielectric (parylene) layer of various thickness and a 100-nm layer of fluoropolymer (Teflon AF1600) is studied both theoretically and experimentally. A theoretical consideration of the EW effect, taking into account the detailed structure of the electrical double layer, and a model for the saturation of the contact angle observed at high applied potentials are presented. The theoretical treatment presented here confirms that the EW effect is, in the most part, due to polarization of the dielectric, as opposed to purely a rearrangement of the double layer, and explains why no dependence of the EW effect on the electrolyte concentration has been observed. The theory is only applicable over a limited potential range, and we have developed a model that predicts the potential at which this limit occurs. This is the potential at which the SL surface energy becomes zero. The limiting potential is dependent only on the bulk liquid and solid properties and is thus fixed for a given system. Experimentally, the limiting potential corresponds to the onset of contact-angle saturation, although variations in the exact angle of saturation are not uncommon due to the kinetic effects involved in a real system as opposed to a strict thermodynamic analysis. The model predicts that for an EW device in which an aqueous droplet can be forced to completely wet a hydrophobic surface, a surface with basically the same surface energy as the liquid is required.

Electrodynamic coupling of high and low latitudes: Simulations of shielding/overshielding effects

Abstract: The penetration of the electric field and associated magnetic perturbations from high latitudes to low latitudes is studied with the Magnetosphere-Thermosphere-lonosphere-Electrodynamics General Circulation Model (MTlEGCM) of Peymirat et al. [1998] in response to variations of the polar cap potential drop. For a sudden decrease of the polar cap potential of ∼ 40 kV, the model reproduces the well-known overshielding phenomenon corresponding to a sudden reversal of the low-latitude electric field. For quasi-periodic oscillations of the polar cap potential drop of ∼ 40 min period, the model predicts that the poleward electric field and the eastward Hall current in the auroral zone lag slightly in phase (< 1 min), while the eastward electric field and current at the magnetic equator are advanced slightly in phase, with respect to the potential-drop oscillations. These phase differences are interpreted as the consequence of the succession of shielding and overshielding episodes induced by the response of the region-2 field-aligned currents to the polar cap variations. The phase differences among the polar cap potential and the auroral and equatorial electric fields and currents increase with the plasma sheet pressure. The amplitude of the associated magnetic perturbations is very dependent on the distribution of the potential along the polar cap boundary. The model predictions are tested against the observations of Kobea et al. [this issue] for two events, one representing simple overshielding and the other associated with polar cap potential oscillations. The model underestimates the decay time of the magnetic perturbations by a factor of 2 during the overshielding event, and the model gives results compatible with the observations during the second event. The disagreements may be due to limitations of the model and uncertainties of the input parameters.

Calculation of electric field and potential distribution along nonceramic insulators considering the effects of conductors and transmission towers

Abstract: Calculation of the electric field and potential distribution is extensively performed in the design and development of nonceramic insulators for high voltage transmission applications. This paper applies a two-dimensional (2-D) and a three-dimensional (3-D) electric field analysis program to calculate the field distribution of the insulators under clean and dry conditions. A three-phase 345 kV transmission tower with nonceramic suspension insulators (two I-strings and one V-string) was analyzed with each phase as an individual case. The effects of conductors, tower configurations and grading devices on the electric field and potential distribution along the insulators were investigated. The calculation results show that the factors investigated have different levels of effects on the distribution.

Charged local defects in extended systems

Abstract: The conventional approach to treat charged defects in extended systems in first principles calculations is via the supercell approximation using a neutralizing jellium charge. I explicitly show that errors in the resulting electrostatic potential surface are comparable to a band gap energy in semiconductors, for cell sizes typically used in simulations. I present a method for eliminating divergence of the Coulomb potential in charged supercell calculations that correctly treats the electrostatic potential in the local vicinity of a charged defect, via a mixed boundary condition approach.

