Applied Ac Field Research Articles

Effect of the AC electric field on the dielectric permittivity of lead magnesium niobate relaxor

Abstract The effect of the amplitude of the applied AC field on the dielectric permittivity of PbMg1/3Nb2/303 relaxor ceramics is investigated. It is demonstrated that increasing amplitude has the same effect as decreasing frequency on the maximum in the temperature dependence of the real part of the permittivity. It is shown that the obtained results are consistent with the predictions of the model employing the analogy with the domain wall motion and cannot be interpreted in terms of a thermally activated reorientation of the local spontaneous polarisation.

DOUBLE PHASE CONJUGATE MIRROR AT 1.55 μm BETWEEN SINGLE MODE FIBERS WITH CdZnTe: V

We present experimental results of a double phase conjugate mirror (DPCM) in a crystal of Cd 0.96 Zn 0.04 Te:V operating at 1.55 μm. Elimination of conical diffraction has been obtained by cylindrical lenses and has allowed to measure a coupling efficiency between monomode fibers versus different experimental parameters. An oscillation of the diffracted beam direction at the frequency of the applied ac field has been observed. Perturbation of the grating period that satisfy the Bragg matching can be due to mechanical vibrations resulting from the piezoelectric effect. In a ring phase conjugate mirror (PCM) configuration, experimental results of the conversion efficiency and of the phase of the diffracted beam are also presented.

Evidence for domain-type dynamics in the ergodic phase of the PbMg1/3Nb2/3O3 relaxor ferroelectric.

It was observed that the dielectric permittivity of ${\mathrm{PbMg}}_{1/3}$${\mathrm{Nb}}_{2/3}$${\mathrm{O}}_{3}$ (PMN) relaxor ceramics increases with increasing amplitude of the applied ac field. A strong nonlinearity occurs at temperatures where the frequency dispersion of the dielectric response is observed. A similarity was found between the effects of the amplitude and frequency on the permittivity. It was shown that, taken together, these data evidence that the relaxation phenomena in PMN are controlled by domain-type dynamics rather than thermally activated flips of the local spontaneous polarization. \textcopyright{} 1996 The American Physical Society.

A study of the nonlinearity in magnetic response of

We have carried out low-field ac measurements of harmonic susceptibilities and ac magnetization of a single-phase partially melt-textured high-temperature superconductor over a wide range of temperatures, field amplitudes and measuring frequencies. Both the fundamental and third-harmonic susceptibility versus temperature and ac magnetization at fixed temperatures close to exhibit strong ac field amplitude and small frequency dependences. In the third-harmonic susceptibility, we observed a double-peak feature in the vicinity of as a function of the applied ac field. This strong field dependence is associated with depinning of vortices at some B and T in different superconducting matrices, which involves different ac loss mechanisms. Experimental results are compared with predictions of some existing theoretical models.

Dynamic holographic interferometry by photorefractive crystals for quantitative deformation measurements

A holographic interferometer that uses two-wave mixing in a photorefractive (Bi12SiO20) crystal under an applied ac field is described. The interferometer uses a repetitive sequence of separate record and readout times to obtain quasi real-time holographic interferograms of vibrating objects. It is shown that a good signal-to-noise ratio of the interferometer is obtained by turning off the object illumination and the applied ac field during readout of the hologram. The good signal-to-noise ratio of the resulting holographic interferograms enables phase measurement, which allows for quantitative deformation analysis.

Electro-fused isolated wheat (Triticum aestivum L.) gametes develop into multicellular structures

The electrofusion-mediated fertilization of single egg cells of wheat with isolated individually selected wheat sperm cells was successfully carried out for the first time. On average the fusion frequency was 30% but under optimal conditions it was possible to reach as much as 55%. Two days after electric fusion 60% of the fusion products started to divide, 88.5% of them forming multicellular structures and in a few cases microcalluses. The culture of single unfertilized egg cells with or without the application of AC field and electric pulses induced no cell division. The egg cells and fusion products were cultured in a maize feeder-cell system.

