Abstract Body

We are interested in understanding and controlling the process-microstructure-property of REBCO bulk superconductors. For high-performance, low-cost and reliable-fabrication, a variety of approaches have been newly developed in our lab, such as

1) New seed materials of Mg-NdBCO films combined superheating nature and Mg-doping effect have been proved to have higher thermal stability for the growth of high TpREBCO bulk superconductors.

2) Aiming to refined and uniform RE211, a novel TSMG approach was developed, in which instead of RE211+RE123, modified precursor powders (MPP, RE2O3, and BaxCuyOz) were employed. As a result, massive numbers of small-sized RE211 were in-situ derived from the homogeneous nucleation. Furthermore, MPP is used to combine with a compositional graded structure, effectively suppressing the inherent enlargement and segregation of RE211 particles.

3) Two novel seeding strategies for creating incomplete crystallographic shapes of REBCO crystals, which possess self-repairing capability, leading to rapid surface growth. One is in situ self-assembly seeding, by which self-reparability promotes REBCO growth, while the other is vertically-connected seeding, by which self-reparability triggers REBCO nucleation.

4) For recycling failed and hard‐to‐melt REBCO bulks, a two‐step process, the re-melting plus quenching and re-growing was applied. More recently, REBCO buffer-crystals, which used to be discarded, were directly utilized as seeds for producing REBCO bulks.

Abstract Body

We reported previously that a high-quality REBaCuO bulk (about 20 mm in diameter and 10 mm in thickness) can be obtained using a high-density REBCO precursor prepared by the spark plasma sintering (SPS) method [1]. However, to prepare a precursor larger than 60 mm in diameter, a large SPS apparatus must be required, which results in a high cost. On the other hand, fine powder is well known to offer a highly dense precursor, for which it is another disadvantage that compression defects often occurs during pelletization [2]. Therefore, in this paper, we aimed to granulate Ag-GdBCO powder using an acrylic resin powder as a binder and produce a high-quality REBCO bulk.

The Ag-GdBCO powder was ball-milled and mixed using stainless steel media. Stearic acid as a lubricant and an acrylic resin acting as a binder were added during this process. No compaction defects were found in the green compacts made using the granulated Ag-GdBCO granules, which resulted in the high-quality bulk successfully. In the presentation, we will also report on the detailed degreasing conditions of the precursor and the trapped magnetic field characteristics of the obtained Ag-GdBCO bulk.

References

[1] H. Hakoishi et al., IEEE Trans. Appl. Supercond. 33 (2023) 6800405
[2] S. Nariki et al., Physica C 463-465 (2007) 308

Keywords: Ag-GdBCO single grain bulk, Granulation, Degreasing, Trapped field

Abstract Body

High T_c superconducting (HTS) bulk LREBa₂Cu₃O_y (LRE: Nd, Sm, Eu, Gd) "LRE-123" superconductors, with critical temperatures (T_c) above the boiling point of liquid nitrogen (77.3 K) have numerous technological and industrial applications. Typically, these bulk superconductors are fabricated employing top-seeded melt growth (TSMG) or infiltration growth (TSIG) processes. Recent experiments have shown that TSIG overcomes the limitations of TSMG_[1]. Among the HTS bulk LRE-123 superconductors, ternary LRE-Ba₂Cu₃O_y superconductors, such as (Nd,Eu,Gd)Ba₂Cu₃O_y and (Sm,Eu,Gd)Ba₂Cu₃O_y are renowned for their superior superconducting performance. These superconductors exhibit levitation at liquid oxygen temperature (90.2 K) and show irreversibility at 15 T at 77 K, H // c-axis _[2,3]. However, fabricating these ternary LRE-Ba₂Cu₃O_y compounds requires specific oxygen-controlled environments due to RE/Ba substitutions, which can degrade superconducting performance when fabricated in air.  In this work, we successfully fabricated ternary (Sm,Eu,Gd)Ba₂Cu₃O_y bulk in the air using the TISG process, achieving enhanced superconducting performance. Initially, we used various liquid sources, such as REBa₂Cu₃O_y + Ba₃Cu₅O₈ (1:1) (RE= Sm, Gd, Y and Er), to fabricate the bulk. The bulk obtained using YBa₂Cu₃O_y+ Ba₃Cu₅O₈ exhibited a sharp transition width (∆T_c) of 3.88 K, representing a 52.5% improvement compared to the bulk obtained from ErBa₂Cu₃O_y+ Ba₃Cu₅O₈. To further enhance the transition width, we systematically enriched the (Sm,Eu,Gd)₂BaCuO₅ precursor powder with various weight percentages of BaO₂ to control the RE/Ba substitutions. With optimized BaO₂ in the secondary phase, we achieved a T_{(c, onset)} of > 94 K and ∆T_c < 1 K. The enhanced bulk sample showed a 71.24 % improvement over the reference sample. These results facilitate the fabrication of high-performance ternary (Sm,Eu,Gd)Ba₂Cu₃O_y bulk superconductors in air, broadening their applications in various industrial and technological fields.