Flexural vibration analysis of sandwich beam coupled with piezoelectric actuator

Abstract: This paper provides a basic mechanics model for the flexural analysis of a sandwich beam coupled with a piezoelectric layer. The Euler beam model for a long and thin beam structure is employed, together with the electric potential satisfying the surface free charge condition for free vibration analysis. The distribution of the piezoelectric potential is obtained by including the Maxwell equation in the formulation. Based on the results of vibration analysis, it is shown that the dynamic characteristic of the entire structure is related to the position of the piezoelectric layer. More importantly, the mode shape distribution of the electric potential in the piezoelectric layer in the longitudinal direction is related to the transverse displacement, or more accurately the curvature, of the sandwich beam, and the latter is dependent on the boundary conditions. Hence, the commonly adopted assumption of uniform electric potential in the longitudinal direction needs to be carefully re-examined. The distribution of electric potential obtained serves as a guide for selecting the trial function for the mode shapes of the electric potential required in numerical methods, such as FE modelling, for coupled piezoelectric structures.

Spin configurations of a carbon nanotube in a nonuniform external potential

Abstract: We study, theoretically, the ground state spin of a carbon nanotube in the presence of an external potential. We find that when the external potential is applied to a part of the nanotube, its variation changes the single electron spectrum significantly. This, in combination with Coulomb repulsion and the symmetry properties of a finite length armchair nanotube, induces spin flips in the ground state when the external potential is varied. We discuss the possible application of our theory to recent measurements of Coulomb-blocked peaks and their dependence on a weak magnetic field in armchair carbon nanotubes. (c) 2000 The American Physical Society.

Effects of an external magnetic field, and of oblique radio-frequency electric fields on multipactor discharge on a dielectric

Abstract: This paper analyzes, separately, the effects of an external magnetic field, the rf magnetic field, and of an oblique rf electric field, on multipactor discharge on a dielectric. Using Monte Carlo simulation, we obtain the susceptibility diagram in terms of the magnetic field, the rf electric field, and the dc charging field for various dielectric materials. We find that a magnetic field parallel to either the rf electric field or the dc electric field does not qualitatively change the susceptibility diagram. However, an external magnetic field perpendicular to both the rf electric field and the dc electric field can significantly affect the susceptibility diagram. Thus oriented magnetic fields lower the upper susceptibility bound when the magnetic field strength is approximately equal to Bres[T]=0.036f(GHz), where f is the rf frequency. Both the lower and upper susceptibility boundary may be raised significantly by a large external magnetic field, B≫Bres. Susceptibility to single surface multipactor is greatest when the rf electric field is nearly parallel to the dielectric, but is dramatically decreased for angles of obliqueness greater than approximately 5°–10°. The rf magnetic field does not affect the lower boundary, but may extend the upper boundary greatly.

Charged local defects in extended systems

Abstract: The conventional approach to treat charged defects in extended systems in first principles calculations is via the supercell approximation using a neutralizing jellium charge. I explicitly show that errors in the resulting electrostatic potential surface are comparable to a band gap energy in semiconductors, for cell sizes typically used in simulations. I present a method for eliminating divergence of the Coulomb potential in charged supercell calculations that correctly treats the electrostatic potential in the local vicinity of a charged defect, via a mixed boundary condition approach.

Electric field variability associated with the Millstone Hill electric field model

Abstract: Joule heating that is generated at high latitudes in the thermosphere because of the magnetospherically imposed electric potential is proportional to the average of the square of the electric field (E field). Most theoretical Joule heating computations use only average electric fields, resulting in heating that is proportional to the square of the average E field. The computation of the average of the square of the E field requires knowledge about the statistical characteristics of E field variability associated with the average electric field model. In this paper we present the variability associated with the Millstone Hill bin-averaged empirical E field model [Foster et al. 1986] and discuss the implications of variability as an upper atmosphere energy source. We rebinned the radar plasma drift measurements from Millstone Hill, Massachusetts, in magnetic latitude and local time as a function of auroral activity and calculated the average electric fields and the variability associated with them as reflected in the bin standard deviations. We present the E field patterns and the associated variability for both quiet and disturbed geomagnetic conditions for the four seasons. We show that for an electric field model with a Gaussian distribution of small-scale variability around the mean, the average field and the variability have equal contributions to Joule heating generation.

