Field Trapping Ability Research Articles

Comparison of the Field Trapping Ability of MgB2 and Hybrid Disc-Shaped Layouts.

Superconductors have revolutionized magnet technology, surpassing the limitations of traditional coils and permanent magnets. This work experimentally investigates the field-trapping ability of a MgB2 disc at various temperatures and proposes new hybrid (MgB2-soft iron) configurations using a numerical approach based on the vector potential (A→) formulation. The experimental characterization consists in measurements of trapped magnetic flux density carried out using cryogenic Hall probes located at different radial positions over the MgB2 sample, after a field cooling (FC) process and the subsequent removal of the applied field. Measurements were performed also as a function of the distance from the disc surface. The numerical modelling of the superconductor required the evaluation of the critical current density dependence on the magnetic flux density (Jc(B)) obtained through an iterative procedure whose output were successfully validated by the comparison between experimental and computed data. The numerical model, upgraded to also describe the in-field behavior of ARMCO soft iron, was then employed to predict the field-trapping ability of hybrid layouts of different shapes. The most promising results were achieved by assuming a hollow superconducting disc filled with a ferromagnetic (FM) cylinder. With such a geometry, optimizing the radius of the FM cylinder while the external dimensions of the superconducting disc are kept unchanged, an improvement of more than 30% is predicted with respect to the full superconducting disc, assuming a working temperature of 20 K.

Review of synthesis of high volumetric density, low gravimetric density MgB2 bulk for potential magnetic field applications

This review paper summarizes and analyses the current literature on the synthesis of dense MgB2 bulk with high density and by both the modified solid reaction method and the Mg infiltration method. MgB2 bulk shows Telsa-level magnetic field-trapping ability in small volumes (1–5 cm3) at 15–20 K, which is beneficial for multiple applications, particularly where light weighting is beneficial. The review also compares the properties of MgB2 to rare earth barium cuprate bulk magnets and NbTi solenoid magnets and discuess potential applications such as magnetic lenses, compact NMR, and magnetic shielding.

Electromagnetic properties of curved HTS trapped field stacks under high-frequency cross fields for high-speed rotating machines

Superconducting electric propulsion systems, characterized by high power densities and efficiencies, provide a possibility to zero carbon emission for future aviation. Stacks of high temperature superconducting (HTS) coated conductors (CCs) have become an alternative for high field magnets applied to superconducting machines, given their excellent field trapping ability and thermal stability. High-frequency ripple fields always exist in high-speed electric machines. Most research work regarding HTS trapped field stacks (TFSs) was focused on their magnetization methods and amplitude of trapped flux density; however, their performance in the high-frequency environment remains unclear. Despite several numerical models established for flat HTS TFSs, a comprehensive analysis of curved ones is still lacking, which possess geometrical applicability for cylindrical rotating shafts. Aimed at exploring the electromagnetic properties of curved HTS TFSs applied to high-speed rotating machines, a 3D numerical model considering both the multilayer structure and the Jc (B) dependence of HTS CCs has been built. Current and magnetic flux density distributions, as well as loss properties of a curved HTS TFS have been studied in detail, under perpendicular and cross fields with varying frequencies ranging from 50 Hz to 20 kHz. Results have shown that, the widely adopted two-dimensional-axisymmetric models are inapplicable to study the electromagnetic distributions of TFSs because of the emergence of the electromagnetic criss-cross defined in this paper. High-frequency ripple fields can drive induced current towards the periphery of the HTS TFS due to the skin effect, leading to a fast rise of AC loss and even an irreversible demagnetization of the stack. This paper has qualified and quantified the high-frequency electromagnetic behaviours of curved HTS TFSs, providing a useful reference for their loss controlling and anti-demagnetization design in high-speed propulsion machines.

