Effect Of Polarity Reversal Research Articles

Two dimensional elastic dynamically focused beam migration in transversely isotropic (TI) media

The anisotropic ray tracing equations of elastic waves are derived for calculating the central ray's travel time, trajectory, and parameters P and Q. Based on the anisotropic ray tracing algorithm, we proposed a transversely isotropic elastic dynamically focused beam migration for two dimensional media. Based on the two dimensional Kirchhoff-Helmholtz integral of elastic waves, the elastic imaging weight coefficients are used to reduce the crosstalk noise. Then, based on the previous research, we introduced a sign function to eliminate the effect of polarity reversal in qPqSV wave migration. Through model tests, it can be found that the proposed method can process the multicomponent seismic data and image the complex structures for anisotropic media. The optimized dynamic ray tracing algorithm is superior to the conventional one in computational efficiency. Furthermore, the proposed method eliminates the limitations of the Gaussian beam imaging. This method not only does not affect the accuracy of shallow imaging, but also enhances the energy focusing and the amplitude of deep layers. The model tests have shown the accuracy and effectiveness of our proposed method.

Investigating the effect of polarity reversal of the applied current on electrochemical degradation of per-and polyfluoroalkyl substances

Advanced electrochemical oxidation methods have demonstrated significant potential in degrading per- and polyfluoroalkyl substances (PFAS). However, the widespread application of these techniques is impeded by the long term performance and the limited service life of electrodes. This study introduces a novel approach by investigating the impact of polarity reversal (PR) of the applied current on the electrochemical degradation of perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), 6:2 fluorotelomer sulfonate (6:2 F TS), and aqueous film-forming foam (AFFF) samples. A laboratory-scale electrochemical cell equipped with two titanium sub-oxide (Ti4O7) electrodes is used for the experimental investigation. The results indicate 6-h direct current (DC) operation causes fouling and ageing of the electrode (anode) such as the formation of titanium oxide (TiO2). The application of PR effectively mitigates fouling and aging, leading to an enhanced mass transfer and an improved long-term defluorination of PFOA, PFOS, 6:2 FTS, and AFFF samples. The investigation also suggests that PR can help to reduce the energy consumption by ∼13%. Overall, the application of PR emerges as a promising and practical strategy to augment the electrochemical degradation of PFAS, addressing critical issues related to electrode stability and longevity.

Effect of polarity reversal on floc formation and rheological properties of a sludge formed by the electrocoagulation process

Anode fouling is one of the key limiting factors to the widespread application of electrocoagulation (EC) for treatment of different types of contaminated water. Promising mitigation strategy to fouling is to operate the process under polarity reversal (PR) instead of direct current (DC). However, the PR operation comes at the cost of process complexity due to the alternation of electrochemical and chemical reactions. In this study, we systematically investigated the link between evolving fouling layer during DC and PR close to iron and aluminum electrodes and morphological and rheological properties of the formed sludge. By operando visualization of EC process, we demonstrate that during PR operation, precipitation of the iron and aluminum species occurs close to the anode interface, resulting in flocs with higher porosity and lower density than those formed under DC conditions. However, rheological investigation revealed that the PR conditions resulted in a sludge with more pronounced solid-like signature, but this enhancement in its viscoelastic properties is closely related to a period of the current's polarity reversal. We attribute this unexpected result to higher shear rate and collision of particles during PR conditions.

How does periodic polarity reversal affect the faradaic efficiency and electrode fouling during iron electrocoagulation?

Electrocoagulation (EC) is a promising electrochemical water treatment technology. However, a major challenge to sustaining effective long-term EC operation is controlling the precipitation of materials on the electrodes, commonly referred to as fouling. Periodically reversing electrode polarity has been suggested as an in-situ fouling mitigation strategy and is often implemented in EC field applications. However, the utility of this approach has not been investigated in detail. In this study, the effect of polarity reversal (PR) on the performance of EC using iron electrodes was examined under different water chemistry conditions and at a range of reversal frequencies. It was observed that the faradaic efficiency in PR-EC was always lower than that in the EC systems operated with a direct current (i.e., DC-EC). It was also observed that the faradaic efficiency progressively decreased as the current reversal frequency increased, with the faradaic efficiency dropping as low as 10% when the PR interval was 0.5 min. Results from fouling layer, chronopotentiometric, and cyclic voltammetric investigations indicated that the decrease in the faradaic efficiency was caused by (i) increased electrode fouling by iron precipitates and (ii) electrochemical side reactions at the electrode-electrolyte interface. The extent of these effects was dependent on the solution chemistry; oxyanions and sulfide were found to be particularly detrimental to the performance of PR-EC, causing severe electrode fouling while decreasing the faradaic efficiency. Fouling could be mitigated by increasing the solution convection rate, resulting in a shear on the electrode surface that removed iron and other electrochemically reactive species from the electrodes.

