Bulk superconductors can trap high magnetic fields exceeding that of conventional permanent magnets as a manifestation of a macroscopic quantum effect. In recent years, a new record trapped magnetic field has been achieved, and the applicability to various devices has been vigorously studied. In this review, the trends in the research and development of superconducting bulk materials, including flux pinning control and mechanical reinforcement for REBCO, magnetization techniques, new materials (MgB2 and IBSC), and modeling and simulation, are summarized.

This is a viewpoint on the letter by S. Namba et al 2019 (Supercond. Sci. Technol. 32 12LT03).

This paper describes the development of RE-Ba-Cu-O (RE:Y or rare earth elements) bulk superconductors (“bulks”). HTS bulks have a great potential to pioneer a new application field of superconductors. The high-quality HTS bulks developed by Nippon Steel Corporation are called “QMG”, which is a single-grained material of REBa2Cu3Ox (RE123) with RE2BaCuO5 (RE211) inclusions finely dispersed in the superconducting matrix. The name QMG originally comes from the initial process, “Quench and Melt-Growth”. However, no quench step is required in the advanced QMG process, and thus “Q” now means “Quality” instead of “Quench”. The key point of QMG technologies is the simultaneous control of the crystal growth and the micro-structure. Although it is believed to be very difficult to produce large single-grained HTS bulks exceeding 80 mm in diameter, we have succeeded in enlarging QMG up to 150 mm using the RE compositional gradient method. These ultra-large HTS bulks have been used for the superconducting magnetic bearing (SMB) of the flywheel energy storage system in Yamanashi, Japan, contributing to non-contact support of a heavy 4,000 kg rotating flywheel. In addition to SMB, other promising application are HTS bulk magnets for ship propulsion motors, wind or tidal power generators, and desktop cryogen-free NMR/MRI. HTS bulks are also an ideal material for current leads. Different bulk properties are required for different applications. We have therefore customized QMG bulks depending on application requirements: Dy-QMG for current leads, Eu-QMG for NMR/MRI, and Gd-QMG for other HTS bulk applications.

