Alternating Transport Current Research Articles

Theoretical study of the line defect in χ3-borophene: structures, electronic properties, direct-current and alternating-current transport properties

Two-dimensional material χ3-borophene is a stable borophene allotrope and has been synthesized in experiment. Combining density functional theory with non-equilibrium Green’s function method, in the present study we theoretically investigated the structures, stabilities, electronic properties, and the direct-current (DC) and alternating-current (AC) transport properties of its line defects. The obtained results suggest that along the zigzag direction there exist some special line defects that can enhance the stability of χ3-borophene and the six-coordination B atoms play a key role for the enhanced stability. The line defects may change the atomic orbital components of the Dirac cone of χ3-borophene and have profound influences on the electronic properties. For the DC transport, under high bias voltages along a given transport direction the line defects may lead to constant current phenomena. For the AC transport, under different frequency ranges the line defects have different AC responses with respect to the external time-dependent field, exhibiting complicated AC transport behaviors. Furthermore, for the low-frequency range, the equivalent circuits of χ3-borophene and its line defects were suggested, which will be beneficial for designing borophene-based functional nanodevices.

Computation of AC loss of HTS coils wound by coated conductors with different widths

The second-generation high-temperature superconductor (HTS) tape is a promising material for the HTS application. However, the extremely large aspect ratio of the HTS coated tape leads to high power dissipation in the environment of a time-varying magnetic field. Reducing the width of HTS coated tape is a feasible way to decrease the alternating current (AC) loss of devices composed of HTS tapes. In this study, a numerical model of the HTS coil group composed of six sub-coils based on the T-A formulation is presented in which each HTS sub-coil is wound with original or narrow width Y-Ba-Cu-O (YBCO) tape. The 1/3 and 1/4 narrow width tapes are used to represent the tapes, which are divided into three and four equal parts, respectively. Then, we calculate the AC losses in 1/3 and 1/4 narrow width HTS coil groups in different cases. The estimated results of AC loss are compared with those of the HTS coil group wound by HTS tapes with original width. The numerical results show that AC transport losses of 1/3 and 1/4 narrow width HTS coil groups are smaller than that of the original HTS coil group for the applied high current. Furthermore, with the decrease in tape width, the reduction of AC loss becomes more significant. In contrast with the original width HTS coil group, the magnetization losses of the 1/3 and 1/4 narrow width HTS coil groups will decrease at the high magnetic field. In addition, the influences of harmonic components on AC loss are also considered.

An experimental study on the effect of fatigue loading on electrical impedance in open-hole carbon nanofiber-modified glass fiber/epoxy composites

Fiber-reinforced composites (FRCs) are valued for their high specific properties. However, FRCs are vulnerable to difficult-to-predict damage growth and complex failure modes thereby motivating the need for robust structural health monitoring (SHM). Self-sensing via nanofiller-induced piezoresistivity has much potential for this. Research in piezoresistivity to date has overwhelmingly focused on the direct current (DC) response. This is important because alternating current (AC) has important benefits such as improved data density, potential for greater sensitivity, and reduced power requirements. In light of the potential of AC-based methods, this work explores AC transport responsiveness to high-cycle fatigue loading in carbon nanofiber (CNF)-modified glass fiber/epoxy laminates via equivalent circuit analyses. These results show that equivalent circuit behavior is significantly affected by fatigue-induced damage accumulation suggesting a mechanism for monitoring fatigue. However, clear circuit parameter evolution-load cycle trends were not observed suggesting further work is needed to fully realize the potential of AC-based monitoring.

Frequency-dependent alternating current piezoresistive switching behavior in self-sensing carbon nanofiber composites

Carbon nanofillers have received much attention for piezoresistive-based self-sensing of deformation in polymeric, cementitious, and ceramic composites. To date, direct current (DC) conductivity has been overwhelmingly favored for self-sensing. This is important because alternating current (AC) methods, though much less studied, have advantages over DC methods. Therefore, we herein explore AC conductivity-strain relations for polymeric carbon nanofiber (CNF) composites. It was found that Jonscher’s power law accurately describes AC conductivity as a function of both normal and shear strain. This provides a framework by which macroscale AC piezoresistivity can be characterized. Further, it was observed that the coefficients of this power law are non-linear in strain. During this testing, it was also observed that the CNF/epoxy exhibits a frequency-dependent piezoresistive switching behavior. At low frequencies, the material exhibits positive piezoresistivity. At higher frequencies, however, the material exhibits negative piezoresistivity. A state of zero piezoresistivity also exists between these cases. Computational micro-modeling revealed that this piezoresistive switching behavior is the consequence of inter-CNF AC transport behaving like a parallel resistor-capacitor arrangement and strain affecting the parallel or tunneling resistance. This novel switching behavior opens the door to many exciting possibilities for frequency-selective piezoresistive behavior in next-generation carbon nanofiller-based sensors.

