AC Magnetization Loss Research Articles

Bifunctional CoFe2O4/ZnO Core/Shell Nanoparticles for Magnetic Fluid Hyperthermia with Controlled Optical Response

Conjugation of optical and magnetic responses in a unique system at the nanoscale emerges as a powerful tool for several applications. Here, we fabricated bifunctional CoFe2O4-core/ZnO-shell nanoparticles with simultaneous photoluminescence in the visible range and ac magnetic losses suitable for hyperthermia. The structural characterization confirms that the system is formed by a ≈7 nm CoFe2O4 core encapsulated in a ≈1.5-nm-thick semiconducting ZnO shell. As expected from its high anisotropy, the magnetic losses in an ac magnetic field are dominated by the Brown relaxation mechanism. The ac magnetic response of the core/shell system can be accurately predicted by the linear response theory and differs from that one of bare CoFe2O4 nanoparticles as a consequence of changes in the viscous relaxation process due to the effect of the magnetostatic interactions. Concerning the optical properties, by comparing core/shell CoFe2O4/ZnO and single-phase ZnO nanoparticles, we found that the former exhibits a broade...

Influence of the Striation Process and the Thickness of the Cu-Stabilization on the AC Magnetization Loss of Striated REBCO Tape

Rare earth-based coated conductors have large ac magnetization losses, which hinder their use in a variety of applications. A common way to reduce these losses is by striating the superconductor into narrow filaments by means of laser ablation. Practical superconductors also need a sufficiently thick copper stabilization. The best way of creating copper-stabilized filamentized coated conductors is still to be found. In this paper, the ac magnetization losses of differently prepared 12 mm wide coated conductors manufactured by SuperOx were measured. The samples differ in terms of thicknesses of copper stabilization (5 and 10 μm), number of filaments (from 10 to 60), and order for copper deposition (filaments produced before or after electroplating the tape). The ac loss measurements were performed at 77 K in a wide range of frequencies of the external magnetic field, which allows determining the onset of coupling currents and the importance of the coupling losses. Complementary dc measurements of the transverse resistivity between filaments help to understand the path of the coupling currents. Thanks to the variety of tested samples and operating conditions, this paper provides a direct comparison of the two striation processes in commercially available high-temperature superconductor coated conductors.

AC loss characteristics of CORC® cable with a Cu former

High-temperature superconductors from the REBCO (RE = rare earth) family have attained industrial production and their performance is continuously being enhanced. However, cabling technology for high-current (kA range) cables for magnet technology is still challenging and there are only few cable concepts available (CORC®, Roebel cable, twisted stack cable). Each of them exhibits different characteristics. In this paper we experimentally investigate CORC® cable produced in-house utilizing a copper tube former. Such a former offers a central cooling channel for partial or complete cable cooling by forced flow of coolant. We focus mainly on AC loss due to transporting AC current, an external applied AC magnetic field and their simultaneous action. In the case of transporting AC current we found indications that a large part of the total loss has its origin in eddy currents due to an axial magnetic field. For the investigation of magnetization AC loss, we prepared several samples with different configurations. In this case we found direct evidence for increasing AC loss due to losses in the metallic former. However, we also found that at low field amplitudes the magnetization AC loss of the complete cable is lower than the loss in the bare former. This is caused by shielding of the magnetic field by a superconductor, which was also confirmed by numerical simulations.

AC Loss Reduction in Multifilamentary Coated Conductors With Transposed Filaments

Filamentization has been shown to be an effective method in reducing magnetization ac loss of RE-Ba-Cu-O (REBCO) coated conductors. We have used a laser striation method followed by selective electroplating of copper to fabricate fully stabilized multifilamentary REBCO tapes with 12, 24, and 46 filaments. While expected levels of ac loss reduction has been confirmed in short segments of such multifilamentary tapes, electric coupling between the filaments of long tapes needs to be suppressed by transposition of the filaments. In this work, a technique was developed and implemented to reduce ac losses in REBCO multifilamentary coated conductors through a new design that allows us to transpose the filaments without any mechanical twisting. The process consists of patterning REBCO tapes by laser ablation followed by partial insulation and bonding. A twofold reduction of ac magnetization losses was achieved in the transposed multifilamentary REBCO tape when subject to an external perpendicular magnetic field at frequencies between 50 and 200 Hz and peak field values up to 88 mT. The filament-to-filament resistance and its effect on the coupling have also been investigated.