Electroosmotic Flow through an Annulus.

Abstract: The electro-osmosis through an annulus is investigated. The electric potential and flow velocity profile are obtained by solving the linearized Poisson–Boltzmann equation and the Stokes equation. Both the thin and thick double layer limits are analyzed. Under the condition of thin double layer, the electro-osmotic mobility can be described by the Helmholtz–Smoluchowski equation with a geometry-dependent correction factor. There exist net flows even for zero area-averaged surface charge density due to the curvature differences between the inner and outer walls. The flow direction is determined by the sign of the charge on the inner cylinder. We also found that under certain circumstances the flow direction in an annulus is opposite to that in a capillary with the same sign of the net charge.

On the electric potentials inside a charged soft hydrated biological tissue: streaming potential versus diffusion potential.

Abstract: The main objective of this study is to determine the nature of electric fields inside articular cartilage while accounting for the effects of both streaming potential and diffusion potential. Specifically, we solve two tissue mechano-electrochemical problems using the triphasic theories developed by Lai et al. (1991, ASME J. Biomech Eng., 113, pp. 245-258) and Gu et al. (1998, ASME J. Biomech. Eng., 120, pp. 169-180) (1) the steady one-dimensional permeation problem; and (2) the transient one-dimensional ramped-displacement, confined-compression, stress-relaxation problem (both in an open circuit condition) so as to be able to calculate the compressive strain, the electric potential, and the fixed charged density (FCD) inside cartilage. Our calculations show that in these two technically important problems, the diffusion potential effects compete against the flow-induced kinetic effects (streaming potential) for dominance of the electric potential inside the tissue. For softer tissues of similar FCD (i.e., lower aggregate modulus), the diffusion potential effects are enhanced when the tissue is being compressed (i.e., increasing its FCD in a nonuniform manner) either by direct compression or by drag-induced compaction; indeed, the diffusion potential effect may dominate over the streaming potential effect. The polarity of the electric potential field is in the same direction of interstitial fluid flow when streaming potential dominates, and in the opposite direction of fluid flow when diffusion potential dominates. For physiologically realistic articular cartilage material parameters, the polarity of electric potential across the tissue on the outside (surface to surface) may be opposite to the polarity across the tissue on the inside (surface to surface). Since the electromechanical signals that chondrocytes perceive in situ are the stresses, strains, pressures and the electric field generated inside the extracellular matrix when the tissue is deformed, the results from this study offer new challenges for the understanding of possible mechanisms that control chondrocyte biosyntheses.

Damped dust oscillations as a plasma sheath diagnostic

Abstract: Dust can be suspended in a plasma sheath under certain conditions. An analysis of the dust trajectory as it approaches its equilibrium suspension height can provide a description of the spatial variation of the potential in the sheath. We describe an experiment in which we track such trajectories and calculate their oscillation frequencies, equilibrium heights and damping constants. These three measured parameters are then interpreted in such a manner as to reveal the neutral drag force on the dust and the curvature of the sheath electric potential in which the dust moves. We then calculate the charge on the dust particles suspended in the plasma sheath. We also show that, to a high degree of accuracy, the sheath potential predicted by several numerical models in the literature as well as by our experimental results is parabolic, i.e. the sheath field is quite linear.

Competitive Electrowetting of Polymer Surfaces by Water and Decane

Abstract: The present study deals with electrocapillarity effects in the three-phase system water−decane−polymer. This system consists of a decane droplet which adheres under water on a polymer surface. We applied an electric potential between an electrode beneath the polymer film and a platinum electrode immersed in the water. The polymer film acts as insulator: no steady electrical current flows between the electrodes. When the voltage is increased, the contact angle of decane and water changes, in some cases by up to 100°. The shape of the electrowetting curve (contact-angle cosine versus voltage plot) strongly depends on the type of the polymer. A saturation at higher voltages, connected with a hysteresis, is interpreted in terms of charge carrier injection into the polymer surface. Other deviations of the experimental results from the Lippmann−Young parabola concern a broadening with the polyolefins, a strong shift, and apparent reduction to one branch with two fluoropolymers, and for PET and a heavily oxidiz...

