Ac Field Research Articles

A Multi-Mode High-Efficiency Fault-Tolerant Permanent Magnet Machine

This article proposes a new multi-mode high-efficiency fault-tolerant permanent magnet machine with an inner stator to place AC field windings. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis current in the field windings enables the proposed machine to be driven individually or together with the armature windings. On the other hand, the flux can be regulated by applying the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> -axis current to the field windings. As a result, multiple operation modes are realized. The proposed machine can run in different modes at different torque and speed operating conditions, implementing the multi-zone high-efficiency operation. Meanwhile, a new fault-tolerant control method can be applied in the proposed machine by combining the mode switching and compensation current in the healthy windings, thus improving the efficiency under fault-tolerant operation. Moreover, the multi-zone high-efficiency regions of the proposed and conventional machines are investigated. The proposed and traditional fault-tolerant methods for the proposed machine are compared. The results reveal that the proposed machine offers a larger high-efficiency region and is capable of stronger fault tolerance. Furthermore, since the double stators in the proposed machine need to operate individually, a swallow-shaped rotor with auxiliary salient poles is adopted to reduce outer torque ripple and inner torque ripple. Finally, the experiments are performed to verify the predicted performances.

Electrochromic reverse-mode PDLCs as smart windows

ABSTRACT The reduction of energy consumption is according to the EU Parliament one of the most important actions to reduce the use of fossil fuels and the emission of carbon dioxide to contrast climate changes. Smart windows, and in particular PDLCs, are an easy way to increase the energy efficiency of buildings and promote energy saving. They have found application as large area surfaces able to change on-demand their transmittance in a continuous manner from an opaque to a transparent state in many fields such as interior design, architecture, and automotive. In this work, we report the preparation and characterisation of a reverse mode PDLC, in which the nematic mixture of a liquid crystal and an ionic liquid allows the on-demand modification of both the transmittance and colour of the device. The transparent and uncoloured OFF state, which is the suggested state from a safety and energy saving point of view, can be modulated into an opaque and/or coloured state by the application of an AC and/or DC electric field.

Application of the Lock-In Technique in Magnetoelectric Coupling Measurements of the PZT/Terfenol-D Composite

This paper presents a study of magnetoelectric (ME) properties of the PZT/Terfenol-D composite with a varying number of layers. The composite consists of piezoelectric and magnetostrictive phases that are mechanically coupled. The purpose of this setup is to gain control over the electric polarization of a material via an external magnetic field. Unlike most similar composites, our samples utilize a commercial piezoelectric patch instead of pure PZT. At present, researchers face two main problems regarding magnetoelectric materials: (i) the effect is observed far below room temperature for single-phase materials, and (ii) the ME coupling is too weak to be commercially viable. Our research was carried out via the lock-in technique on two PZT/Terfenol-D samples we synthesized. Relatively strong room-temperature magnetoelectric coupling between piezoelectric and magnetostrictive phases was observed for both samples. Two types of characteristics were investigated: (i) ME voltage versus magnetic AC field frequency, and (ii) ME voltage versus magnetic DC field. We detected multiple, grouped signal peaks ascribed to different resonance modes. Uniquely, the peaks form band-like characteristics which might be an important step in bringing the materials closer to wider commercial use.

Influence of a bias dc field and an ac field amplitude on the dynamic susceptibility of a moderately concentrated ferrofluid.

In this paper, we study the effect of a bias dc field on the dynamic response of a moderately concentrated ferrofluid to an ac magnetic field of arbitrary amplitude. The ferrofluid is modeled by an ensemble of interacting moving magnetic particles; the reaction of particle magnetic moments to ac and dc magnetic fields occurs according to the Brownian mechanism; and the ac and dc magnetic fields are parallel. Based on a numerical solution of the Fokker-Planck equationfor the probability density of the orientation of the magnetic moment of a random magnetic particle, dynamic magnetization and susceptibility are determined and analyzed for various values of the ac field amplitude, the dc field strength, and the intensity of dipole-dipole interactions. It is shown that the system's magnetic response is formed under the influence of competing interactions, such as dipole-dipole, dipole-ac field, and dipole-dc field interactions. When the energies of these interactions are comparable, unexpected effects are observed: the system's susceptibility can either increase or decrease with increasing ac field amplitude. This behavior is associated with the formation of nose-to-tail dipolar structures under the action of the dc field, which can hinder or promote the system's dynamic response to the ac field. The obtained results provide a theoretical basis for predicting the dynamic properties of ferrofluids to improve their use in biomedical applications, such as, in magnetic induction hyperthermia.