Local electromagnetic properties and hysteresis losses in uniformly and non-uniformly wound superconducting racetrack coils

A noteworthy physical dependence of the hysteresis losses with the axial winding misalignment of superconducting racetrack coils made with commercial second generation high temperature superconducting (2G-HTS) tapes is reported. A comprehensive study on the influence of the turn-to-turn misalignment factor on the local electromagnetic properties of individual turns is presented by considering six different coil arrangements and ten amplitudes for the applied alternating transport current, Ia, together with an experimentally determined function for the magneto-angular anisotropy properties of the critical current density, Jc(B,θ), across the superconducting tape. It has been found that for moderate to low applied currents Ia≤0.6Ic0, with Ic0 the self-field critical current of individual tapes, the resulting hysteretic losses under extreme winding deformations can lead to an increase in the energy losses of up to 25% the losses generated by a perfectly wound coil. High-level meshing considerations have been applied in order to get a realistic account of the local and global electromagnetic properties of racetrack coils, including a mapping of the flux front dynamics with well defined zones for the occurrence of magnetization currents, transport currents, and flux-free cores, which simultaneously has enabled an adequate resolution for determining the experimental conditions when turn-to-turn misalignments of the order of 20–100 μm in a 20 turns 4mm wide racetrack coil can lead not only to the increment of the AC losses but also to its reduction. In this sense, we have shown that for transport current amplitudes Ia>0.7Ic0, a slight reduction in the hysteresis losses can be achieved as a consequence of the winding displacement, which is at the same time connected with the size reduction of the flux-free core at the coil central turns. Our findings can be used as a practical benchmark to determine the relative losses for any 2G-HTS racetrack coil application, unveiling the physical fingerprints that possible coil winding misalignments could infer.

Electric Field and Energy Losses of Rounded Superconducting/Ferromagnetic Heterostructures at Self-Field Conditions

The ac losses induced by an alternating transport current in type-II superconductors is a well-known phenomenon, which still attracts much attention because of its intrinsic relevance for the proper development of practical applications. In the case of single core superconducting cables of cylindrical cross-section, it is possible to find exact analytical solutions at self field conditions, and it has been believed for nearly two decades that the use of an ideal soft ferromagnetic sheath with negligible magnetization losses will not affect the electromagnetic properties of the superconducting wire, and on the contrary due to the shielding magnetic properties of the ferromagnet, the total ac losses of the SC wire have to be reduced or as maximum they must be equal to the one for the bare superconductor at self-field conditions, what contraries the experimental evidences that show a non-negligible increase on the ac losses. In this paper, we explain the physical nature of this mysterious increase in the ac losses for rounded superconducting/ferromagnetic heterostrutures, which for the sake of generality, has been solved within the critical state theory, and a magnetic multipolar expansion that enables the direct coupling of the magnetostatic properties of the superconductor and an ideal soft ferromagnet. A significant increase in the transient electric field during excitation period has been observed, which might have utter implications on the adequate selection of insulation materials for superconducting/ferromagnetic heterostructures.

The effect of alternating current on the current states of a quantum interferometer shunted by a superconducting inductance

The patterns of reversible changes in the critical current and discrete current states of a structure in the form of a superconducting quantum interferometer shunted by superconducting inductance, as a result of passing an alternating transport current through the structure and applying an external alternating magnetic field simultaneously with a direct transport current, was established. A new type of discrete stationary state was discovered during the transition of the interferometer to the resistive state caused by the combined action of direct and alternating transport currents.

