Showing papers on "Space charge published in 2007"

Carbon materials for supercapacitor application.

Abstract: The most commonly used electrode materials for electrochemical capacitors are activated carbons, because they are commercially available and cheap, and they can be produced with large specific surface area. However, only the electrochemically available surface area is useful for charging the electrical double layer (EDL). The EDL formation is especially efficient in carbon pores of size below 1 nm because of the lack of space charge and a good attraction of ions along the pore walls. The pore size should ideally match the size of the ions. However, for good dynamic charge propagation, some small mesopores are useful. An asymmetric configuration, where the positive and negative electrodes are constructed from different materials, e.g., activated carbon, transition metal oxide or conducting polymer, is of great interest because of an important extension of the operating voltage. In such a case, the energy as well as power is greatly increased. It appears that nanotubes are a perfect conducting additive and/or support for materials with pseudocapacitance properties, e.g. MnO2, conducting polymers. Substitutional heteroatoms in the carbon network (nitrogen, oxygen) are a promising way to enhance the capacitance. Carbons obtained by one-step pyrolysis of organic precursors rich in heteroatoms (nitrogen and/or oxygen) are very interesting, because they are denser than activated carbons. The application of a novel type of electrolyte with a broad voltage window (ionic liquids) is considered, but the stability of this new generation of electrolyte during long term cycling of capacitors is not yet confirmed.

Anode-interface localized filamentary mechanism in resistive switching of TiO2 thin films

Abstract: The filamentary resistance switching mechanism of a Pt∕40nm TiO2∕Pt capacitor structure in voltage sweep mode was investigated. It was unambiguously found that the conducting filaments propagate from the cathode interface and that the resistance switching is induced by the rupture and recovery of the filaments in the localized region (3–10nm thick) near the anode. The electrical conduction behavior in the high resistance state was well explained by the space charge limited current (SCLC) mechanism that occurs in the filament-free region. The various parameters extracted from the SCLC fitting supported the localized rupture and formation of filaments near the anode.

Conical Forward THz Emission from Femtosecond-Laser-Beam Filamentation in Air

Abstract: We attribute a strong forward directed THz emission from femtosecond laser filaments in air to a transition-Cherenkov emission from the plasma space charge moving behind the ionization front at light velocity. Distant targets can be easily irradiated by this new source of THz radiation.

Tuning magnetoresistance between positive and negative values in organic semiconductors.

Abstract: Magnetic-field-dependent injection current, namely magnetoresistance, is readily observable in organic semiconductor devices. This provides a non-contact approach to tune organic optoelectronic properties by using a magnetic field. Here, we demonstrate that this magnetoresistance can be changed between positive and negative values by adjusting the dissociation and charge reaction in excited states through changing the bipolar charge injection in organic light-emitting diodes. This finding reveals that the magnetic-field-dependent generation of secondary charge carriers from the dissociation and charge reaction affects the injection current by forming further space charges at the organic–electrode interfaces and therefore accounts for the tunable magnetoresistance. Furthermore, the dissociation and charge reaction have opposite dependences on magnetic field in the generation of secondary charge carriers, consequently leading to negative and positive magnetoresistance, respectively. As a result, adjusting the dissociation and charge reaction in excited states provides a convenient pathway to tune the magnetoresistance in organic semiconductors.

Electro-osmotic slip and electroconvective instability

Abstract: Electric conduction from an electrolyte solution into a charge selective solid, such as ion exchange membrane or electrode, becomes unstable when the electrolyte concentration near the interface approaches zero owing to diffusion limitation. The sequence of events leading to instability is as follows: upon the decrease of the interface concentration, the electric double layer at the interface transforms from its common quasi-equilibrium structure to a different, non-equilibrium one. The key feature of this new structure is an extended space charge added to the usual one of the quasi-equilibrium electric double layer. The non-equilibrium electro-osmotic slip related to this extended space charge renders the quiescent conductance unstable. A unified asymptotic picture of the electric double-layer undercurrent, encompassing all regimes from quasi-equilibrium to the extreme non-equilibrium one, is developed and employed for derivation of a universal electro-osmotic slip formula. This formula is used for a linear stability study of quiescent electric conduction, yielding the precise parameter range of instability, compared with that in the full electroconvective formulation. The physical mechanism of instability is traced both kinematically, in terms of non-equilibrium electro-osmotic slip, and dynamically, in terms of forces acting in the electric double layer.