Occurrence of Flux Jumps in MgB2 Bulk Magnets during Pulse-Field Magnetization

The magnetic flux capturing of MgB2 bulk magnets made by spark plasma sintering process has been precisely investigated to clarify the mechanism of flux motions during the pulse-field magnetization processes. The field trapping ratio B T/B P was evaluated as a key parameter of field trapping ability which strongly relates to the heat generation due to the rapid flux motion in the samples. The time dependence of magnetic flux density revealed the actual flux motion which penetrated the samples. The trapped fields B T and field trapping ratios B T/B P of various samples were classified into three regions of ‘no flux flow’, ‘fast flux flow’ and ‘flux jump’ according to the generation of heat and its propagation. A flux jump was observed late at 280 ms from the beginning of PFM process, while the field penetration B P showed its peak at 10 ms. Considering the heat propagation speed, the long-delayed flux jump should be attributed to the macroscopic barriers against the heat propagation to the surface centre of bulk magnet.

Enhancement of trapped field in single grain Y–Ba–Cu–O bulk superconductors by a modified top-seeded melt-textured growth

The modified top-seeded melt-textured growth technique for fabricating single grain Y–Ba–Cu–O (YBCO) bulk superconductors with high field-trapping ability by using modified precursor pellets was reported. The modified precursor pellets are composed of different precursor powders YBa2Cu3O (Y123) + x mol% Y2BaCuO5 (Y211) + 1 wt% CeO2 without any further chemical doping. The modified YBCO bulks up to 25 and 34 mm in diameter were successfully fabricated from the modified precursor pellets. Microstructural observation results showed that the modified YBCO bulk exhibited a homogeneous distribution of Y211 phase particles, which was qualitatively explained by the solute diffusion growth model in combination with the trapping/pushing theory. As a result, it is notable that the peak trapped field values of 0.91 T (maximum 0.96 T) and 1.2 T (maximum 1.28 T) at 77 K were achieved for 25 and 34 mm modified YBCO bulks, respectively. In a word, the results from present work are very helpful to understand the melt growth mechanism and to further improve the properties of YBCO bulk superconductors for practical applications.

Performance and applications of quench melt-growth bulk magnets

This paper describes the progress in quench melt-growth (QMG) bulk magnets, developed by the Nippon Steel & Sumitomo Metal Corporation, which consist of single crystalline RE123 phase and finely dispersed RE211 particles. QMG bulks can trap high magnetic fields. The field-trapping ability of QMG bulks is largely increased with an improvement in its Jc and size, promising the realization of various applications such as flywheel energy-storage systems, ship motors, NMR/MRI spectrometers, wind-power generators and so on. Intensive research has revealed that the optimal RE element is different depending on application requirements. Gd-QMG bulk is the most promising material for several high-field engineering applications. The trapped magnetic field of Gd-QMG bulk 60 mm in diameter at 77 K is twice as large as that of Y-QMG bulk with a similar size due to its excellent Jc properties. The large Gd-based QMG bulks up to 150 mm in diameter are fabricated by incorporating the RE compositional gradient method. Compact NMR/MRI spectrometers are one of the promising applications of bulk superconductors. Eu-QMG bulks are suitable for NMR magnets. NMR applications require extremely homogeneous magnetic fields. In the Eu-system, the small paramagnetic moment of a Eu ion compared to a Gd ion improves the field homogeneity in the bulk. For the application of current leads, Dy-based QMG is available by utilizing a low thermal conductivity.

Influence of layered precursor pellets on the growth and properties of Y-Ba-Cu-O bulk superconductors by top-seeded melt-textured growth

