Showing papers on "Electric potential published in 2004"

Electrochemomechanical Energy Conversion in Nanofluidic Channels

Abstract: When the Debye length is on the order of or larger than the height of a nanofluidic channel containing surface charge, a unipolar solution of counterions is generated to maintain electrical neutrality A pressure-gradient-driven flow under such conditions can be used for ion separation, which forms the basis for electrochemomechanical energy conversion The current−potential (I−φ) characteristics of such a battery were calculated using continuum dynamics When the bulk concentration is large and the channel does not become a unipolar solution of counterions, both the current and potential become small On the other hand, when bulk concentration is so much smaller, the mass diffusion becomes the rate-controlling step and the potential drops rapidly in the high current density region When the Debye length of the solution is about half of the channel height, the efficiency is maximized

Poole-Frenkel electron emission from the traps in AlGaN/GaN transistors

Abstract: Defect-related localized electronic states in AlGaN/GaN transistors give rise to commonly observed charge trapping phenomena. Electron dynamics through the trapping centers is strongly affected by electric fields, which can exceed values of 10(6) V/cm during device operation. The field-assisted emission characteristics provide a unique way to determine the physical properties of the trapping centers. We present a detailed study of the effects of electric field and temperature on the rate of electron emission from the barrier traps in AlGaN/GaN high-electron-mobility transistors. We demonstrate that for temperatures above 250 K, the Poole-Frenkel (PF) emission is the dominant mechanism for electrons to escape from the trapping centers. The emission rate increases exponentially with the square root of the applied field consistent with the decrease of the apparent activation energy predicted by the PF model. We find that the observed trapping center is described by the attractive long-range Coulomb potential with the zero-field binding energy of similar to0.5 eV. (C) 2004 American Institute of Physics.

Electron Tunneling at the TiO2/Substrate Interface Can Determine Dye-Sensitized Solar Cell Performance

Abstract: The electric potential distribution in dye-sensitized solar cells plays a major role in the operation of such cells. Models based on a built-in electric field which sets the upper limit for the open circuit voltage (Voc) and/or the possibility of a Schottky barrier at the interface between the mesoporous wide band gap semiconductor and the transparent conducting substrate have been presented. We show that I−V characteristics in the dark and upon illumination are very well explained by electron tunneling, rather than transport over a Schottky barrier, at this interface. Our calculations, based on tunnel currents, show that a discontinuity of the conduction band at the TiO2/FTO interface, rather than a built-in electric field, suffices for efficient electron transfer through this interface, and, thus, for efficient operation of this type of solar cell. Clearly, this will hold only if the photoinduced electrostatic potential barrier between the transparent conducting substrate and the mesoporous wide band ga...

Liquid crystal display and driving method thereof

Abstract: A liquid-crystal display including a liquid-display panel having a first electrode and a second electrode, with a liquid crystal therebetween, and a driver for supplying a first electric potential to the first and the second electrodes with a duty that is set to have a ratio of a display-on period to a set AC driving period exceeding 50%, and, during a display-off period of the set AC driving period, the driver supplies a second electric potential to the first and the second electrode, the second electric potential having an opposite polarity to the first electric potential and a greater level than that of the first electric potential Consequently, the liquid-crystal display and the driving method thereof can prevent the transmittance loss and thus increase brightness

Two-dimensional trapping of dipolar molecules in time-varying electric fields.

Abstract: Simultaneous two-dimensional trapping of neutral dipolar molecules in low- and high-field seeking states is analyzed. A trapping potential of the order of 20 mK can be produced for molecules such as ${\mathrm{ND}}_{3}$ with time-dependent electric fields. The analysis is in agreement with an experiment where slow molecules with longitudinal velocities of the order of $20\text{ }\text{ }\mathrm{m}/\mathrm{s}$ are guided between four 50 cm long rods driven by an alternating electric potential at a frequency of a few kHz.

Numerical Modeling and Performance Study of a Tubular SOFC

Abstract: A quasi-2D model was proposed and made available for numerical studies on the performance of a single tubular solid oxide fuel cell (SOFC) under practical operating conditions. The model takes account of the air and fuel flow velocity fields, ohmic and thermodynamic heat generation, convective heat-transfer, mass transfer of participating chemical species including the electrochemical processes, and the electric potential and electric current in the electrodes and electrolyte. Numerical computation was carried out to test the proposed model for a single unit cell having a specific geometry being operated at a few different thermal and composition conditions for the inlet fuel and air flows. Obtained numerical results show that the quasi-two-dimensional approximation adopted in the model to mitigate the computational cost effectively can work reasonably well. At low electric current density, the cell terminal voltage was overpredicted. In order to improve the model on this point, the simple treatment adopted for the activation and concentration polarization in the model must be replaced by a more sophisticated approach in future studies. Discussions were further given conceming the obtained results for the overall cell performance and the detailed features of the velocity, thermal, and mass-transfer fields in the cell in addition to the local electrochemical characteristics. It is suggested that the air flow convective heat-transfer is important as a cooling means and that overpotential due to concentration polarization is more serious for the cathode side than for the anode side. All the presented results including the electricity conversion efficiency were observed to agree reasonably well with the popularly accepted cell performance.