Polymeric HVDC Cable Design and Space Charge Accumulation. Part 1: Insulation/Semicon Interface

Abstract: From theory and experiments, it can be deduced that materials for DC applications should not accumulate a large amount of space charge if accelerated degradation of the insulation system is to be avoided. Therefore, the characterization of DC insulation must take into account the evaluation of space charge accumulation. This cannot be done exhaustively without taking a system approach considering both the semiconductive material and the insulation, in particular, the properties of the semicon/insulation interface. The latter interface, in fact, plays a major role in space charge injection/accumulation in the insulation bulk. Having analyzed different semiconductive and insulating materials candidate for HVDC cable applications, the best solution to be exploited for HVDC cable design would be the combination showing a high threshold for space charge accumulation, a small rate of charge accumulation as a function of electric field and a small activation energy, i.e., a space charge amount less dependent on temperature. Therefore, space charge measurements will provide important information to cable material manufacturers with the aim of tailoring insulation and semicon specifically for HVDC application and, thus, improving the reliability of polymeric cables.

Field-induced resistive switching based on space-charge-limited current

Abstract: Polycrystalline (Ba,Sr)(Zr,Ti)O3 thin films sandwiched between two Pt electrodes have been revealed to exhibit hysteretic current-voltage (I-V) characteristics and resistive switching at room temperature. High- and low-resistance states, as well as a less abrupt state transition, occur during the voltage cycle. The maximum ratio between these two resistance states is about 230. Analyses of I-V behaviors have been executed, and it is proposed that space-charge-limited-current conduction in higher voltage region caused by asymmetric electron trapping centers is responsible for such transition of resistance states.

Gate coupling and charge distribution in nanowire field effect transistors.

Abstract: We have modeled the field and space charge distributions in back-gate and top-gate nanowire field effect transistors by solving the three-dimensional Poisson's equation numerically. It is found that the geometry of the gate oxide, the semiconductivity of the nanowire, and the finite length of the device profoundly affect both the total amount and the spatial distribution of induced charges in the nanowire, in stark contrast to the commonly accepted picture where metallic dielectric properties and infinite length are assumed for the nanowire and the specific geometry of the gate oxide is neglected. We provide a comprehensive set of numerical correction factors to the analytical capacitance formulas, as well as to numerical calculations that neglect the semiconductivity and finite length of the nanowire, that are frequently used for quantifying carrier transport in nanowire field effect transistors.

Charge injection process in organic field-effect transistors

Abstract: The charge injection process in top-contact organic field-effect transistors was energetically observed with displacement of the Fermi level as a result of scanning the gate voltage. Doping of charge-transfer molecules into the metal/organic interface resulted in low interface resistance, which unveiled the bulk transport of the injected charges from the contact metal into the channel. The authors found that the bulk transport clearly obeys the Meyer-Neldel rule, according to which the exponential density of states near the band edge limits the charge injection.

Negative capacitance in organic semiconductor devices: Bipolar injection and charge recombination mechanism

Abstract: The authors report negative capacitance at low frequencies in organic semiconductor based diodes and show that it appears only under bipolar injection conditions. They account quantitatively for this phenomenon by the recombination current due to electron-hole annihilation. Simple addition of the recombination current to the well established model of space charge limited current in the presence of traps yields excellent fits to the experimentally measured admittance data. The dependence of the extracted characteristic recombination time on the bias voltage is indicative of a recombination process which is mediated by localized traps.

Fast ignition by laser driven particle beams of very high intensity

Abstract: Anomalous observations using the fast ignition for laser driven fusion energy are interpreted and experimental and theoretical results are reported which are in contrast to the very numerous effects usually observed at petawatt-picosecond laser interaction with plasmas. These anomalous mechanisms result in rather thin blocks (pistons) of these nonlinear (ponderomotive) force driven highly directed plasmas of modest temperatures. The blocks consist in space charge neutral plasmas with ion current densities above 1010A∕cm2. For the needs of applications in laser driven fusion energy, much thicker blocks are required. This may be reached by a spherical configuration where a conical propagation may lead to thick blocks for interaction with targets. First results are reported in view of applications for the proton fast igniter and other laser-fusion energy schemes.

Electric field and electron orbits near a triple point

Abstract: Triple point, defined as the junction of metal, dielectric, and vacuum, is the location where electron emission is favored in the presence of a sufficiently strong electric field. In addition to being an electron source, the triple point is generally regarded as the location where flashover is initiated in high voltage insulation, and as the vulnerable spot from which rf breakdown is triggered. In this paper, we focus on the electric field distribution at a triple point of a general geometry, as well as the electron orbits in its immediate vicinity. We calculate the orbit of the first generation electrons, the seed electrons. It is found that, despite the mathematically divergent electric field at the triple point, significant electron yield most likely results from secondary electron emission when the seed electrons strike the dielectric. The analysis gives the voltage scale in which this electron multiplication may occur. It also provides an explanation on why certain dielectric angles are more favorabl...

