Abstract Body

REBa₂Cu₃O_y (REBaCuO) superconducting bulk materials, where RE denotes Yttrium and rare-earth elements, are subjected to electromagnetic force and thermal stress in devices. As the bulk size and critical current increase, these forces and stresses also rise, making the improvement of mechanical properties, such as fracture strength, crucial for developing high-performance superconducting devices. REBaCuO bulk materials, which are single-grain, are typically fabricated through melt-processing using a seed crystal. Conventional REBaCuO bulk materials often contain pores that degrade mechanical properties. However, a new fabrication technique, called the infiltration growth technique, can produce REBaCuO bulk materials with low porosity.  The mechanical properties of REBaCuO bulk materials fabricated using the infiltration growth technique have not been extensively investigated. In this study, tensile tests were conducted on specimens cut from YBaCuO bulk material fabricated by the infiltration growth technique to evaluate their mechanical properties. The effects of porosity on mechanical properties were examined in relation to the fracture mechanism.

Keywords: YBaCuO, Single-Grain bulk, Infiltration growth, Mechanical properties

Abstract Body

The design technology of high magnetic levitation YBaCuO superconducting bulk material can be immediately applied to superconducting power devices. In order to remove defects such as pores and cracks in the crystal when producing a superconducting bulk magnet and to improve the mechanical and electrical properties of the superconductor, a Y1.6Ba2.3Cu3.3O7-y superconducting bulk magnet with artificial holes was produced using SmBa2Cu3O7-y as a seed and introduced with artificial holes. In addition, the pores are reduced due to the pores, and unlike the bulk without holes, the surface area that supplies oxygen is increased during the oxygen annealing process, and in the structure of the bulk without holes, it is difficult for oxygen to bind to the interior, but through the surface area exposed by the artificial holes, it is easy to bind to oxygen and thus has a structure in which superconductivity is enhanced. This shows that the maximum magnetic levitation force of the bulk without artificial holes of 40 mm size is 45.95 N and that of the bulk with artificial holes is 116.228 N, which shows that the magnetic levitation force increases by 33% due to the nine artificial holes. After creating the artificial holes, the empty holes are bonded with BiPdCd alloy to transmit the physical impact, thereby reinforcing the insufficient mechanical strength. Through this process, it is possible to enhance the durability of the Y1.6Ba2.3Cu3.3O7-y superconducting bulk magnet by dispersing the physical impact exposed to the artificial holes. This research was supported by Korea Electric Power(Grant number: R22XO05-01).

Keywords: superconducting bulk, artificial hole, spike structure

Abstract Body

In a previous study, we achieved a record-high trapped field of B_T=1.61 T at the center of the surface of an MgB₂ bulk composite at 20 K by double-pulsed field applications using a split-type coil [1]. The MgB₂ bulk composite consisted of the stacking of MgB₂ ring bulk sandwiched by two thin oxygen-free copper (OFC) ring plates from both sides and a soft-magnetic iron (yoke) cylinder inserted into the bore of stacking. The yoke is well known to improve a trapped field by the pulsed-field magnetization (PFM), because it slows the intrusion velocity of vortices and then traps them effectively. Although the OFC plates were expected to act as a path of heat generated by the extensive vortex dynamics during PFM, we need to clarify the role of the OFC-plates to achieve the further enhanced trapped field. In this paper, we carried out the simulation study for the trapped field properties of the stacking of the OFC-MgB₂-OFC cylindrical composite and found that the OFC plates did not act as the heat path but disturbed the intrusion of vortices due to the eddy current. The present results are in contrast to those by another group [2], the difference in which we will also discuss in the presentation.

References

[1] T. Hirano et al., Supercond. Sci. Technol. 33 (2020) 085002
[2] V. Cientanni et al., Supercond. Sci. Technol. 34 (2021) 114003

Acknowledgment

This work was partly supported by JSPS KAKENHI Grant Number JP21H01788 and the research grant program of The Futaba Foundation.

