Alternating-current Field Research Articles

A Study on an Energy-saving and High-efficient Pack Boriding Technology for Tool and Die Steels

Alternating current field (ACF) enhanced pack boriding was carried out on three kinds of tool and die steels of T7, T12 and W18Cr4 V at low and moderate temperatures. A column-shaped electrode was positioned in the center of a pack container, the wall of the container was taken as another electrode. Samples and the boriding agent were set between the electrodes. The test results revealed that the ACF with a current of 4 A and a voltage of 30∼50 V enhanced the boriding of those tool steels. Compared with the conventional pack boriding, the case thickness of the sample was increased by more than 70% by ACF enhanced pack boriding. The case microstructure was improved to some extent. With the new boriding technology, the treating temperature can be lowered, the soaking time can be shortened, and the utilization ratio of agents can be increased. Preliminary analysis has been made on mechanisms of ACF's effects to the pack boriding.

Effective response in nonlinear spherical coated composites under external AC and DC electric fields

By using the perturbation method, effective nonlinear direct current (DC) and alternating current (AC) responses of nonlinear composites with spherical coated inclusions randomly embedded in a host medium are studied under the action of an external electric field Ea = E0 + E1 sin ωt + E3 sin 3ωt with different amplitudes and frequencies. The local potentials of composites at all harmonics are given in the inclusion particles and the host regions. All effective nonlinear responses to composites and the relationship between the effective nonlinear responses at all harmonics are also deduced for the spherical coated inclusions in a dilute limit.

Self-sustained firing activities of the cortical network with plastic rules in weak AC electrical fields

Both external and endogenous electrical fields widely exist in the environment of cortical neurons. The effects of a weak alternating current (AC) field on a neural network model with synaptic plasticity are studied. It is found that self-sustained rhythmic firing patterns, which are closely correlated with the cognitive functions, are significantly modified due to the self-organizing of the network in the weak AC field. The activities of the neural networks are affected by the synaptic connection strength, the external stimuli, and so on. In the presence of learning rules, the synaptic connections can be modulated by the external stimuli, which will further enhance the sensitivity of the network to the external signal. The properties of the external AC stimuli can serve as control parameters in modulating the evolution of the neural network.

Manipulating single annealed polyelectrolyte under alternating current electric fields: Collapse versus accumulation

Effective manipulation and understanding of the structural and dynamic behaviors of a single polyelectrolyte (PE) under alternating current (AC) electric fields are of great scientific and technological importance because of its intimate relevance to emerging bionanotechnology. In this work, we employ fluorescence correlation spectroscopy (FCS) to study the conformational and AC-electrokinetic behaviors of a model annealed PE, poly(2-vinyl pyridine) (P2VP) under both spatially uniform and non-uniform AC fields at a single molecule level. Under spatially uniform AC-fields, we observe a gradual and continuous coil-to-globule conformational transition (CGT) of single P2VP at varied AC-frequency when a critical AC-field strength is exceeded, in contrast to the pH-induced abrupt CGT in the absence of AC-fields. On the contrary, under spatially non-uniform AC-fields, we observe field-driven net flow and accumulation of P2VP near high AC-field regions due to combined AC electro-osmosis and dielectrophoresis but surprisingly no conformational change. Thus, distinct AC-electric polarization effect on single annealed PE subject to AC-field homogeneity is suggested.

Electric field distribution and effective nonlinear AC and DC responses of graded cylindrical composites

The perturbation method is used to study the localization of electric field distribution and the effective nonlinear response of graded composites under an external alternating-current (AC) and direct-current (DC) electric field Eapp = E0(1 + sin ωt). The dielectric profile of the cylindrical inclusions is modeled by function ∊i(r) = Ckrk (r ≤ a), where r is the radius of the cylindrical inclusion, and Ck, k, a are parameters. In the dilute limit, the local potentials and the effective nonlinear responses at all harmonics are derived. Meanwhile, the general effective nonlinear responses are also derived and compared with the effective nonlinear responses at harmonics under the AC and DC external field. It is found that the effective nonlinear AC and DC responses at harmonics can be calculated by those of the general effective nonlinear of the graded composites under the external DC electric field. Moreover, the obtained local electrical fields show that the electrical field distribution in the cylindrical inclusions is controllable, and the maximum of the electric field inside the cylinder is at its center.

