Quadratic electro-optic properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 transparent ceramics under both DC and AC bias

The basic quadratic electro-optic properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) transparent ceramics have been studied under both DC and AC electric field bias. The contribution of piezoelectric resonance to electro-optic effect for this quadratic electro-optic ceramics material has been experimentally demonstrated and theoretically analyzed. It is found that, at the piezoelectric resonance frequencies, the piezoelectric induced electro-optic effect dominates and leads to a dramatically high sensitivity for weak electric signal detection. About 20 dB signal-to-noise ratio is attained when detecting AC electric field strength of 1 V/m with optimized DC bias. Besides, the effects of AC frequency and amplitude on halfwave voltage V(π) of PMN-PT are investigated.     

Nonlinear eddy current effects in ferromagnetic materials in the presence of DC magnetic fields

Eddy current effects in nonlinear ferromagnetic materials in the presence of a supplementary DC magnetic field are analyzed. The analysis is for a one-dimensional configuration, a semi-infinite plate magnetized homogeneously in a direction parallel to its surface. The nonlinear magnetization characteristic of the ferromagnetic plate material is assumed to be of a step-function form. Solutions of the electric field intensity on the plate surface, the penetration depth of eddy currents in the plate, the eddy current losses, and surface impedance in the presence of a DC magnetic field are obtained. A connection is found between the values obtained in simultaneous DC and AC magnetization conditions and the values obtained in the case of only AC magnetic field excitation.<>     

Lorentz force actuation of a heated atomic force microscope cantilever

We report Lorentz force-induced actuation of a silicon microcantilever having an integrated resistive heater. Oscillating current through the cantilever interacts with the magnetic field around a NdFeB permanent magnet and induces a Lorentz force that deflects the cantilever. The same current induces cantilever heating. With AC currents as low as 0.2 mA, the cantilever can be oscillated as much as 80 nm at resonance with a DC temperature rise of less than 5 °C. By comparison, the AC temperature variation leads to a thermomechanical oscillation that is about 1000 times smaller than the Lorentz deflection at the cantilever resonance. The cantilever position in the nonuniform magnetic field affects the Lorentz force-induced deflection, with the magnetic field parallel to the cantilever having the largest effect on cantilever actuation. We demonstrate how the cantilever actuation can be used for imaging, and for measuring the local material softening temperature by sensing the contact resonance shift.     

Transient response analysis of a Rosen-type piezoelectric transformer and its applications

This paper investigates the characteristics of voltage transient response, such as the maximum voltage, direct current (DC) time constant, alternating current (AC) time constant and oscillation frequency, and their applications for a Rosen-type piezoelectric transformer (PT). The transient response is induced immediately after an AC voltage connected to the PT is switched off. For the applications, the maximum voltage is used to elucidate how to cause an electrical shock for users of the PT under open-circuit operation. The DC time constant and the AC time constant are used to estimate the equivalent resistance of the mechanical loss and the dielectric resistance of the dielectric loss, respectively. Also, the AC time constant is used to estimate the quality factor of the PT. Additionally, the oscillation frequency is used as an antiresonant frequency of the PT. In order to verify the above characteristics and applications, both an equivalent circuit with initial conditions and a drive system with a control switch interposed between the PT and its AC voltage source are adopted to derive the transient response and measure its characteristics. Effects of the load resistance of the PT and the switching-off time of the voltage source on the transient response are measured and discussed.     

Thermocapillary convection in a liquid bridge subjected to interfacial cooling

Influence of heat loss through interface on a supercritical three-dimensional thermoconvective flow in a long liquid bridge is numerically investigated under terrestrial conditions. A flow in a high Prandtl number liquid surrounded by an ambient gas of constant temperature is simulated for the large aspect ratio, Γ=1.8. It is shown that the heat loss plays a significant role in the flow dynamics. It modifies both the flow and the liquid temperature field. Moreover, for the relatively large aspect ratio and a high Prandtl number liquid the heat loss from interface leads to destabilization of the flow.     