Abstract Body

Magnesium diboride (MgB₂) is one of the most promising superconductors, known for its ease of fabrication, tunability, low cost, and light weight. Enhancing its critical current density is essential for its practical applications. A recent study explored the effects of 54 different dopants on the superconducting transition temperature and critical current density of bulk MgB₂ [1]. The study clarified that dopants and additives play a crucial role in improving the performance of bulk MgB₂. In this research, we investigate the impact of combining niobium diboride (NbB₂) with magnesium diboride, as NbB₂ was selected due to its similar lattice parameter to MgB₂ while reducing the c-axis parameter. Samples were prepared through a solid-state sintering process in an argon atmosphere, with a temperature plateau of 775°C for 3 hours. The compositions used were 1Mg + 2B + x(Nb₂O₅), with x ranging from 0 to 0.005. X-ray diffraction analysis confirmed that Nb₂O₅ reacted completely with magnesium to form MgO, along with niobium metal and Nb_1-xB₂. The MgO content in the MgB₂ samples increased from 9.84 wt% in the reference sample to 15.02 wt% in the sample with MgB₂(Nb₂O₅)_0.005. Despite this, all samples, both with and without Nb₂O₅, exhibited a superconducting onset transition temperature of around 37.9 K. The critical current density (J_c), estimated using Bean's critical state model, showed a decrease from 267 to 255 kA/cm² at 20 K in a self-field as the dopant concentration increased. However, with minimal additon, such as in MgB₂(Nb₂O₅)_0.001, J_c remained stable in a self-field at 20 K and improved under an external field of 2 T at 20 K, increasing from 27 to 30 kA/cm². The reduced performance with higher dopant levels may be attributed to the increased MgO content, which could overshadow the benefits of niobium addition.

References

[1] Simon G and Muralidhar M (2024) Alloys and Compounds Communications, 100023.

Keywords: MgB₂, Nb₂O₅ addition, X-ray diffraction, Critical current density

Abstract Body

In this comprehensive study, we enhanced the superconducting properties of bulk MgB₂ using in-situ techniques through three key approaches: boron processing, silver addition, and advanced spark plasma sintering (SPS) techniques. Both boron processing and silver addition were executed using solid-state sintering, while advanced sintering methods were later employed to further optimize the material. Initially, our efforts were concentrated on sieving crystalline boron following ball milling, which reduced powder size and improved uniformity.  This process led to enhanced inter-grain connectivity and microstructural homogeneity, as confirmed by scanning electron microscope (SEM) analysis and contributed to the high purity of the MgB₂ phase, as verified by x-ray diffraction (XRD). As a result, the critical current density (J_c) increased by 116% at 20 K, reaching 156 kA/cm².  In the subsequent phase, silver (Ag) was added into the boron precursor. By optimizing the silver content, we enhance J_c by creating additional pinning centers within the Ag-Mg phase ^[1], as confirmed by XRD results. Incorporating 4.5 wt.% Ag led to the highest J_c values reported for MgB₂, achieving 227 kA/cm² at 20 K and up to 379 kA/cm² at 7 K under self-field conditions.  Finally, we applied spark plasma sintering (SPS) to densify MgB₂ samples. This technique allowed us to achieve 95% of the theoretical density ^[2], resulting in J_c values of 407 kA/cm² at 10 K and 261 kA/cm² at 20 K (see Fig.1).  These improvements represent a 67% improvement over traditional solid-state sintering of undoped MgB₂, demonstrating the significant impact of combining boron processing with SPS in enhancing the superconducting properties of bulk MgB₂.

References

[1] M. Muralidhar, K. Inoue, M. R. Koblischka, A. Murakami, M. Murakami, Effects of silver addition on critical current densities and mechanical properties in bulk MgB², Adv. Eng. Mater. 17 (2015) 831-838.
[2] Y. Xing, P. Bernstein, M. Miryala, J. G. Noudem, High critical current density of nanostructured MgB2 bulk superconductor densified by spark plasma sintering, Nanomaterials. 12(15) (2022) 2583.

Acknowledgment

Malik Shadab, one of the authors, acknowledges SIT for its financial support during his doctoral program.

Figure 1. Comparison of J_c at 20 K for bulk MgB₂ using normal boron precursor versus processed boron, silver addition, and advanced sintering methods.

Keywords: Bulk MgB₂, Ball milling, Sieving, Ag addition, Microstructural analysis, Critical current density (J_c)

Abstract Body

Since bulk magnets can easily generate magnetic fields of 2 tesla or higher, they are being considered for applications such as generators for large wind turbines and electric propulsion systems for aircraft and large ships. Pulsed-field magnetization (PFM) is a practical method for industrial applications because it can magnetize a bulk in any location using general-purpose equipment. On the other hand, since the performance of the bulk is not fully demonstrated in terms of the trapped field, it is necessary to improve the amplitude of trapped field and to simplify magnetization process. In previous studies, it has been found that magnetic flux penetrates from areas with low characteristics and is trapped in areas with high characteristics. However, since recent improvements in bulk fabrication methods have improved the uniformity of the bulk, the trapped field becomes larger if the magnetic flux can penetrate, but strong magnetic shielding makes it difficult for the magnetic flux to penetrate into the bulk. We have focused on the shape of the soft-iron yoke used in PFM and have examined the possibility of improving the magnetization efficiency by changing the shape of the yoke. Originally, our aim was to reduce the weight of equipment by reducing the amount of soft-iron, but we understood that changing the shape of the yoke distorts the applied field distribution, which may affect the magnetic-flux trapping characteristics. This paper investigates numerically how the applied field changes by changing the shape of the soft-iron yoke and how the applied field distribution affects the trapped field characteristics.

