Polarity Of Applied Voltage Research Articles

In Operando Study of Microsupercapacitors with Gel Electrolytes Using Nano‐Beam Synchrotron X‐ray Diffraction

AbstractSynchrotron radiation X‐ray diffraction (XRD) with nanoscale beam size was used here for in situ and in operando study of micro‐supercapacitors (MSC) with gel electrolyte and MXene Ti3C2Tx electrodes. The electrode structure was characterized as a function of applied voltage and distance from the gap separating electrodes using microscopic cells with cylindrical shape designed for transmission mode XRD. The devices with gel electrolytes based on H2SO4 (with H2O/PVA and DMSO/PVA) showed stable performance with no changes in MXene structure under voltage swaps between positive and negative values. Experiments with KI‐based electrolytes demonstrated changes of MXene structure correlated with decrease of energy storage parameters under conditions of increased operation voltage starting from 0.8 V. The optimal performance of the MSCs was observed when the MXene structure remained unchanged upon switching the applied voltage polarity. The changes of inter‐layer distance of MXene upon swap of applied voltage correlate with decrease of device performance and are undesirable for stable operation of MSC's. We also tested feasibility of X‐ray fluorescence (XRF) for characterization of electrolyte ion migration in MSCs using 2D element mapping. Irreversible sorption of iodine by MXene was found using XRF mapping of charged electrodes using standard in‐plane MSC device and KI electrolyte.

Voltage-induced modulation of the magnetic exchange in binuclear Fe(III) complex deposited on Au(111) surface.

We present a complete computational study devoted to the deposition of a magnetic binuclear complex on a metallic surface, aimed to obtain insight into the interaction of magnetically coupled complexes with their supporting substrates, as well as their response to external electrical stimuli applied through a surface-molecule-STM molecular junction-like architecture. Our results not only show that the deposition is favorable in two of the four studied orientations, but also, that the magnetic coupling is only slightly perturbed once the complex is adsorbed. We observe that the effects of the applied bias voltage on the magnetic coupling strongly depend on the molecule orientation with respect to the surface and the voltage polarity. Further analysis shows that this behavior is attributable to the stabilization/destabilization of the d-type singly occupied orbitals of the iron centers, reinforced by the strong local electric fields and induced charge densities only present in certain orientations of the deposited molecule and applied voltage polarity.

Numerical investigation of the role of linear and nonlinear forces in determining the direction of electric wind caused by atmospheric pressure DC corona discharge

In this study, the force generated by atmospheric positive and negative corona discharges was investigated using a simulation of a wire–cylinder configuration. We provided new insight into the atmospheric corona discharge by introducing a nonlinear force on the charged particles in the vicinity of the wire electrode. To elucidate the origin of both forces in corona discharges, we performed 2D simulations via COMSOL Multiphysics and MATLAB software. It was observed that the direction of nonlinear force is always from the wire to the cylinder regardless of the applied voltage polarity. It was illustrated that the corresponding nonlinear force of the positive corona is larger than that of the negative corona discharge. However, the span of the nonlinear force is greater in the negative corona discharge. The numerical simulation results showed that, in addition to the linear force (Coulomb force), a strong nonlinear force is generated around the wire electrode (powered electrode) that plays a complementary role in the production of electric wind caused by corona discharge. As this nonlinear force is limited to the vicinity of the wire electrode, it is possible to ignore the nonlinear force with a good approximation in the calculation of the total electrohydrodynamic force, but this force cannot be ignored in the process of forming the electric wind.

PLLA scaffolds with controlled surface potential and piezoelectricity for enhancing cell adhesion in tissue engineering

The effect of the surface potential of biomaterials on cell attachment and development in regenerative medicine is still an unexplored area driving many regeneration processes, especially in piezoelectric bone tissue. Within this study, we electrospun poly(l-lactide) scaffolds constructed of fibers with either higher (−600 mV) or lower (−300 mV) surface potential, which is controlled by applied voltage polarity during their production. Interestingly, this way, the piezoelectric performance of PLLA fibers can be enhanced. The direct measurement of the PLLA fiber surface potential using Kelvin probe force microscopy (KPFM) showed a good correlation with the zeta potential analysis. The piezoelectricity of PLLA fibers was verified with piezoresponse force microscopy (PFM), indicating that it can be enhanced by applying the positive voltage polarity to the nozzle during electrospinning. Importantly, the cell adhesion assay showed a significant effect of the higher surface potential of PLLA fibers on osteoblasts behavior and creating a favorable bioelectric microenvironment. We observed enhanced initial adhesion of cells in the first 5 h with no additional effect of surface potential on proliferation, morphology, or collagen production. It was demonstrated that electrospun PLLA fibers with tunable surface potential and piezoelectricity are excellent for constructing bone tissue engineering scaffolds.