Electro-osmotic streaming on application of traveling-wave electric fields.

Abstract: We describe ac electro-osmotic flow of an aqueous electrolyte on application of a traveling-wave electric field Depending on the frequency of the applied traveling wave, the interaction of the electric double layer charge and the tangential electric field leads to fluid flow in the direction of the traveling wave We have derived two theoretical models that describe this flow as a function of the amplitude of the applied electric potential, the signal frequency, and the material properties of the system The first is based on a capacitative model and is limited to frequencies much lower than the double layer relaxation frequency The second is an analytical solution of the electrokinetic equations and is also valid at higher frequencies We provide experimental evidence that streaming takes place on application of a traveling wave of potential by tracing the movements of fluorescent latex beads over a spiral electrode structure Streaming takes place at applied potentials low enough for the method to be easily integrated into lab-on-a-chip devices

Systematic characterization of low-pressure capacitively coupled hydrogen discharges

Abstract: This paper presents a systematic characterization of pure hydrogen capacitively coupled discharges, produced in a parallel plate cylindrical setup. A two-dimensional, time-dependent fluid model is used to describe the production, transport, and destruction of electrons and positive ions H+, H2+, and H3+, at different frequencies (13.56–60 MHz), pressures (0.2–8 Torr), rf applied voltages (50–450 V) and geometric dimensions (1.6–12.8 cm radii and 1.6–6.4 cm interelectrode distances). A good agreement is found between calculation results and experimental measurements for the coupled electrical power, the plasma potential, and the self-bias potential, at various frequencies and rf applied voltages. However, the model generally underestimates the electron density with respect to its measured values. The paper discusses different space-time events, such as the development of double-ionization structures or the occurrence of field inversion and field reversal phenomena. The dependencies on pressure and frequency of the time-average electric field distribution are analyzed and related to the electron displacement within space-charge sheaths. This study is later used to understand the variations of the hydrogen dissociation rate, with changes in discharge operating conditions. The influence of reactor dimensions on the spatial profiles of the plasma potential, the rf electric field, the electron density, and the electron mean energy are analyzed in terms of discharge symmetry. An investigation of the space-time averaged rf electric field variations, with changes in the applied voltage, pressure, and geometric dimensions is carried out. These variations are shown to follow a universal similarity curve, if an adequate normalization is used when plotting the rf electric field as a function of pressure. This innovative representation of rf discharges allows a univocal definition of a reactor working point, for given operating conditions.

Thin electron current sheets and their relation to auroral potentials

Abstract: [1] Using the two-dimensional, steady-state Vlasov theory derived in a previous paper, we further investigate properties of thin current sheets that result from exact particle motion. The self-consistent quasi-neutral structure of thin current sheets generally requires the presence of an electric potential, which is constant along field lines. We establish that potential differences of up to the ion thermal potential kTi/e may be present when the current sheet thickness becomes comparable to or smaller than a typical ion Larmor radius. We further explore the possible association of thin current sheets with the perpendicular electric fields that are part of the U-shaped potential structures above auroral arcs. For sufficiently small scales, the converging perpendicular electric field corresponds to two oppositely directed thin Hall current sheets. We construct such double current sheet models and confirm that they result in potentials with the postulated shape and, for sufficiently small scales, in substantial potential differences (in fractions of the ion thermal potential kTi/e). The formation of such double current sheets, found recently in simulations of an earthward collapsing entropy-depleted magnetic flux tube (“bubble”), may provide a further link between bursty bulk flows, bubbles, and auroral features.