Fouling minimization with nanofluidic membranes; How electric field may help

Researchers have given water purification substantial consideration in recent years as the availability of fresh water supplies has become a worldwide problem. Water treatment is the typical use for membranes. Enhancing membrane performance is thus a much sought after area of study. In this study, we examine a nanochannel that is embedded in a nanofluidic membrane, located between two reservoirs containing KCl electrolyte solution with equal concentrations. The neutral component present in the source reservoir is bovine serum albumin (BSA). A negatively charged polyelectrolyte layer coats the inside of the nanochannel, making the mass transfer more manageable and efficient. By applying an electric field and inducing electroosmotic flow, the electrolyte passes through the nanochannel without a change in concentration, while BSA is concentrated in the feed reservoir. Fouling performance was evaluated by measuring the flux of fluid passing through the nanochannel, the concentration difference CK+-CCl-, the electroosmotic velocity of the flow, and the time average volume flux. The BSA rejection was also monitored as a goal function. The difference between the electrolyte and the soft layer's physical–chemical properties was varied, and the parameters of electrolyte concentration, surface charge density of the soft layer, and type and frequency of the applied electric field, were all altered to determine their impact on membrane fouling. To do this, the Poisson-Nernst-Planck, continuity, and Navier-Stokes equations were numerically solved using the finite element method in an unsteady state. The findings demonstrate that fouling in the nanochannel can be reduced by increasing the charge density of the soft layer and lowering the electrolyte concentration. Using an AC field at intermediate frequencies is far more successful than a DC field in reducing fouling. Nevertheless, the AC field does not outperform the DC field in fouling reduction at either low or high frequencies tested.

Dynamic resistance and voltage response of a REBCO bifilar stack under perpendicular DC-biased AC magnetic fields

The dynamic resistance of REBCO (REBa2Cu3O7-d, RE stands for rare earth), coated conductors (CCs) is a key parameter in many high-temperature superconductor applications where CCs carry DC currents exposed to AC and DC magnetic fields, such as field-triggered persistent current switches, flux pumps, and fault current limiters. In this work, dynamic resistance and dynamic voltage have been studied via experiments and finite element method (FEM) simulations in a REBCO bifilar stack at 77 K, under combined AC and DC magnetic fields with different magnitudes, frequencies, and waveforms. Our results show some distinct features of dynamic resistance and voltage from those under pure AC magnetic fields. With an increasing DC magnetic field, the dynamic resistance exhibits an obvious linearity with the applied AC magnetic field, and becomes less dependent on the AC field frequency. The fundamental frequency of the dynamic voltage under a DC magnetic field becomes the same as that of the applied AC field, which completely differs from the pure AC field case where the fundamental frequency doubles. For the first time, instantaneous threshold field (B th) values are obtained from the dynamic voltage, which are substantially different in the field-increasing and field-decreasing processes. These key differences are attributed to the dominant role of DC magnetic fields in determining the critical current of the superconductor, which significantly dwarfs the influence of AC fields. These new discoveries may help researchers better understand the electromagnetism of superconductors and be useful for relevant applications.

Inverted Sedimentation of Active Particles in Unbiased ac Fields.

Gaining control over the motion of active particles is crucial for applications ranging from targeted cargo delivery to nanomedicine. While much progress has been made recently to control active motion based on external forces, flows, or gradients in concentration or light intensity, which all have a well-defined direction or bias, little is known about how to steer active particles in situations where no permanent bias can be realized. Here, we show that ac fields with a vanishing time average provide an alternative route to steering active particles. We exemplify this route for inertial active particles in a gravitational field, observing that a substantial fraction of them persistently travels in the upward direction upon switching on the ac field, resulting in an inverted sedimentation profile at the top wall of a confining container. Our results offer a generic control principle that could be used in the future to steer active motion, direct collective behaviors, and purify mixtures.