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number 24K07458.

Keywords: REBCO bulk, pulsed-field magnetization, soft-iron yoke, applied field

Abstract Body

We have investigated the critical current density (J_C) properties in magnetic fields under high pressure by pressurizing FeTe_XSe_1-X bulk single crystals up to 1.3 GPa. This material has the simplest structure among Fe-based superconductors, and the pressure dependence of critical temperature (T_C) is remarkable. As a sample preparation method, we used a melting method with two-step heat treatment to produce high-quality FeTe_XSe_1-X bulk single crystals. We prepared samples with different composition ratios (X=0.55, 0.6, and 0.65) and checked the change of J_C for each sample using a high-pressure cell. X=0.6 sample showed that J_C was improved by a factor of about 2 with a pressure of 0.24 GPa under 4.2K and 4T. By clarifying the difference in pressure properties depending on the composition ratio, we discuss the flux pinning mechanism of FeTe_XSe_1-X bulk single crystals.

Keywords: Fe-based superconductor, critical current density, flux pinning, high pressure, composition ratios

Abstract Body

The practical order of critical current density J_c, which is 10⁵ A/cm² at 10 T and 4.2 K, for superconducting wires and tapes towards the high-field applications, can be achieved by the iron-based superconductors (IBSs) like hot-pressed Ag-sheathed Ba_1-xK_xFe₂As₂ (Ag-Ba122) [1] and hot-isostatic-pressed Cu/Ag composite-sheathed Ba_1-xK_xFe₂As₂ (Cu/Ag-Ba122) [2] tapes. They prove the development potential and advantages for the IBS tapes fabricating by the low-cost powder-in-tube (PIT) method. However, the extra techniques of hot pressing and hot isostatic pressing during the PIT process increase the complexity and bring the restrictions on fabricating long wires and tapes while greatly improving the J_c. Fortunately, the development of stainless steel/Ag composite-sheathed Ba_1-xK_xFe₂As₂ (SS/Ag-Ba122) tapes, which are led by an innovative fabrication strategy and have made a new record of 2.2 × 10⁵ A/cm² at 10 T and 4.2 K for IBSs [3], give another possibility of large J_c without using hot pressing or hot isostatic pressing. This study investigated the magnetic field and angular dependence of J_c, J_c-B and J_c-θ, for the high-performance SS/Ag-Ba122 tapes, trying to find out the origin of J_c improvement and give suggestions to the future fabrication through showing their special J_c properties comparing to the flat-rolled Ag- and Cu/Ag-sheathed ones.

The J_c-B was measured from 0 T to 18 T (B^inc) and from 18 T to 0 T (B^dec) at both B⊥tape (θ = 0°) and B//tape surface (θ = 90°) as shown in Fig. 1. The magnetic field hysteresis that J_c at B^dec is larger than J_c at B^inc below certain magnetic field B_equal is observed like that in the Ag- and Cu/Ag-Ba122 tapes, but the value of B_equal is only 2~3 T, much lower than 8~10 T for the previous flat-rolled Ba122 tapes. It shows that SS/Ag-Ba122 tapes have relatively small magnetic field hysteresis in J_c, indicating the improved grain boundaries. In case of decreasing B for both angles, J_c drops when B≈ 0 T. In case of increasing B, the J_c behaviors are different and more complicated. At θ = 0°, J_c increases from 0 T to 0.2 T, then turns to reduce until 18 T. At θ = 90°, J_c reversely reduces from 0 T to 0.2 T, then turns to increase to get a peak at 2 T (B_peak) and finally back to reduce until 18 T. The low field J_c behaviors can be explained by Gurevich and Cooley’s theory [4] and have been widely observed in the Cu/Ag-Ba122 tapes in our previous studies, although  the B_peak (~ 6.5 T) is higher. Lower B_peak of SS/Ag-Ba122 also suggests the better connection between grains and a better grain texture.

The J_c-θ was measured from 0° to 90° at magnetic fields 1 ~ 18 T. The angular hysteresis in J_c is negligibly small even at 1 T. The J_c anisotropy, J_c(90°)/ J_c(0°) < 1 is similar to that for the previous high-performance Ba122 tapes. The J_c shows single peak around θ = 0° at all the measured magnetic fields up to 18 T, which are different in the Cu/Ag-sheathed Ba122 tapes [6].

Considering the extremely high-J_c for SS/Ag-Ba122 comparing to the previous Ba122 tapes, the differences and changes in the J_c properties may be related to the factors of the J_c improvement. In the presentation, the comparison among different Ba122 tapes and the detailed analysis will be made and discussed.