Recent progress in the application of the oxide bulk superconductor (QMG TM) has been reported. QMG is the high-Tc bulk superconductor originally developed by Nippon Steel & Sumitomo Metal Corporation. Regarding the ring-shaped QMG bulk magnet, we describe the generation of a 10 T class high magnetic field that was made possible by developing new reinforcement technology. QMG consisting of a RE (rare earth element) -based oxide superconducting material as the main component now enables the production of large high-performance materials by single crystallization techniques using the RE substitution effect. As a result, although it had potentially high superconducting properties of a bulk magnet functioning as a 10 T class powerful magnet, the material may become cracked due to the electromagnetic force generated when magnetized. By developing a new reinforcing method, we succeeded in preventing this cracking and generated a high magnetic field of more than 10 T. 1. Preface About 80% of the superconductivity technology market is constituted by magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) spectroscopy, and it is a strong magnetic field application that reaches several tesla by the superconducting magnets used in these devices. 1) After succeeding in liquefying helium in 1908, Heike Kamerlingh Onnes observed superconductivity for the first time at 4.2 K in his research using mercury and the ultralow temperature liquid helium as a refrigerant in 1911. As many researchers do today, he is said to have been working on the use of superconducting materials for the production of wire and the application to magnets, as part of the application of the superconductivity phenomenon involving the complete disappearance of materials' electrical resistance. 2) At present, MRI and NMR spectroscopy both using superconducting magnets are essential for diagnosis systems used in the medical field and analyzing systems in the drug discovery, analysis, and other fields, respectively. In this sense, superconducting magnets are everywhere now, having penetrated society. Since the first observation of superconductivity, along with the increase in practical application of the phenomenon, the tenacious search for superconductors that exhibit superconductivity at higher temperatures has made progress. Following the discovery of an oxide superconductor by Bednorz and Müller in 1986, high-temperature superconducting substances that exhibit superconductivity at the boiling point of liquid nitrogen (77 K) were found one after another over the next couple of years. 3–5) With such development, Nippon Steel & Sumitomo Metal Corporation started the R&D of oxide superconducting materials. We promoted the R&D with our focus on not only the search for materials with higher critical temperatures (critical temperature: temperature at which materials exhibit the state of superconductivity [Tc]) than those of already discovered types, but also on properties involved in critical current density (Jc: maximum current value that can flow while the state of superconductivity is maintained), which is one of the most important parameters for the application of a superconducting material. This effort succeeded in producing a single crystal superconducting material, named QMG TM, excellent in the Jc property. 6, 7) This material, with a critical temperature of approx. 90 K, is a compound oxide consisting of yttrium (Y) and other rare earth elements (REs), barium (Ba), and copper (Cu). Its main component is a superconducting substance expressed as REBa2Cu3O7–x. As shown in Fig. 1, the QMG bulk microstructure is characterized by the approx. 1-μm insulator particles represented by RE2BaCuO5(211) dispersed in the single crystal phase of REBa2Cu3O7–x(123). Figure 2 * Senior Researcher, Dr. Eng., Materials Research Lab., Advanced Technology Research Laboratories 20-1 Shintomi, Futtsu City, Chiba Pref. 293-8511 NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT No. 117 DECEMbER 2017 37 shows the appearance of a QMG bulk as a monocrystalline solid in which the 123 phase is grown from a seed crystal. On the surface of a QMG bulk when it has just fully grown, crystal habit lines with four-fold rotational symmetry extending from the seed crystal can be observed; this is a trace of the growth generally progressed under the condition that the seed crystal is put in contact with axis c such that the seed crystal's normal line and axis c are in the same direction. As described above, the QMG bulk has both macrostructural and microstructural characteristics: a monocrystalline material without grain boundaries, which prevent the flow of the superconducting current, constituting the material and dispersed fine 211 particles responsible for the magnetic flux pinning function, respectively. The product name of “QMG” derives from the Quench and Melt Growth method, which was used when the first microstructure was produced. 7, 8) After the first production, an improved Quench and Melt Growth method involving the composition substitution for REs and the use of seed crystals with a higher decomposition temperature enabled the production of a monocrystalline material, thus establishing the basic production method. 9–11) With the excellent Jc property as indicated by the material structure characteristics described above, R&D to expand the application of QMG is currently underway. In addition to the current lead 12) that has already been put into practical use and the magnetic bearing member 13) for power storage flywheels that is now at the verification testing stage, the use of QMG to produce bulk magnets 14) is making progress; the method involves passing permanent currents in a QMG bulk to trap magnetic fluxes in it, thereby forcing the QMG bulk to function as a permanent magnet. During the earliest days of the bulk magnet application development, the low temperature environment had been created using liquid nitrogen. In recent years, the advancement of refrigerator technologies has allowed a small refrigerator to cool QMG bulk to low temperatures at 50 K or below. By magnetizing the cooled QMG bulk to force it to have a strong magnetic field from several tesla to over 10 tesla, a compact magnet with strong magnetic field (permanent magnet) can be produced. 15–17) Figure 3 shows the principle of magnetization. After placing a QMG bulk in a magnetic field in the normal conducting state, the QMG bulk is cooled to undergo transition into the superconducting state. Then, superconducting currents are electromagnetically induced in the QMG bulk by reduction of the external magnetic field. The QMG bulk is magnetized by the permanent currents. Since the Jc property of QMG is sufficiently high, a strong magnetic field exceeding 10 tesla can be generated. However, QMG does not have sufficient strength as a magnet. Bulk magnet cracking due to the large electromagnetic force applied for magnetization has been an issue of QMG. Such material cracking due to magnetizing using a powerful magnetic field was first reported in association with a magnetization test during which a strong magnetic field was used to create a superconducting magnet at 40 K. 18) Since then, a method to prevent cracking by containing the electromagnetic force has been developed. 19) The method involves precision machining of the circumference as enabled by the advancement of machining technologies and adhering a metal ring to the material in order to make the material fit in the metal ring at room temperature. When being cooled, the ring shrinks to suppress the electromagnetic force, thereby preventing the material from cracking. Although it depends on the size of samples, for materials with a 60-mm outside diameter, this reinforcement method enabled magnetization at the 5 tesla level. NeverFig. 1 Microstructure of QMG Fig. 2 Appearance of QMG Fig. 3 Magnetization procedure of QMG bulk magnet