Computational study on the discharge dynamics of atmospheric pressure He plasma driven by high frequency AC voltage

A two dimensional self-consistent fluid model has been established to investigate the discharge dynamics of double-ring electrode He atmospheric pressure plasma jet (APPJ) driven by high frequency AC voltage. The difference of the internal stream and external jet and the influence of the change of applied voltage polarity on plasma discharge characteristics has been discussed. It has been discovered that the capacitive breakdown characteristic of the double ring electrode significantly enhances the intensity of the APPJ. The discharge intensity of the external jet is stronger than that of the internal stream and the propagation speed of the external jet is faster than that of the internal stream due to the ionization and Penning ionization of N2 and O2. Therefore, the density of reactive species in the external jet is greater than that in the internal stream. When the negative voltage is applied to the downstream electrode, the propagation direction of the internal stream changes to the downstream electrode. The ionization of the external jet is also concentrated near the downstream electrode and in the streamer head. The radial propagation distance of the external jet on the dielectric surface continues to increase and the peak value of the radial electric field is concentrated at the streamer head. When the applied voltage changes from negative to positive, the propagation direction of the internal stream turns to the upstream electrode and the upstream jet is formed above the electrode. At the beginning of the positive cycle, the radial propagation distance of the external jet is shortened due to the effects of the electron attachment of O2 and the radial electric field. With the increase of applied voltage, the ionization in the streamer head gradually increases, which promoted the radial propagation of external jet.

Electric discharge initiation in water with gas bubbles: A time scale approach

High voltage nanosecond pulse driven electric discharges in de-ionized water with an argon bubble suspended between two electrodes were experimentally investigated. Two electrode configurations were used to temporally resolve the time scales of the discharge from the applied voltage rise time (7 ns), through the end of the first pulse (∼30 ns), and longer (>50 ns). We found that, in positive and negative applied voltage polarities, discharge initiates in the water at the tip of the anode. The discharge in the water rapidly extends (∼104 m/s) to the apex of the bubble and light emitted from inside the bubble begins to form. The steep rate of rise of the applied voltage (dV/dt<4 kV/ns) and the short time for the development of discharge in the water suggest that cavitation is a likely mechanism for discharge initiation and propagation in water. In addition, the short duration of the applied voltage pulse results in only a partial Townsend discharge inside the bubble.

Photoswitchable Binary Nanopore Conductance and Selective Electronic Detection of Single Biomolecules under Wavelength and Voltage Polarity Control.

We fabricated photoregulated thin-film nanopores by covalently linking azobenzene photoswitches to silicon nitride pores with ∼10 nm diameters. The photoresponsive coatings could be repeatedly optically switched with deterministic ∼6 nm changes to the effective nanopore diameter and of ∼3× to the nanopore ionic conductance. The sensitivity to anionic DNA and a neutral complex carbohydrate biopolymer (maltodextrin) could be photoswitched "on" and "off" with an analyte selectivity set by applied voltage polarity. Photocontrol of nanopore state and mass transport characteristics is important for their use as ionic circuit elements (e.g., resistors and binary bits) and as chemically tuned filters. It expands single-molecule sensing capabilities in personalized medicine, genomics, glycomics, and, augmented by voltage polarity selectivity, especially in multiplexed biopolymer information storage schemes. We demonstrate repeatedly photocontrolled stable nanopore size, polarity, conductance, and sensing selectivity, by illumination wavelength and voltage polarity, with broad utility including single-molecule sensing of biologically and technologically important polymers.

Polarity dependent electrowetting for directional transport of water through patterned superhydrophobic laser induced graphene fibers

The possibilities of the precise control of wetting properties of a series of laser-induced graphene (LIG) films consisting of microscale air pockets on top of nano-scale surface roughness using electrowetting are demonstrated. By application of a marginal DC bias (∼2 V), water can efficiently wet as well as can be pumped through the superhydrophobic LIG substrates. Interestingly, the electrowetting phenomenon is strongly dependent on the applied voltage polarity and it causes an abrupt wetting transition from superhydrophobic (contact angle ∼152°) Cassie state to superhydrophilic (contact angle ∼7°) Wenzel state on the LIG films. By analyzing the voltage polarity dependent electrowetting results with an equivalent electrical circuit model at the solid-liquid interface, and considering the hierarchical dual surface roughness (micro-nano scale), the transition between the “slippy” Cassie state and the “sticky” Wenzel states is explained. Furthermore, we demonstrate that the unique structural characteristics of the custom-designed micropatterned LIGs, with precisely tailored surface energy by simple post-annealing treatment, enable easy preparation of superhydrophobic LIG films. The approach to prepare stable superhydrophobic LIG with voltage polarity dependent wetting mode transition is used here to controllably transport of water through 3D porous LIG surfaces.