Effect of polarity‐reversal voltage on charge accumulation and carrier mobility in silicone rubber/silicon carbide composites

Polarity-reversal voltage in high-voltage direct current transmission system would cause serious electric field distortion, accelerating charge injection. The effect of polarity reversal on charge transport characteristics in silicone rubber (SiR)/silicon carbide (SiC) composites with nonlinear conductivity at different temperatures is studied. The charge transport characteristics of silicone rubber/silicon carbide composites under unipolarity voltage and polarity-reversal voltage at 30°C, 70°C and 90°C are obtained by the isothermal surface potential decay method, and the carrier mobility and trap characteristics of the composites are further calculated. Studies have shown that a large amount of hetero-charge remains in silicone rubber materials under polarity-reversal voltage, which in turn leads to intensive charge neutralisation and severe electric field distortion. The silicon carbide doped composites exhibit excellent performance in suppressing the accumulation of residual charges and weakening the charge neutralisation under polarity-reversal voltage. In addition, high temperature enhances the conductivity of silicone rubber/silicon carbide composites and reduces the accumulation of residual charges, which restrains the distortion of the local electric field under polarity-reversal voltage. This research shows that the application of non-linear conductivity material is an effective measure to solve the problem of cable insulation under polarity-reversal voltage.

Interface coupling and charge doping in graphene on ferroelectric BiAlO3(0001) polar surfaces.

The ferroelectric field-effect transistors (FeFETs) based on graphene/ferroelectric (Gr/FE) hybrid systems have been attracting a lot of attention in recent years. The interface interaction and charge transfer between graphene and the ferroelectric substrates are important factors that determine the performance of graphene-based FeFETs. According to our intuitive sense, the electrostatically doped carriers in graphene on the ferroelectric positive and negative surfaces should be n-type and p-type, respectively. In the present work, however, by employing first-principles density functional theory (DFT) calculations, we reveal that an unusual charge doping effect occurs in graphene on the thermodynamically preferred ferroelectric BiAlO3(0001) polar surfaces. The graphene on the BiAlO3(0001) positive surface is electrostatically doped p-type, while the BiAlO3(0001) negative surface induces n-type carriers in the graphene overlayer. Further analysis demonstrates that, although the electrostatic doping effect in the Gr/FE system depends on the polarization direction of the ferroelectric substrate, the resultant carrier type and density in graphene are determined by the specific band arrangement between graphene and the ferroelectric polar surface. In addition to the graphene-based FeFETs, our results predict that the Gr/BiAlO3(0001) systems can be used to fabricate graphene p-n homojunctions by engineering the domain pattern in the ferroelectric substrate.

Nano-Domains Produced through a Two-Step Poling Technique in Lithium Niobate on Insulators.

We proposed a two-step poling technique to fabricate nanoscale domains based on the anti-parallel polarization reversal effect in lithium niobate on insulator (LNOI). The anti-parallel polarization reversal is observed when lithium niobate thin film in LNOI is poled by applying a high voltage pulse through the conductive probe tip of atomic force microscope, which generates a donut-shaped domain structure with its domain polarization at the center being anti-parallel to the poling field. The donut-shaped domain is unstable and decays with a time scale of hours. With the two-step poling technique, the polarization of the donut-shaped domain can be reversed entirely, producing a stable dot domain with a size of tens of nanometers. Dot domains with diameter of the order of ∼30 nm were fabricated through the two-step poling technique. The results may be beneficial to domain-based applications such as ferroelectric domain memory.