This paper describes the recent progress in QMG bulks in Nippon Steel and Sumitomo Metal Corporation. QMG is a high-Jc bulk material, which consists of a single crystalline RE123 phase with finely dispersed RE211 particles. QMG bulks have a significant potential for high-field engineering applications, since they can trap high magnetic fields depending on their J c and size. Gd-QMG bulks are very attractive for various bulk applications such as flywheel energy storage systems, ship motors, wind power generators, etc. The trapped magnetic field of Gd-QMG bulks 60 mm in diameter at 77 K is twice as large as that of Y-QMG bulks with a similar size due to their excellent J c properties. An enlargement in the diameter of the bulk leads to the enhancement of the trapped magnetic field. The large Gd-based QMG bulks up to 150 mm in diameter were fabricated by incorporating the RE compositional gradient method. Compact nuclear magnetic resonance (NMR)/MRI spectrometers are one of the promising applications of bulk superconductors. Eu-QMG bulks are suitable for NMR magnets. NMR applications require extremely homogenous magnetic fields. In the Eu-system, the small paramagnetic moment of the Eu ion compared to the Gd ion improves the field homogeneity in the bulk.

Recent studies have shown that ferromagnetic materials can be used together with bulk high temperature superconductors in order to improve their magnetic trapped field. Remarkably, it has also been pointed out that ferromagnets can help in reducing the crossed field effect, namely the magnetization decay that is observed under the application of AC transverse magnetic fields. In this work, we pursue a detailed study of the influence of the geometry of the ferromagnetic part on both trapped fields and crossed field effects. The magnetic properties of the hybrid superconducting/soft ferromagnetic structures are characterized by measuring the magnetic moment with a bespoke magnetometer and the local magnetic field density with Hall probes. The results are interpreted by means of 2D and 3D numerical models yielding the distribution of the superconducting currents as a function of the ferromagnet geometry. We examine in details the distortion of the shielding superconducting currents distribution in hybrid structures subjected to crossed magnetic fields. These results confirm the existence of an optimum thickness of the ferromagnet, which depends on the saturation magnetization of the ferromagnetic material and the current density of the superconductor. A hybrid structure providing an efficient protection against the crossed magnetic field while maintaining the magnetic induction along the axis of the structure is suggested. The limitations of the 2D modelling in this configuration are discussed.

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. When such superconducting (SC) "trapped field magnets" are combined to a ferromagnetic (FM) disc, the total magnetic moment is increased with respect to that of the superconductor alone. In the present work, we study experimentally the magnetic behaviour of such hybrid FM/SC structures when they are subjected to cycles of applied field that are orthogonal to their permanent magnetization, i.e. a "crossed-field" configuration. Experimental results show that the usual "crossed-field demagnetization" caused by the cycles of transverse field is strongly reduced in the presence of the ferromagnet.

High-temperature superconducting (HTS) bulk materials of RE-Ba-Cu-O (RE: Y or rare-earth elements) exhibit excellent J c properties, even at 77 K, leading to current leads with large current capacity and superior magnetic field tolerance. The thermal conductivity of HTS bulk materials is two orders of magnitude smaller than that of the conventional current lead material of copper, and Dy-Ba-Cu-O has particularly low thermal conductivity in the RE-Ba-Cu-O family. Unlike HTS wires, HTS bulks have no metal sheath such as silver or silver alloys with high thermal conductivity, resulting in the reduction of heat leak through the current lead, even in the compact size. Although they are a kind of brittle ceramic or oxide, HTS bulks can be transformed into robust current leads by reinforcing them with the support cover made of glass-fiber-reinforced plastics with low thermal conductivity and high strength. In addition to excellent mechanical stress tolerance, HTS bulk current leads also have a robust structure that minimizes thermal stress, which was experimentally demonstrated by a rapid cooling test to liquid nitrogen over 100 cycles. Due to distinctive advantages such as compactness, robustness, and superior field tolerance, more than 500 pieces of HTS bulk current leads have been successfully provided so far.

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 J c 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 J c 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.