{\rm N}_{2}(A\,^{3}\Sigma _{\rm u}^{+}) density measurement in a dielectric barrier discharge in N2 and N2 with small O2 admixtures

Abstract: This paper deals with the measurement of metastable state density in a dielectric barrier discharge in nitrogen and nitrogen with small admixtures of oxygen, operating in a Townsend-like discharge regime. The measurement is made by optical?optical double resonance-LIF, calibrated by a method based on the measurement of the ratio of nitrogen second positive system and NO-? emissions, and of NO density by LIF. A metastable density of the order of 1013?cm?3 was found in a nitrogen diffuse discharge. Addition of small oxygen concentrations to the discharge drives a transition to the filamentary regime that appears to be caused not by a marked decrease of the metastable density in the discharge but rather by a considerable increase of its quenching rate. Such an increase, due to collision quenching by O2 and O, strongly reduces the survival of the metastable between two discharge pulses. These observations are consistent with the idea that the diffuse regime can be due to a space charge memory effect due to the nitrogen triplet metastable, which is cancelled by the introduction of oxygen in the gas feed.

Anomalous polarization inversion in ferroelectrics via scanning force microscopy

Abstract: Local poling of ferroelectrics by the sharp conducting tip of a scanning force microscope (SFM) is studied experimentally and theoretically. The formation of the inverse domain under the SFM tip, where the polarization is oriented in the direction opposite to that of the poling field, is reported for bulk ferroelectrics (single crystals of solid solutions PbZn1/3Nb2/3O3–PbTiO3). This finding confirms earlier results on ferroelectric thick films, thus proving the universality of the anomalous polarization inversion in ferroelectric media. It is shown that the inverse domain grows with the increase of the poling voltage and duration and remains stable for a long time after the removal of electric field. The growth process is described by a dynamic model assuming that the appearance of inverse domains is due to a local internal electric field directed against the poling one. This field is attributed to the space charge formed beneath the SFM tip due to injection of charge carriers and their subsequent drift and trapping. Poling voltage and poling time dependences of the domain size are correctly described by the model. Implications of the anomalous polarization inversion for the domain engineering and dense data storage in ferroelectrics are discussed.

Induced electrokinetic transport in micro-nanofluidic interconnect devices.

Abstract: Hybrid micro-nanofluidic interconnect devices can be used to control analyte transfer from one microchannel to the other through a nanochannel under rest, injection, and recovery stages of operation by varying the applied potential bias. Using numerical simulations based on coupled transient Poisson-Nernst-Planck and Stokes equations, we examine the electrokinetic transport in a gateable device consisting of two 100 microm long, 1 microm wide negatively charged microchannels connected by a 1 microm long, 10 nm wide positively charged nanochannel under both positive and negative bias potentials. During injection, accumulation of ions is observed at the micro-nano interface region with the positive potential and depletion of ions is observed at the other micro-nano junction region. Net space charge in the depletion region gives rise to nonlinear electrokinetic transport during the recovery stage due to induced pressure, induced electroosmotic flow of the second kind, and complex flow circulations. Ionic currents are computed as a function of time for both positive and negative bias potentials for the three stages. Analytical expressions derived for ion current variation are in agreement with the simulated results. In the presence of multiple accumulation or depletion regions, we show that a hybrid micro-nano device can be designed to function as a logic gate.

Production of space charge at the boundaries of layer clouds

Abstract: [1] The ionosphere-Earth current density Jz creates space charge at the upper and lower boundaries of layer clouds. This occurs because clouds have an order of magnitude lower conductivity than the clear air at the same altitude, and as the current density flows through the boundaries, it creates a gradient of electric field that must be satisfied by the accumulation of space charge, according to Gauss's law. We have modeled the production of space charge and its partition between charges on droplets, aerosol particles, and ions and performed sensitivity tests for the variation of a number of relevant atmospheric parameters. We find typical droplet charges of 50–100 elementary charges, positive at cloud top and negative at cloud base, consistent with recent observations. The charges are of sufficient magnitude to suggest measurable electrical effects on scavenging of ice-forming nuclei and cloud condensation nuclei. The results are relevant to the modeling of solar or internally forced changes of Jz and space charge on cloud microphysics as a possible cause of small effects on weather and climate.