Investigation of charge transport and breakdown properties in XLPE/GO nanocomposites part 2: Effect of polarity reversal

In this paper, the effect of polarity reversal on the space charge behaviors and breakdown properties of cross-linked polyethylene (XLPE)/graphene oxide (GO) nanocomposites are studied. Five kinds of XLPE based nanocomposites doped with different combinations of GO quantum wells (QWs) and the polycyclic aromatic compounds (PAC) were prepared by mechanical blending. The dynamic space charge distributions and breakdown strength of the XLPE samples under polarity reversal were measured. The experimental results indicate that the injection and accumulation of space charges in XLPE show polarity dependence under polarity reversal. The breakdown strength of the XLPE samples decreases with the increase of the pre-applied hetero-polarity electric field. Both the GO QWs and PAC can inhibit charge transport and improve the breakdown properties of XLPE, and the synergistic effect between the two dopants is observed. As a result, XLPE samples modified by the PAC functionalized GO QWs show a minimum space charge accumulation and the highest breakdown strength under polarity reversal. The quantum chemistry calculation method was employed to clarify the charge transport characteristics in XLPE dielectrics, and the relationship between space charge distributions and breakdown properties of the nanocomposites under polarity reversal was discussed.

EuS/Hf0.5Zr0.5O2 Bilayers as a Prospective Multiferroic System

AbstractComposite bilayer multiferroics combining ferroelectric (FE) and ferromagnetic (FM) thin‐film materials in a heterostructure and exhibiting magnetoelectric (ME) coupling effect are of great scientific and technological interest. In particular, electronically driven ME coupling implies that the FE polarization orientation affects the magnetic properties of FM at the interface with FE. Unlike metals, where the electric field penetrates over distances of 1–2 unit cells only, magnetic semiconductors, particularly doped EuS, with a ≈10 nm screening length appear a viable alternative. In addition, EuS exhibits a metal–insulator transition, thus offering new functionalities in nanoelectronics. Meanwhile, ultrathin polycrystalline films of doped HfO2, such as Hf0.5Zr0.5O2 (HZO), stabilized in the noncentrosymmetric orthorhombic phase, are identified as a novel class of robust FE materials. In this work, FM EuS integrated with FE HZO in a bilayered structure is promoted as a prospective composite multiferroic. The functionality of both ultrathin FM‐EuS and FE‐HZO layers as well as their compatibility in a capacitor configuration is demonstrated. The comprehensive information on the structural, chemical, and electronic properties of EuS/HZO interface endorses it as a promising medium for magnetoelectric coupling phenomena, particularly, the effect of polarization reversal in FE‐HZO on the magnetic and transport properties in EuS.

Electrocoagulation with Polarity Reversal for Treatment of Produced Water

Electrocoagulation (EC) is a cost-effective and reliable technology to treat water and wastewater and has the ability to remove many types of contaminants. Studies have shown that EC operated with aluminum or iron electrodes exhibits higher treatment efficiencies than traditional chemical coagulation with aluminum sulfate or ferric chloride salts [1], [2]. EC involves the in-situ generation of metal hydroxide coagulant by the electrochemical dissolution of sacrificial metal anodes using a direct current, combined with generation of hydroxide ions at the cathode. However, material precipitation on the electrodes associated with long term operation is a major problem hindering the scale up of EC [3]. The growth of electrode surface layers increases passivation, which reduces the treatment efficiency and increases operating costs [4]. Polarity reversal during electrocoagulation, i.e. intermittently changing the direction of the current, is a method that can remove passivation layers on the electrodes formed during direct current operation [5]. The main goal of this study was to investigate the effect of polarity reversal on the reaction and electrode fouling mechanisms as well as the performance of electrocoagulation for the treatment of SAGD produced water.Total organic carbon and silicon removal efficiencies were measured to evaluate treatment performance. Laser scanning confocal microscopy was used to monitor the pH distribution close to electrodes as well as the formation of solid products in an electrocoagulation cell. Customized polycarbonate bench scale reactors were used to study the relationship between coagulant production, polarity reversal frequency, solution composition, and flowrate. The cycle time of the polarity reversals was varied from 5 to 600 s, and the Reynolds numbers was varied between 30 to 200. The Faradaic efficiencies for the coagulant dissolution at different operating conditions were determined by digesting the solid products followed by elemental analysis.The evolution of pH during polarity reversal revealed that at higher frequencies or higher flow rates, the thickness of the interfacial pH boundary layer was lower. The quantification of pH was used to study the effect of pH on passivation layer stability. It was found that at higher frequencies, Faradaic efficiencies were lower for EC with iron electrodes, whereas increased efficiencies were observed for aluminum electrodes due to increased susceptibility to non-Faradaic corrosion. EC using aluminum electrodes (Al-EC), employing polarity reversal at all frequencies led to a reduction in cell voltage and therefore a reduction in the required energy for treatment.