Electrical Response of Flow, Diffusion, and Advection in a Laboratory Sand Box

Abstract: Self-potential monitoring (SPM) is one of the most promising geophysical methods for hydrologic applications, since any change in subsurface water flow, chemistry, or thermodynamics can induce an electrical response. However, major difficulties may arise because different couplings (e.g., electrokinetics and electrodiffusion) can occur simultaneously. We performed laboratory experiments to isolate the electric response of flow during conditions of constant composition, of ionic diffusion of NaCl in stagnant fluid, and of advective transport of NaCl and KCl. For this purpose, fluid flow and/or salt diffusion were generated in a rectangular sand box, and the resulting electric potential differences were measured between custom-made, small, unpolarizable electrodes. In pure electrokinetic experiments (i.e., flow of water with constant salinity), the electric signal was proportional to the hydraulic gradient and to the salinity, in agreement with previous experimental and theoretical results. The other experiments showed that diffusive and advective transport of salt (i.e., in stagnant and flowing fluid conditions, respectively) can generate significant electric potential differences. Monitoring these potential differences allows determination of the motion of the concentration front in the sand box.

Charge pump circuit

Abstract: A leakage path through a parasitic diode in a charge transfer MOS transistor is cut off to prevent increase in the power consumption and loss of control of a charge pump circuit. A first charge transfer MOS transistor and a second charge transfer MOS transistor are N-channel type and are connected in series with each other. A ground electric potential VSS is supplied to a source of the first charge transfer MOS transistor as an input electric potential, and an output electric potential is obtained from an output terminal connected with a drain of the second charge transfer MOS transistor. A back gate of the first charge transfer MOS transistor is set by a first switching circuit to either an electric potential at a connecting node between the first and the second charge transfer MOS transistors or the ground electric potential VSS.

Local Molecular Dynamics with Coulombic Interactions

Abstract: We propose a local, $\mathcal{O}(N)$ molecular dynamics algorithm for the simulation of charged systems. The long ranged Coulomb potential is generated by a propagating electric field that obeys modified Maxwell equations. On coupling the electrodynamic equations to an external thermostat we show that the algorithm produces an effective Coulomb potential between particles. On annealing the electrodynamic degrees of freedom the field configuration converges to a solution of the Poisson equation much like the electronic degrees of freedom approach the ground state in ab initio molecular dynamics.

Nonuniform radio-frequency plasma potential due to edge asymmetry in large-area radio-frequency reactors

Abstract: In small area capacitive reactors, the rf and dc components of the plasma potential can be assumed to be uniform over all the plasma bulk because of the low plasma resistivity. In large area reactors, however, the rf plasma potential can vary over a long range across the reactor due to rf current flow and the nonzero plasma impedance. A perturbation in rf plasma potential, due to electrode edge asymmetry or the boundary of a dielectric substrate, propagates along the resistive plasma between capacitive sheaths. This is analogous to propagation along a lossy conductor in a transmission line and the damping length of the perturbation can be determined by the telegraph equation. Some consequences are the following: (i) The spatial variation in sheath rf amplitudes causes nonuniform rf power dissipation near to the reactor sidewalls. (ii) The surface charge and potential of a dielectric substrate can be negative and not only positive as for a uniform rf plasma potential. The variation of sheath dc potential across a dielectric substrate causes nonuniform ion energy bombardment. (iii) The self-bias voltage depends on the plasma parameters and on the reactor and substrate dimensions-not only on the ratio of electrode areas. (iv) The nonuniform rf plasma potential in presence of the uniform dc plasma potential leads to nonambipolar dc currents circulating along conducting surfaces and returning via the plasma. Electron current peaks can arise locally at the edge of electrodes and dielectric substrates. Perturbations to the plasma potential and currents due to the edge asymmetry of the electrodes are demonstrated by means of an analytical model and numerical simulations. (C) 2004 American Institute of Physics.

Effect of varying electric potential on surface-plasmon resonance sensing.

Abstract: The high sensitivity of surface-plasmon resonance (SPR) sensors allows measurements of small variations in surface potentials to be made. We studied the changes of the SPR angle when an oscillating electric potential was applied to a gold film on which surface plasmons were excited. The shifts of the SPR resonance angle were observed for various aqueous solutions as an adjacent medium. A model that takes into account the redistribution of charges at the double layer near the metal-liquid interface as well as the oxidation of the gold film was developed. It was found that a change in the electronic density at voltages below the oxidation potential and, in addition, the oxidation of the gold surface above this potential are the main mechanisms that account for the observed dependences. It was shown that relatively slow oxidation-reduction processes can explain the observed hysteresis effect. Application of these techniques to studies of dielectric properties and conformational changes of polar biomolecules, such as tubulin, are discussed.

