Frequency Of Ac Field Research Articles

Chaotic magnetization dynamics in single-crystal thin-film structures

The nonlinear dynamics of homogeneously precessing magnetization in perpendicularly magnetized single-crystal films has been investigated in a wide range of ac field frequencies on the basis of a numerical solution to the Landau-Lifshitz equation and construction of the spectrum of Lyapunov exponents. The conditions for implementing and specific features of chaotic dynamic modes are revealed for films of three basic crystallographic orientations: (100), (110), and (111). It is shown that chaotic precession modes can be controlled using external magnetic fields. Time analogs of the Poincare section of chaotic mode trajectories are considered.

Dielectrophoresis of ionized gas microbubbles: Dipole reversal due to diffusive double-layer polarization

Gas bubbles generated by electrolysis reactions are shown to exhibit anomalous induced ac dipoles and dielectrophoretic behavior that cannot be described by classical Maxwell–Wagner theory. Normal charging and screening of conducting ionized gas in the gas-phase double layer are shown to render the bubble insulating at low ac field frequencies to affect negative dielectrophoresis. This screening effect couples with dielectric polarization at high frequencies to produce no crossover frequency for small bubbles and two crossover frequencies for bubbles larger than a critical size of 40 μm. A double-layer theory accurately captures the two crossover frequencies and critical bubble size behavior.

Dielectrophoresis and AC-Induced Assembly in Binary Colloidal Suspensions

Dielectrophoretic behaviors and assembly of a binary suspension in aqueous media are examined in the presence of nonuniform alternating current (AC) electric field. A peculiar low-frequency threshold and dielectrophoresis (DEP) crossover frequency determine the applicable frequency window for binary assembly under positive DEP, which can be effectively tuned by medium conductivity and particle size, suggesting that the dynamic double-layer effect is responsible for the interfacial polarization of micrometer to submicrometer-sized particles in aqueous suspensions. Strong effects of AC-field frequency, medium conductivity, and size ratio on binary assembly morphology have been observed. A frequency-medium conductivity phase diagram is obtained to illustrate the morphological transition of assembled colloidal aggregates from segregated, ordered assemblies to inverted segregation with the appearance of amorphous phases upon increasing frequency and/or medium conductivity, which is a direct consequence of the competition between DEP and hydrodynamic mobility. Significantly, our results demonstrate a rapid method to form hybrid nanostructured materials.

Effect of impurities on the steady component of the current in a quantum wire under the joint action of ac and dc fields

The current flowing along a cylindrical quantum wire with a superlattice in the case of the simultaneous application of dc and ac fields is calculated. It is assumed that the wire contains impurity centers, whose ionization results in the generation of nonequilibrium carriers in the conduction band. It is found that the dependence of the steady component of the current on the ac-field frequency is a step-like function. It is shown that the distance between steps depends on the conduction miniband width and the transverse quantum confinement parameters and is independent of the impurity-level depth.

Electroconvection in a Homeotropic Bent-Rod Nematic Liquid Crystal Beyond the Dielectric Inversion Frequency

We characterize the structural transitions in an initially homeotropic bent-rod nematic liquid crystal excited by ac fields of frequency f well above the dielectric inversion point fi. From the measured principal dielectric constants and electrical conductivities of the compound, the Carr-Helfrich conduction regime is anticipated to extend into the sub-megahertz region. Periodic patterned states occur through secondary bifurcations from the Freedericksz distorted state. An anchoring transition between the bend Freedericksz (BF) and degenerate planar (DP) states is detected. The BF state is metastable well above the Freedericksz threshold and gives way to the DP state, which persists in the field-off condition for several hours. Numerous +1 and -1 umbilics form at the onset of BF distortion, the former being largely of the chiral type. They survive in the DP configuration as linear defects, nonsingular in the core. In the BF regime, not far from fi, periodic Williams-like domains form around the umbilics; they drift along the director easy axis right from their onset. With increasing f, the wave vector of the periodic domains switches from parallel to normal disposition with respect to the c vector. Well above fi, a broadband instability is found.