Switching effects in Ag2S–Ag3AsS3 quantum dots

Photoluminescence and wavelength-modulated transmission spectra of Ag3AsS3 crystals were investigated at 10 K Ag3AsS3 structures with Ag2S layers were obtained by chemical and electrochemical methods. It was shown that nanolayers with Ag2S quantum dots can be formed on the surface. The current-voltage characteristics, impedance, and photoeffect of these structures were studied depending on the applied voltage polarities. The time dependences of Ag–Ag2S–Ag3AsS3/heterostructure conductivity at different voltages applied to the illuminated contact were studied.

Influence of air impurities on the transition from a symmetric discharge to an asymmetric discharge in an atmospheric pressure helium diffuse dielectric barrier discharge

The transition from a symmetric and single period (SP1) discharge to an asymmetric and single period (AP1) discharge is a typical nonlinear dynamical phenomenon in dielectric barrier discharges (DBDs) at atmospheric pressure. Considering the presence of air impurities in practical applications which is always unavoidable due to the air-tightness of the DBD reactor, a one-dimensional fluid model with 26 species and 154 reactions is developed to thoroughly investigate the influence of air impurities on the transition from the SP1 discharge to the AP1 one in atmospheric DBDs in a helium mixture with air impurities. In our study case, simulation results show that the discharge experiences the transitions from the SP1 discharge to the AP1 one twice when the air impurity content is increased from 1 to 200 ppm. The first transition (appearing around 60–100 ppm) is due to the additional pre-ionization electrons generated by the higher rate of Penning ionization in the pre-ionization phase, whereas the second transition (occurring around 170–200 ppm) is due to the reduction of the residual electron density which is caused by the decrease in the electron production rates and the change in applied voltage polarity. In addition, as the air impurity content exceeds a certain amount, the rates of Penning ionization are the result of the competition between the increase in the mole fractions of N2 and O2 and the decrease in the molar fractions of He* and He2*.

Numerical simulations of stable, high-electron-density atmospheric pressure argon plasma under pin-to-plane electrode geometry: effects of applied voltage polarity

When applying high-voltage direct current to a pin-to-plane electrode geometry with a distance of 2 mm under atmospheric pressure in argon gas, electrical breakdown forms primary then secondary streamers. The polarity of the applied voltage affects this streamer-propagating phenomenon. Properties such as propagation speed, streamer head size, and plasma generation are parameterized at nanosecond scales by computational simulations of a self-consistent, multi-species, multi-temperature plasma fluid modeling approach. For positive polarity on the pin electrode, streamer-head propagation speeds up and streamer head size increases with increasing applied voltages. However, local electron density at the head decreases. For negative polarity, corona-like discharges form around the pin electrode under low applied voltages, and diffusive steamers form under high applied voltages. Secondary streamers re-propagate from the pin after primary streamer propagation, forming a plasma with a high electron density of 1021 m−3 for the positive polarity. We show that low-voltage operations with positive polarity are useful for stable high-electron-density discharges under atmospheric pressure argon.

Investigation on dielectric and mechanical properties of epoxy reinforced with glass fiber and nano-silica composites

The composite materials of different filler concentration were fabricated by reinforcing the glass fiber and silica nanoparticles into epoxy to understand the variation in surface wettability, water droplet initiated discharge, dielectric parameters, surface charge, space charge and mechanical properties. Nanofiller inclusion has shown the enhanced surface properties viz. water droplet initiated corona inception voltage (CIV) and hydrophobicity. CIV is high under negative DC voltage followed with positive DC and AC voltage. It is realized that the dielectric constant is increased with the addition of nano-silica. The surface potential has decayed exponentially irrespective of the applied voltage polarity and the type of the sample, whereas, the 3 wt% sample showed the highest decay rate for an applied voltage. Silica filled composites have shown better space charge accumulation and decay characteristics initially and then deteriorated rapidly when the filler concentration is increased. The optimised filler concentration is found considering the minimal space chare accumulation, decay rate and the optimised electric field for the space charge analysis. Further, the optimised field conditions were devised for the optimum filler concentration for the prolonged life span and reliability of the insulation material. The mechanical storage modulus has increased with the addition of nanofiller. The plasma temperature calculated from the LIBS spectra has shown a direct correlation with the surface hardness of the material.