Optical, magnetic, and microwave properties of Ni/NiO nanoparticles

In this research, the optical, magnetic, and microwave properties of NiO and Ni/NiO nanoparticles have been studied. The absorbance spectra of the samples show the electronic d–d excitations with energy band gap of about 3.8 eV. The magnetization measurement confirms the existence of ferromagnetic phase at room temperature, which could be originated from the uncompensated surface spins or ferromagnetic clusters in the antiferromagnetic ground state of NiO nanoparticles. The microwave parameters such as ac conductivity, skin depth, electric and magnetic loss tangents, attenuation constant, and reflection loss have been calculated. While both magnetic and dielectric relaxation processes have been observed in the complex permeability and permittivity, the microwave absorption is mainly attributed to the dielectric relaxation processes.

Magnetic AC Loss of Monofilament Sr 0 . 6 K 0 . 4 Fe 2 As 2 /Ag Tape

Measurements of magnetic AC losses in applied magnetic fields have been made on monofilament Sr0.6K0.4Fe2As2/Ag tape in the temperature range between 15 and 30 K. The magnetic AC losses are obtained by an M-H hysteresis loop-based method. That is, AC losses were determined from the area of the hysteresis loop measured using a vibrating sample magnetometer (VSM) under an externally applied cyclic field up to 7 T. Temperature dependence critical currents of the tapes are measured as a function of magnetic field up to 10 T. The experimental losses are compared with those based on the critical-state model and those based on the modified critical-state model. There is a good agreement with the second model over a wide field range. Also, measured AC losses with varying field ramp rate are compared with each other.

Magnetic AC loss of a mono-Sr0.6K0.4Fe2As2 tape/Ag in perpendicular field

The magnetic AC losses of monofilament Sr0.6K0.4Fe2As2/Ag tapes are measured in the temperature range between 20K and 30K both in perpendicular and parallel field. The loss, measured by the standard magnetization technique, is determined from the area of the hysteresis loop using a vibrating sample magnetometer (VSM) in a cyclic field of amplitude up to 7 T. The results in perpendicular field are compared to that of the parallel-field loss and theoretical calculation of magnetization loss at various temperatures. There is a reasonable agreement between the theoretical model and the experimental results even in high field. The magnetic critical current density ( Jc) of the tape, obtained by the magnetic hysteresis measurements M(H), are investigated in two field directions and in the temperature range from 5K to 30K. The comparison between the magnetic Jc in both field directions and the transport Jc of the tape are also done at various temperatures and fields. The anisotropy of Jc (Γ=Jcab/Jcc) is very small.

Filamentary MgB2Wires With Low Magnetization AC Losses

This paper describes the development of MgB2 wires with low magnetization ac losses made by in situ or internal magnesium diffusion methods. A strong contribution of eddy currents was found in a wire with a highly conductive copper sheath, but a GlidCop sheath showed no eddy current loss addition. For minimized hysteresis loss, the filaments have to be small enough, which, however, reduces the critical current. Twisting of filaments was used to achieve a reduction in coupling loss; however, this induced mechanical strain into the filaments and decreased the critical current. Cabling of MgB2 strands decreased the coupling loss without degradation. Interfilament resistivity was tuned using different barriers (Nb, Ti, and NbTi), which allowed for an effective reduction in the coupling loss and resulted in a shift of the ac loss maximum into high frequencies.