Effects of cell orientation and electric field frequency on the transmembrane potential induced in ellipsoidal cells

The transmembrane potential (Δϕ) induced by external electric fields is important both in biotech applications and in new medical therapies. We analyzed the effects of AC field frequency and cell orientation for cells of a general ellipsoidal shape. Simplified equations were derived for the membrane surface points where the maximum Δϕ is induced. The theoretical results were confirmed in experiments with three-axial chicken red blood cells (a:b:c=6.66 μm:4.17 μm:1.43 µm). Propidium iodide (PI) staining and cell lysis were detected after an AC electropermeabilization (EP) pulse. The critical field strength for both effects increased when the shorter axis of a cell was parallel to the field, as well as at higher field frequency and for shorter pulse durations. Nevertheless, data analysis based on our theoretical description revealed that the Δϕ required is lower for the shorter axis, i.e. for smaller membrane curvatures. The critical Δϕ was independent of the field frequency for a given axis, i.e. the field strength had to be increased with frequency to compensate for the membrane dispersion effect. Comparison of the critical field strengths of PI staining in a linear field aligned along semi-axis a (142 kV m− 1) and a field rotating in the a–b plane (115 kV m− 1) revealed the higher EP efficiency of rotating fields.

Simulation on dielectric susceptibility and domain evolution of relaxor ferroelectrics

Dielectric susceptibility and domain evolution of the relaxor ferroelectrics have been simulated using the Monte Carlo method upon the Potts–Ising model. The grain size effect and the applied ac field frequency effect on the dielectric susceptibility were theoretically investigated. We found that the dielectric susceptibility increases and the T m (the temperature at which the dielectric susceptibility reaches the maximum) shifts to lower temperature with increasing average grain size or decreasing frequency. In addition, we obtained the value of the relaxation parameter γ estimated from the linear fit of the modified Curie–Weiss law; its changing trend with increasing average grain size or increasing frequency was well consistent with the experimental observation. From the results of the domain pattern evolution process, we observed the differences between relaxor ferroelectrics and normal ferroelectrics subjected to an applied ac field.

AC application of second generation HTS wire

For the production of Second Generation (2G) YBCO High Temperature Superconductor wire American Superconductor uses a wide-strip MOD-YBCO/RABiTSTM process, a low-cost approach for commercial manufacturing. It can be engineered with a high degree of flexibility to manufacture practical 2G conductors with architectures and properties tailored for specific applications and operating conditions. For ac applications conductor and coil design can be geared towards low hysteretic losses. For applications which experience high frequency ac fields, the stabilizer needs to be adjusted for low eddy current losses. For these applications a stainless-steel laminate is used. An example is a Low Pass Filter Inductor which was developed and built in this work.

Ac field-induced Fano resonances in a parallel-coupled double quantum dot system

The effects of an ac electric field on the Fano resonance in a parallel-coupled double quantum dot system are investigated theoretically. The field can induce the photon-assisted Fano resonances for both symmetrical and asymmetrical parallel configurations. The magnitude and position of the photon-assisted Fano peak can be tuned by the ac field strength and frequency, respectively. Furthermore, the Fano resonance can appear with increasing the field frequency for both the symmetrical and asymmetrical configurations. This provides an efficient mechanism to control the Fano resonance. The photon–electron pumping effects for the symmetrical and asymmetrical cases are also studied in the weak- and strong-coupling regime.

AC Electric Field-Induced Alignment and Long-Range Assembly of Multi-Wall Carbon Nanotubes Inside Aqueous Media

The present work examines the behavior of multiwall carbon nanotubes (MWCNT) inside AC electric fields created by three-dimensional electrodes. The response of carbon nanotubes stably suspended in water with the aid of a nonionic surfactant is monitored by combining microscopic observations with on-line measurements of the suspension resistivity. It is found that polarization effects induced by the externally applied AC electric field on MWCNTs can cause their unidirectional orientation and end-to-end contact that result in formations of spatially distributed, long-range, three-dimensional and electrically conducting structures that span the entire gap between the electrodes. The length of the formed structures, which in the present case was approximately 30 times larger than that of an individual carbon nanotube, can be controlled by adjusting the spacing between the electrodes. The influence of main experimental parameters, namely, MWCNT concentration, applied voltage, AC field frequency, and electrode surface topography on the suspension behavior is experimentally examined. Results are demonstrated for applied voltage values, AC field frequencies, and carbon nanotube concentrations in the range 4-40 Vptp, 10 Hz-5 MHz, and 0.001-2.0 wt%, respectively. While higher electric field strengths accelerate the formation of aligned structures, higher frequency values were found to result in suspensions that exhibit smaller electrical resistivity. Carbon nanotube dispersions exposed to an AC electric field exhibit a 100-fold or more decrease in their electrical resistivity, even when carbon nanotube concentrations as low as 0.005 wt% are used.