References

[1] H. Huang et al., Supercond. Sci. Technol., 31, 015017, 2018.
[2] S. F. Liu et al., Sci. China Mater., 64, 2530-2540, 2021.
[3] Y. W. Ma, EUCAS2023, 4-MO-FM2-03S, 2023. Paper to be published.
[4] A. Gurevich and L.D. Cooley, Phys. Rev. B., 50, 13563, 1994.
[5] J. Y. Luo et al., to be submitted.
[6] J. Y. Luo et al., IEEE Trans. Appl. Supercond. 33, 8200405, 2023.

Figure 1. The magnetic field dependence of J_c at (a) B⊥tape (θ = 0°) and (b) B//tape (θ = 90°) for the SS/Ag-Ba122 tape.

Keywords: Iron-based superconductor, critical current density, anisotropy, hysteresis

Abstract Body

Iron-based superconductors have received extensive attention in superconductivity mechanisms and high-field applications due to their ultra-high upper critical fields and low anisotropy. A major goal in the application research of iron-based superconductors is the fabrication of superconducting wires and tapes with high critical current density (J_c) and low cost. The use of Cu/Ag composite sheath can reduce the use of expensive silver, thereby reducing the cost of the tapes. In practical research, it was found that the content of K element has a great influence on the transport J_c of Cu/Ag composite tapes. In this study, the effects of K content on the transport J_c were systematically studied by adding additional K element in the preparation of (Ba, K) Fe₂As₂ precursor powder. The highest transport J_c was measured for Cu/Ag composite sheathed tapes prepared using precursor powders with a K ratio of 0.46 (K-0.46) at initial K ratios ranging from 0.42 to 0.54. K-0.46 tape sintered at 740 °C for 3 h achieved a critical current density of 5.8 × 10⁴ A cm^-2 at 10 T and 4.2 K. The properties of tapes with different K contents were characterized, including grain crystallinity, core density, c-axis texture and elemental distribution. The results show that that the precursor powder with a K ratio of 0.46 is the best choice for the preparation of high-performance Cu/Ag composite tapes.

Figure 1. Flow chart for tape preparation

Keywords: Iron-based superconductors, content of K element, critical current density, pre-composite, Cu/Ag composite tapes

Abstract Body

Iron-based superconductor (IBS) is one of the promising candidates for high field superconducting magnets, due to their high upper critical field and low anisotropy. For large-scale accelerator magnets, a compact cable containing tens or hundreds of tapes with high current carrying compacity is necessary. Recently, the IBS cable has been successfully fabricated at the Institution of High Energy Physics, Chinese Academy of Sciences (IHEP, CAS). The cable’s structure is similar to the conventional Roebel cable, but it is implemented by in-plane bending of stacked IBS tapes without punching. There are nine strands transposed in the cable, and each strand contains four seven-filamentary IBS tapes with three stainless steel tape in the middle to increase the mechanical property. The length of the cable is about 6 m, and the current carrying compacity of a short sample which is cut from the cable is tested at different magnetic fields and 4.2 K. The cable’s parameters, fabrication process, performance test, as well as results analysis are reported in this paper.

Abstract Body

We have made Magnesium diboride (MgB₂) wires by decomposing Mg(BH₄)₂ with Mg and B as precursors.  MgB₂ is an intermetallic compound with a superconducting critical temperature (T_c) of 39 K, enabling MgB₂ superconductor to be applied into large-scale applications around 20 K, liquid helium-free MRI. Mg(BH₄)₂, as a promising storage hydrogen material, can decompose into nanoscale Mg, amorphous B powders and nanosize the MgB₂ grains. These nanoscale powders reduce the porosity in MgB₂ filament and form MgB₂ nanograin, which could improve its grain boundary pinning and J_c. Herein, different ratios (0% ~ 100%) of decomposition products of Mg(BH₄)₂ powders mixed with Mg and amorphous B powders are innovatively used as precursors. After heat treatment, the phase, microstructure and superconducting properties of these MgB₂ wires were investigated to reveal the variations introduced by different proportion of decompositions. The J_c of these wires increase at first and then decrease with the increase of proportions, verifying that the decomposed Mg(BH₄)₂ effectively improve the superconducting transport performance of MgB₂ wires.