Effect of Polarity Reversal on Performance of Synchronous Induction Coil Launcher

In this article, in order to improve the muzzle velocity and energy conversion efficiency of the synchronous induction coil launcher (SICL), the polarity reversal problem is studied. The circuit model of a six-stage SICL is built for low-speed launching, a 40-stage SICL is built for medium- and high-speed launching, and the validity of the circuit model is verified by the transient field coupled model based on finite element software. For the six-stage launcher, if the trigger timing is unchanged and the polarity of the last three stages is just reversed, the armature will bear greater braking force and the armature temperature rises sharply, eventually leading to a significant decrease in muzzle velocity. By analyzing the variation of the driving currents and the induced eddy currents in the armature, the reason for the significant braking force is discussed. Based on this, we propose a method to significantly improve the launch performance by delaying the coils with reversed polarity. For the 40-stage launcher, maintain the polarity of the first 20 stages unchanged and reverse the polarity of the latter 20 stages. In addition, since the number of turns and the trigger time are calculated by the adaptive method, the braking effect is slowed down by increasing the coil pitch of the 20th and 21st stages. The simulation results show that the polarity reversal can also lift the downward acceleration, thus improving the energy conversion efficiency.

Generation of the Sun’s Radial Magnetic Field by Global Hydrodynamic Flows

A theory on the emergence of global hydrodynamic flows of matter and the generation of global magnetic fields in the solar convective zone is proposed. The unstable profile of differential rotation plays a crucial role in it: all hydrodynamic flows on the Sun (poloidal circulation, torsion oscillations, and spatiotemporal variations of the poloidal flow) are generated due to the loss of stability of differential rotation. None of the models known to us, wherein torsion oscillations and variations of the meridional circulation are typically calculated separately and are thus considered to be independent flows, reproduce this result. The calculations within our model suggest, on the contrary, that the indicated flows are actually toroidal and poloidal components of a single, 3-dimensional global flow. The decisive role of torsion oscillations in the generation of the radial alternating magnetic field is highlighted in the present study. It is demonstrated by numerical simulation that the time-varying radial magnetic field on the surface of the Sun reaches its maximum at the poles, where it changes polarity with a period of ~22 years. This process may be identified with the observed effect of polarity reversal of the polar field within the Hale magnetic cycle. It is demonstrated that the lines of zero values (polarity reversal) of the surface radial magnetic field pass through the maxima of the velocity modulus of zonal flows (torsion oscillations). In addition, the lines of magnetic polarity reversal of the radial field and the maximum velocities of surface zonal flows drift from the poles to the equator. It is noted that the obtained results on the latitudinal evolution of surface zonal flows correlate with the behavior of deep zonal flows determined by processing helioseismological data.

Polarization reversal of scattered thermal dust emission in protoplanetary disks at submillimetre wavelengths

Investigation of the polarized light of protoplanetary disks is key for constraining dust properties, disk morphology, and embedded magnetic fields. However, different polarization mechanisms and the diversity of dust grain shapes and compositions lead to ambiguities in the polarization pattern. The so-called “self-scattering” of thermal, re-emitted radiation in the infrared and millimetre and submillimetre wavelengths is discussed as a major polarization mechanism. If the net flux of the radiation field is in the radial direction, it is commonly assumed that the polarization pattern produced by scattering in a protoplanetary disk shows concentric rings for disks seen in face-on orientation. We show that a change of 90° of the polarization vector orientation may occur and mimic the typical pattern of dichroic emission of dust grains aligned by a toroidal magnetic field in disks seen close to face-on. Furthermore, this effect of polarization reversal is a fast-changing function of wavelength and grain size, and is thus a powerful tool to constrain grain composition and size distribution present in protoplanetary disks. In addition, the effect may also provide unique constraints for the disk inclination, especially if the disk is seen close to face-on.