It is well known that a fine and homogeneous distribution of Y2BaCuO5 (Y211) phase particles in single-grain Y-Ba-Cu-O (YBCO) bulk superconductors is essential for improving field-trapping ability. However, the size and concentration of Y211 phase particles in the fully melt-processed superconducting bulk increase significantly with the distance from the seed, which results in the accumulation of Y211 phase particles and the degradation of superconducting properties. In this paper, we report a new method of fabricating single-grain YBCO using layered precursor pellets. Using the top-seeded melt-textured growth process, single-grain YBCO bulk superconductors of about 22 mm in diameter and 9 mm in thickness were fabricated from layered precursor pellets and standard precursor pellets, respectively. The layered precursor pellets consist of precursor powders with 40 mol% Y211 at the top, 30 mol% Y211 in the middle and 20 mol% Y211 at the bottom of the whole pellets, while standard precursor pellets are prepared from precursor powders with only 40 mol% Y211. The growth morphology, microstructure and magnetic flux properties of the layered samples and standard samples were comparatively studied. The results proved that the layered precursor pellets allow a sufficient growth in the c-growth sector and a more uniform distribution of the Y211 phase in the matrix. The distribution of Y211 phase particles is qualitatively explained by the prevalent trapping/pushing theory. The trapped field at 77 K reaches 0.8 T, nearly 29% higher than the standard sample. The present results are very valuable for further improving the properties of YBCO bulk superconductors.

The processing and properties of single grain Y–Ba–Cu–O fabricated from graded precursor powders

The preparation of single grain, Y–Ba–Cu–O (YBCO) bulk superconductors by top-seeded melt-growth (TSMG) usually involves precursor powders that contain a uniform distribution of the constituent YBa2Cu3O7−δ (Y-123) and Y2BaCuO5 (Y-211) phase compounds. However, it has been observed that the concentration of Y-211 particles in the fully melt processed superconducting bulk increases significantly with distance from the seed, which results in a degradation of superconducting properties towards the edge and bottom of the sample. Here we investigate the effect of preparing bulk YBCO superconductors by TSMG using spatially graded Y-211/Y-123 precursor powder. The graded precursor bulks were prepared with a maximum composition of 40 wt% Y-211 in the vicinity of the seed, which decreased to 30 wt% and then 20 wt% towards the bottom and edge of the green body. Standard samples were melt processed from precursor powders containing 30 wt% Y-211 to enable comparison. The field trapping ability, Tc and Jc, of three graded and two standard samples were investigated and compared statistically. The distribution of Y-211 particles along different growth directions of the samples was analysed, and any crystallographic misorientation was investigated. The observed distribution of Y-211 particles in YBCO is explained qualitatively by trapping/pushing theory, and its correlation with the superconducting properties of the melt processed bulk samples has been analysed. Finally, the practical feasibility of the graded technique is evaluated.

Magnetic and Thermal Properties of HTS Bulk Magnet in the Pulsed-Field Magnetizing Process

In order to enhance the field-trapping ability of high Tc superconducting melt-textured bulk materials which act as quasi-permanent magnets when they capture the external magnetic fields, it is important to enhance the mechanical toughness of the materials to stand the stress induced by the magnetic repulsive force and thermal expansion. We adopted a dense Dy123-based bulk material with reduced void concentration in the experiment. Since the heat generations due to the flux motions in the samples results in the degradation of Jc, the time evolutions of the trapped magnetic fields and the temperature rises during and after the pulsed-field magnetizing processes were precisely measured at the same time to evaluate the penetrating flux motions and the heat generations in the sample. A single and couple of the magnetic pulsed fields with various intensities were successively applied to the sample at 30 K. The single magnetic field application exhibited a peak effect in the trapped-field behavior and tended to decline due to the heat generation. In the iterative pulsed-field application, the behaviors of the trapped fields and the temperature changes were found to be inverse between the first and the second pulsed-field applications. This implies that the flux penetration behavior into the sample magnet at the second field application is strongly restricted by the presence of former trapped fields which were formed by the first field applications.