Phthalocyanine arrangements on Ag(100): From pure overlayers of CoPc and F16CuPc to bimolecular heterostructure.

We have utilized scanning tunneling microscopy (STM) and low energy electron diffraction to determine the structural properties of two types of metal-phthalocyanines (MPcs), i.e., cobalt-phthalocyanine (CoPc) and hexadecafluorinated copper-phthalocyanine (F16CuPc) on the Ag(100) surface. For coverage close to one monolayer, both systems form long-range ordered structures with square unit cells. The size and rotation of the unit cell with respect to the silver lattice depend on the chemical composition of MPc. Both types of molecules prefer adsorption with around a 30° angle between the molecular axis and the [011] silver direction. The CoPcs mainly arrange in a (5 × 5)R0 phase; however, two additional local arrangements, a and a (7 × 7)R0, were detected by STM. The F16CuPcs form a structure. The co-adsorption of CoPc and F16CuPc on the Ag(100) surface in a 1:1 ratio leads to the formation of a compositionally ordered chessboard-like structure. During filled states imaging, the different appearance of the central part of each MPc allows us to distinguish CoPcs from F16CuPcs. Regardless of the applied voltage polarity, the ligands of F16CuPcs appear brighter than the ligands of CoPcs.

A critical re-examination on conduction processes in gas-insulated DC devices at low electric fields

The established theories behind low-field ion-drift currents assume that the charge generation from natural ionization is the main source of the charge-carriers, and that it defines the current flowing through the insulation gas in gas-insulated direct current (DC) devices. This charge generation is assumed to be independent of the applied electric field in the cases where the electric field is sufficiently below the onset of microdischarges at the interfaces. However, the results of a few previous studies have suggested the presence of a field-dependent current contribution from some conduction processes, which might significantly accelerate the decay of the surface charges distributed on the insulator surfaces. In this paper, an extensive study on low-field charge transport is presented, and the influences of the insulation volume, electric field, and gas pressure on low-field charge transport are discussed. In accordance with the common structure employed in gas-insulated devices, Al 2 O 3 -filled epoxy resin is used in the solid parts of the device and sulfur hexafluoride (SF 6 ) is used for gaseous insulation. At electric fields of approximately 1800 V/m, a gas current that is up to 30 times higher, when compared to the conduction currents caused by the charge generation from natural ionization, is observed. This enhanced current reveals a further conduction process acting in a limited range of low electric fields and is characterized to be independent of the applied voltage polarity. With an increase in the gas pressure, the current amplitude is found to increase linearly and higher electric field is required to reach its maximum. At electric fields below 200 V/m, the conduction current reverses its pressure dependency and increases with decreasing gas pressure. The reported findings strongly indicate that the enhanced low-field charge transport in gas-insulated devices may be owing to electrophoretic conduction.

Electron density in surface barrier discharge emerging at argon/water interface: quantification for streamers and leaders

Optical emission spectroscopy, fast intensified CCD imaging and electrical measurements were applied to investigate the basic plasma parameters of surface barrier discharge emerging from a conductive water electrode. The discharge was generated at the triple-line interface of atmospheric pressure argon gas and conductive water solution at the fused silica dielectrics using a sinusoidal high-voltage waveform. The spectroscopic methods of atomic line broadening and molecular spectroscopy were used to determine the electron densities and the gas temperature in the active plasma. These parameters were obtained for both applied voltage polarities and resolved spatially. Two different spectral signatures were identified in the spatially resolved spectra resulting in electron densities differing by two orders of magnitude. It is shown that two discharge mechanisms take a place: the streamer and the leader one, with electron densities of 1014 and 1016 cm−3, respectively. This spectroscopic evidence is supported by the combined diagnostics of electrical current measurements and phase-resolved intensified CCD camera imaging.

Effect of Electric Field in the Stabilized Premixed Flame on Combustion Process Emissions

The effect of the AC and DC electrical field on combustion processes has been investigated by various researchers. The results of these experiments do not always correlate, due to different experiment conditions and experiment equipment variations. The observed effects of the electrical field impact on the combustion process depends on the applied voltage polarity, flame speed and combustion physics. During the experiment was defined that starting from 1000 V the ionic wind takes the effect on emissions in flue gases, flame shape and combustion instabilities. Simulation combustion process in hermetically sealed chamber with excess oxygen amount 3 % in flue gases showed that the positive effect of electrical field on emissions lies in region from 30 to 400 V. In aforementioned voltage range carbon monoxide emissions were reduced by 6 % and at the same time the nitrogen oxide emissions were increased by 3.5 %.