Polar response and Fréedericksz transition in cybotactic nematic fluids of unsymmetrical bent-core molecules

The appearance of polar order in nematic fluids is of broad interest rejuvenating affluent concepts of physics and chemistry. In bent-core nematics (BCNs), ferroelectric switching is a challenging quest of considerable interest after its primary observation in 1,2,4-oxadiazole-based BCNs. Herein, we observed induced polar switching in the cybotactic nematic phase (Ncyb) with a polarization value of ∼185 nC/cm2 in an unsymmetrical bent-core compound 8-F-OH. Temperature-dependent SAXS/WAXS analysis and dielectric measurements revealed nano-sized cybotactic clustering within the nematic phase. Electric-field-induced Fréedericksz transitions (FT) in the Ncyb phase at different frequencies and temperatures were investigated. Compound 8-F-OH exhibited two domains that formed and competed when the system was driven by an ac field. Thus, the build-up process of FT involved the reorientation of molecules and the transport of domains, where each had a distinctive characteristic time. We envisage that these detailed investigations can pave a bright way toward the development of advanced-level optical and electro-optical applications of BCN systems.

Impact of Processing Parameters on Contactless Emulsification via Corona Discharge.

A contactless emulsification method is presented using corona discharge. The corona discharge forms using a pin-to-plate configuration, creating a non-uniform electric field. This results in a simultaneous electrohydrodynamic (EHD) pumping of silicone oil and an electroconvection of water droplets that accelerate and submerge inside the oil, leading to a continuous water-in-oil (W/O) emulsion formation process. The impact of the oil viscosity and corona generating AC and DC electric fields (i.e., voltage and frequency) on the characteristics of the emulsions is studied. The emulsification power consumption using the AC and DC electric fields is calculated and compared to traditional emulsion formation methods. While using the DC electric field results in the formation of uniform emulsions, the AC electric field is readily available and uses less power for the emulsification. This is facile, contactless, and energy-efficient for the continuous formation of W/O emulsions.

Complex AC Magnetic Susceptibility as a Tool for Exploring Nonlinear Magnetic Phenomena and Pinning Properties in Superconductors.

The versatile AC magnetic susceptibility technique offers a detailed insight into the complex electrodynamic phenomena in superconductors. In the present study, we outline the key effects related to the temperature, AC field amplitude and frequency variations of the fundamental and harmonic components for an investigation of the vortex dynamics in a flux-grown FeSe crystal. By means of higher harmonic (nonlinear) analysis, we have explored certain atypical, asymmetric features in the AC magnetic response. These effects were identified through the detection of an even (second) harmonic and an unusual temperature shift in the odd (third) harmonic, possibly due to the complex interactions related to the composite superconducting/magnetic morphology of the crystal. Using the high-frequency sensitivity of the third harmonic, the basic functional dependencies of the pinning activation energy, as the main mixed state parameter, were determined with the implementation of the Kim-Anderson Arrhenius relation in the framework of the collective creep theory.

Coexistence of Bloch and Parametric Mechanisms of High-Frequency Gain in Doped Superlattices

The detailed theoretical study of high-frequency signal gain, when a probe microwave signal is comparable to the AC pump electric field in a semiconductor superlattice, is presented. We identified conditions under which a doped superlattice biased by both DC and AC fields can generate or amplify high-frequency radiation composed of harmonics, half-harmonics, and fractional harmonics. Physical mechanisms behind the effects are discussed. It is revealed that in a general case, the amplification mechanism in superlattices is determined by the coexistence of both the phase-independent Bloch and phase-dependent parametric gain mechanisms. The interplay and contribution of these gain mechanisms can be adjusted by the sweeping AC pump strength and leveraging a proper phase between the pump and strong probe electric fields. Notably, a transition from the Bloch gain to the parametric gain, often naturally occurring as the amplitude of the amplified signal field grows, can facilitate an effective method of fractional harmonic generation in DC–AC-driven superlattices. The study also uncovers that the pure parametric generation of the fractional harmonics can be initiated via their ignition by switching the DC pump electric field. The findings open a promising avenue for the advancement of new miniature GHz–THz frequency generators, amplifiers, and dividers operating at room temperature.

Effect of DC and AC electric fields on crack healing behavior in 8 mol% yttria‐stabilized cubic zirconia polycrystal

AbstractThe effect of the flash event (FE) on microcrack healing behavior in 8 mol% yttria‐stabilized zirconia was examined at healing temperatures of 1040 and 1230°C under the direct and alternating (DC and AC) electric fields. The crack healing behavior changed depending on the factors of the electric field, healing temperature, and crack length. Although the crack healing proceeded with the temperature, the healing rate increased with the crack length, suggesting that the external energy stored as crack surface energy would provide a driving force for the crack healing. Although the crack healing occurs even under the static annealing without the electric field, the healing rate was accelerated by FE significantly more under the AC field than under the DC field. The microcracks with a length of ≈20 μm were fully healed at 1230°C only for 10 min by the FE treatment under the AC field, and the flash healing behavior was four times faster than that of the static annealing. These results suggest that the enhanced healing behavior cannot be explained only by thermal effects, and the accelerated diffusivity caused additionally by nonthermal effect under FE might contribute to the enhanced healing behavior, especially in the AC electric field.