References

[1] Agarwal, A. G., & Miryala, M. (2024). Ceram. Int., 50 (17), 31559-66
[2] Muralidhar, M. (2002). Phys. Rev. Lett., 89, 237011-1
[3] Muralidhar, M., et al. (2003). Appl. Phys. Lett., 83 (24), 5005-5007

Acknowledgment

Akash Garg Agarwal acknowledges financial support from SIT for the doctoral program.

Keywords: Top-seeded infiltration growth; Ternary Bulk LRE-123; RE/Ba substitution; Critical temperature.

Abstract Body

Due to the helium shortage, demand for cost-effective MgB₂ superconductors is growing. MgB₂ offers high performance without requiring helium cryogenics, even though it operates at lower temperatures than YBa₂Cu₃O_y superconductors [1]. Its advantages include simpler fabrication, lower-cost materials, and strong superconducting properties. The cost and performance of MgB₂ superconductors are influenced by the production of the nano boron powders initially added during manufacturing. We have successfully optimized the UX-600 ultrasonication process for producing low-cost nano boron. By refining parameters such as power, time, and temperature, we reduced the nano boron particle size from 508 nm to 128 nm. This optimization resulted in a significant improvement in critical current performance, increasing from 268 kA/cm² to 461 kA/cm² at 20K and in a self-field—a 72% enhancement, as reported in our latest patent. Furthermore, the critical current was measured at an impressive 657 kA/cm² at 10K and in a self-field, which can be attributed to the fine nano particle formation, as confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses. These advancements mark the highest J_c value reported in the literature for bulk MgB₂ material produced through sintering. The quality of the MgB₂ material was further validated by X-ray diffraction and magnetization-temperature (M-T) measurements. The improved critical current performance is explained by the combination of power, time, and sedimentation processes, which enabled the control of commercial boron particle size to the nano scale for MgB₂ production.

References

[1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Superconductivity at 39K in magnesium diboride, Nature 410 (2001) 63-64.

Keywords: Bulk MgB₂ , ultrasonication, nano boron, critical current density

Abstract Body

FeSe based superconductors became an applicational research hotspot among iron-based superconductors for the merits of their simplest lattice and nontoxic starting materials. However, there exists a big gap between the superconducting properties of FeSe wires/tapes and single crystals, the reason of which is the complicate phase evolution mechanism of Fe-Se binary system, concluding weak-link of intergrain and excess Fe phenomenon. Therefore, a new dual coordination effect has been adopted to solve these problems via exploring an easy and efficiency chemical elements doping.

A dual-oscillation effect was put forward in F-doped Fe(Se,Te) sample to explain the enhancement of flux pinning, which changes the hexagonal phase from harmful phase to effective pinning center and significantly enhances the activation energy. The Cl doping and hydrostatic pressure process were combined to enhance critical current density in polycrystalline Fe(Se,Te). Cl doping could introduce the point-like secondary phase as pinning center and hydrostatic pressure could strengthen the pinning competence of these point defects. As the result, the critical current density was increased up to 100 times via this dual coordination. An easy and efficiency Ag/O co-doping method was adopted for Fe(Se,Te) to enhance the intergrain connection and removing the excess Fe at the same time,. As a result, the critical current density in both self-filed and high field were greatly increased.