Abstract Body

REBa₂Cu₃O_y (RE=rare earth, Y, Gd, etc., REBCO) coated conductors are narrowed and twisted to reduce the AC losses [1]. Since it is practically important to investigate the mechanical properties, such as bending and twisting, of the critical current (I_c) for such wires, we have been studying the effect of torsion on I_c. In this study, we have investigated the in-plane critical current density (J_c) distribution of REBCO coated conductors (CCs) with low I_c due to torsion and observed the microstructure by Scanning Electron Microscopy (SEM) to confirm the relation between the defect structure and I_c. For the I_c measurement for 2 mm-width REBCO CCs, we cut it 100 mm in length and soldered it with copper wire for voltage leads. The REBCO CCs were then clamped using a pair of Cu blocks, which were used as current leads. The torsion angle was applied by rotating the upper Cu block manually. We measured current-voltage curves by the four-probe method using pulse current, ensuring accurate results and I_c estimation. The measurement was terminated when I_c reached approximately 40% of the initial I_c. Then, to identify the location of I_c reduction, trapped magnetic field distribution and in-plane J_c distribution were measured by Scanning Hall-probe Microscopy. After that, the surface microstructures were observed by SEM to confirm whether defects equivalent to the percentage of I_c reduction were generated. As a result, longitudinal cracks up to 180 µm in the width direction at the tape edge were observed, and the cracks are connected to each other. Exfoliation of crystal grains also confirmed. Due to these defects, the effective width in the tape-width direction was 800 µm at the narrowest part, which was about 40% of the 2 mm-width of the REBCO coated conductors. This is comparable to the I_c reduction rate, suggesting that the continuous cracks and exfoliation cause I_c reduction. The details will be discussed.

References

[1] N. Amemiya, “AC Losses in Tc Superconductors-AC Loss Characteristics of Tapes and Power Transmission Cables-” J. Cryo. Soc. Jpn. Vol. 45 （2010） 376

Acknowledgment

The results of this research were obtained as a result of NEDO's commissioned work and JSPS Grant-in-Aid for Scientific Research JP22H02021, JP22H02019.

Keywords: REBa₂Cu₃O_y, Critical current, Torsion dependence, In-plane J_c distribution

Abstract Body

REBa₂Cu₃O_y (REBCO, RE = Rare earth) high-temperature superconducting tapes have high critical temperature and high critical current density (J_c) characteristics in magnetic fields and are expected to be applied to superconducting devices such as high-field magnets and superconducting motors. Since local defects limit the J_c property in REBCO tapes, in which the superconducting layer is formed as a thin film with a few micrometers thickness, it is important to clarify the cause of the J_c limitation in the superconducting thin films. Therefore, we have been studying magneto-optical imaging (MOI), which has high spatial resolution among magnetic field distribution measurements. In this study, to evaluate the temperature and magnetic field dependence of the in-plane J_c distribution in the REBCO thin film with metal stripes, we obtained and analyzed magneto-optical images acquired under various conditions using MOI. We observed the trapped magnetic field distribution for a 5 mm × 5 mm superconducting thin films with metal stripes that divide the film into five sections in the plane. The number of pixels for MOI observation was 2048 × 2048 pixel and the signal depth was 16 bits. MOI observations were performed at each external magnetic field by cooled the sample to a predetermined temperature. The external magnetic field was varied from 0 T to 0.42 T in 0.02 T intervals and then demagnetized to 0 T. First, we considered the calibration for the conversion from signal intensity to magnetic field. As a result, the obtained magnetic field distribution was confirmed to be in good agreement with the theoretical one. Then we calculated the J_c distribution from the magnetic field distribution obtained by the experiment. The J_c was confirmed to be in general agreement with the values calculated from the theoretical formula of Brandt et al [1][2]. Those results indicated that the five filaments were electrically separated and the in-plane J_c was highly uniform without local defects. The temperature and magnetic field dependence of the J_c will be discussed.

References

[1] E. H. Brandt, et al., Europhys. Lett. 22 (1993) 735
[2] Y. Mawatari, J. Cryo. Soc. Jpn 44 (2009) 2

Acknowledgment

This work was supported in part by the Joint Research Hub Program of NIMS and by JSPS Grants-in-Aid for Scientific Research JP22H02019 and JP22H02021.

Keywords: Magneto-Optical Imaging, REBa₂Cu₃O_y, Critical current density distribution

Abstract Body

The depairing current density J_d is the theoretical limit of the current density that can flow in the superconducting state [1]. Although the typically observed critical current density is often considerably smaller than J_d, critical current densities comparable to J_d have actually been observed in superconducting nanostrips with thin thickness and narrow width.

In this study, we numerically investigate the dependence of the critical current density J_c in type-II superconducting films on the magnetic field B_a and on the film thickness d_s. Here, the applied magnetic field is parallel to the film surfaces, and we consider the case of longitudinal (transverse) magnetic field parallel (perpendicular) to the transport current. We restrict to the case of extreme type-II superconductors (λξ ≫ 1, where λ is the penetration length and ξ is the coherence length) in the Meissner state where no quantized magnetic flux exists, and we numerically solve the nonlinear London equation derived from the one-dimensional Ginzburg-Landau equation [2].