Effect of Polarization Reversal in Ferroelectric TiN/Hf0.5Zr0.5O2/TiN Devices on Electronic Conditions at Interfaces Studied in Operando by Hard X-ray Photoemission Spectroscopy.

Because of their compatibility with modern Si-based technology, HfO2-based ferroelectric films have recently attracted attention as strong candidates for applications in memory devices, in particular, ferroelectric field-effect transistors or ferroelectric tunnel junctions. A key property defining the functionality of these devices is the polarization dependent change of the electronic band alignment at the metal/ferroelectric interface. Here, we report on the effect of polarization reversal in functional ferroelectric TiN/Hf0.5Zr0.5O2/TiN capacitors on the potential distribution across the stack and the electronic band line-up at the interfaces studied in operando by hard X-ray photoemission spectroscopy. By tracking changes in the position of Hf0.5Zr0.5O2 core-level lines with respect to those of the TiN electrode in both short- and open-circuit configurations following in situ polarization reversal, we derive the conduction band offset to be 0.7 (1.0) eV at the top and 1.7 (1.0) eV at the bottom interfaces for polarization, pointing up (down), respectively. Energy dispersive X-ray spectroscopy profiling of the sample cross-section in combination with the laboratory X-ray photoelectron spectroscopy reveal the presence of a TiOx/TiON layer at both interfaces. The observed asymmetry in the band line-up changes in the TiN/Hf0.5Zr0.5O2/TiN memory stack is explained by different origin of these oxidized layers and effective pinning of polarization at the top interface. The described methodology and first experimental results are useful for the optimization of HfO2-based ferroelectric memory devices under development.

Effect of polarity reversal on the enhanced electrokinetic remediation of 2,4-D-polluted soils: A numerical study

This article presents a numerical study of the transport phenomena involved in the electrokinetic remediation of soils polluted with polar pesticides. 2,4-Dichlorophenoxyacetic acid is used as a representative of this pesticide type. A one-dimensional configuration with two facing electrodes placed in electrolyte compartments and a cathodic overflow pollutant extraction system has been used for that purpose. The conventional electrokinetic remediation process is evaluated by keeping the electrode polarity constant, and to obtain acceptable yields, it is necessary to extend the treatment for more than 250 days. The application of periodic polarity reversals is proposed to improve these results. This strategy maximises the pesticide concentration in the cathodic compartment, thus maximising the pollutant extraction rate. The results show that by applying polarity reversals over 6-h periods, it is possible to accelerate the treatment, thus improving its overall efficiency up to 94.5% compared with the treatment using a constant electrode polarity.

Effect of polarity reversal on space charge properties of CB/LDPE composite under DC field

Voltage polarity reversal is a special condition in HVDC transmission system. In this study, low density polyethylene (LDPE) and nano-CB/LDPE composites films were manufactured, and the space charge distribution was measured under DC voltage with and without the polarity reversal respectively by pulsed electro-acoustic (PEA) method to reveal the doping effect of nano-CB on the space charge distribution. According to the experimental results of LDPE, before the polarity reversal, homo-charges are injected and accumulate around the electrodes, and these space charges cannot dissipate completely after the voltage polarity being reversed which resulting in more serious electric field distortion. According to the test results of CB/LDPE after voltage polarity reversal, the density of shallow trapped charges decreases whereas the density of deep trapped charges increases, and the average charge density is reduced. The LDPE with 0.03wt% of CB nanoparticles shows sound performance on inhibiting the space charge accumulation after the voltage polarity reversal.

Domain wall orientation and domain shape in KTiOPO4 crystals

Domain shape evolution and domain wall motion have been studied in KTiOPO4 (KTP) ferroelectric single crystals using complementary experimental methods. The in situ visualization of domain kinetics has allowed revealing: (1) qualitative change of the domain shape, (2) dependence of the domain wall velocity on its orientation, (3) jump-like domain wall motion caused by domain merging, (4) effect of domain shape stability. The model of domain wall motion driven by generation of elementary steps (kink-pair nucleation) and subsequent kink motion is presented. The decrease in the relative velocity of the approaching parallel domain walls is attributed to electrostatic interaction. The effect of polarization reversal induced by chemical etching is observed. The obtained results are important for the development of domain engineering in the crystals of KTP family.