Influence of Critical Current Density Distribution on Transport AC Losses in Superconducting Wire in a DC Magnetic Field

In typical application-like conditions, the inhomogeneous distribution and anisotropy of critical current density must be considered simultaneously in transport AC loss calculation. In this paper, we derive the analytical formulas of transport AC losses for high-temperature superconductors (HTSs) with linear and quadratic distribution of critical current density under applied DC magnetic field. The influence of the inhomogeneous distribution and anisotropy of critical current density has been analyzed. The results show that the impact of the distribution form on transport AC loss is more obvious under applied DC magnetic field. And the influence of applied DC magnetic field will increase as the distribution form becomes steeper.     

Effect of AC Magnetic Field on the Levitation Force of YBCO Bulk above NdFeB Guideway

In the present High Temperature Superconducting (HTS) maglev vehicle system, the nonuniformity of the magnetic field along the movement direction above the NdFeB guideway is inevitable due to the assembly error and inhomogeneous of the material property of the NdFeB magnet. In order to investigate the influence of the nonuniformity on the levitation performance of the HTS bulk, an electromagnet supplied by AC current is used to simulate the nonuniformity of the external magnetic field. The levitation force of the HTS bulk is measured when applying AC currents to the electromagnet coils. Experimental results indicate that the levitation force changes abruptly and then oscillates after applying AC external magnetic field, and the levitation force is attenuated by the AC magnetic field after withdrawing the AC field. Moreover, the oscillation amplitude and the attenuation rate of the levitation force increase with the amplitude of the AC external magnetic field.     

Experimental Presentation of Microwave Absorption due to Shaking of JV by AC Magnetic Field in Bi2212 and Bi2223

The Josephson Vortex dynamics in high anisotropy superconductors Bi2212 and Bi2223 induced by AC magnetic field collinear to DC magnetic field and parallel to the layers is studied via their interaction with microwave field. Experimental results as function of DC magnetic field, AC magnetic field and temperature are presented. The AC induced microwave dissipation is larger than dissipation without AC field. The results are explained by the theoretical interpretation reported recently due to shaking by the AC field that depins the JV (Shaltiel et al., Phys. Rev. B 77, 014508, 2008). Similar behavior in these two compounds as a function of the variables involved discloses that the shaking effect should be observed in any high anisotropy superconductors. It shows that the AC field interacting with JV has an active and not the usual passive modulating role of the DC field in EPR dissipation experiments. The technique can be used to investigate JV dynamics and JV phase diagram.     

Further developments in oxide crystal growth using gas lasers

It is shown that a DC energised CO₂-N₂-He laser system leads to a lower loss of material by evaporation than a comparable AC system when applied to oxide crystal growth. The reasons for this behaviour are discussed and a method for minimising evaporation using an AC system is described.     

Oscillation Characteristics of Levitation Force of YBCO Bulk Exposed to AC Magnetic Field above NdFeB Guideway

In the present High Temperature Superconducting (HTS) maglev vehicle system, the nonuniformity of the magnetic field along the movement direction above the NdFeB guideway is inevitable due to the assembly error and inhomogeneity of the material property of the NdFeB magnet. In order to investigate the influence of the nonuniformity on the levitation performance of the HTS bulk, the experiment involved an electromagnet, which is supplying AC current to simulate the nonuniformity of the external magnetic field. The levitation force of the HTS bulk is measured when applying AC current on the electromagnet coils. The results indicate that the levitation force abruptly changes and oscillates after applying AC external magnetic field. The effect of the amplitude of the AC magnetic field on the levitation force is studied; the result shows that the oscillation amplitude of the levitation force increases with the amplitude of the AC external magnetic field and is independent of the Field Cooling Height (FCH) of the bulk.     

The Effect of Dy Doping on the Magnetic Behavior of YBCO Superconductors

The effect of dysprosium (Dy) doping on yttrium barium copper oxide (YBCO) prepared by conventional solid-state reaction method has been investigated by means of XRD, AC susceptibility, and DC magnetization measurements. AC susceptibility measurements for sintered YBCO pellets have been performed as a function of temperatures at constant frequency and AC field amplitude in the absence of a DC bias field. DC magnetization measurements were done at 5, 20, and 77 K upon zero field cooling (ZFC) process. The magnetization measurements showed a paramagnetic behavior existing at high magnetic fields. The magnetic field dependence of critical current density of the samples has been estimated from DC magnetization data. The partial Dy substitution for Y on YBCO superconductors improves the bulk critical current density at high magnetic fields and at high-temperature regions (higher than 20 K).     