Field trapping property of HTS bulk magnet with reduced voids in pulsed field magnetizing process

The field trapping ability of melt-textured bulk superconductors is usually restricted by the mechanical properties because of the magnetic stress induced in the samples during the activation processes which give them intense magnetic fields of several T Therefore it is of crucial importance to enhance the toughness of the materials by means of reinforcing techniques. In this experiment, we adopted a dense Dy-based HTS bulk sample fabricated by means of controlling the concentration of voids inside during the heat treatment. A single magnetic pulse fields up to 7.66 T have been applied in the temperature range down to 30.6 K with use of GM refrigerator and the profiles of pulsed fields and the resultant trapped fields are discussed with respect to their performances. It was found that the trapped field grows as elongating rise time of the pulsed field and lowering initial temperature. The densely fabricated Dy123 sample was activated up to 1.98 T at 31 K in the same manner with no substantial difference from the porous Gd123 samples when the magnetic pulse field of 7.66 T was applied. The temperature rises up to 20 K are not substancial in comparison to those of the Gd123 compounds which exhibit the temperature rises more than 30 K at 40 K by applying 4.64 T This is attributed to the fact that the Jc value of the dense Dy123 are inferior to that of Gd123, which reflectes the field trapping ability.

Effect of magnetic particle additions on flux pinning in bulk Y-Ba-Cu-O superconductors

Abstract In order to increase the critical current density of high temperature superconductors, it is commonly employed to disperse nonsuperconducting particles as flux pinning centers into the superconductive matrix. For Y-Ba-Cu-O bulk superconductors Y2BaCuO5 (Y211) phase particles are known to be effective pinning centers. It has been proposed that magnetic particles are more efficient pinning centers, since they will have a stronger positive interaction with quantized fluxoids. A combined addition of Y211 and magnetic metal fine powders has been reported to be effective in improving flux pinning performances. Therefore, in this study we aimed at improving the pinning forces of Y-Ba-Cu-O bulk superconductors by introducing the Fe-Al-Si-based alloys that are ferromagnetic. We also studied the effect of the addition of γ-hematite, which is non-magnetic, in order to compare the contribution to the flux pinning enhancement. For comparative study, we measured the trapped field in single-domain bulk superconductors and found that a small addition of Fe-Al-Si based ferromagnetic powders and γ-hematite improved field trapping ability.

Superconducting properties of Y-Ba-Cu-O bulk with BaSnO3 addition

Dispersion of non-superconducting phase particles is commonly employed in melt textured REBa2Cu3O7-δ (RE123; RE: rare earth element) bulk superconductors in order to improve the critical current density. It has been clarified that the interfaces of RE2BaCuO5 (RE211) and RE123 matrix contribute to the flux pinning, and therefore, the critical current density can be enhanced due to the refinement of RE211 particles. In recent years additions of RE2Ba4CuMOy (M=Zr, Nb, Ta, Mo, U, Sn, and Re) particles have been investigated in an attempt to improve the pinning performance. While the common particle size is around 1-5 μm for the RE211, the size of RE2Ba4CuMOy particles is much finer and typically 50∼300 nm, implying that the pinning may be improved with the dispersion of RE2Ba4CuMOy particles. In this study, we have studied the effects of the BaSnO3 addition to Y2BaCuO5 (Y211) powders. It was found that two different compounds of Y2Ba4CuSnOy and Y2BaCuO5 phase particles were co-produced by adding BaSnO3 to Y211. These powders were added to the precursor and subjected to the melt-textured process to grow bulk Y-Ba-Cu-O superconductors of 20 mm diameter with the top-seed melt growth process. The finished bulk samples were observed with optical microscopy and subjected to trapped field measurements with Hall sensors. The trapped field measurements showed that BaSnO3 addition was effective in improving the field trapping ability.

Reinforcement of bulk Y-Ba-Cu-O superconductors by using Fe-Mn-Si-Ni shape memory alloy rings