The dependence of the critical current density J_c on the longitudinal magnetic fields B_a for various film thicknesses d_s is shown in Fig. 1. For thin films (d_s ≲ λ), J_c is close to J_d near B_a=0 and decreases nonlinearly and slowly with increasing B_a. For thick films (d_s ≫ λ), on the other hand, J_c < J_d at B_a= 0, and J_c decreases rapidly with increasing B_a. The B_a dependence of J_c for d_s≫λ fits well with the theoretical J_c=J_c0 [1-(B_a / B_c)² ]^1/2. Where J_c0 is the zero-field critical current density and B_c is the thermodynamic critical magnetic field.

References

[1] V. L. Ginzburg and L. D. Landau, Zh. Eksp. Teor. Fiz. 20, 1064 (1950).
[2] P. G. de Gennes, Solid Stat. Commun. 3, 127 (1965).

Acknowledgment

This work has been supported by JSPS KAKENHI, GrantNo. JP20K05314.

Fig. 1 Dependence of the critical current density J_c on the longitudinal magnetic field B_a, (a) where J_c is scaled by J_d and B_a is scaled by B_c and (b) where J_c is scaled by J_c0 and B_a is scaled by the superheating field B_sh

Abstract Body

The REBCO (Rare-Earth BCO) tapes have the ability to carry a high current under a high magnetic field. Their use makes it possible to realize superconducting magnets used to generate steady ultra-high magnetic field. For example, a 40 T magnet is under development in the United States [1] and a 33 T Cryogen-free Superconducting Magnet (CSM) is currently built in Japan [2]. Both projects use REBCO tapes to realize the innermost coil of the magnets. To design these systems, the Ic evaluation is required under such high magnetic field and beyond because the magnet itself experiences a higher magnetic field than the nominal magnetic field.

Obtaining these data is a challenge. On one hand, the sample space inside steady magnets generating more than 20 T is generally highly constrained, making it unpractical to measure Ic for full width REBCO tapes due to the excessive losses from carrying significantly high transport current. On the other hand, the available DC magnetic field in user magnet is also limited to 45T [3][4].

To measure Ic of 4 mm wide REBCO tapes under ultra-high magnetic field at low temperature, we propose to use a 1 kA pulsed current measurement system [5] under a pulsed magnetic field [6] with an objective greater than 30 T. The novelty of this work is the application of the combination of pulsed current measurement under pulsed magnetic field for REBCO tapes with high current capacity[7][8][9]. This contribution presents a detailed description of the complete system with a special focus on the sample preparation, noise reduction, synchronization between measurement and pulsed magnetic field generation.

References

[1] H. Bai et al., "The 40 T Superconducting Magnet Project at the National High Magnetic Field Laboratory," in IEEE Transactions on Applied Superconductivity, vol. 30, no. 4, pp. 1-5, June 2020, Art no. 4300405, doi: 10.1109/TASC.2020.2969642

[2] Badel et al., "Conceptual Design of a 33 T Cryogen-Free Magnet REBCO Insert: Mechanical Aspects and Protection Against Thermal Runaway," in IEEE Transactions on Applied Superconductivity, vol. 34, no. 5, pp. 1-5, Aug. 2024, Art no. 4301205, doi: 10.1109/TASC.2024.3367620.

[3] M.D. Bird, S. Bole, I. Dixon, Y.M. Eyssa, B.J. Gao, H.J. Schneider-Muntau, The 45T hybrid insert: recent achievements, Physica B: Condensed Matter, Volumes 294–295, 2001, Pages 639-642, ISSN 0921-4526, https://doi.org/10.1016/S0921-4526(00)00734-1.

[4] Z. Fang et al., "Test Operations of the Upgraded Hybrid Magnet at CHMFL," in IEEE Transactions on Applied Superconductivity, vol. 34, no. 5, pp. 1-5, Aug. 2024, Art no. 4300705, doi: 10.1109/TASC.2024.3350581

[5] Y. Tsuchiya, I. Sakai, K. Mizuno, Y. Kohama, Y. Yoshida and S. Awaji, "Critical Current Measurements of HTS Tapes Using Pulsed Current in High Fields at Low Temperatures," in IEEE Transactions on Applied Superconductivity, vol. 33, no. 5, pp. 1-5, Aug. 2023, Art no. 8001105, doi: 10.1109/TASC.2023.3261265.

[6] Kohama, Yoshimitsu, and Koichi Kindo. "Generation of flat-top pulsed magnetic fields with feedback control approach." Review of Scientific Instruments 86.10 (2015).

[7] Wozniak, Mariusz & Hopkins, Simon & Glowacki, B. A.. (2012). Characterisation of a MgB2 wire using different current pulse shapes in a pulsed magnetic field. Przeglad Elektrotechniczny. 88. 29-31.