Modulate the direct-current and alternating-current transport properties of magnetic γ-graphyne heterojunctions by chemical modification

Using density functional theory and the non-equilibrium Green's function method, we theoretically investigated the direct-current (DC) and alternating-current (AC) quantum transport properties of magnetic γ-graphyne heterojunctions. For the DC case, we found that the γ-graphyne heterojunction has rich transport properties such as spin-filtering and magnetoresistance effects. As the marginal H atoms of the heterojunction are replaced by O atoms, an outstanding dual spin-filtering phenomenon appears and the magnetoresistance is enhanced. Meanwhile, after chemical modification, the heterojunction exhibits a noticeable rectification effect. For the AC case, depending on the frequency, the total and spin AC conductances can be capacitive, inductive, or resistive. At some given frequencies, the signs of the imaginary parts of the AC conductances for two different spins are opposite; thus, the two spin currents have opposite AC responses. A significant photon-assisted tunneling effect was found in the heterojunctions at high frequency range. More interestingly, after chemical modification in a wide frequency range, the imaginary part of the AC conductance changes the sign, indicating that the AC transport properties of the γ-graphyne heterojunction can be effectively modulated by chemical methods.

Power dissipation in HTS coated conductor coils under the simultaneous action of AC and DC currents and fields

This paper presents a comprehensive alternating current (AC) loss study of a circular high temperature superconductor (HTS) coated conductor coil. The AC losses from a circular double pancake coil were measured using the electrical method. A 2D axisymmetric H-formulation model using the FEM package in COMSOL Multiphysics has been established to match the circular geometry of the coil used in the experiment. Three scenarios have been analysed: Scenario 1 with AC transport current and DC magnetic field (experiment and simulation); Scenario 2 with DC transport current and AC magnetic field (simulation); and Scenario 3 with AC transport current and AC magnetic field (simulation and experimental data support). The angular dependence analysis on the coil under a magnetic field with different orientation angle θ has been carried out for all three scenarios. For Scenario 3, the effect of the relative phase difference Δφ between the AC current and the AC field on the total AC loss of the coil has been investigated. In summary, a current/field/angle/phase dependent AC loss (I, B, θ, Δφ) study of a circular HTS coil has been carried out. The obtained results provide useful indications for the future design and research of HTS AC systems.

Investigation and comparison of AC losses on stabilizer-free and copper stabilizer HTS tapes

This paper presents the measurement and simulation of Alternating Current (AC) losses on the Stabilizer-free and Copper Stabilizer High Temperature Superconducting (HTS) Tapes: SuperPower SF12100 and SCS12050. The AC loss measurement utilised electrical method to obtain overall losses with AC transport currents. The 2D H-formulation by COMSOL Multiphysics has been used to simulate the real geometry and multi-layer HTS tapes. Ferromagnetic AC losses of substrate have been assumed to be ignored as the substrates of SF12100 and SCS12050 are non-magnetic. Hysteresis AC losses in the superconducting layer, and eddy-current AC losses in copper stabilizer, silver overlayer and substrate were concerned in this investigation. The measured AC losses were compared to the AC losses from simulation, with 3 cases of different AC frequency 10, 100, and 1000 Hz. The eddy-current AC losses of copper stabilizer at frequency 1000 Hz were determined from both experiment and simulation. The estimation of AC losses with frequency at 10,000 Hz was also carried out using simulation method. Finally, the frequency dependence of AC losses from Stabilizer-free Tape and Copper Stabilizer Tape were compared and analysed.

Transport Properties of Melt-Cast-Processed Bi-2212 Bulk Superconductor Bars

Bulk materials of high-critical-temperature superconductors (HTSs) are promising for current leads and fault-current limiters because of negligibly small Joule heating under current transport and very low thermal conductivity. We consider that a Bi-2212 bulk superconductor prepared by the melt-casting process (MCP) is one of the best materials for these applications. In addition, MCP is well known to be a simple and economic process, and also served as a method for obtaining any sizes of HTS bulks consent to requirements for practical applications. In this paper, we present a study on the transport and quench properties of Bi-2212 bulk superconductors doped with $\hbox{K}_{2}\hbox{CO}_{3}$ . First, the MCP samples doped with $\hbox{K}_{2}\hbox{CO}_{3}$ were analyzed by using an X-ray diffractometer and X-ray fluorescence spectrometry. In addition, the critical temperatures of the MCP samples doped with $\hbox{K}_{2}\hbox{CO}_{3}$ were observed. Then, we have investigated the relations between the critical current value and the doping quantity of $\hbox{K}_{2}\hbox{CO}_{3}$ . The value of the direct-current critical current was determined at 77 K, and the self-fields derived from the electric field versus the transport current characteristics were obtained using a criterion of 1 $\mu\hbox{V/cm}$ . Finally, the alternating-current transport properties of the MCP sample were measured, and we have discussed the frequency dependence of the critical current with current distributions