Alternating-current losses in silver-sheathed (Bi,Pb)2Sr2Ca2Cu3O10 tapes II: Role of interfilamentary coupling

ac losses at 60 Hz have been measured for silver-sheathed (Bi,Pb)2Sr2Ca2Cu3O10 multifilamentary tapes at 27 and 77 K to investigate the role of interfilamentary coupling. The results are qualitatively well described by the use of the concept of the critical coupling length lc. Further, it is shown that to obtain effective reduction of the coupling loss, a very small twist pitch length (≪1 cm) as well as a substantial reduction in the filament thickness are required for an applied ac field of, for example, 100 rms A/cm at 77 K.

Frequency analysis of the acoustic emission signals from PZT ceramics during the application of AC fields

Abstract Frequency analysis of Acoustic Emission signals in the range of 100–1200 kHz obtained from PZT ceramics during the application of limited and continuous cycles of ac field was undertaken to understand the fatigue behaviour of these ceramics. In the limited cycles experimentation, lower fields yielded higher range of frequency components (200–900 kHz) and higher fields yielded lower range of frequency components (200–500 kHz) at differnt temperatures of 60° C and 160° C. During continuous application of ac field, the initial stages of application yielded higher frequency components (100–850 kHz) and with increasing time of application, the frequency components of acoustic emission signal shifted towards lower frequency (less than 400 kHz). Electrical discharge induced cracking, resulted in the emission of frequency components in the range of 100–275 kHz.

Vortex dynamics in a Bi2Sr2CaCu2O8 crystal with columnar defects.

The frequency-dependent shielding of an applied ac field is used to study the vortex dynamics in a Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} single crystal irradiated with 5.8-GeV Pb ions. At fields below the matching field {ital B}{sub {Phi}}{approx}200 G, the sample exhibits strong pinning due to the irradiation-induced columnar defects. The dynamics in this field regime is shown to be in accurate agreement with the predictions of the Bose-glass theory of Nelson and Vinokur [Phys. Rev. B 48, 13 082 (1993)]. We propose to explain deviations from the Bose-glass predictions at fields greater than {ital B}{sub {Phi}}, and temperatures exceeding 85 K, by vortex decoupling and the appearance of a surface barrier, respectively. The crossover between creep of pancake vortices above {ital B}{sub {Phi}} and Bose-glass creep below {ital B}{sub {Phi}} is marked by a break in the irreversibility line. At all temperatures the irreversibility field is determined by the disappearance of the shielding current due to flux creep. No evidence for critical scaling of the resistivity was found.

Applied AC and DC magnetic fields cause alterations in the mitotic cycle of early sea urchin embryos.

This study demonstrates that exposure to 60 Hz magnetic fields (3.4-8.8 mT) and magnetic fields over the range DC-600 kHz (2.5-6.5 mT) can alter the early embryonic development of sea urchin embryos by inducing alterations in the timing of the cell cycle. Batches of fertilized eggs were exposed to the fields produced by a coil system. Samples of the continuous cultures were taken and scored for cell division. The times of both the first and second cell divisions were advanced by ELF AC fields and by static fields. The magnitude of the 60 Hz effect appears proportional to the field strength over the range tested. The relationship to field frequency was nonlinear and complex. For certain frequencies above the ELF range, the exposure resulted in a delay of the onset of mitosis. The advance of mitosis was also dependent on the duration of exposure and on the timing of exposure relative to fertilization.