Method for determining the junction temperature of alternating current light-emitting diodes

A numerical simulation was used to simulate the temperature distribution during AC and DC operations of an alternating current light-emitting diode (AC LED). The relationship between the junction temperature and the temperature at the centre of the bottom surface of the submount of an AC LED was measured under DC operation. This relationship was confirmed by numerical simulation. The numerical results were consistent with the experimental observations in that the temperature at the centre of the bottom surface of the submount was insensitive to the current variations that occur in an AC LED, probably because of the large mass of the submount. However, it was difficult to measure the temperature oscillation at the junctions in an AC LED, although this oscillation can be clearly seen in the numerical results. Thus, the authors propose a formula for predicting the range of the oscillating junction temperature for an AC LED.     

Angular Dependence of Transport AC Losses in Superconducting Wire with Position-Dependent Critical Current Density in a DC Magnetic Field

Transport AC losses play a very important role in high temperature superconductors (HTSs), which usually carry AC transport current under applied magnetic field in typical application-like conditions. In this paper, we propose the analytical formula for transport AC losses in HTS wire by considering critical current density of both inhomogeneous and anisotropic field dependent. The angular dependence of critical current density is described by effective mass theory, and the HTS wire has inhomogeneous distribution cross-section of critical current density. We calculate the angular dependence of normalized AC losses under different DC applied magnetic fields. The numerical results of this formula agree well with the experiment data and are better than the results of Norris formula. This analytical formula can explain the deviation of experimental transport current losses from the Norris formula and apply to calculate transport AC losses in realistic practical condition.     

Improving power system dynamic performance using wide-area high-voltage direct current damping control

A systematic methodology for placement and integration of wide-area measurements in high-voltage direct current (HVDC) damping modulation is developed for large-scale AC/DC power system. A concept observed centre of inertia (OCOI) of power grid is described. For an interconnected AC/DC power network, the frequency difference between the OCOI of the sub-grids can be used as the input signal for HVDC modulation control when the DC link connects two regional power grids. This input contains more useful information of the interarea oscillation modes than the conventionally used signals. In addition, it takes into account the realistic situation of a large-scale power grid. Simulation results for the wide-area control based on the China Southern Power Grid data justify the feasibility and advantage of the phasor measurement unit application in HVDC modulation and power system stability control.     

3D simulation of AC loss in a twisted multi-filamentary superconducting wire

In the AC loss simulation, it is a huge challenge to model the twisted wire at the filament level, due to the complex structure as well as long-time computation consumption. In this paper, we use 3D finite-element method based on H-formulation to study the AC loss in a twisted superconducting wire. The wire is treated as a homogenous material with the anisotropic conductivity in the filament region. We quantitatively simulate the AC loss induced by the AC transport current and magnetic field profile, and the effect of the twist pitch on the AC loss. In the case of AC transport current, larger pitch length leads to higher loss, and the pitch length effect is contrary to the case of applied magnetic field. The influences of the magnetic field direction and non-uniform current distribution subjected to the strand bending are also investigated. It is observed that, the transverse magnetic field has a more significant influence on the AC loss than the longitudinal magnetic field. The non-uniform current distribution can result in a higher AC loss, compared to a corresponding uniform current distribution.     

Irreversibility Line and Flux Relaxation in Bi:2223/Ag Tape Prepared by CTFF Technique: H//<Emphasis Type="Italic">c</Emphasis> axis

We have studied flux dynamics in Bi:2223/Ag tape prepared by the CTFF technique in the orientation of H//c axis. Irreversibility line (IL) of a Bi:2223/Ag tape has been determined from AC susceptibility and the DC magnetization technique based on FC and ZFC curves. We have observed that the irreversibility temperature determined from the AC susceptibility method locates at a higher temperature than that was determined from the DC magnetization method. This shift could be attributed to the viscous dissipation generated by the oscillation of flux lines. FC magnetization at 20 K and 30 K for the field range from 500 to 2500 Oe are the same and show a plateau implying the same amount of reversible magnetization due to the Meissner current. Remanent magnetization versus temperature curves, which give us information on the flux pinning mechanism of the vortices, are also presented.     

