AC Electric Field Research Articles

Electrohydrodynamic convection instabilities observed in suspensions of cellulose nanocrystals

Cellulose nanocrystals are slender, negatively charged nanoparticles that spontaneously form a cholesteric liquid crystal in aqueous suspension above a critical concentration. When they are suspended in apolar solvents such as toluene using surfactants, the application of an AC electric field leads to the reorientation and then distortion of the cholesteric order until the cholesteric structure completely unwinds into a nematic-like order, typically above 0.4–0.6 kV/cm at 1kHz. In this work, we show that at much higher electric fields (ge 4.6 kV/cm at 1 kHz) the sample develops a periodic pattern that varies with the field amplitude. We ascribed this pattern to electrohydrodynamic convection instabilities. These instabilities usually present complex regimes varying with the field, the voltage, the frequency and the geometry. However, the typical geometry where these instabilities were most documented across the literature differs from the geometry used in this work. This work concludes with possible future experimental investigations to clarify the exact regime of instability responsible for these observations.

Enhanced high‐power performance of Fe‐doped PZMNZT piezoelectric ceramics

AbstractHigh‐power piezoelectric ceramics are typically driven to output vibration velocity (v0) under high AC electric fields. Herein, the Fe2O3 doped 0.125 Pb(Zn1/3Nb2/3) O3–0.075 Pb(Mn1/3Nb2/3)O3–0.8 Pb(Zr0.48Ti0.52)O3(PZMNZT–xFe; x = 0.05–0.35) piezoelectric ceramics were prepared to enhance v0, and the favorable comprehensive electrical properties, such as d33 = 315 pC/N, Qm = 1738, kp = 0.58, kt = 0.48, εr = 1156, tan δ = 0.4%, and Tc = 320°C, were achieved in the PZMNZT–0.15Fe ceramic. Most importantly, the PZMNZT–0.15Fe ceramic presented a reliable v0 of 0.90 m/s, which was 2.25 times of the commercial PZT4 ceramic (∼0.40 m/s). The excellent high‐power performance should be attributed to ordering functional elements such as crystal grains and ferroelectric domains. Overall, this work reveals that the PZMNZT–0.15Fe ceramic is competitive for high‐power applications.

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.

Electrorheological Characteristic of Heavy Oil Emulsions under High-Frequency/High-Voltage AC Electric Field

Electrorheological Characteristic of Heavy Oil Emulsions under High-Frequency/High-Voltage AC Electric Field

Critical coalescence electric intensity of water droplets adsorbing surfactant molecules under electromagnetic synergy field

Electromagnetic synergy is a more effective physical method than a single AC electric field (ACEF) to enhance oil–water separation. However, the critical coalescence conditions and transformation mechanism of droplets adsorbing surfactant molecules in the synergistic electromagnetic field (EMSF) still lack research. Herein, a series of Triton X-100-adsorbed droplets with different surface tensions were prepared, and the critical coalescence electric intensity (Ec) of the droplets under ACEF and EMSF were compared. Micro high-speed experiments revealed that Ec of droplets under EMSF is higher than under ACEF. The electric intensity, Ec, increased with the increase in interfacial tension γ under both ACEF and EMSF. This study provides valuable results for designing high-performance devices by introducing electromagnetic synergy in water-in-oil emulsion treatment. For the equipment with electromagnetic synergy to strengthen oil–water separation, the allowable optimal voltage should be higher than that of the electrostatic coalescer, and the adjustment range is 2.99%−9.57%.

A Boltzmann Electron Drift Diffusion Model for Atmospheric Pressure Non-Thermal Plasma Simulations

We introduce a fluid computational model for the numerical simulation of atmospheric pressure dielectric barrier discharge plasmas. Ion and neutral species are treated with an explicit drift diffusion approach. The Boltzmann relation is used to compute the spatial distribution of electrons as a function of the electrostatic potential and the ionic charge density. This technique, widely used to speed up particle and fluid models for low-pressure conditions, poses several numerical challenges for high-pressure conditions and large electric field values typical of applications involving atmospheric-pressure plasmas. We develop a robust algorithm to solve the non-linear electrostatic Poisson problem arising from the Boltzmann electron approach under AC electric fields based on a charge-conserving iterative computation of the reference electric potential and electron density. We simulate a volumetric reactor in dry air, comparing the results yielded by the proposed method with those obtained when the drift diffusion approach is used for all charged species, including electrons. We show that the proposed methodology retains most of the physical information provided by the reference modeling approach while granting a substantial advantage in terms of computation time.