Effects of electrical conductivity on poling of ferroelectric composites

Composites of lead zirconate titanate ceramic particles dispersed in non-ferroelectric polymers of different electrical conductivities (polyurethane and polyethylene) have been prepared and poled under ac fields of different amplitudes Ea and frequencies f. Our experimental results reveal that the poling efficiency of the composites (as indicated by the observed remanent polarization PCR) is strongly dependent on the frequency of the poling field and the electrical conductivity of the polymer matrix. A low-frequency ac field and a high-conductivity polymer matrix would make the poling become more effective. The phenomena can be understood by the simple theoretical model, in which space charges are allowed to accumulate at the particle–matrix interfaces. The increasing trend of PCR with decreasing f predicted by the model agrees well with the experimental results. It is suggested that the poling process is mainly governed by a crucial parameter ‘relaxation time’ τ of the interfacial charge. If the poling time is longer than τ, the interfacial charge can accumulate at the interfaces to stabilize the aligned dipoles in the ceramic phase, and hence the composite becomes poled. Otherwise, the composite cannot be effectively poled even under a high electric field.

Effect of temperature, pressure, and composition on DC resistivity and AC conductivity of conductive styrene–butadiene rubber–particulate metal alloy nanocomposites

AbstractConductive nanocomposites were prepared using styrene butadiene rubber as the polymer matrix and nanosized powder of copper–nickel (Cu–Ni) alloy as the filler. The filler loading was varied from 0 to 40 phr. The electrical conductivity of filled polymer composites is due to the formation of some continuous conductive networks in the polymer matrix. Atomic force microscopy was used to determine the particle size of the nanofiller and its nature of dispersion in the rubber matrix. The DC volume resistivity was measured against the loading of the nanofiller to check the percolation limit. The effect of temperature, applied pressure, time duration under constant compressive stress on the DC resistivity and AC conductivity of the composites with different filler loading were investigated. The change in DC resistivity and AC conductivity against temperature of these composites exhibited positive coefficient of temperature. With the change in applied pressure and time duration under constant compressive stress the DC resistivity undergoes an exponential decrease. The effect of AC field frequency on the AC conductivity was investigated. POLYM. COMPOS. 28:696–704, 2007. © 2007 Society of Plastics Engineers

Using nonlinear ac electrokinetics vortex flow to enhance catalytic activities of sol-gel encapsulated trypsin in microfluidic devices.

A novel microstirring strategy is applied to accelerate the digestion rate of the substrate N(alpha)-benzoyl-L-arginine-4-nitroanilide (L-BAPA) catalyzed by sol-gel encapsulated trypsin. We use an ac nonlinear electrokinetic vortex flow to stir the solution in a microfluidic reaction chamber to reduce the diffusion length between the immobilized enzyme and substrate in the solution. High-intensity nonlinear electroosmotic microvortices, with angular speeds in excess of 1 cms, are generated around a small ( approximately 1.2 mm) conductive ion exchange granule when ac electric fields (133 Vcm) are applied across a miniature chamber smaller than 10 mul. Coupling between these microvortices and the on-and-off electrophoretic motion of the granule in low frequency (0.1 Hz) ac fields produces chaotic stream lines to stir substrate molecules sufficiently. We demonstrate that, within a 5-min digestion period, the catalytic reaction rate of immobilized trypsin increases almost 30-fold with adequate reproducibility (15%) due to sufficient stirring action through the introduction of the nonlinear electrokinetic vortices. In contrast, low-frequency ac electroosmotic flow without the granule, provides limited stirring action and increases the reaction rate approximately ninefold with barely acceptable reproducibility (30%). Dye molecules are used to characterize the increases in solute diffusivity in the reaction reservoir in which sol-gel particles are placed, with and without the presence of granule, and compared with the static case. The solute diffusivity enhancement data show respective increases of approximately 30 and approximately 8 times, with and without the presence of granule. These numbers are consistent with the ratios of the enhanced reaction rate.