[8] K Rogacki et al 2002 Supercond. Sci. Technol. 15 1151

[9] Maxime Leroux, Fedor F. Balakirev, Masashi Miura, Kouki Agatsuma, Leonardo Civale, and Boris Maiorov, Phys. Rev. Applied 11, 054005

Acknowledgment

NEDO, Kakenhi, EMIC, Chemicon

Keywords: HTS tapes, critical current, pulsed current measurement, pulsed magnetic field

Abstract Body

In order to improve the flux pinning properties of GdBa₂Cu₃O_7-δ(Gd123) superconductors, the introduction of BaMO₃ (BMO, M=Hf, Zr, etc.) as artificial pinning centers (APCs) is widely known [1][2]. However, because of the large lattice misfit of BaMO₃ with the base material and its tendency to coarsen the crystals, the c-axis of Gd123 is not oriented when more than 3 vol% of BaMO₃ is introduced. In this study, we tried to introduce the superconductor Gd₂CuO₄(Gd214) (T_C=18 K) as an APC, which has not been reported before. The reason for this is that Gd214 has a smaller lattice misfit with the base material, so more APCs can be expected to be introduced compared to the conventional BaMO₃. As a result, we succeeded in introducing 6.23 vol% of Gd214 without any loss of crystallinity of the base material, and the maximum J_C at 4.2 K was obtained in the film with 2.02 vol% of Gd214, with values of 8.13 MA/cm² at 0.5 T and 3.05 MA/cm² at 3 T. The values were 1.99 and 2.21 times higher than those of the non-doped film, respectively.

References

[1] T. Yoshihara et al., “BaMO₃ (M = Zr, Hf) Doped REBCO Tapes Fabricated by Fluorine-Free MOD,” IEEE Trans. Appl. Supercond., vol. 33, no, 5, Aug. 2023, Art. no. 6600205.

[2] N. Kobayashi, O. Miura and, R. Kita, "Improvement of flux pinning properties for hafnium-doped Gd123 thin films fabricated by fluorine-free MOD method", IEEE Trans. Appl. Supercond., vol. 27, no. 4, Jun. 2017, Art. no. 8001104.

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number JP 24KJ1858

Keywords: RE123, BMO, fluorine-free metal organic deposition, critical current density, high-J_C superconductor.

Abstract Body

We have fabricated Gd123 thin films co-doped with several kind of RE₂CuO₄ (RE214: RE= Gd, Nd, Sm, Eu) by the FF-MOD method and investigated their superconducting properties. We will report that the introduction of the superconductor RE214 (T_C=18.5 K) as an APC into RE123 improved the J_C-B properties for the first time [1]. In this study, in order to improve the J_C-B properties, we tried to refine the RE214 crystals by co-doping several kinds of RE214 having different lattice constants and growing them independently in the base material. As a result, for the same volume fraction introduction, the J_C-B property improved with increasing the kind of RE214 introduced at the same time, and all films were improved over non-doped films. At 4.2K and 6.9T, the  was 0.782 MA/cm² with the addition of Sm214 alone, and 0.877 MA/cm² with the addition of three types (Gd214, Nd214, Sm214), representing increases of 1.07 times and 1.19 times, respectively, compared to the non-doped film. Moving forward, we expect further improvement in superconducting properties by altering the combinations of  in, their doping ratios, and the sintering conditions.

References

[1] R.Ishii, K.Sato, O.Miura,” Improvement of J_C-B properties by co-doping with optimized combinations of BaMO3 (M=Sn, Zr, Ce) for FF-MOD Gd123 thin film”, The Applied Superconductivity Conference2024, Utah(U.S.A), September 2024.

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number JP 24KJ1858

Keywords: RE123,RE214, fluorine-free metal organic deposition, critical current density, high-T_C superconductor.

Abstract Body

To improve the J_C-B properties of FF-MOD RE123 thin films, it is effective to introduce the normal-conducting precipitate BaMO₃ (M= Ce, Zr, Hf, etc.) as an artificial pinning centers by doping elemental M into the starting solution. Furthermore, it has been reported that by co-doping with different M simultaneously, BMO having different lattice constants is independently introduced into the base material, resulting in refinement and better J_C-B properties than when single M doping. In this study, we combined this M-co-doping strategy with the introduction of Gd214 pin, which we recently reported its effectiveness.