Operational Characteristics of HTS Coils With Flux Diverters in Semipersistent Mode Under Alternating Magnetic Field

This paper deals with the effect of magnetic flux diverters on the current decay behaviors of high-temperature superconducting (HTS) coils which operate in semipersistent current mode (PCM) under alternating magnetic field. In superconducting synchronous machines, the HTS field coil can be designed to operate in PCM to obtain a steady magnetic field during operation. Due to the alternating magnetic field from the armature winding, the HTS field winding experiences an external magnetic field. As a result, the additional magnetization ac loss in the field coil would be induced, and it affects the current decay characteristics. In order to improve the current decay behavior of the HTS field coil, the use of flux diverter made of magnetic material can be considered. In this paper, we experimentally investigated the current decay characteristics of HTS field coils with magnetic flux diverters under alternating magnetic field. The measurement setup, which is composed of HTS field coil and armature coil, was fabricated. The measurement test of the current decay on the HTS field coil in PCM operation was carried out as functions of the amplitude and frequency of the armature coil.

Simultaneous Magnetic Shielding and Magnetization Loss Measurements of YBCO Cylinders at Variable Temperatures Under Cryogenic Helium Gas Circulation

A novel experimental setup for simultaneous measurements of magnetic shielding characteristics and magnetization ac loss measurements is described. The measurement system consists of a controllable helium circulation system that enables cooling of the samples to temperatures as low as 15 K and simultaneously allows calorimetric ac loss measurements. The simultaneous measurement of shielding characteristics and ac losses permits estimation of losses in realistic operating conditions of high-temperature superconducting magnetic shields. It is shown that if the extent of losses is high compared with the cooling efficiency, the magnetic shields could quench. The data on magnetic shielding efficacy along with the corresponding magnetization losses are presented.

Magnetization ac loss reduction in HTS CORC® cables made of striated coated conductors

High temperature superconductors (HTSs), like for instance REBCO (RE = rare earth) coated conductors, are of high potential for building large superconducting magnets. Some magnets, such as accelerator magnets, require the use of superconducting cables to allow fast ramping, and low magnetization loss to mitigate field quality issues. One of the methods to lower ac loss is to divide the superconducting layer in the tape into filaments. In this paper, conductors with copper stabilization for practical applications are laser scribed into narrow filaments. Striated tapes are then wound into conductor on round core (CORC®) cables. The critical current and magnetization ac loss of single tapes were measured. We found that the stabilizing copper layer causes difficulties for laser scribing. The degradation of the critical current is more pronounced than in the case of non-stabilized tapes. The selection of the number of filaments is therefore a compromise between critical current degradation and reduction of ac loss. Based on the results obtained from single tape experiments, the optimum number of filaments in 4 mm wide tapes was chosen, and CORC® cables with 2, 3 and 4 layers of tapes with and without filaments were manufactured. Magnetization ac loss measurements at 77 K showed a reduction of ac loss in the cables with filaments. This reduction corresponds almost to the number of filaments. Measurement at different frequencies also showed that the coupling loss in CORC® cables with a short twist-pitch is relatively small in comparison to hysteretic loss.

AC Magnetization Loss and Transverse Resistivity of Striated YBCO Coated Conductors

It is well known that the separation of thin (RE)BCO superconducting films into electrically isolated stripes (striation process) leads to significant reduction of the magnetization losses. However, in practice, achieving the theoretically predicted loss reduction is quite complicated, due to imperfect separation of the stripes: techniques used for striation leave resistive bridges between the stripes, and coupling currents are free to flow. Very little is known about the precise paths of the coupling currents, other than the fact that the transverse resistivity may play a major role. In this paper, we investigate the magnetization ac loss and the transverse resistivity profile on samples with different numbers of filaments and with different thicknesses of the stabilization layer. The reduction of stabilization layer thickness leads to better control of the laser grooves and substantially suppresses coupling loss. The total loss in those tapes was reduced significantly and is very close to the theoretical expectation.

Hysteretic Magnetization Losses of HTSC Tapes in Coaxial AC and DC Magnetic Fields

In the work we present experimental results of ac magnetization losses measurements performed in the presence of a dc-bias magnetic field. Both ac and dc magnetic fields were coaxial and directed perpendicularly to the flat surface of sample tapes. The HTS tapes used in our experiments are commercially available composite tapes, namely a BSCCO-2223/Ag multifilamentary tape and an YBCO-123 coated conductor tape. The measurements were carried out with a sinusoidally varying magnetic fields at frequency range of about 21-113 Hz, and amplitudes up to 100 mT. The superimposed a dc-bias magnetic field was of the order of up to 50 mT. At a certain magnetic field amplitude, and at a certain dc bias magnetic field, a distinguish minimum in ac losses is observed. Occurrence of the ac loss minima differs considerably for two studied sample tapes.