DC electric field generation and distribution in magnetized plasmas

Very large DC and AC electric fields cannot be sustained between conducting electrodes because of volume gas breakdown and/or surface field emission. However, very large potential fields are now routinely generated in plasma structures, such as laser generated wake in unmagnetized plasmas. In magnetized plasmas, large DC fields can also be sustained and controlled perpendicular to the magnetic field, but the metallic end plates limiting the plasma, terminating the magnetic field lines, and usually providing the voltage drop feed between the field lines impose severe restrictions on the maximum field. However, it is shown that very large radial DC voltage drops can be sustained by injecting waves of predetermined frequencies and wave vectors, traveling along the azimuthal direction of an axially magnetized plasma cylinder, or by injecting fast neutral particles beams along this azimuthal direction. The large conductivity along the magnetic field lines and the small conductivity between the field lines then distribute this voltage drop. The global power balance and control parameters of wave and beam generated large DC electric fields in magnetized plasmas are identified, described, and analyzed.

Observation of Generalized Dynamic Localizations in Arbitrary‐Wave Driven Synthetic Temporal Lattices

AbstractDynamic localization (DL) refers to a wavefunction confinement effect of electrons in periodic lattices under an ac electric field driving. Recent studies have transplanted DLs from electronic to photonic systems using periodically curved waveguide lattices, where the ac electric field is mimicked by the waveguide's bending curvature. However, due to the severe limitations of bending losses and poor tunability for highly curved, arbitrarily shaped waveguides, present studies mainly focus on DL under the simplest sinusoidal‐wave driving; its extension to an arbitrary‐wave driving remains challenging. Here, by constructing a synthetic temporal lattice with a coupled fiber‐loop circuit, the limitations of spatially curved waveguides are circumvented and generalized DLs are experimentally demonstrated under arbitrary‐wave driving. First, by applying band‐collapse criteria, the general conditions for the driving field's symmetry to achieve DLs are revealed. Then, two representative even‐function driving fields of square‐ and triangle‐waveforms are chosen, and wave‐packet revivals are observed as signatures of DLs. As a counter‐example, the breakdown of DL is also verified using odd‐function driving field of sawtooth‐waveform. The work reveals the conditions for realizing generalized DLs and experimentally verifies them using synthetic lattice schemes. This paradigm may find potential applications in versatile temporal‐waveform reshaping, pulse manipulations for optical communications, and signal processing.

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.

Electrocoalescence Behavior of Droplets Dispersed with Na2CO3 in Oil under the Electromagnetic Synergy Field.

Electromagnetic synergy is a more effective physical method than a single AC electric field (ACEF) to enhance oil-water separation. However, the electrocoalescence behavior of droplets dispersed with salt ions in oil under the synergistic electromagnetic field (EMSF) still lacks research. Herein, the evolution coefficient of liquid bridge diameter (C1) characterizes the growth rate of the liquid bridge diameter, a series of Na2CO3-dispersed droplets with different ionic strengths were prepared, and C1 values of droplets under ACEF and EMSF were compared. Micro high-speed experiments revealed that C1 under ACEF is larger than C1 under EMSF. In particular, when σ = 100 μS·cm-1and E = 629.73 kV·m-1, C1 under the ACEF is 15% larger than C1 under EMSF. Additionally, the theory of ion enrichment is put forward, which explains the influence of salt ions on ζ potential and total surface potential in EMSF. This study provides guidance for designing high-performance devices by introducing electromagnetic synergy in water-in-oil emulsion treatment.

Electrothermoplasmonic flow in gold nanoparticles suspensions: Nonlinear dependence of flow velocity on aggregate concentration

Efficient mixing and pumping of liquids at the microscale is a technology that is still to be optimized. The combination of an AC electric field with a small temperature gradient leads to a strong electrothermal flow that can be used for multiple purposes. Combining simulations and experiments, an analysis of the performance of electrothermal flow is provided when the temperature gradient is generated by illuminating plasmonic nanoparticles in suspension with a near-resonance laser. Fluid flow is measured by tracking the velocity of fluorescent tracer microparticles in suspension as a function of the electric field, laser power, and concentration of plasmonic particles. Among other results, a non-linear relationship is found between the velocity of the fluid and particle concentration, which is justified in terms of multiple scattering-absorption events, involving aggregates of nanoparticles, that lead to enhanced absorption when the concentration is raised. Simulations provide a description of the phenomenon that is compatible with experiments and constitute a way to understand and estimate the absorption and scattering cross-sections of both dispersed particles and/or aggregates. A comparison of experiments and simulations suggests that there is some aggregation of the gold nanoparticles by forming clusters of about 2–7 particles, but no information about their structure can be obtained without further theoretical and experimental developments. This nonlinear behavior could be useful to get very high ETP velocities by inducing some controlled aggregation of the particles.