Two-dimensional analytical Mott-Gurney law for a trap-filled solid

Abstract: The letter presents a two-dimensional analytical model of the space charge limited (SCL) current injection in a solid with exponentially distributed trap energy state. By considering that the electrons are injected from an infinitely long emission strip of width W, the one-dimensional SCL current density is enhanced by a factor of 1+F(4∕π)∕(W∕L), where F=1∕(l+2) measures the mean position of the injected electrons in the solid of length L, and l is the ratio of the distribution of the traps to the free carriers. The analytical formula is verified by using a two-dimensional device simulator.

DC conduction and electrical breakdown of MgO/LDPE nanocomposite

Abstract: To understand basic electric properties of nano-sized magnesium oxide (MgO)/low-density polyethylene (LDPE) nanocomposite under DC voltage application, the volume resistivity, the space charge distribution, the breakdown strength and the short circuit tree were investigated. By the addition of nano-sized MgO filler, both the DC breakdown strength and the volume resistivity of LDPE increased. At the average DC electric field of about 120 kV/mm, a positive packet-like space charge was observed in LDPE without MgO nano-filler, whereas a little homogeneous space charge was observed in MgO/LDPE nanocomposite material at the front of electrode. The initiation probability of short circuit tree is decreased by addition of MgO nano-filler. And also, the length of short-circuit tree in the film with MgO nano-filler is shorter than that without MgO nano-filler. From these results, it is confirmed that the addition of MgO nano-filler leads to the improvement of DC electrical insulating properties of LDPE.

Influence of thermal treatment and residues on space charge accumulation in XLPE for DC power cable application

Abstract: The effects of cross-linking by-products (residues) quantity and thermal treatment on space charge accumulation and decay in manufacturer modified XLPEs for DC power cable application have been investigated using the pulsed electro-acoustic technique. The threshold stress for space charge generation among the modified and reference XLPEs during voltage-ramping was found to show considerable variation and to depend upon the material and the amount of residue present. However, the modified XLPE material was found to exhibit a higher threshold for space charge accumulation than the reference XLPE whatever the conditions. De-gassed samples were found to exhibit the highest threshold stress, with that of the modified de-gassed XLPE accumulating no space charge at all even after 24 h stressing at 70 kV. In general heterocharge regions were formed when the residues were present and homocharge or no charge was formed when the residues were removed by degassing. Differences were also found in the space charge decay following short-circuit (volts-off), with the decay of heterocharge being rapid, whereas that of homocharge being slow. The charge accumulations have been tentatively explained by the mechanisms of ion-pair separation when residues are present, and interfacial injection when residues are absent. Decay of the heterocharge is governed by ion-pair displacement just as the generation and will have a similar time scale. In contrast homocharge decay will be governed by charge de-trapping and extend to time scales well beyond that of injection for charge in the deepest traps

Maxwell-Wagner space charge effects on the Pb(Zr,Ti)O3–CoFe2O4 multilayers

Abstract: Electrical properties of Pb(Zr053Ti047)O3∕CoFe3O4 (PZT/CFO) multilayers (MLs) were investigated in the light of Maxwell-Wagner space charge effects and the data differ from those of BiFeO3∕CoFe3O4 nanocomposites These MLs are of special interest as weak field sensors of magnetism These data fit the Maxwell-Wagner space charge model of Catalan et al [Appl Phys Lett 77, 3078 (2000)] Very large temperature shifts in dielectric loss peak (>200K) are indicative of high space charge density at the PZT/CFO interfaces These conclusions are confirmed by spurious hysteresis loop that are similar to those of highly conducting specimens

Electrical characterization of metal/pentacene contacts

Abstract: We report on the results of electrical characterization experiments on metal/pentacene contacts. The metals explored were gold, silver, and platinum. Two kinds of devices were fabricated and tested: symmetric metal/pentacene/metal structures and nonsymmetric metal/pentacene/indium-tin-oxide structures. The rectification ratio of the latter devices was found to depend strongly on the metal and ranged between 2 and 103 for applied voltages of ±1V. Schottky barrier heights were extracted from the reverse current activation energy measurements over the temperature range of ∼240–340K. Numerical device modeling using the experimentally determined Schottky barrier heights yielded results that are consistent with the experimental current-voltage characteristics, and indicated the presence of a non-negligible (trapped) positive space charge in the pentacene films.

Mass storage in space charge regions of nano-sized systems (Nano-ionics. Part V).