Exact two-electron wave packet dynamics of H2 in an intense laser field: Formation of localized ionic states H+H−

Abstract: We have developed an efficient grid method that can accurately deal with the electronic wave packet dynamics of two-electron systems in three-dimensional (3D) space. By using the dual transformation technique, we remove the numerical difficulties arising from the singularity of the attractive Coulomb potential. Electron–electron repulsion is incorporated into the wave packet propagation scheme without introducing any approximations. The exact electronic dynamics of H2 is simulated for the first time. At small internuclear distances (e.g., R=4 a.u.), an ionic component characterized by the structure H+H− is created in an intense laser field E(t) (intensity>1013 W/cm2 and λ≈720 nm) because an electron is transferred from the nucleus around which the dipole interaction energy for the electron becomes higher with increasing |E(t)|. The localized ionic structure is identified with the H− anion at the nucleus around which the dipole interaction energy becomes lower. Tunneling ionization proceeds via the formati...

Electric field distribution in polymer light-emitting electrochemical cells

Abstract: We use electroabsorption spectroscopy and modeling studies to probe the electric field in light-emitting electrochemical cells. At room temperature and constant applied bias, the steady-state internal field is zero for a range of biases. However, when the ions are frozen in place by cooling under steady bias, and the bias is subsequently changed, the profile of the electric potential resembles a typical p-n junction.

Diffusiophoretic Mobility of Spherical Particles at Low Potential and Arbitrary Double-Layer Thickness

Abstract: The diffusiophoretic motion of a charged spherical particle in an unbounded solution of a symmetrically charged electrolyte with a uniform prescribed concentration gradient is analytically studied. The electrokinetic equations which govern the electric potential profile, the ionic concentration distributions (or electrochemical potential energies), and the velocity field in the fluid phase surrounding the particle are linearized by assuming that the system is only slightly distorted from equilibrium. Using a regular perturbation method, these linearized equations are solved for a rigid dielectric sphere with its surface charge density (or ζ potential) as the small perturbation parameter. An analytical expression for the diffusiophoretic velocity of the colloidal particle in closed form is obtained from a balance between its electrostatic and hydrodynamic forces. This expression, which is correct to the second order of the surface charge density or ζ potential of the particle, is valid for an arbitrary val...

Electric potential produced by a dipole in a homogeneous conducting sphere

Abstract: The potential produced by a dipole in a homogeneous conducting sphere is useful in a simulation study. The current available solutions still suffer from some shortcomings. In this communication, a closed solution is developed for the precise calculation of the potential anywhere in the spherical model.

Surface potential at surface-interface junctions in SrTiO 3 bicrystals

Abstract: An analytical approach to the quantification of electrostatic force microscopy and scanning surface potential microscopy images of systems with electric potential inhomogeneity has been developed in order to determine the interface potential in a donor-doped \ensuremath{\Sigma}5 ${\mathrm{SrTiO}}_{3}$ grain boundary. The voltage dependencies of the electrostatic force gradient and surface potential verify the solutions. The distance dependencies of force gradient and surface potential were used to quantify the potential at the grain boundary-surface junction and the depletion width. Both measurements yield the same properties despite the difference in imaging mechanisms. The interface potential is shown to result from local charge rather than from a local variation in dielectric constant. The amount of charge implied by an interface potential of 30 mV would render detection of associated defects impossible, but these results represent a lower limit in intrinsic bulk grain boundary potential.

Nonlinear model for coherent electric field structures in the magnetosphere

Abstract: A new pseudo-three-dimensional electron hole in a magnetized plasma is possible when the low-frequency ion dynamics is taken into account. The newly found nonlinear Bernstein-Greene-Kruskal stationary solution, whose parallel phase velocity ranges between almost zero and the electron thermal speed, has the form of a cylinder that is tilted relative to the magnetic field. These structures are interpreted as three-dimensional electron holes coupled with hydrodynamic vortices, and provide a possible theoretical explanation for the POLAR and FAST satellite observations of coherent structures characterized by bipolar spikes of the parallel electric field and large perpendicular ion kinetic energies.