Self-consistent multi-arcs dynamic model for high voltage polluted insulators

This paper presents a self-consistent multi-arcs dynamic model for high voltage polluted insulator under impulse voltage. This model is an improvement of that one developed for single arc; it is based on an electrical equivalent circuit consisting of a series of different partial arcs and residual layer resistance. It takes into account the configuration of insulator, applied voltage polarity and the instantaneous changes of discharges parameters. The proposed model enables to compute the flashover voltage and leakage current and to show the influence of dry bands on the voltage distribution along the surface and furthermore on the electrical performance of polluted insulator. It is validated using a simple plane model in the case of uniform and non-uniform polluted bands. The computed flashover voltage and leakage current are found in good agreement with those measured experimentally.

Charge injection in thin dielectric layers by atomic force microscopy: influence of geometry and material work function of the AFM tip on the injection process

Charge injection and retention in thin dielectric layers remain critical issues for the reliability of many electronic devices because of their association with a large number of failure mechanisms. To overcome this drawback, a deep understanding of the mechanisms leading to charge injection close to the injection area is needed. Even though the charge injection is extensively studied and reported in the literature to characterize the charge storage capability of dielectric materials, questions about charge injection mechanisms when using atomic force microscopy (AFM) remain open. In this paper, a thorough study of charge injection by using AFM in thin plasma-processed amorphous silicon oxynitride layers with properties close to that of thermal silica layers is presented. The study considers the impact of applied voltage polarity, work function of the AFM tip coating and tip curvature radius. A simple theoretical model was developed and used to analyze the obtained experimental results. The electric field distribution is computed as a function of tip geometry. The obtained experimental results highlight that after injection in the dielectric layer the charge lateral spreading is mainly controlled by the radial electric field component independently of the carrier polarity. The injected charge density is influenced by the nature of electrode metal coating (work function) and its geometry (tip curvature radius). The electron injection is mainly ruled by the Schottky injection barrier through the field electron emission mechanism enhanced by thermionic electron emission. The hole injection mechanism seems to differ from the electron one depending on the work function of the metal coating. Based on the performed analysis, it is suggested that for hole injection by AFM, pinning of the metal Fermi level with the metal-induced gap states in the studied silicon oxynitride layers starts playing a role in the injection mechanisms.

Comparative study of electrical breakdown properties of deionized water and heavy water under pulsed power conditions.

A comparative study of electrical breakdown properties of deionized water (H2O) and heavy water (D2O) is presented with two different electrode materials (stainless steel (SS) and brass) and polarity (positive and negative) combinations. The pulsed (∼a few tens of nanoseconds) discharges are conducted by applying high voltage (∼a few hundred kV) pulse between two hemisphere electrodes of the same material, spaced 3 mm apart, at room temperature (∼26-28 °C) with the help of Tesla based pulse generator. It is observed that breakdown occurred in heavy water at lesser voltage and in short duration compared to deionized water irrespective of the electrode material and applied voltage polarity chosen. SS electrodes are seen to perform better in terms of the voltage withstanding capacity of the liquid dielectric as compared to brass electrodes. Further, discharges with negative polarity are found to give slightly enhanced discharge breakdown voltage when compared with those with positive polarity. The observations corroborate well with conductivity measurements carried out on original and post-treated liquid samples. An interpretation of the observations is attempted using Fourier transform infrared measurements on original and post-treated liquids as well as in situ emission spectra studies. A yet another important observation from the emission spectra has been that even short (nanosecond) duration discharges result in the formation of a considerable amount of ions injected into the liquid from the electrodes in a similar manner as reported for long (microseconds) discharges. The experimental observations show that deionised water is better suited for high voltage applications and also offer a comparison of the discharge behaviour with different electrodes and polarities.

Investigating Pulsed Discharge Polarity Employing Solid-state Pulsed Power Electronics

The power electronics technique has become a key technology in solid-state pulsed power supplies. Since pulsed power applications have been matured and found their way into many industrial applications, moving toward energy efficiency is gaining much more interest. Therefore, finding an optimum operation condition plays an important role in maintaining the desired performance. Investigating the system parameters contributed to the generated pulses is an effective way in improving the system performance further ahead. One of these parameters is discharge polarity, which has received less attention. In this article, the effects of applied voltage polarity on plasma discharge have been investigated in different mediums at atmospheric pressure. The experiments have been conducted based on a high-voltage DC power supply and a high-voltage pulse generator for point-to-point and point-to-plane geometries. Furthermore, the influence of electric field distribution is analyzed using finite-element simulations for the employed geometries and mediums. The experimental and simulation results have verified the important role of the applied voltage polarity, employed geometry, and medium of the system on plasma generation.