Abstract Bulk Y-Ba-Cu-O superconductors are brittle ceramics and their tensional strengths are very low. Therefore, reinforcement of the bulk superconductor is needed for practical applications. Pre-compression load has been shown to be effective in enforcing the bulk superconductors. Fe-Mn-Si alloys exhibit shape-memory effects and the size change due to the shape recovery is large. Therefore, the Fe-Mn-Si shape memory alloy rings will function as effective reinforcement material for the bulk superconductors. We prepared Y-Ba-Cu-O bulk superconductors with top-seeded melt-growth process and the Ni added Fe-Mn-Si (Fe-Mn-Si-Ni) alloy ring which exhibited better shape memory performances than Ni-free Fe-Mn-Si alloys. The ring was extended by inserting a steel rod and heated to 623K. The amount of shape recovery strain was about 2%. Based on these results, the TSMG-processed bulk YBa- Cu-O superconductor 39.0 mm in diameter was inserted into the Fe-Mn-Si-Ni ring whose inner diameter was 39.3 mm at room temperature. With heating to 623K, the Fe-Mn-Si-Ni ring shrank and firmly encapsulated the bulk Y-Ba-Cu-O superconductor. Cracking was not observed in the bulk superconductor. It was interesting to note that the trapped magnetic field of the Y-Ba-Cu-O superconductor at 77K was increased from 2,550 G to 3,795 G through Fe-Mn-Si-Ni ring reinforcement. These results clearly show that the reinforcement treatment with Fe-Mn-Si-Ni alloy ring or pre-compression load is effective in improving the field trapping ability in addition to thee improvement of the mechanical properties.

Growth of large bulk Y–Ba–Cu–O with multi-seeding

Large bulk Y–Ba–Cu–O superconductors, 62 mm in diameter and 12 mm in thickness, havebeen fabricated by a cold seeding method in which four seeds are placed on top of theprecursor at room temperature prior to melt processing. The advantage of a cold seedingtechnique is that the desired seeding location can be arranged at room temperature.However, due to the reaction of the seed with the precursor at high temperatures,controlled single-grain growth has been difficult. In the present study, we carefullyinvestigated the influence of the processing parameters such as the maximumheating temperature, the dwelling time at this temperature, the temperatureto start slow cooling and the slow cooling rate on the grain growth behavior.Through the optimization of these processing parameters, we have succeeded ingrowing large bulk Y–Ba–Cu–O with four large grains. The sample with 1 wt% CeO2 addition showed almost the same field trapping ability as a single-domain sample.

Processing and properties of large grain Y–Ba–Cu–O containing Y2Ba4CuWOy and Ag second phase inclusions

Second phase nanoinclusions of composition Y2Ba4CuWOy (Y-24W1) have been incorporated into the YBa2Cu3O7−δ (Y-123) superconducting matrix to form effective artificial flux pinning sites. A remarkable improvement in critical current density Jc has been observed at both self- and higher applied fields in these samples, in contrast to the effect that the presence of Y2BaCuO5 (Y-211) second phase inclusions has on Jc, particularly at low field. The field trapping ability of samples containing the Y-24W1 phase has been improved by adding silver to the nanocomposite precursor composition in an attempt to reduce the formation of c-axis microcracks in the sample microstructure. The combined effect of the Y-24W1 artificial defects and the addition of silver produce enhanced trapped fields in these samples.

Fabrication of high performance Y-123/Y-24Nb1/Ag single grain composites

Y 2Ba 4CuMO (Y-2411, where M is a transition metal element) has been recognized as a potential candidate for enhanced flux pinning in Y–Ba–Cu–O (YBCO) bulk superconductors. However, the non-uniform distribution of Y-2411 in the superconducting phase matrix and the presence of a– c micro-cracks in the sample microstructure limit the magnetic field trapping ability of bulk YBCO containing these inclusions. Recent attempts to add Ag to the YBCO matrix have been observed to enhance the microstructural properties of Y-2411 containing materials. In this work we report the fabrication of high performance YBCO composite containing Y-24Nb1 (Y 2Ba 4CuNbO y ) and metallic Ag. The average value of critical current density, J c, is compared for samples containing different concentrations of Y-24Nb1. The microstructural features of these samples have been compared with a YBCO single grain processed with Y-24Nb1 and metallic Ag.