This work provided some new solutions to tune the non-superconducting hexagonal phase, improve intergrain connection, and remove the excess Fe in Fe(Se, Te) polycrystalline, which could play an important role in application of FeSe based wires and tapes in the future.

Abstract Body

The superconducting synchronous rotating machines exhibit high torque at low speeds, making them suitable for gearless ship propulsion and wind turbines. Because they have multiple field poles, we want to efficiently magnetize the high-temperature superconducting (HTS) bulk material by pulsed field magnetization (PFM) [1]. However, as is well known, HTS bulk materials trap the magnetic field non-uniformly by PFM and have a low total magnetic flux with a low trapped magnetic flux density. So, improving the trapped magnetic field properties of HTS bulk materials is a key technology to the practical application of superconducting power devices.

The reason why HTS bulk material does not obtain a uniform and large trapped magnetic field distribution by PFM is that the temperature of the bulk material rises significantly as the flux jump occurs. Flux jumps should increase the trapped magnetic flux density because they activate flux motion, but more than that, they degrade the trapped magnetic field properties of the HTS bulk material. It is technically difficult to construct an electrical circuit that generates a voltage of several hundred volts or more and a current of several thousand amperes, which, operating at low losses, generates a strong magnetic field that exceeds the trapped flux density by several times. Therefore, it has been believed that the problem associated with PFM cannot be drastically improved. However, active control of power through advances in power electronics freely shapes the pulsed magnetic field waveform generated by the magnetizing coil with low loss. So-called waveform-controlled pulse magnetization (WCPM) controls the generation of flux jumps by applying a pulse field waveform with a shape different from the conventional LCR transient to the HTS bulk, thereby improving the trapped magnetic field properties [1]. Nevertheless, since the magnetic resistance of the magnetic circuit around the HTS bulk is reduced by the flux jumps generated, the range of conditions for achieving high field trapping in sequence-controlled WCPM is narrow, especially the lower the HTS bulk material is cooled to a low temperature, the more difficult it is to achieve. On the other hand, WCPM with feedback control, although technically challenging, can actively control the occurrence of flux jumps because the applied flux density can be adjusted to match the transient behavior of the magnetic flux in the HTS bulk material. With a negative feedback WCPM using as input the intrusion flux density measured at the surface of the HTS bulk material, we achieved a trapped flux density of about 4 T in a ø45 GdBCO bulk cooled to 40 K [2]. In this presentation, we will show these latest pulse magnetization technologies and their results [3].

References

[1] T. Ida et al., Materials preparation and magnetization of Gd-Ba-Cu-O bulk high-temperature superconductors, Supercond. Sci. Technol. 29 [5] (2016) 054005.
[2] T. Ida et al., Pulse field magnetization of GdBCO without rapid decrease in magnetic flux density, IEEE Trans. Appl. Supercond. 34 [3] (2024) Article#: 6800906.
[3] N. Kawasumi et al., Measurement of dynamic magnetic flux density distribution at the surface of HTS bulk, IEEE Trans. Appl. Supercond. 34 [3] (2024) Article#: 6800604.

Acknowledgment

The authors thank A. A. Caunes, H. Imamichi and N. Kawasumi for their help. This work was supported in part by JSPS KAKENHI under Grant 20K21044 (2020–2022) and Grant 21H01541.

Keywords: PFM, WCPM, GdBCO, Synchronous rotating machine

Abstract Body

Due to their exceptional ability to trap magnetic flux lines, large bulk, melt-textured (RE)Ba₂Cu₃O_7-x superconductors (RE = rare earth element) can be used as powerful permanent magnets. Such permanent magnets are desired in a range of large-scale applications including e.g. rotating machines, magnetic levitation or undulators. The present work focuses on using such magnets for generating large magnetic field gradients and forces over a centimeter scale, a typical size for designing magnetic drug delivery systems [1,2]. Previous works have focused on the field gradients and forces generated by one bulk superconductor magnetized permanently or several bulk superconductors magnetized with parallel magnetization directions [3,4]. Such configurations can be achieved using stationary bulks placed in a time-varying magnetic field having a fixed direction. In the present work we focus on the magnetic field gradient between two bulk superconductors with anti-parallel magnetization directions, as shown schematically in figure 1(a). The goal of our work is to explore experimentally the amplitude of the magnetic field gradient that can be reached in practice in such anti-parallel configuration. The work includes (i) designing an experimental system for magnetizing simultaneously the two bulks at high field and then rotating one of them, (ii) designing the system to be able to measure the flux density distribution between the bulks and (iii) determining experimentally the improvement of the gradient compared to that of a single bulk.