Impedance spectroscopy and conduction mechanism analysis of bulk nanostructure Prussian blue pellets

The structure and morphology of the fabricated Prussian Blue (PB) pellets are examined using FTIR, XPS, XRD and FESEM techniques. The morphology of the crystalline Prussian blue (PB) revealed a nanostructure with an average crystallite size of 91 nm. The frequency (42 Hz–1 MHz) and temperature (303–393K) of the Prussian Blue (PB) in Bulk form are dependent. In the medium region (1 kHz–100 kHz), the electric dipoles relax and start to lag the applied ac electric field signal and the estimated activation energy of 0.247 eV. The investigated Nyquist plots (Z″ versus Z′) of the Prussian Blue (PB) samples at different temperatures (303–393 K) are interpreted by an equivalent parallel RgCg circuit. The bulk resistance of Prussian Blue samples was estimated at various temperatures, and its behaviour exhibited temperature dependence; generally, it follows the Nearest Neighbor Hopping (NNH) model. According to Meyer–Neldel rule, the calculated activation energy is 0.565 ± 0.001 eV. The evidence of localized states is investigated, and its density, N(EF) ∼ 5.47 × 1018 eV−1 cm−3. The ac conductivity, σac, merges into a plateau of two parts: the first is a frequency-independent dc conductivity, σdc, which is strongly temperature-dependent. The rest of the plateau at higher frequencies showed the temperature-insensitive ac conductivity obeying Jonscher's power law. The values of the exponent s were investigated to figure out the intimate conduction mechanism in Prussian Blue (PB) samples; it follows the correlated barrier hopping (CBH) theoretical model. The hopping distance and energy have values ∼ unity and ≥ kBT, respectively. The experimental results revealed a linear relationship between hopping distance and energy against temperature, with the first decreasing and the second increasing linearly.

Charging Characteristics of Micrometer-Sized Metal Particles and Their Effects on Partial Discharge of Insulating Oil in Flowing

In this study, the motion behavior of micron-sized metal particle swarm in case of flow-electric field coupling is observed in a non-contact manner based on the experiments. According to the results of observation, the charging characteristics of metal particle swarm in insulating oil flow under DC electric field are analyzed in detail, and the effects of electric field intensity, voltage type and oil flow velocity on the charging process and motion of particle swarm are studied. The obtained results show that the applied electric field will cause the streamlines of velocity of the metal particles to oscillate along its direction, where such effect becomes more obvious as the electric field intensity increases. The increase of oil flow velocity can weaken the influence of electric field and make the streamlines of velocity tend to be flat. The oscillation effect of streamlines of velocity under AC electric field is much weaker than that of DC electric field. In addition, based on the measured kinetic parameters of particles, it is found that the charge quantity of the moving particles in the gap of electrodes has the characteristics of temporal and spatial variations, and the particles have a variety of charging modes, including collisional charging, charge transfer, neutralization and accumulation. High velocity of flow and the alternating electric field hinder the charge transfer between particles, resulting in a decrease in the overall charge of the particle swarm. Finally, from the perspective of micro-discharge and electric field distortion, we explain the influence mechanism of flow velocity and voltage type on the discharge characteristics of oil flow containing metal impurities.

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.

Dynamic Behavior and Residence Characteristics of Space Charge in Oil-Paper Insulation Under Polarity–Reversal Electric Field

As one of the core components of the direct current (dc) transmission system, the converter transformer bears not only the alternate current (ac) electric field but also complex electric fields, such as polarity reversal. In the process of polarity reversal, the phenomenon of space charge residence occurs in the oil-paper insulation inside the converter transformer, which distorts the internal electric field, leading to insulation deterioration or even failure. However, studies on the dynamic characteristics of space charge during the reversal process and the mechanism of insulation degradation are not comprehensive. Therefore, in this study, a test pulse matching circuit was developed to accurately match the test pulse with the polarization voltage of polarity reversal and perform the multi-point rapid test of the space charge transient characteristics during polarity reversal. The space charge tests (of the unaged specimen) were performed at the polarity–reversal time of 60, 30, and 10 s, and the influence of polarity–reversal time on the charge residence characteristics of oil-paper insulation was compared and analyzed. Additionally, space charge tests were performed on oil-paper insulation samples with accelerated electro–thermal–mechanical aging treatment for 0, 2.5, 5, 10, and 15 days to compare and analyze the influence of oil-paper insulation aging degree on charge residence characteristics under the action of multi-physical field coupling. Research results indicate that the shorter the reversal time, the shorter the time required for the charge to dissipate, and the more the charge resides, the stronger its enhancement effect on the electric field. As the aging degree of the sample increases, the dissipation rate of the resident charge becomes slower during the reversal process, and the resident charge increases, so that the maximum electric field intensity inside the sample increases under the same polarization electric field. The study of the charge residence characteristics and the dynamic charge behavior during the polarity inversion process has important guiding significance for clarifying the deterioration mechanism of the converter transformer oil-paper insulation under polarity–reversal electric field.