Change in the microhardness of nonmagnetic crystals after their exposure to the Earth’s magnetic field and AC pump field in the EPR scheme

Changes in the microhardness of ZnO, triglycine sulfate (TGS), and potassium acid phthalate (KAP) crystals after their exposure to crossed ultralow magnetic fields, i.e., the Earth’s field B Earth ≈ 50 μT and the alternating-current field \(\tilde B \approx 3 \mu {\rm T}\) orthogonal to it, have been revealed. In ZnO crystals, the microhardness increases, whereas in TGS and KAP, it decreases. A maximum change (10–15%) is reached within 1–3 h after magnetic treatment; then, the microhardness gradually recovers to its initial value for the first day. After a sufficient pause, the effect is completely reproduced under the same conditions. The resonant frequency of the pump field \(\tilde B\) corresponds to the EPR condition with a g-factor close to two. The magnetic memory exhibits a strong anisotropy: for each of the crystals, a direction is found, which, being coincident with the Earth’s magnetic field vector B Earth, causes complete or partial suppression of the effect. In ZnO and TGS crystals, these are symmetry axes 6 and 2, respectively. In the KAP crystal, it is the direction in the cleavage plane orthogonal the 2 axis. Possible physical mechanisms of the observed phenomena have been discussed.

Electrophoretic deposition under modulated electric fields: a review

Classical electrophoretic deposition (EPD) relies on continuous direct current (CDC) to deposit charged particles on electrodes. In recent decades, modulated electric fields such as pulsed direct current (PDC) and alternating current (AC) have been investigated. This paper reviews EPD under these modulated electric fields and major applications of the deposited microstructures. The paper starts with a short overview of EPD principals such as the electrical double layer of the charged particle, electrophoretic mobility and main suspension parameters including zeta potential, particle size, conductivity, viscosity and stability of the suspension. The EPD mechanisms from the earliest model reported by Hamaker and Verwey to latest models including Sarkar and Nicholson model and influence of the electrohydrodynamics and electroosmosis as well as electrode surface and its electrochemical double layer on the deposition process have been briefly discussed. Two categories of modulated electric fields, PDC and AC fields have been addressed with their advantages and disadvantages. It is found that compared to CDC, PDC offers the advantage of: i) reducing the coalescence between gas bubbles induced by water electrolysis from aqueous suspensions, hence yielding deposition of smooth and uniform coatings, ii) reducing aggregation and disaggregation of nanometer sized particles, leading to formation of uniform and homogenous deposits and, iii) PDC generates low change in pH near the electrode, thus it is convenient for deposition of biochemical and biological species in their highly active states. The main disadvantage of PDC over CDC lies in the decrease of the deposition yield. The latter can be more pronounced if low time-pulses are used. Various categories of AC signals including symmetrical fields with no net DC component and asymmetrical AC signals without and with net DC component have been discussed. Overall, the deposition rate under AC fields increases with polarization time and amplitude. With respect to frequency, the deposition rate increases with frequency up to certain value then drops at elevated frequencies. It is noted that deposition under AC signals offers the possibility to produce superior quality coatings from aqueous suspensions because electrolysis of water as well as particle orientation during the deposition could be controlled. From the application standpoint, PDC and AC, offers new application perspectives such as in biotechnology. Because under modulated electric fields, EPD can now be accomplished from aqueous suspensions with low water electrolysis rates, a variety of biochemical and biological species can be deposited to yield highly active layers suitable for a wide range of applications including biosensors, biofuel cells and bioreactors.

Fabrication of Aligned-Carbon-Nanotube-Composite Paper with High and Anisotropic Conductivity

A functional carbon-nanotube (CNT)-composite paper is described in which the CNTs are aligned. This “aligned-CNT composite paper” is a flexible composite material that has CNT functionality (e.g., electrical conductivity) despite being a paper. An advanced fabrication method was developed to overcome the problem of previous CNT-composite papers, that is, reduced conductivity due to random CNT alignment. Aligning the CNTs by using an alternating current (AC) field was hypothesized to increase the electrical conductivity and give the paper an anisotropic characteristic. Experimental results showed that a nonionic surfactant was not suitable as a CNT dispersant for fabricating aligned-CNT composite paper and that catechin with its six-membered rings and hydrophilic groups was suitable. Observation by scanning electron microscopy of samples prepared using catechin showed that the CNTs were aligned in the direction of the AC field on the paper fibers. Measurement of the electric conductivity showed that the surface resistance was different between the direction of the aligned CNTs (high conductivity) and that of verticality (low). The conductivity of the aligned-CNT-composite paper samples was higher than that of nonaligned samples. This unique and functional paper, which has high and anisotropic conductivity, is applicable to a conductive material to control the direction of current.