The experimental system relies on a bespoke insertion tool designed [5] to be accommodated in the sample chamber of a commercial Quantum Design (QD) Physical Property Measurement System (PPMS). Unlike commercially available insertion tools (e.g. the QD ‘horizontal rotator’) the system developed in-house allows the rotation of relatively large size samples, e.g. a cube of 6 mm side. The insertion tool combines a driving shaft operating in translation with a rack and pinion system to convert the translation motion into a rotational one. The final mechanism is shown in figure 1(b) and allows for a rotation of 208°, hence enabling the direction of magnetization to be easily reversed by 180°. The mechanical motion is achieved in the vacuum-sealed chamber of the PPMS with low helium gas pressure. Experiments can therefore be carried out at various cryogenic temperatures set by the PPMS.

In order to measure the flux density distribution, a home-made gradient measuring probe was designed, developed and calibrated. The probe contains 13 aligned miniature Hall sensors and is shown in figure 1(b). The connecting wires required for reading the Hall sensor signals are connected to the existing 12-pin connector placed at the bottom of the PPMS sample chamber and an external instrumentation feedthrough. The exact distance between the active areas of the Hall sensors was determined by a calibration experiment carried out prior to placing the measuring probe in the insertion tool. The calibration experiment involved moving a permanent magnet along a straight line and recording the positions of the different sensors.

Experiments were carried out on cubic (RE)Ba₂Cu₃O_7-x bulk superconductors (6 mm side), permanently magnetized using a zero-field cooled (ZFC) procedure [5]. The measured flux density B_z(z) along the line z joining the centers of the faces when the magnetizations are anti-parallel is shown in figure 1(c) at 65 K. Also plotted is the flux density measured when only one bulk sample (either 1 or 2) is used. The anti-parallel configuration is found to result in an increased magnetic flux density gradient ∂B_z(z)/∂z in the region between the bulks. Other experiments were carried out with various DC background fields and at other temperatures.

In summary, this work shows that it is possible to characterize experimentally the magnetic field gradient between two bulk superconductors with anti-parallel magnetization directions, and to investigate the resulting increase in the magnetic flux density gradient. Perspectives using bulk superconductors of larger size will be given. The results of this study are helpful to understand how the rotation of magnetized bulk superconductors can be used to achieve larger gradients and forces for practical applications.

References

[1] A. Arsenault et al. IEEE Trans. Appl. Supercond. 33 4401409 (2023)
[2] Senapati et al. Signal Transduct Target Ther. 3 7 (2018)
[3] F. Mishima et al. IEEE Trans. Appl. Supercond. 17 2303 (2007)
[4] S. Nishijima et al. IEEE Trans. Appl. Supercond. 18 874 (2008)
[5] M. Houbart et al. Supercond. Sci. Technol. 37 095009 (2024)

Acknowledgment

This work was supported by the Fonds de la Recherche Scientifique − FNRS under grant CDR n° J.0218.20 (35325237). Michel Houbart was recipient of a FRS-FNRS Research Fellow grant.

Figure 1. (a) Schematic illustration of magnetized bulk superconductors with anti-parallel magnetization directions. (b) Bespoke insertion tool designed to be inserted in a PPMS system in order to allow the rotation of large magnetized superconducting samples. The magnetic field gradient measuring probe is placed in the gap between the holders for Sample 1 and Sample 2 (c) Distribution of the magnetic flux density B_z at 65 K using either only one bulk sample (Sample 1 or Sample 2) or two bulk superconductors with anti-parallel magnetization directions (Samples 1+2)

Keywords: bulk superconductor, trapped-field magnet, rotating superconductor.