Numerical characterization of transient electrohydrodynamic deformation and coalescence of single-core double emulsion droplets by AC field dielectrophoresis

Considering its good biocompatibility, the water-in-oil-in-water type compound double emulsion (DE) droplet serves as an ideal microreaction vessel upon their on-demand electro-coalescence. By coupling phase field description to fluid dielectrophoresis, transient numerical simulation is conducted to characterize the dynamic feature of surface charge wave and electric stress induced at various phase interfaces of single-core DE drops subjected to external AC electric fields. A novel prolate/prolate electro-deformation mode is discovered by numerical analysis for A/B/A type two-phase droplet when both the conductivity and permittivity of oil shell are much smaller than that of water phase. Based upon this, AC electrostatic attraction between two opposing single-core DE droplets is then investigated in a broad parametric space. Collision of two adjacent oil shells is always followed by the dielectrophoretic integration of corresponding water cores wrapped within them, leading to three different possible modes of inter-droplet electro-coalescence under distinct core/shell radius ratio. Our numerical analysis herein unveils for the first time the hitherto unknown spatial-temporal electrohydrodynamic behavior of single-core DE droplets driven by Maxwell-Wagner structural polarization, during their deformation and coalescence as a consequence of an active interplay among those induced droplet dipoles.

The effect of aligning graphene‐oxide nanoplatelets on moisture absorption and thermal stability of polymeric nanocomposites

AbstractThis article investigates the effect of aligning graphene‐oxide nanoplatelets (GONPs) on the moisture absorption and thermal stability of polymeric nanocomposites. Various nanocomposite specimens were fabricated using different GONP contents to improve water uptake resistance and thermal stability by aligning GONPs using an AC electric field during manufacturing. The results showed that aligning 0.3 wt% GONPs resulted in the most significant improvements, with the reductions of 76% and 54% observed in the water uptake of nanocomposites at the initial and saturation stages, respectively. The thermal stability of the nanocomposite specimens was also investigated using thermogravimetric analysis, in which nanocomposites containing 0.3 wt% randomly dispersed GONPs demonstrated improved performance compared with the neat epoxy specimen. However, aligning GONPs resulted in a marginal effect on the thermal stability parameters of nanocomposites. Moreover, Fourier transform infrared spectroscopy was conducted to investigate the chemical nature and hydrophilicity of the specimens. In addition, the freeze‐fractured surfaces of the neat and nanocomposite specimens were investigated using scanning electron microscopy. The results indicated the role of nanoparticle alignment in enhancing the performance of nanocomposites under hygrothermal conditions in terms of water uptake, which is of high importance in the marine industry.

Photo-assisted spin transport in double quantum dots with spin–orbit interaction

We investigate the effect of spin–orbit interaction on the intra- and interdot particle dynamics of a double quantum dot (QD) under ac electric fields. The former is modeled as an effective ac magnetic field that produces electric-dipole spin resonance transitions, while the latter is introduced via spin-flip tunneling amplitudes. We observe the appearance of non-trivial spin-polarized dark states (DSs), arising from an ac-induced interference between photo-assisted spin-conserving and spin-flip tunneling processes. These DSs can be employed to precisely measure the spin–orbit coupling in QD systems. Furthermore, we show that the interplay between photo-assisted transitions and spin-flip tunneling enables the system to operate as a highly tunable spin filter. Finally, we investigate the operation of the system as a resonant flopping-mode qubit for arbitrary ac voltage amplitudes, allowing for high tunability and enhanced qubit control possibilities.

Surface Creation Using Free Abrasives Controlled by AC Electric Field

Surface Creation Using Free Abrasives Controlled by AC Electric Field

Dielectric characteristics and simulation of dielectric strength of binary poly (vinylidene fluoride) nanocomposites

This work presents the polyvinylidene fluoride (PVDF) nanocomposites with conducting MXene nanosheets and semiconducting titanium dioxide (TiO2) fillers fabricated using a solution casting method. The dielectric constant of PVDF/MXene nanocomposites was higher than PVDF/TiO2 nanocomposites, where slightly more loss was associated with the incorporation of conducting MXene filler at a given weight percentage. The obtained dielectric constant is 16.2 at 1 kHz for PVDF/MXene composite films. Scanning electron microscopy and X-ray diffraction were used measurement of the morphology and crystallinity of composite films. In addition, the AC electric field and polarization distribution of both composites were analyzed using COMSOL software, outcomes indicate a good fit of experimental results. This research provides the path for preparing dielectric composites for electronic applications.