Pulsed volume discharge in a nonuniform electric field at a high pressure and the short leading edge of a voltage pulse

Abstract: It is shown that a volume discharge is formed in a nonuniform electric field for the short leading edge of a voltage pulse and nanosecond pulse duration without any additional preionisation source in various gases at pressures higher than atmospheric (6 atm in helium and 3 atm in nitrogen). Lasing at atomic transitions in Xe is obtained in an Ar—Xe mixture under a pressure of 1.2 atm for an active length of 1.5 cm. A record-high specific power input (more than 0.8 GW cm-3 under a pressure 1 atm in air) is realised in the volume discharge stage. The volume discharge is formed due to preionisation of the discharge gap by fast electrons accelerated due to amplification of the electric field in the cathode region and in the gap. In a nonuniform electric field, volume discharge is realised under a quasistationary voltage from 10 to 180 kV across the gap, at a pulse repetition rate of up to 160 Hz and for various discharge gap geometries.

Radial electric field and transport near the rational surface and the magnetic island in LHD

Abstract: The structure of the radial electric field and heat transport at the magnetic island in the large helical device (LHD) are investigated by measuring the radial profile of the poloidal flow with charge exchange spectroscopy and measuring the time evolution of the electron temperature with ECE. A vortex-like plasma flow along the magnetic flux surface inside the magnetic island is observed when the n/m = 1/1 external perturbation field becomes large enough to increase the magnetic island width above a critical range (15–20% of minor radius) in LHD. This convective poloidal flow results in a non-flat space potential inside the magnetic island. The sign of the curvature of the space potential (∂2Φ/∂r2, where Φ is the space potential) depends on the radial electric field at the boundary of the magnetic island. The heat transport inside the magnetic island is studied with a cold pulse propagation technique. The experimental results show the existence of radial electric field shear at the boundary of the magnetic island and a reduction in heat transport at the boundary and inside the magnetic island.

A Coupled Zig-Zag Third-Order Theory for Piezoelectric Hybrid Cross-Ply Plates

Abstract: A new zig-zag coupled theory is developed for hybrid cross-ply plates with some piezoelectric layers using third-order zig-zag approximation for the inplane displacements and sublayer wise piecewise linear approximation for the electric potential. The theory considers all electric field components and can model open and closed-circuit boundary conditions. The deflection field accounts for the transverse normal strain due to the piezoelectric d 33 coefficient. The displacement field is expressed in terms of five displacement variables (which are the same as in FSDT) and electric potential variables by satisfying exactly the conditions of zero shear stresses at the top and bottom, and their continuity at layer interfaces. The governing equations are derived from the principle of virtual work. Comparison of the Navier solutions for the simply-supported plates with the analytical three-dimensional piezoelasticity solutions establishes that the present efficient zig-zag theory is quite accurate for moderately thick plates.

Evolution and memory effects in the homogenization limit for electrical conduction in biological tissues

Abstract: We study a problem set in a finely mixed periodic medium, modelling electrical conduction in biological tissues The unknown electric potential solves standard elliptic equations set in different conductive regions (the intracellular and extracellular spaces), separated by a dielectric surface (the cell membranes), which exhibits both a capacitive and a nonlinear conductive behaviour Accordingly, dynamical conditions prevail on the membranes, so that the dependence of the solution on the time variable t is not only of parametric character As the spatial period of the medium goes to zero, the electric potential approaches in a suitable sense a homogenization limit u0, which keeps the prescribed boundary data, and solves the equation This is an elliptic equation containing a term depending on the history of the gradient of u0; the matrices B0, A1 in it depend on the microstructure of the medium More exactly, we have that, in the limit, the current is still divergence-free, but it depends on the history of the potential gradient, so that memory effects explicitly appear The limiting equation also contains a term ℱ keeping trace of the initial data

Temperature gradient in Hall thrusters

Abstract: Plasma potentials and electron temperatures were deduced from emissive and cold floating probe measurements in a 2 kW Hall thruster, operated in the discharge voltage range of 200–400 V. An almost linear dependence of the electron temperature on the plasma potential was observed in the acceleration region of the thruster both inside and outside the thruster. This result calls into question whether secondary electron emission from the ceramic channel walls plays a significant role in electron energy balance. The proportionality factor between the axial electron temperature gradient and the electric field is also significantly smaller than might be expected by fluid models.

Stable plasma configurations in a cylindrical magnetron discharge

Abstract: Transition between different plasma configurations is studied in a system with negative biased cylindrical target in crossed E×B fields. It was found that the diffuse plasma torus formed around the cylindrical target in relatively small magnetic field (0.02T on target surface) changes the shape with magnetic field to form a thin disk with a width lower than 1cm when target voltage is less than −400V. The target current decreases sharply when the magnetic field reaches some critical value. When the target voltage exceeds 400V, the target current increases with the magnetic field and the plasma has always toroidal shape. The plasma behavior can be understood taking in account the interaction of the drift currents and the magnetic field.