Influence of Oxygenation in Copper Stabilized Multifilamentary 2G HTS Tapes Made by Selective Electroplating and Laser Ablation

Striation and selective electroplating of copper stabilizer layer has been proven to be an effective way of reducing the hysteretic AC loss of second generation high temperature superconducting (2G-HTS) tapes. This method consists of a top down laser material removal (ablation) followed by post-oxygenation to create resistive oxide layer on the grooves and selective electroplating of a thick Cu stabilizer layer on the filaments. In such a filamentization technique, scalability, the degree of coupling loss contribution and critical current (I c ) degradations are the main concerns. In this study, we have investigated the post-oxygenation temperature effect on the I c degradation and coupling loss contribution. The results showed that post-oxygenation temperature has immense effect on the I c degradation and also alters the coupling loss contribution to the total magnetization AC losses. The coupling loss contribution was found to be less in samples oxygenated at higher temperatures; however, the degradation of I c was found to be higher. In addition, preliminary experiments were conducted to scale-up this newly-developed technique. Initially, 10 m long fully-filamentized tape with thick Cu stabilizer layer was fabricated and at least 5 times reduction in AC loss was observed.

Three-Dimensional Numerical Simulations of Twisted Stacked Tape Cables

Different magnet applications require compact high current cables. Among the proposed solutions, the Twisted Stacked Tape Cable (TSTC) is easy to manufacture and has very high tape length usage efficiency. In this kind of cables the tapes are closely packed, so that their electromagnetic interaction is very strong and determines the overall performance of the cable. Numerical models are necessary tools to precisely evaluate this interaction and to predict the cable's behavior, e.g. in terms of effective critical current and magnetization currents. For this purpose, we developed a fully three-dimensional model of a TSTC, which not only takes into account the twisted geometry of these cables, but is also able to account for the contact resistances of the current terminations. The latter can have profound influence on the way the current is partitioned among the tapes, especially on short laboratory prototypes. In this paper, we first use the numerical model to compute the critical current and the magnetization AC loss of a twisted tape, showing the differences with the case of a straight tape. Then, we use it to calculate the current distribution in a TSTC cable, comparing the results with those experimentally obtained on a cable composed of four straight stacked tapes. The results show the ability of the model to simulate twisted conductors and constitutes a first step toward the simulation of TSTC in high-field magnet applications. The presented modeling approach is not restricted to the TSTC geometry, but may be used for any cable configuration with periodical translational symmetry.

Numerical modeling of twisted stacked tape cables for magnet applications

In view of high-temperature superconductor (HTS) magnet applications, the concept of Twisted Stacked Tape Cable (TSTC) made of HTS coated conductors is very promising because of the easy manufacturing process and of the very high tape length usage efficiency. For the use of these cables in magnet applications, where the cables carry high current while subjected to the strong magnetic field generated by the rest of the magnet, the possibility of being able to calculate in detail current and field distributions is very welcome, particularly for evaluating the cable’s performance during the charge of the magnet. The numerical modeling of this kind of cable is particularly challenging because of the twisted geometry. In this paper, we use a 3-D finite element model to compute the magnetization AC losses of a twisted superconductor and current repartition among the tapes in a cable composed of four HTS coated conductor tapes. The utilized model is able to simulate not only twisted geometries, but also the contact resistance of the electrical terminations used to inject the current. The latter can importantly influence the current repartition between the tapes, especially in short samples. The model is also able to take into account the angular dependence of the critical current on the local magnetic field, whose relative orientation with respect to the tape needs to be locally evaluated as a consequence of the twisted geometry.