Structure and physicochemical properties of thin film photosemiconductor cells based on porphine derivatives

Photosemiconductor thin films based on two organic porphine derivatives have been investigated. These compounds have different pendent groups; the film morphology, along with the specific fabrication technique, is determined to a great extent by these groups. The films have been fabricated by vacuum sputtering and using the Langmuir−Schaefer method. According to the atomic force microscopy (AFM) data, the Langmuir−Schaefer films are more homogeneous than the sputtered ones. It is shown that the sputtered films based on substituted porphine have a looser stacking than the initial analog. A spectroscopy study revealed a bathochromic shift of the Soret band in the Langmuir−Schaefer films–sputtered films series. This shift is explained by the increase in the concentration and size of molecular aggregates in sputtered films. It is shown that a polycrystalline C60 fullerene film deposited onto an amorphous substituted porphine layer improves the photoelectric characteristics of the latter. Both the time stability of the photodiode structure and its ampere‒watt sensitivity increase (by a factor of 10 in the transition regime). The steady-state current does not change. The effect of polarity reversal of the photovoltaic signal is observed in a planar С60‒substituted metalloporphine heterostructure, which is similar to the pyroelectric effect. The polarity reversal can be explained by the contribution of the trap charge and discharge current at the interface between the amorphous photosemiconductor and crystalline photosemiconductor to the resulting photoelectric current.

Remediation of PCE contaminated clay soil by coupling electrokinetics with zero-valent iron permeable reactive barrier

In the present study, the electrokinetic technology was coupled with a permeable reactive barrier (PRB), composed of micro-scale Fe to treat a clay soil contaminated by perchloroethylene (PCE). A non-ionic surfactant, Triton X-100, was selected as the solubility enhancing agent. A series of batch tests, having 10-day duration, were carried out to evaluate the fate, transport and removal of PCE. This study also demonstrates the effectiveness of the non-uniform EK/ZVIPRB system for the treatment of the PCE-contaminated, low permeability soils. A bench scale system was used, which contained 400 g of PCE contaminated soil (0.6 g PCE kg−1 soil) and a PRB containing 30 g of iron particles. Seven lab scale tests were conducted using a potential gradient of 1 V cm−1 for studying the influence of different variables like surfactant, the inclusion of PRB in the soil and the effect of polarity reversal. Results showed that the combination of EK and ZVIPRB could enhance the overall removal of PCE from soils by 40 % as compared to EK alone. Among the variables, the surfactant was seen to be essential for the movement of the pollutant through the PRB, and the PCE removal improved significantly on reversing the electrode polarity in the EK-PRB test (73 % in anode, 64 % in cathode). The best results showed an 80 % PCE removal efficiency after 10 days.

Effect of polarity-reversal on electrokinetic enhanced bioremediation of Pyrene contaminated soil

Polycyclic aromatic hydrocarbons (PAHs) contaminated soil was studied under electrokinetic enhanced biodegradation with polarity-reversal. The effects of polarity reversal were investigated on electric current, pH, microbial counts and the spatial distribution of pyrene biodegradation rate during the application of electro-bioremediation. Three types of treatment were conducted: PYR-degrading microbial treatment only (Bio), electro-bioremediation without polarity-reversal (EK-Bio), electro-bioremediation with polarity-reversal at an interval of 2hours (EK-Bio-PR). After 42 days, EK-Bio-PR maintained a neutral soil pH (7.20) and relatively stable electric current. After running for 17 days, the electric current stabilized around 10mA in EK-Bio-PR, and which was 5 times than that in KE-Bio. Polarity-reversal enhanced microbial counts, and improved the PYR degradation. The bacterial counts reached up to 4.8×1011cfu/g and the best PYR degradation efficiency was 55.9% in EK-Bio-PR at the end of experiment. Thus, the application of electrokinetics with periodic polarity-reversal enabled better microbial growth and biodegradation, which further improved the overall removal of PAHs pollutant.