Numerical analysis of transport AC loss in HTS slab with thermoelectric interaction

This paper presents an investigation of the AC loss in high temperature superconducting slabs, when applying an alternating transport current of sinusoidal form. The proposed model describing this AC loss is built upon the following coupled physics: the Maxwell’s equations governing the electromagnetic fields, the intrinsic nonlinear relations, and the thermal diffusion equation governing the temperature fields. The numerical results demonstrated that the temperature of the whole slab (interior or exterior) increases monotonously with the frequency and amplitude of the applied alternating current. After considering the local temperature changing effect, it is found that most of the magnetic induction, the critical current density, and the local current density in the slab periodically vary with time with non-sinusoidal form, while the period is either the same or half of that of the applied current, which fully exhibits a typical characteristic of nonlinear systems. Further, the AC loss varying with the amplitude and frequency of the applied current has the power law as almost same as the analytical results based on Bean’s critical state model at low amplitude I0. However, for the case of high current amplitude I0, the numerical results gradually deviate from the analytical results and the local temperature changing has a significant effect on the transport loss.

Calorimetric AC loss measurement of MgB2 superconducting tape in an alternating transport current and direct magnetic field

Applications of MgB2 superconductors in electrical engineering have been widely reported, and various studies have been made to define their alternating current (AC) losses. However, studies on the transport losses with an applied transverse DC magnetic field have not been conducted, even though this is one of the favored conditions in applications of practical MgB2 tapes. Methods and techniques used to characterize and measure these losses have so far been grouped into ‘electrical’ and ‘calorimetric’ approaches with external conditions set to resemble the application conditions. In this paper, we present a new approach to mounting the sample and employ the calorimetric method to accurately determine the losses in the concurrent application of AC transport current and DC magnetic fields that are likely to be experienced in practical devices such as generators and motors. This technique provides great simplification compared to the pickup coil and lock-in amplifier methods and is applied to a long length (∼10 cm) superconducting tape. The AC loss data at 20 and 30 K will be presented in an applied transport current of 50 Hz under external DC magnetic fields. The results are found to be higher than the theoretical predictions because of the metallic fraction of the tape that contributes quite significantly to the total losses. The data, however, will allow minimization of losses in practical MgB2 coils and will be used in the verification of numerical coil models.

Alternating-Current Transport Properties in Nd0.7Sr0.3MnO3 Ceramic with Secondary Phases

Nd0.7Sr0.3MnO3 ceramics with secondary phases were prepared by ball-milling and post heat- treatment at 1623 K for 3, 7 and 13 h, respectively. The results from x-ray diffraction and energy dispersed spectroscopy show that some secondary phases are introduced and grow gradually with sintering time. These secondary phases have significant effects on the ac transport. For all the samples, the real part of impedance (Zr) decreases with increasing frequency and the Zr peak moves to a higher temperature. Interestingly, for a given frequency the Zr peak decreases with sintering time. However, for samples B and C which were sintered for a longer time than sample A, an additional Zr peak appears at a higher temperature and gradually increases with sintering time. The reposition of trapped charges in phase/grain boundaries or secondary phases is supposed to be responsible for the unusual relaxation process.

Effects of Frequency on AC Conductivity and Magnetoresistance in Doped La0.65Ca0.35MnO3 Manganites

The effects of frequency on the alternating-current (AC) conductivity of polycrystalline La0.65Ca0.35MnO3 compounds prepared by solid-state reaction with various heat treatments have been studied. The AC conductivity and magnetoresistance (MR) are affected by the heat treatment temperature. The MR is enhanced at and below the transition temperature (TC). The AC transport behavior of this system shows pronounced negative MR even at small applied magnetic fields for temperatures well below TC. It is observed that such effects originate from spin-dependent scattering at grain boundaries as a consequence of spin disorder at the interfaces of the boundaries and spin scattering within the grains. The real part of the AC conductivity σ′ exhibits a pronounced frequency dependence, measured in the frequency range f < 100 kHz. The minimum occurs at a frequency that is temperature dependent. The MR below TC and the frequency dependence of the AC electrical conductivity σ′(f) are interpreted as being consequences of cluster and grain boundary effects.