Optically stimulated electron-hole resonance in photorefractive CdTe with a low frequency ac field

We report the results of an experimental and theoretical study of electron-hole competition in CdTe:Ge photorefractive crystal with an incoherent auxiliary illumination and alternating low-frequency field. Resonant two-wave mixing (TWM) gain enhancement is studied that has been found depending on the wavelength and intensity of the incoherent illumination. We show that a low-frequency ac field can be used for an effective TWM gain enhancement under conditions appropriate to the electron-hole resonance. A time oscillation of the photorefractive gain concerned with the ac field is studied experimentally, and self-generation of time subharmonics is reported.

Autoresonance parametric excitation of localized oscillations of magnetization in a ferromagnet by an AC field of a variable frequency

Dynamics of localized magnetic inhomogeneities in the form of breathers has been studied in an easy-plane ferromagnet in an applied ac magnetic field of a variable frequency. Conditions have been determined for an autoresonance parametric excitation of a breather in the presence of dissipation with its amplitude growing in time from small values to approximately unity. The possibility has been shown of not only its resonance growth but also of a resonance depression of nonlinear oscillations of localized magnetic inhomogeneities.

Ac susceptibility of a melt-textured YBa2Cu3Ox ring closed by a strong-coupling contact

The complex ac susceptibility χ=χ′−jχ″ of a melt-textured YBa2Cu3Ox ring closed by a strong-coupling contact is measured after zero-field cooling to 77K as a function of the ac field amplitude Hm and frequency f. The resulting χ(Hm,f) is similar to that derived from a power-law relation between the local current density and electric field but with maximum χ″(Hm) increasing steadily with increasing f, which is explained by the creep of Abrikosov-Josephson vortices along the contact driven by the London force of supercurrents and the Lorentz force of the induced normal currents.

Nonlinear dynamic magnetization regimes in (100) ferrite-garnet films

To study the nonlinear dynamics of a uniformly precessing magnetization in perpendicularly magnetized (100) ferrite-garnet films, equations of motion are numerically solved over a wide ac field frequency range. Bifurcation changes in the magnetization precession and the states of dynamic bistability have been detected. The conditions of high-amplitude regular and stochastic dynamic regimes are revealed, and the possibilities of controlling these precession regimes by applied magnetic fields are shown.

Precise DNA placement and stretching in electrode gaps using electric fields in a microfluidic system

Control over the placement of stretched deoxyribonucleic acid (DNA) molecules in a microfluidic system is a critical requirement for molecular nanotechnology. A technique is developed where a large number of DNA molecules can be immobilized specifically at one end to the electrode tip and stretched in a microchannel using high frequency ac fields. λ-DNA molecules are immobilized and stretched using 100kHz ac fields in a 100μm wide and 75μm deep Si microchannel. Using a floating electrode in between two biased electrodes, stretched T2 DNA molecules are immobilized across a 5μm wide electrode gap by electric field and hydrodynamic flow.

AC susceptibility of Sr 2RuO 4–Sr 3Ru 2O 7 eutectics: Dependence on AC field strength and frequency

The low-field AC susceptibility of a eutectic crystal of the Sr 2RuO 4–Sr 3Ru 2O 7 system exhibits superconducting transition features at three temperatures. We investigate the dependence on AC field strength from 0.021 to 2.1 Oe-rms and frequency from 17 to 3 kHz. The two lower-temperature peaks in the imaginary part originate in the Sr 3Ru 2O 7 part. The lowest-temperature peak is exceedingly sensitive to the field amplitude and shifts to lower temperatures with increasing amplitude. By contrast, it slightly shifts to higher temperatures with increasing frequency. These results support that the sensitive features involve weak links forming the screening paths.

Magnetic history dependence of the AC susceptibility of GdSr2RuCu2Oz

AbstractThe AC susceptibility of GdSr2RuCu2Oz samples has been measured as function of the temperature, at different AC field amplitudes (from 1 to 16 G) and frequencies (in the range [10‐103 Hz]), applied parallel to the major axis of the sample, and without any additional DC magnetic field. In particular we have analysed the temperature region above the Superconducting critical temperature Tc, and around the magnetic transition temperature Tm. Our attention has been focused on the differences between the curves measured after cooling the sample in zero AC field (Zero AC Field Cooling or ZAFC) and those measured after cooling in presence of a non zero AC field (AC Field Cooling or AFC). Clear differences between the ZAFC and AFC susceptibility curves appear in the temperature range between Tc and Tm, and they depend on the measuring field amplitude and frequency. Finally, indications of a spin glass behaviour in Ru‐1212 system observed from its AC magnetic response are reported (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)