Nonlinear motion of magnetic vortex under alternating-current magnetic field: Dynamic correction of a gyrovector and a damping tensor of the Thiele’s equation

We investigated a forced oscillation motion of a magnetic vortex structure on a ferromagnetic nanodisk under an alternating-current (AC) external magnetic field using micromagnetic simulations. With variations of the AC field amplitude and the frequency, it has been revealed that the forced oscillation motion could become nonlinear and significantly deviated from the description of the original Thiele’s equation. We propose a simple model with a dynamic correction of a gyrovector and a damping tensor in solving the Thiele’s equation, which is valid even in case of the nonlinear magnetic vortex core motion.

Self-assembled dendritic nanowires of Au–Pt alloy through electrodeposition from solution under AC fields

Au–Pt dentritic nanowires (DNWs) bridging a pair of 10–40μm spacing electrodes were successfully self-assembled from the corresponding ions solution via deposition of an alternating current (AC). The morphology and the composition of Au–Pt DNWs were characterized by scanning electron microscopy (SEM) and energy-dispersion X-ray (EDX) spectrometer. In the electrolyte of various molar ratios of [AuCl4]−/[PtCl6]2−, Au–Pt DNWs with 8–95at% Pt were fabricated through applying a constant voltage 8–16Vpp and varied frequencies of 300Hz for nucleation and 1MHz for growth. The DNWs have the diameter range of 100–200nm. The X-ray diffraction (XRD) results confirmed that the DNWs of 20 or 80at% Pt have the alloy phase structure. Au–Pt DNWs are promising nanomaterials of chemical sensors, bioelectronic circuits and nanoelectronic interconnection.

Linear stability analysis of electrohydrodynamic instabilities at fluid interfaces in the “small feature” limit

The endeavour to effectively harness interfacial electrohydrodynamic instabilities, to create small patterns, involves reducing the wavelength of the instability. This can be accomplished by decreasing the separation between the electrodes which may not always be possible. One may therefore have to reduce the surface tension or increase the applied voltage at a fixed electrode spacing. This can result in the wavelength of the pattern becoming of the same order as the electrode separation. Pease and Russel (J. Chem. Phys. 118, 3790 (2003)) were the first to argue that the commonly used Thin-Film Approximation (TFA) that involves an asymptotic expansion in the small parameter δ = (ε(0) φ(0)(2)/(γh(0)))(1/2) (where ε (0) is the permittivity of vacuum, φ (0) is the root mean square value of the applied potential, γ is the surface tension and h (0) is the thickness of the thin film) need not always be valid and γ may not be small in experiments. Higher-order corrections to the TFA might therefore be necessary. We extend the Direct Current (DC) field analysis of Pease and Russel to an Alternating Current (AC) field. AC field has been suggested as an effective way of controlling the wavelength of electrohydrodynamic instabilities at fluid-fluid interfaces. Infact, the perfect and leaky dielectric limits can be realised in the same fluid at very high and very low electric field frequencies, respectively. Recently, Roberts and Kumar (J. Fluid Mech. 631, 255 (2009)) carried out an analysis using TFA to investigate AC-field-induced instabilities at air-polymer interfaces. We propose a Generalized Model (GM), without the lubrication approximation, and carry out detailed comparison with the TFA. We consider the top fluid to be air, a perfect dielectric, and the bottom fluid to be a perfect or a leaky dielectric. The analysis is carried out for both DC and AC fields, and the deviation from TFA is expressed in terms of the parameter B = γh(0)/(ε(0) φ(0)(2) ) = δ(-2). We discuss variation of the wavelength of the fastest growing mode with frequency of the applied field for any arbitrary value of B, unlike the analysis of Roberts and Kumar which is restricted to B ≫ 1(δ ≪ 1) . We also revisit the analysis of Pease and Russel for instabilities under DC field and present the results in terms of the single parameter, B.