3T3 cell motility and morphology before, during, and after exposure to extremely‐low‐frequency magnetic fields

Automated image cytometry techniques were used to measure motility and morphology in 3T3 fibroblasts exposed to extremely-low-frequency (ELF) magnetic fields. Cell motility and morphology were measured as a function of time before, during, and after 3-4 hour exposures to vertically oriented, 100 microTRMS sinusoidal magnetic fields at various frequencies in the 10-63 Hz range. Sham exposures were also carried out. No static DC fields were applied, but the geomagnetic field was almost vertical and, therefore, had a large component (28.3 microT) parallel to the applied AC field. The morphology and motile behavior of the cells were characterized by mathematically defined descriptors, which were calculated and averaged for the exposure period as well as for control periods that preceded and followed the exposure period. Each experiment involved the tracking of 100 cells that were subjected to one of the test frequencies (unless a sham exposure was being conducted). Statistical analysis of the results showed that even small changes of 10-20% could be significant at the P < .05 level. Changes on this order were measured in a significant proportion of the experiments. However, because such results were seen for both the sham-exposed and the ELF-exposed cells, and because the range of values that was obtained for the sham exposures was the same as that obtained for the ELF exposures, we concluded that there was no evidence to show that any of the measured changes were attributable to the applied ELF magnetic field.

A SQUID-based ac susceptometer

We have developed a high-resolution ac susceptometer that uses a rf superconducting quantum interference device to directly measure the flux coupled into a superconducting detection coil from a sample’s changing magnetic moment in an applied ac field. The system operates in a frequency range from 0.01 to 1500 Hz and an applied ac field range of 0.1–400 μT with a sensitivity of about 5×10−12 A m2 for magnetic moment measurement, and at a reduced sensitivity down to 0.001 Hz. The instrument is based on an existing dc magnetometer system and uses that system’s temperature control and dc superconducting magnet to allow operation over a temperature range from 2 to 400 K and in applied dc fields of ±5.0 T. During a measurement all operations are controlled automatically by computer from a menu-driven software system, with user input required only on initiation of a measurement sequence. Both real and imaginary components of the ac susceptibility can be determined.

Study of the effect of transport current and combined transverse and longitudinal fields on the AC loss in NET prototype conductors

AC losses in cables carrying DC as well as AC transport currents at different DC background fields up to 2T have been measured on three types of Nb/sub 3/Sn subcables in a new test facility. In this facility it is possible to apply sinusoidal transverse AC fields up to dB/dt=5T/s and longitudinal AC fields up to dB/dt=30T/s separately and simultaneously. The AC loss is measured with a calorimetric method. Simultaneously applied transverse and longitudinal fields can result in a loss which exceeds the added contributions of the separate applied AC fields. Within the measured range it is about correct (within 10%) to add the loss components due to DC transport current up to 10 kA and both applied transverse and longitudinal AC fields. The measured total loss is always above the sum of the loss components. >

Calculation of quasi-static electromechanical coupling coefficients for electrostrictive ceramic materials

The quasi-static coupling coefficients, k/sub 13/ and k/sub 33/, for electrostrictive ceramics are computed analytically. The calculation is based on a three-dimensional constitutive relation that models both electrostriction and nonlinear dielectric behaviors. The results show that the coupling factors depend on the amplitudes of the applied ac field and the dc bias, as well as the mechanical prestress. For an actuator without bias voltage or prestress, the coupling coefficients approach an asymptotic value with increasing electric field. The primary coefficients, k/sub 13/ and k/sub 33/, for a lead magnesium niobate, Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/BaTiO/sub 3/(PMN-PT-BT), based relaxor ferroelectric are computed as an example. The results show that the coupling coefficients for PMN-PT-BT materials are roughly comparable with those of existing piezoelectrics. These coefficients are important parameters for material section and power source design for transducer devices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Frequency-dependent photorefractive response in the presence of applied ac electric fields