Alternating-Current Transport Properties of the Interface between Nd0.7Sr0.3MnO3 Ceramic and a Ag Electrode

Electrical transport properties of the interface between a Nd0.7Sr0.3MnO3 ceramic and a Ag electrode are investigated using the ac impedance over a wide temperature and frequency ranges. The ac impedance measurements give the compressed semicircle arcs at different temperatures, which are used for the analysis of different contributions to electrical transport based on an electrical equivalent circuit. A significant interface-dependent electroresistance effect of 530% is clearly developed around the metal-insulator transition temperature 130 K, which is confirmed as the interface-layer dependent Curie temperature by the plot of interfacial conductance with frequency at different temperatures.

Proposal of new structure of MgB2 wires with low AC loss for stator windings of fully superconducting motors located in iron core slots

The new structure of MgB2 monofilamentary wires for stator windings of fully superconducting motors is proposed to reduce their AC losses in iron core slots for the application of an alternating transport current. In order to validate the proposed structure of wire for loss reduction, numerical calculations are carried out by means of a finite element method using edge elements formulated with a self-field due to currents induced in an analysis region. It is assumed that the voltage–current characteristics of the MgB2 superconductor are given by Bean’s critical state model, in which the critical current density is independent of the local magnetic field. The influences of wire structures on the AC losses are discussed quantitatively toward the optimum design of stator windings in fully superconducting motors with the MgB2 wires.

Dependence of Transport-Current Losses in ${\rm MgB}_{2}$ Superconducting Wire on Temperature and Frequency

AC losses of a monofilamentary MgB2 wire in a self-field are experimentally evaluated at several temperatures and frequencies. The alternating transport currents are applied to the straight sample wire, and the AC losses are observed electrically with a lock-in amplifier. The temperature of the sample wire was adjusted by a conduction cooling with gaseous helium, and the stability was below ±0.1 K near a fixed temperature. The temperature dependence of the transport-current losses was roughly explained with a theoretical expression based on the Bean model, though the experimental results of the transport-current losses slightly depend on the frequency.

Numerical and theoretical evaluations of AC losses for single and infinite numbers of superconductor strips with direct and alternating transport currents in external AC magnetic field

AC losses in a superconductor strip are numerically evaluated by means of a finite element method formulated with a current vector potential. The expressions of AC losses in an infinite slab that corresponds to a simple model of infinitely stacked strips are also derived theoretically. It is assumed that the voltage-current characteristics of the superconductors are represented by Bean’s critical state model. The typical operation pattern of a Superconducting Magnetic Energy Storage (SMES) coil with direct and alternating transport currents in an external AC magnetic field is taken into account as the electromagnetic environment for both the single strip and the infinite slab. By using the obtained results of AC losses, the influences of the transport currents on the total losses are discussed quantitatively.

AC losses in monofilamentary MgB2 round wire carrying alternating transport currents

AC losses in a monofilamentaryMgB2 round wire with niobium and copper metal sheaths and carrying alternating transportcurrents are evaluated at several temperatures and frequencies. First, the transport currentlosses are observed electrically using a lock-in amplifier. Experimental results show that theAC losses decrease with an increase in the temperature if the amplitude of the transportcurrent normalized by the corresponding critical current is maintained constant. On theother hand, the AC losses increase slightly with the frequency. Next, the AC losses arecalculated numerically by a finite difference method. The numerical results for thesuperconductor filament show a good agreement with the results of the conventionaltheoretical expression formulated using the Bean model over a wide range of currentamplitudes. It is also found that the AC losses in the niobium sheath are negligible whereasthose in the copper sheath are comparable with those in the superconductor. Onthe basis of the numerical calculations, an expression is analytically derived forestimating the eddy current loss occurring in a metal sheath. The derived expressionwell reproduces the AC loss properties of both the copper and niobium sheaths.