Role of Electric Field on Surface Wetting of Polystyrene Surface

The role of surface charge in fluid flow in micro/nanofluidics systems as well as the role of electric field to create switchable hydrophobic surfaces is of interest. In this work, the contact angle (CA) and contact angle hysteresis (CAH) of a droplet of deionized (DI) water were measured with applied direct current (DC) and alternating current (AC) electric fields. The droplet was deposited on a polystyrene (PS) surface, commonly used in various nanotechnology applications, coated on a doped silicon (Si) wafer. With the DC field, CA decreased with an increase in voltage. Because of the presence of a silicon oxide layer and a space charge layer, the change of the CA was found to be lower than with a metal substrate. The CAH had no obvious change with a DC field. An AC field with a positive value was applied to the droplet to study its effect on CA and CAH. At low frequency (lower than 10 Hz), the droplet was visibly oscillating. The CA was found to increase when the frequency of the applied AC field increased from 1 Hz to 10 kHz. On the other hand, the CA decreased with an increasing peak-peak voltage at or lower than 10 kHz. The CAH in the AC field was found to be lower than in the DC field and had a similar trend to static CA with increasing frequency. A model is presented to explain the data.

Innovative Calorimetric AC Loss Measurement of HTSC for Power Applications

The applications of high-temperature superconductors (HTS) in electric power components have been widely reported and various studies have been made to define their alternating current (AC) losses-a key design parameter for many practical high power electrical engineering applications. However, very few studies over the range 25 to 45 K have been conducted even though this is one of the favored temperature ranges for cost-effective applications of HTS. Methods and techniques used to characterize and measure these losses have been so far grouped into `electrical' and `calorimetric' approaches with external conditions set to resemble the application conditions. In this paper, we present an approach using the calorimetric method to accurately determine losses in the superimposed AC and DC fields likely to be experienced in practical devices such as Fault Current Limiters. This technique provides great simplification compared to pick-up coil and lock-in amplifier methods and is applied to a lower temperature range. The preliminary loss data at 40 K will be presented in applied AC magnetic fields with DC fields up to 1T. The data of losses obtained on this sample will allow the estimation and minimization of losses in practical high power HTS coils and will be used in the verification of numerical coil models.

AC Field-Induced Polymer Electroluminescence with Single Wall Carbon Nanotubes

We developed a high-performance field-induced polymer electroluminescence (FPEL) device consisting of four stacked layers: a top metal electrode/thin solution-processed nanocomposite film of single wall carbon nanotubes (SWNTs) and a fluorescent polymer/insulator/transparent bottom electrode working under an alternating current (AC) electric field. A small amount of SWNTs that were highly dispersed in the fluorescent polymer matrix by a conjugate block copolymer dispersant significantly enhanced EL, and we were able to realize an SWNT-FPEL device with a light emission of approximately 350 cd/m(2) at an applied voltage of ±25 V and an AC frequency of 300 kHz. The brightness of the SWNT-FPEL device is much greater than those of other AC-based organic or even inorganic ELs that generally require at least a few hundred volts. Light is emitted from our SWNT-FPEL device because of the sequential injection of field-induced holes and then electron carriers through ambipolar carbon nanotubes under an AC field, followed by exciton formation in the conjugated organic layer. Field-induced bipolar charge injection provides great material design freedom for our devices; the energy level does not have to be aligned between the electrode and the emission layer, and the balance of the carrier injected and transported can be altered in contrast to that in conventional organic light-emitting diodes, leading to an extremely cost-effective and unified device architecture that is applicable to all red-green-blue fluorescent polymers.

Unfolding collapsed polyelectrolytes in alternating-current electric fields

We investigate the unfolding of single polyelectrolyte (PE) chains collapsed by trivalent salt under the action of alternating-current (AC) electric fields through computer simulations and theoretical scaling. The results show that a collapsed chain can be unfolded by an AC field when the field strength exceeds the direct-current (DC) threshold and the frequency is below a critical value, corresponding to the inverse charge relaxation/dissociation time of condensed trivalent counterions at the interface of the collapsed electrolyte. This relaxation time is also shown to be identical to the DC chain fluctuation time, suggesting that the dissociation of condensed polyvalent counterion on the collapsed PE interface controls the polyelectrolyte dipole formation and unfolding dynamics under an AC electric field.

Generation of VLF Mode Instability by Generalized Distribution Function in the Presence of Parallel AC Electric Field in Uranus

VLF (very low frequency) mode instability with parallel AC electric field was studied for generalized loss-cone distribution with an index j, which is reducible to bi-Maxwellian, loss-cone and delta function for j = 0,1 and ∞. The particle trajectories and dispersion relation are obtained through a kinetic approach and method of characteristic solutions. The calculations are compared with the observations of low frequency waves of Voyager 2. The growth rate of plasma parameters suited to magnetosphere of Uranus is obtained. It is inferred that the magnitude as well as frequency of AC (alternating current) field increases the growth rate and widen the band width significantly. In addition to temperature anisotropy, particles in plasma having generalized loss-cone distribution provide an additional source of energy.