We consider the process of nonstationary enhancement of the photorefractive effect through the application of time-varying (square-wave) electric fields. By examining the full second-order equation governing the photorefractive response, we show that this response is strongly dependent on the frequency of the applied ac field and exhibits a maximum when the period of oscillation is between the photorefractive response time, τP, and the charge-recombination time in the medium, tR. Furthermore, we show that the diffraction efficiency, the gain, and the phase of the response fluctuate in tandem with the oscillating applied field, and we examine the dependence of these fluctuations on the driving frequency. These frequency-dependent phenomena are described both by numerical integration of the charge-transport equations and through an analytic solution thereof. From this analysis we obtain simple expressions for the magnitude of the efficiency, the gain, and the phase fluctuations and the conditions for maximum average diffraction efficiency. In particular, we show that the oscillation period yielding the maximum response behaves as a weighted geometric average of the response and the recombination times of the form T = constant × (τP2tR)1/3. These results are compared with those of earlier analyses of ac-field enhancement and are then verified in a BSO crystal to which ac fields over a range of frequencies are applied.

Giant static dipole moment change on electronic excitation in highly oriented J-aggregates

A large change in the static dipole moment upon electronic excitation was unexpectedly observed in highly oriented one-dimensional J-aggregates of 1,1′-diethyl-2,2′-quinocyanine bromide (PIC-Br) prepared by a method developed by our group. Difference absorption spectra due to the Kerr effect were induced by changes in a static dipole moment and a polarizability separated by the dual-phase lock-in detection. The change in the static dipole moment associated with the transition from the ground state to the exciton state was measured with an applied ac field ( F ) of 300 Hz and polarization ( e ) parallel and perpendicular to the one-dimensional axis ( d ) of the oriented J-aggregates, and obtained in each configuration to be 18 ( F || d / e || d ), 12 ( F ⊥ d / e || d ), 27 ( F || d / e ⊥ d ), and 14 ( F ⊥ d / e ⊥ d ) D.

Nonlinear theory of two- and four-wave interaction by means of a stationary grating recorded by different-frequency waves

Two- and four-wave interactions by means of a photorefractive grating recorded by a moving interference pattern are investigated theoretically. The primary stationary grating is recorded by two waves of different frequency in an externally applied ac field. Secondary shifted and nonshifted gratings are recorded by frequency-degenerate interference; their influence is analyzed qualitatively. The depletion of all the waves is taken into account.

Theory of spatial subharmonic instability under hologram recording in a photorefractive crystal with applied AC field

A theory describing threshold conditions for spontaneous generation of spatial subharmonics in a photorefractive crystal with an externally applied square-wave ac voltage is presented. Predicted results are in good agreement with the experimental data.

AC magnetic susceptibility and flux noise in simple Josephson-coupled superconducting clusters

We have studied a simple model of granular superconductors which includes an ensemble of simple loops formed by superconducting grains, with weak intergrain junctions. Thermally activated phase slippage transitions between stable states of the N-contact loop are shown to determine the hysteretic behavior in an applied AC field. The imaginary part of the AC magnetic susceptibility, χ″, and the power spectral density of magnetic flux noise, S Ф, have been calculated. We have shown the importance of two-level potential configurations of the system. The 1/ƒ-like flux noise has been found to arise from the superposition of random flux transitions in two-level systems with statistically distributed parameters.

Temporal evolution of the electron energy distribution function in oxygen and chlorine gases under dc and ac fields

An analysis of the temporal evolution of the electron energy distribution function (EEDF) and the electron swarm parameters in oxygen and chlorine gases is presented. The spatially homogeneous time-dependent Boltzmann equation is solved for dc and radio-frequency ac electric fields by a finite-element method. A comparison is made of the swarm parameters obtained for the following three cases: (a) under the actual ac field; (b) assuming that the EEDF follows faithfully the applied ac field [quasi-steady-state (QSS) approximation]; and (c) using an ‘‘effective’’ dc field (effective dc approximation). It is shown that the effective dc approximation is not applicable to either oxygen or chlorine for frequencies &amp;lt;10 MHz; however, the QSS approximation is justified for chlorine discharges at &amp;lt;13.56 MHz. This has important implications for reducing the computation time in modeling the bulk plasma of glow discharge reactors. It is also shown that atomic chlorine resulting from molecular dissociation has a significant effect on the swarm parameters, especially for large degrees of gas dissociation.