Impact of the Electric Polarizability on the Transport and Collective Dynamics of Metallodielectric Janus Particles.

The objective of this article is to investigate how the electric polarizability manifests on the propulsion and collective dynamics of metallodielectric Janus particles by comparing the velocity spectra under rotating and nonrotating AC fields. Janus particles were fabricated by depositing sequential layers of titanium and SiO2 on spherical cores. Model systems of known polarizability were created by varying the thickness of titanium or by adjusting the concentration of electrolyte. We found that the spectra for propulsion velocity displayed features (amplitude and transition frequencies) that were closely matched in the electrorotation spectra. That is, the transition frequency from dielectric- to metal-side forward matched closely the peak in counterfield rotation, while the minima in propulsion velocity matched the transition frequency from counterfield to cofield rotation. Furthermore, based on electroorientation measurements for prolate Janus ellipsoids, we conclude that the propulsion velocity of spherical Janus particles reflects the real part of their polarizability. Solutions of the Poisson-Nernst-Planck equations confirm the thickness of the metal cap facilitates adjusting the behavior from metal- to dielectric-like. These traits translate into different collective behaviors, such as the ability to traverse or become part of a lattice of nonpatchy silica particles. Overall, these results provide experimental evidence to either challenge or refine existing electrokinetic models of propulsion.

Vortex dynamics induced by scanning SQUID susceptometry

We measured the local magnetic response of a niobium thin film by applying a millitesla-scale AC magnetic field using a micron-scale field coil and detecting the response with a micron-scale pickup loop in a scanning superconducting quantum interference device (SQUID) susceptometry measurement. Near the film's critical temperature, we observed a step-like nonlinear and dissipative magnetic response due to the dynamics of a small number of vortex-antivortex pairs induced in the film by the local applied AC field. We modeled the dynamics of the measurement using a combined two-dimensional London-Maxwell and time-dependent Ginzburg-Landau approach, allowing us to construct a detailed real-space picture of the vortex motion causing the observed dissipative response. This work pushes scanning SQUID susceptometry of two-dimensional superconductors beyond the regime of linear response and lays the foundation for microscopic studies of vortex dynamics and pinning in superconducting devices and more exotic materials systems.

The Effect of a DC Magnetic Field on the AC Magnetic Properties of Oleic Acid-Coated Fe3O4 Nanoparticles.

The AC magnetic properties of a sample of Fe3O4 nanoparticles coated with oleic acid have been investigated with the help of AC susceptibility measurements. In particular, several DC magnetic fields have been superimposed on the AC field, and their effect on the magnetic response of the sample has been analysed. The results show the presence of a double peak structure in the imaginary component of the complex AC susceptibility measured as a function of the temperature. A preliminary evaluation of the Mydosh parameter for both peaks gives the information that each one of them is associated with a different state of interaction between nanoparticles. The two peaks evolve both in amplitude and position when the intensity of the DC field is changed. The field dependence of the peak position shows two different trends, and it is possible to study them in the framework of the currently existing theoretical models. In particular, a model of non-interacting magnetic nanoparticles has been used to describe the behaviour of the peak at lower temperatures, whereas the behaviour of the peak at higher temperatures has been analysed in the framework of a spin-glass-like model. The proposed analysis technique can be useful for the characterisation of magnetic nanoparticles used in several types of applications, such as biomedical and magnetic fluids.