Alternating current electrical field effects on lettuce (Lactuca sativa) growing in hydroponic culture with and without cadmium contamination

The aim of the present study was to investigate the effect of a 10 Hz alternating current (10 Hz 1 V cm−1) and a 50 Hz alternating current (50 Hz 1 V cm−1) on the lettuce plant (Lactuca sativa) growing in a hydroponic (soil-free) culture. Thirty lettuce plants were pre-germinated, and then 15 of them were treated with cadmium solution (CdCl2) of 5 mg/L in concentration. Ten plants (five plants with Cd and five plants without Cd) were subjected to a 10 Hz alternating current (AC) electrical field; 10 plants were subjected to a 50 Hz AC field. The rest of the plants were used as a control. The lettuce plants were harvested after a growth of 60 days. The chlorophyll content, biomass and metal content of the lettuce plants were determined. The biomass of the plants growing in non-contaminated medium was 28 and 106% higher under the 10 and the 50 Hz AC fields respectively compared to the control. Although the plant biomass was reduced by the presence of Cd in the growth medium, the biomass of the plants growing in Cd contaminated medium was 40 and 63% higher respectively for 10 and 50 Hz AC field compared to the plant growing in Cd contaminated medium without electrical treatment. Increased uptake of Cd in the plant shoot was found with the 50 Hz AC field. Significant accumulation and uptake of Cu in plant roots and shoots was found under both electrical treatments.

Effect of magnetic field on electrical properties of nanocrystalline poly(vinylidene fluoride) samples

AbstractBACKGROUND: The electrical properties of nanocrystalline poly(vinylidene fluoride) (PVDF) samples of 20 µm in thickness were measured in terms of thermally stimulated current (TSC), conduction current and dielectric constant after application of a magnetic field.RESULTS: TSC shows the release of trapped charges inside the material that enhances the current with magnetic field. The reason for the polarity reversal of the current with reversal of the magnetic field polarity is due to the change in spin of electrons depending upon the direction of the magnetic field.CONCLUSION: The magnetic field causes trapping of charge carriers in different traps, as the reason for the increase of activation energy with increasing field. The flow of conduction current at constant temperature in magnetically polarized PVDF is governed by Poole–Frenkel and Schottky–Richardson mechanisms. The decrease in dielectric constant at a certain alternating current (AC) frequency and magnetic field with temperature is caused by magnetic polarization in addition to the AC field. Copyright © 2009 Society of Chemical Industry

Transport of biomolecules in asymmetric nanofilter arrays

We propose a theoretical model for describing the electric-field-driven migration of rod-like biomolecules in nanofilters comprising a periodic array of shallow passages connecting deep wells. The electrophoretic migration of the biomolecules is modeled as transport of point-sized Brownian particles, with the orientational degree of freedom captured by an entropy term. Using appropriate projections, the formulation dimensionality is reduced to one physical dimension, requiring minimal computation and making it ideal for device design and optimization. Our formulation is used to assess the effect of slanted well walls on the energy landscape and resulting molecule mobility. Using this approach, we show that asymmetry in the well shape, such as a well with one slanted and one vertical wall, may be used for separation using low-frequency alternating-current fields because the mobility of a biomolecule is different in the two directions of travel. Our results show that, compared to methods using direct-current fields, the proposed method remains effective at higher field strengths and can achieve comparable separation using a significantly shorter device.

Dielectric and dielectrophoretic properties of DNA

The physical properties of DNA are quite important for molecular genetics as well as for its nanotechnological applications. Studying the interactions of alternating current (AC) electric fields with deoxyribonucleic acid (DNA) allows one to draw conclusions about these properties. These interactions are usually investigated in two different ways. In dielectric spectroscopy, a DNA solution is placed in a homogeneous AC field and electronic parameters are measured over several frequency decades in the Hz to GHz range. These electronic data are then interpreted on the basis of physico-chemical models as a result of certain phenomena on the molecular level. In dielectrophoretic studies, a DNA solution is exposed to an inhomogeneous AC field and the spatial response of few or single molecules is monitored by optical or scanning force microscopy. This response can involve translation, elongation and orientation of the molecular strings. In this review, a survey is given of the literature dealing with the dielectric and dielectrophoretic properties of DNA as well as with applications of DNA dielectrophoresis.