Electroosmotic Fluid Motion and Late-Time Solute Transport for Large Zeta Potentials

Abstract: Analytical and numerical methods are employed to determine the electric potential, fluid velocity, and late-time solute distribution for electroosmotic flow in a tube and channel at ζ potentials that are not necessarily small. The electric potential and fluid velocity are in general obtained by numerical means. In addition, new analytical solutions are presented for the velocity in a tube and channel in the extremes of large and small Debye layer thickness. The electroosmotic fluid velocity is used to analyze late-time transport of a neutral nonreacting solute. Zero- and first-order solutions describing axial variation of the solute concentration are determined analytically. The resulting expressions contain eigenvalues representing the dispersion and skewness of the axial concentration profiles. These eigenvalues and the functions describing transverse variation of the concentration field are determined numerically using a shooting technique. Results are presented for both tube and channel geometries ove...

Experimental study of potential structure in a spherical IEC fusion device

Abstract: The spherical inertial-electrostatic confinement (SIEC) concept is designed to focus and accelerate ions and electrons radially inward towards the center of a negatively biased, highly transparent spherical grid. The converging ions create a high-density plasma core where a high fusion rate occurs. In addition, under proper conditions, the ion and electron flows create a space-charge induced "double potential" well (a negative potential well nested inside a positive potential well). This structure traps high-energy ions within the virtual anode created by the double potential, providing a high fusion density in the trap volume. The present experiment was designed to verify double potential well formation and trapping by a measurement of the radial birth profile of energetic (3-MeV) protons produced by D-D fusion reactions in a deuterium discharge. This experiment was designed to operate at high perveance (0.4 to 1.4 mA/kV/sup 3/2/), where formation of a double well is predicted theoretically. Additional steps to aid well formation included: use of the unique Star mode of operation to obtain ion beam focusing down to -1.6 H the ballistic limit and the incorporation of a second electrically "floating" grid (in addition to the focusing/accelerating cathode grid) to reduce the ion radial energy spread to 0.34 mA/kV/sup 3/2/. As the perveance increased, the depth of the double well also increased. At the maximum perveance studied, 1.38 mA/kV/sup 3/2/ (corresponding to 80 mA and 15 kV), the negative potential well depth, corresponding to the measured proton-rate density, was estimated to be 22%-27% of the applied cathode voltage. This represents the first conclusive demonstration of double well formation in an SIEC, since prior measurements by other researchers typically yielded marginal or negative results.

An empirical model of the ionospheric electric potential

Abstract: We present an empirical model of the ionospheric electric potential based on output from the assimilative mapping of ionospheric electrodynamics technique (AMIE). The model is derived using a multivariable linear regression analysis technique to relate the potential at each grid point to the interplanetary magnetic field Byand Bzcomponents. AMIE output over a one week period was used to construct the model. The results of the model are very similar to other electric potential models; namely, the background pattern and the model response to Byand Bzare typical. The unique aspect of this model is the use of the linear response to changes in IMF for the forward prediction of the potential pattern. We show an example of how the model can significantly improve predictions of the potential when it is coupled to a real-time specification model such as the real-time version of AMIE.

Ion optic components for mass spectrometers

Abstract: Apparatus and methods are disclosed for manipulating charged particles. The charged particles are directed from a source thereof into a zone. A first electrical potential is generated in the zone. Simultaneously, a second electrical potential is generated outside the zone. The second electrical potential penetrates into the zone and combines with the first electrical potential to form an oscillating electric potential field having predetermined characteristics sufficient to manipulate the charged particles. The manipulating of the charged particles includes, e.g., transporting, collisional cooling, collisional induced dissociating and collisional focusing. In one embodiment an apparatus comprises a hollow first element and a hollow second element. The second element is disposed within the first element. The second element has at least two openings in a wall thereof. The openings are elongated and radially disposed with respect to the axis of the second element. The length of the openings is at least about 20% of the length of the second element The first element and the second element each are adapted independently to receive a voltage to generate within the second element an electric potential having predetermined characteristics. The apparatus and methods of the invention have particular application to the field of mass spectrometry.