Alternating Current Photovoltaic Effect

It is well known that the photovoltaic effect produces a direct current (DC) under solar illumination owing to the directional separation of light-excited charge carriers at the p–n junction, with holes flowing to the p-side and electrons flowing to the n-side. Here, it is found that apart from the DC generated by the conventional p–n photovoltaic effect, there is another new type of photovoltaic effect that generates alternating current (AC) in the nonequilibrium states when the illumination light periodically shines at the junction/interface of materials. The peak current of AC at high switching frequency can be much higher than that from DC. The AC cannot be explained by the established mechanisms for conventional photovoltaics; instead, it is suggested to be a result of the relative shift and realignment between the quasi-Fermi levels of the semiconductors adjacent to the junction/interface under the nonequilibrium conditions, which results in electron flow in the external circuit back and forth to balance the potential difference between two electrodes. By virtue of this effect, the device can work as a high-performance broadband photodetector with extremely high sensitivity under zero bias; it can also work as a remote power source providing extra power output in addition to the conventional photovoltaic effect.     

Numerical Evaluation of Levitation Force Oscillation of HTS Bulk Exposed to AC Magnetic Field over NdFeB Guideway

The levitation force of the YBCO bulk over an NdFeB guideway used in the high-temperature superconducting (HTS) maglev vehicle system is oscillated by the application of the AC external magnetic field. In our previous work, we interpreted that the oscillation is due to the shielding current fluctuation caused by fluctuant external magnetic field. In this paper, based on the Bean model, an analytical model is adopted to evaluate the levitation force. Comparing with the experimental results, the calculated results show good matching. The model can reveal the oscillation characteristics of the levitation force of HTS bulk which is being exposed to AC external magnetic field. Therefore, the levitation force oscillation of the HTS bulk in the maglev vehicle system can be evaluated by this numerical method.     

Impact of AC and DC Electric Fields on the Microstructure Evolution in Strontium Titanate

Herein, the impact of AC and DC electric fields on microstructure evolution in strontium titanate is investigated. The focus is on nonthermal effects by using current-blocking electrodes. The seeded polycrystal technique allows investigating the impact of a DC electric field on grain growth for different grain-boundary orientations and the impact of the surrounding atmosphere. As in previous studies, faster grain growth is observed at the negative electrode. This effect is stronger for the (100) orientation and in reducing atmosphere. In AC electric field at 1450 °C, a low-enough frequency results in faster grain growth at both electrodes. These findings agree well with previous studies, where an electromigration of oxygen vacancies is found to cause a local reduction at the negative electrode, resulting in less space charge, less cationic segregation, and a higher grain-boundary mobility. At 1500 °C, AC electric fields are found to cause a complete grain growth stagnation at very small grain sizes. This behavior is unexpected; the physical reasons are not clear. Herein, a brief study of sintering in DC electric field reveals slightly faster sintering if a field is applied.     

Pumping Current Noise in Ballistic Nanostructures: Acoustoelectric Current in a Point Contact

We consider current noise in a non-biased ballistic nanostructure irradiated by an AC field, which is assumed to be a stationary random process to imitate time averaging in the experiment. The AC field creates a current, which contains DC and AC components, the latter vanishing after time averaging. Correspondingly, the correlator describing current noise has two contributions. The first contribution to it is responsible for the fluctuations of the DC current. These fluctuations happen because of electron inelastic transitions between scattering states. Transitions agitated by the thermal bath lead to a thermal noise spectrum, which disappears at zero temperature; its magnitude is renormalized compared to equilibrium. Transitions initiated by the AC field lead to a noise spectrum, which is a convolution of the thermal one and that of the AC field. The magnitude of this noise is proportional to the power of the AC field and is finite at zero temperature. The second contribution to the noise correlator is due to fluctuations of the AC current. This noise does not depend on temperature, and its spectrum reproduces that of the AC field.