Abstract Body

REBa₂Cu₃O_y (RE=Rare Earth, REBCO) superconducting tapes are being studied to narrow the tape width to reduce AC losses. Typically, mechanical cutting with a slitting blade and laser cutting are used. However, these methods are processed after the superconducting thin films have been deposited, so there is a risk of tens of micrometers or more damage to the superconducting layers during cutting. Therefore, it is not easy to reduce the separating distances between filaments, which are dead spaces, at the micrometer-level. We have carried out a new method to separate the superconducting films by forming metal stripes of a few µm-width on the substrate before the deposition of superconducting films, where the metallic region acts as an electrically barrier region. In this study, we formed the Zr-stripes with micrometer-level width and investigated their separation state. The Zr-stripe was formed using the photolithography technique and spattering method. The widths at half maximum of the formed Zr-stripes are approximately 4.8 and 2.9 µm. YBa₂Cu₃O_x (YBCO) was then deposited on these substrates by the pulsed laser deposition. Surface observation of both samples by scanning electron microscopy shows lattice-like precipitates above the SrTiO₃ substrate, which may reflect the biaxial orientation of the YBCO. On the other hand, large crystal grains were formed around the Zr-stripe. From the results of the trapped magnetic field distribution by the scanning hall-probe microscope, the magnetic field strength of the metallic region appears to be low, and superconducting areas are separated. DC magnetization measurements by a Physical Property Measurement System showed that both samples were about 0.1 emu in self-field and 60 K, which is about 1/7 of the YBCO thin films without Zr-stripe. This confirms that it is magnetically separated by the Zr-stripe of a few µm-width. The current-carrying properties of the Zr-stripe region are also discussed.

Acknowledgment

This work was supported in part by the Joint Research Hub Program of NIMS and by JSPS Grants-in-Aid for Scientific Research JP22H2019 and JP22H02021.

Keywords: Superconducting tape, REBa₂Cu₃O_y, Zr-stripe

Abstract Body

Narrowing REBa₂Cu₃O_y (REBCO) tapes is effective in reducing AC losses, and mechanical cutting with a slit blade or laser cutting is commonly used.However, these narrowing methods may cause damage to the superconducting layer near the cutting edge. Therefore, we have focused on a separation method by patterning metal stripes on the substrate prior to the REBCO deposition process.In this study, to confirm the effect of Zr on the separation of superconducting films, we formed a wide-width Zr stripe and investigated the superconducting properties near the Zr. Specifically, we formed a Zr stripe with 106 µm in width and 550 nm in height, on a 5 mm square SrTiO₃ substrate by photolithography and sputtering.Then, YBCO films were deposited on the Zr-formed substrate using the pulsed laser deposition method.

First, we observed the trapped magnetic field distribution by Scanning Hall-Probe Microscopy to confirm the separation of the magnetization in the films. The result showed that the magnetic field strength was significantly reduced in the region where Zr was expected to be located.

After that, we fabricated microbridges and measured I-V characteristics using a Physical Property Measurement System. The microbridges of 200 µm in width and 500 µm in length were fabricated perpendicular to the Zr stripe. As a comparison, microbridges without Zr stripe were also fabricated in the same size. The results showed that the I-V characteristics of the microbridge passing over the Zr stripe showed the superconducting transition below T_c,although the I_c was lower than that of the microbridge without Zr stripe. This suggests that a superconducting matrix is formed on the Zr.In addition, temperature and magnetic field dependence of I_c showed that the differences in I_c values of the bridges with and without Zr stripe were more than four times at low magnetic fields, and the differences became smaller at high magnetic fields, indicating that the irreversible magnetic fields were almost the same.This result seems to be consistent with the report [1] that the critical current at low magnetic field decreases as the tilt angle of grain boundaries increases.Therefore, it is possible that REBCO deposited on Zr stripe may be crystalline oriented and connected by low-tilt grain boundaries. The details of the results will be discussed.

References

[1] H. Hilgenkamp et al.; “Grain boundaries in high-T_c superconductors” Rev. Mod. Phys., Vol. 74, No. 2, April 2002

Acknowledgment

This work was supported in part by the Joint Research Hub Program of NIMS and by JSPS Grants-in-Aid for Scientific Research JP22H2019 and JP22H02021.

Keywords: REBa₂Cu₃O_y, Photolithography, Zirconium-stripe, Microbridge

Abstract Body

AC loss is one of the most crucial issues in the high-temperature superconducting (HTS) cables which carry large currents. In recent years, various numerical simulation techniques have been implemented to investigate the AC loss of the HTS cables under applied field and transport current, such as H formulation, T-A formulation, and J integral formulation. However, the H formulation has a problem that it is not easy to set the boundary conditions imposing the transport current in 3D simulation, while the T-A formulation simulates some 3D complex geometries, such as CORC cables, because of the appearance of small gaps, its computational efficiency is comparable to that of H formulation. But the integral method can avoid these two problems very effectively because the air domain is not required. In this paper, we use the T integral formulation with the current density vector T as the state variable, which has fewer unknowns compared to the J integral formulation. However, the integral methods have been considered for a long time to be inappropriate for large-scale problems, due to the dense matrices. To solve this problem, this paper applies the fast multipole method (FMM) to accelerate the T integral formulation, which avoids the generation of dense matrices and improves the computational speed. This paper first compares the computational efficiency of the T integral formulation and the T-A formulation through the 2D interface of the CORC cable and verifies the effectiveness of the T integral formulation in large-scale problems with the help of the FMM. Then, the approach is extended to AC loss calculation for the 3D HTS cables.