Magnetic Properties and AC Losses in AFe2O4(A = Mn, Co, Ni, Zn) Nanoparticles Synthesized from Nonaqueous Solution

Nanosized particles of AFe2O4(A = Mn, Co, Ni, or Zn) spinel ferrites were synthesized by coprecipitation from nonaqueous solutions using nitrate salts as starting reagents. The particles were characterized by X-ray diffraction, transmission electron microscopy, and magnetic measurements. Quasistatic magnetic measurements show superparamagnetic behavior with blocking temperature below room temperature for cobalt, nickel, and zinc spinel ferrite nanoparticles. Characteristic magnetic parameters of the particles including average magnetic moment of an individual nanoparticle and blocking temperature have been determined. The specific loss power which is released on the exposure of an ensemble of synthesized particles to a magnetic field is calculated and measured experimentally. It is shown that among all nanoferrites under study, the ZnFe2O4nanoparticles demonstrate the highest heating efficiency in AC magnetic fields. The key parameters responsible for the heating efficiency in AC magnetic field have been determined. The directions to enhance the SLP value have been outlined.

Magnetization AC losses in MgB2 wires made by IMD process

Magnetization AC losses of MgB2 superconductors with one and four filaments made by an internal magnesium diffusion (IMD) into boron process were measured and analyzed. For AC loss measurement a system based on a calibration-free method was used. Short samples of MgB2 wires were exposed to an external magnetic field with amplitudes up to 0.07 T, frequencies up to 1200 Hz, and a temperature range between 15 K and 40 K. A strong effect of eddy current losses was found in single-core wire containing pure copper sheath, which was proved by the same wire measurement after Cu etching. The impact of coupling current losses in non-twisted four-filament wire and the decoupling effect after twisting were observed. Coupling current losses in a low-frequency region were effectively reduced in agreement with theoretical assumption. The degradation of transport currents due to torsion stress by twisting was taken into account and the normalized AC losses of MgB2 wires made by IMD and powder-in-tube processes were compared. It appears that the IMD process is more perspective for AC applications due to much higher current densities and smaller degradation of current-carrying capability by twisting.

AC Loss and Coupling Currents in YBCO Coated Conductors With Varying Number of Filaments

Striation of high-temperature superconductor coated conductors as a way to reduce their magnetization ac losses has been the subject of intense worldwide research in the past years by several groups. While the principle of this approach is well understood, its practical application on commercial materials to be used in power applications is still far to be implemented due to manufacturing and technological constraints. Recent advances in tape quality and striation technology are now enabling systematic investigations of the influence of the number of filaments on ac loss reduction with a consistency that was not available in the past. In the present work, we demonstrate the technological feasibility of importantly reducing the magnetization losses of commercially available coated conductors by striating them into a high number of filaments (up to 120). The loss reduction exceeds one order of magnitude and does not come at the expense of current-carrying capability: samples with 10 and 20 filaments are unaffected by the striation process, whereas samples with 80 and 120 filaments still retain 80% and 70% of the current-carrying potential, respectively. We also investigate the transverse resistivity between the filaments in order to understand the paths followed by the coupling currents: we found that the coupling current prevalently flows in the metallic substrate, rather than in and out of the filaments. Finally, we use oxidation as a method to reduce the coupling currents and the corresponding losses. The contribution of this work is threefold: 1) it describes the know-how to produce a large number of high-quality striations in commercially available coated conductors, greatly reducing their losses without extensively degrading their transport properties; 2) it provides a comprehensive characterization of the said samples (e.g., measurements in a wide frequency range, transverse resistance profiles, influence of oxidation on dc and ac behavior of the sample); and 3) it provides new insight on the patterns of the coupling currents.

AC Magnetic Properties and Core Loss Behavior of Fe–P Soft Magnetic Sheets

AC magnetic properties and core loss behavior of differently prepared Fe-P sheets have been analyzed in this paper. Furthermore, the results have been compared with non-oriented silicon (Si) steel. The coercivity, magnetic flux density, and permeability measurements in dc magnetic field show Fe-P has better soft magnetic properties than Si steel. The ac magnetic behavior of both the samples has been studied in the frequency range from dc to 1 kHz for a range of flux density values. The above studies indicate that crystalline Fe-P alloys can be the replacement of so-called Si steel for low-power device applications.