Fabrication of textured YBCO bulks with artificial holes

The recently reported hole-patterned YBa2Cu3Oy (Y123) bulks with improved superconducting properties are highly interesting from the aspects of the material qualities and application various points of view. It is well known that the core of plain bulk superconductors needs to be fully oxygenated and some defects like cracks, pores and voids must be suppressed in order that the material can trap high magnetic field or carry high current densities. To minimize the above defects, we have used the infiltration and top seed growth (ITSG) process to prepare regularly perforated YBa2Cu3Oy (Y123) bulk superconductors. This process involves less shrinkage during annealing and a uniform distribution of Y211 inclusions. The texture was evidenced by neutron pole figure measurements. Flux mapping was used to verify the superconducting homogeneity of the samples and to investigate the field trapping ability. Large increase of the trapped field in pulsed magnetization up to 60% in comparison to the hole-free parent sample with the same thermal history has been obtained. In addition, the textured drilled samples were reinforced using resin or metal impregnation and the influence of the different processing steps on the hardness of the materials is investigated.

Temperature rise in melt-textured large grain superconducting bulk magnets during their magnetizing operations

The temperature rises in the melt-processed RE–Ba–Cu–O (RE = Y, Sm, Gd) bulk superconductors were evaluated when the magnetizing operations were performed by the field cooling (FC), the zero field cooling (ZFC) and the pulsed field magnetization (PFM) processes. It is known that the field-trapping ability of the superconducting bulk magnets is restricted by the heat generation due to the flux motion inside the sample not only in the PFM but also even in the FC process. In this paper, the authors discuss the mechanism of heat generation and the flux motions by means of the temperature and resultant trapped field measurement during three magnetizing processes. The sample was cooled in a 5 T static field and then the field was removed with various descending rates (2.53–11.3 mT/s) in FC method. The fields were also applied and then removed with the same rates after setting the sample in the superconducting state in ZFC. These data were compared with the results obtained in the PFM. The temperature evolutions have shown apparent peaks that correspond to the heat generation in their magnetizing processes. The maximum temperature rises have reached 5.9 K in FC and 7.6 K in ZFC operated at 57.6 K and 51.5 K, respectively. The temperature rise exhibited 2 K even when the slowest rate of 2.53 mT/s was applied in FC mode. These data suggest it is crucial to take the heat generation into account in estimating the field-trapping ability of the high performance bulk magnets.

Field trapping property of Gd–Ba–Cu–O bulk superconductor 140 mm in diameter

We have studied field trapping ability of single grain Gd–Ba–Cu–O bulk superconductor 140mm in diameter, which was fabricated by using Gd2BaO4 and Ba–Cu–O phase as raw materials. The distribution of the trapped magnetic field showed single domain without serious weak links, when the bulk was field-cooled in 1T at liquid nitrogen temperature. The maximum trapped magnetic field at 77K and 65K were 2.30T and 3.66T respectively, when a field of 4T was applied. From the distribution measurements of magnetic field, it was confirmed that a large amount of flux can be trapped in this large bulk. It was also confirmed that magnetic field can reach a longer distance in a larger bulk than smaller diameter samples.

Pulsed field magnetization properties for Gd–Ba–Cu–O superconductors impregnated with Bi–Sn–Cd alloy

We present a post-fabrication treatment that improves thermal conductivity of bulk Gd–Ba–Cu–O magnets for pulsed field magnetization (PFM). A small hole about 1mm in diameter was artificially drilled into the center of bulk Gd–Ba–Cu–O 25mm in diameter and 18mm in thickness. An aluminum wire was inserted into the hole, and then the sample was subjected to the impregnation by using Bi–Sn–Cd alloy. We employed PFM to magnetize Gd–Ba–Cu–O bulk superconductors with and without the alloy impregnation treatment. The temperatures of the samples were monitored with thermocouples, and local magnetic field density was measured with a Hall sensor. It was found that the alloy impregnation was effective in suppressing the temperature rise by 4K and in increasing the trapped field by 25% at 44K. The present results show that the alloy impregnation treatment enhanced the thermal conductivity of bulk Gd–Ba–Cu–O, and thereby improved the field trapping ability.