Abstract: The consideration of space charge effects at interfaces results in the prediction of considerable storage anomalies in two-phase materials. They are particularly expected in nano-sized systems building a bridge between chemical and electrostatic storage. These effects are systematically treated in the model of an abrupt planar contact. Several examples of fundamental and/or practical interest are analysed in greater detail.

Radio-frequency discharges in oxygen: I. Particle-based modelling

Abstract: In this series of three papers we present results from a combined experimental and theoretical, particle-based study to quantitatively describe capacitively coupled radio-frequency discharges in oxygen. The particle-in-cell Monte Carlo model on which the theoretical description is based is described in this paper. It treats space charge fields and transport processes on an equal footing with the most important plasma–chemical reactions. For given external voltage and pressure, the model determines the electric potential within the discharge and the distribution functions for electrons, negatively charged atomic oxygen and positively charged molecular oxygen. Previously used scattering and reaction cross section data are critically assessed and in some cases modified. To validate our model, we compare the densities in the bulk of the discharge with experimental data and find good agreement, indicating that essential aspects of an oxygen discharge are captured.

Measurement of the surface potential decay of corona-charged polymer films using the pulsed electroacoustic method

Abstract: In this paper, the pulsed electroacoustic (PEA) technique that allows the determination of space charge in a dielectric material has been used to monitor the electrical potential decay of corona-charged polyethylene films of different thicknesses. To prevent possible disturbance of the surface charge during the PEA measurements, two thin polyethylene films were placed on both sides of the corona-charged sample. Charge profiles measured at different times were used to calculate the potential across the sample. The obtained potential decay was compared with the potential measured using the conventional method. Good agreement has been obtained. More importantly, the charge profile obtained using the PEA technique indicates that bipolar charge injection has taken place.

Hybrid dc–ac electron gun for fs-electron pulse generation

Abstract: We present a new concept of an electron gun for generating subrelativistic few-femtosecond (fs) electron pulses. The basic idea is to utilize a dc acceleration stage combined with an RF cavity, the ac field of which generates an electron energy chirp for bunching at the target. To reduce space charge (SC) broadening the number of electrons in the bunch is reduced and the gun is operated at a megahertz (MHz) repetition rate for providing a high average number of electrons at the target. Simulations of the electron gun were carried out under the condition of no SC and with SC assuming various numbers of electrons in the bunch. Transversal effects such as defocusing after the dc extraction hole were also taken into account. A detailed analysis of the sensitivity of the pulse duration to various parameters was performed to test the realizability of the concept. Such electron pulses will allow significant advances in the field of ultrafast electron diffraction.

Trapping of dielectric particles with light-induced space-charge fields

Abstract: Light-induced space-charge fields in lithium niobate crystals are used to trap and manipulate dielectric particles on the surface of such crystals. Without any external voltage source, strong field gradients are present in the proximity of the crystal surface. These are used to trap particles with diameters in the range between 100 nm and some tens of micrometers.

Radio-frequency discharges in Oxygen. Part 1: Modeling

Abstract: In this series of three papers we present results from a combined experimental and theoretical effort to quantitatively describe capacitively coupled radio-frequency discharges in oxygen. The particle-in-cell Monte-Carlo model on which the theoretical description is based will be described in the present paper. It treats space charge fields and transport processes on an equal footing with the most important plasma-chemical reactions. For given external voltage and pressure, the model determines the electric potential within the discharge and the distribution functions for electrons, negatively charged atomic oxygen, and positively charged molecular oxygen. Previously used scattering and reaction cross section data are critically assessed and in some cases modified. To validate our model, we compare the densities in the bulk of the discharge with experimental data and find good agreement, indicating that essential aspects of an oxygen discharge are captured.

Transient Electrical Response of Dye-Sensitized ZnO Nanorod Solar Cells

Abstract: A dye-sensitized solar cell was build with an electrode consisting of ZnO nanorods with a mean diameter of 80 nm instead of the commonly used nano colloidal TiO2 substrate. The electrical response of the cell under excitation with an alternating current (AC) voltage source was measured and analyzed in a wide frequency range (40 Hz−110 MHz). The results were compared to time-resolved measurements of the photocurrent generated by a nanosecond pulsed light source. Both measurements could be described by the same electrical equivalent circuit that consisted of passive circuit elements only. The slow response of the cell in the ms time range is mainly caused by the large space charge capacitance of the ZnO nanorod electrode. Because of the low impedance of the nanorod electrode, the photo response and the AC response is also influenced by the electrical properties of other interfaces in the cell, such as the counterelectrode. Thus the signal is more complex than the single RC response usually observed for plan...