Investigation of Brushless Doubly-Fed Dual-Stator Machines for Traction Applications

Two brushless doubly-fed dual-stator machines with different inner-stator field windings, namely AC and DC field windings, are proposed and investigated. It is shown the proposed machine with AC field windings exhibits advantages of higher torque density and wider flux-regulation range compared with its DC field winding counterpart. Unlike conventional doubly-fed hybrid-excited machines, the proposed machine consists of AC field windings and hence resulting in more flexible field current control. Its armature and field windings are separately placed in two stators, which can alleviate the space competition between the field and armature slots, as well as significantly enhance the space utilization. Consequently, both flux-regulation capability and torque density can be effectively improved. The topology, operating principle, and mathematical model of the proposed machines are introduced. Then, a sectored flux-weakening control method is proposed and elaborated. Based on finite element method (FEM), the main electromagnetic performances of the proposed machines are comprehensively evaluated. The FEM results manifest the proposed machine with AC field windings shows greatly improved flux-regulation range, higher torque and output power under the proposed sectored flux-weakening method over the whole speed range. Finally, prototype machine of the proposed machine is fabricated, and experimental verifications are performed to validate the feasibility of the proposed machines.

Dielectric Behavior of Cellulosic Insulating Paper Subjected to a Combined AC and DC Electric Field

The effect of high dc-fields on the ac-dielectric response was studied in cellulosic insulating papers. Measurements of the complex permittivity were carried out in a frequency range from several Hz to 1 MHz with a custom-made dielectric test setup. Non-impregnated and oil-impregnated papers were investigated at different water contents. The real and imaginary components of the complex permittivity increased considerably at lower frequencies when a high dc-field was applied simultaneously. The increase depended on the water content of the paper and on the amplitude of the applied electric field. It is suggested that ionic motions in hydrated cellulose caused by a high dc-field had an effect on the complex permittivity in a frequency range determined by the time constants of movements of ionic species.

Influence of the ac magnetic field on the elastic and electric characteristics of a quantum paramagnet

It is shown that electric, piezoelectric, and elastic characteristics of the orthorhombic quantum paramagnet depend on the applied ac magnetic field, collinear to the dc one. The dependence is due to the renormalization of the quadrupole spin susceptibility. In the dynamical regime, the quadrupole susceptibility oscillates with the frequency of the ac field, and with the Rabi-like frequency. In the steady-state regime, the quadrupole susceptibility oscillates with the frequency of the ac field only about the renormalized with respect to the initial one value. That renormalization can change the values of the electric, piezoelectric, and elastic characteristics of the system.

Josephson current-assisted reversal of a single-domain nanoscale ferromagnet driven by cosine chirp pulse

We study the microwave-induced magnetization reversal in two systems, the microwave-driven nanomagnet (NM) and the NM coupled to a Josephson junction (JJ) under the microwave field (NM-JJ-MW). The frequency of the applied cosine chirp pulse changes nonlinearly with time to match the magnetization precession frequency. The coupling between the NM and JJ reduces the magnetization switching time as well as the optimal amplitude of the microwave field as a result of manipulating the magnetization via Josephson-to-magnetic energy ratio G. The reversal effect in NM-JJ-MW is sufficiently robust against changes in pulse amplitude and duration. In this system, the increase of G decreases the possibility of the non-reversing magnetic response as the Gilbert damping increases without further increase in the external microwave field. We also discuss the magnetic response of the NM driven by the ac field of two JJs in which the time-dependent frequency is controlled by the voltage across the junctions. Our results provide a controllable scheme of magnetization reversal that might help to realize fast memory devices.

Harmonic signals of magnetic nanoparticle in suspension for biosensing application: Comparison of properties between Brownian- and Néel-dominant regions

We have studied harmonic signals caused by the nonlinear AC magnetization (M-H) curve of magnetic nanoparticles (MNPs) suspended in solution. First, numerical simulations were performed for wide ranges of the MNP parameters and the excitation fields that are commonly used in biosensing applications. As an example, the third harmonic signal was mainly studied. Because the AC M-H curve for the suspended MNPs is affected by both Brownian and Néel relaxations, we demonstrated the condition that determines the dominant relaxation. We emphasize that the MNP’s behavior changes from Brownian-dominant to Néel-dominant relaxation when the amplitude of the excitation field is increased, even when the MNP parameters are fixed. The properties of the harmonic signals in the Brownian- and Néel-dominant cases were compared, and the differences between the two cases were clarified. The relationships among the higher harmonic signals were also presented. Next, an analytical expression for the third harmonic signal was obtained for both the Brownian- and Néel-dominant cases. In the Néel-dominant case, the alignment of the easy axes caused by the AC field was taken into account. We demonstrated good agreement between the simulation and the analysis results. Finally, harmonic signals were measured using a commercial MNPs sample. Reasonable agreement was obtained between experiment and analysis. The results obtained will be useful for biosensing applications.