In order to develop a compact cryogen-free 4.7-T (200 MHz) NMR magnet, we investigated methods of obtaining a homogeneous magnetic field distribution inside REBCO bulks. In this study we successfully fabricated large single-domain annular GdBCO bulk superconductors 60 mm in diameter and up to 60 mm thick. We also fabricated a large EuBCO bulk 60 mm in diameter, and evaluated the performance of these annular bulks in terms of the NMR magnet. It was found that enhancing Jc-B, increasing bulk thickness and employing an Eu element showing low magnetic permeability were effective in increasing the homogeneity of the magnetic field distribution. Furthermore, it was confirmed that a homogeneous area of magnetic distribution was obtained even at a higher field magnetization of 7 T (300 MHz) at 40 K for stacked GdBCO bulk superconductors 60 mm in diameter and 60 mm thick. Finally, several issues relating to further increasing the trapped magnetic field were discussed.

A compact cryogen-free NMR magnet was developed using annular bulk superconductors. The superconducting bulk magnet is energized using a field-cooling (FC) method operated at 4.7 T. Two magnets with different configurations are proposed and investigated. One magnet consists of three GdBaCuO and two SmBaCuO bulks with outer diameters (OD) of 60 mm, inner diameters (ID) of 16 mm and a total height of 60 mm. The GdBaCuO bulks that have higher relative magnetic permeability (1.01) are sandwiched between the SmBaCuO bulks that have lower permeability (1.0003). The other magnet consists of four EuBaCuO bulks with a permeability of 1.001, OD of 60 mm, ID of 16 mm and height of 70 mm. Using these magnets, we observed a chemical shift of toluene 1H NMR spectra and confirmed that the bulk magnets generated a magnetic field with homogeneity less than 0.5 ppm from the full width at half the maximum (FWHM) of a methyl proton signal of toluene. Magnetic field stability of less than 0.02 μT/hour was achieved in the EuBaCuO bulk magnet.

We have investigated the fabrication and position dependence of superconducting properties of large melt-textured Eu-Ba-Cu-O bulk superconductors for the purpose of application as a compact NMR magnet. We successfully fabricated c-axis oriented single-grain bulk samples up to 65 mm in diameter by melt-processing under a low oxygen partial pressure. The samples of 32 and 47 mm diameters could trap magnetic fields of 1.4 and 1.7 T at 77 K, respectively. Tc in the sample of 33mm diameter is almost similar throughout the sample. In 65 mm-sized sample, however, Tc and Jc lowered in inner region of the sample, leading to lowering of maximum trapped field.

We studied a pulsed field magnetization (PFM) of bulk HTS assembled into a synchronous motor as a field-pole. The PFM is essential to apply bulk HTS inside the machine as a practical technique. In the present study, we developed a PFM technique that is a usage of Controlled Magnetic density Distribution Coil (CMDC). The coil is composed of inner vortex coil and outer solenoid. We successfully obtained the trapped flux density with 1.3 T by the step-wise cooling method with CMDC at 38 K in the motor. The bulk was cooled by a condensed neon. In addition, we studied the PFM for Gd-bulk of 140 mm diameter. By using the CMDC, we obtained the trapped flux density distribution with regular shape. In this paper we report these advanced PFM techniques for a practical machinery applications.

We have investigated the fabrication and the superconducting and field trapping properties of large melt-textured Eu–Ba–Cu–O bulk superconductors for the purpose of application as a compact NMR magnet. We successfully fabricated c-axis oriented single-grain bulk samples up to 65mm in diameter by melt-processing under a low oxygen partial pressure. The samples of 33 and 47mm diameters could trap magnetic fields of 1.4 and 1.7T at 77K, respectively. The position dependence of Tc in the sample of 33mm diameter is very small throughout the sample. In 65mm-sized sample, however, Tc and Jc lowered in inner region of the sample, leading to lowering of maximum trapped field. Trapped magnetic field was improved with keeping for long time at maximum temperature in melt-processing.

We have measured the temperature dependences of the thermal conductivity in the ab-plane, κab(T), and along the c-axis, κc(T), respectively, for the Er–Ba–Cu–O bulk superconductors, which were fabricated by a method of the melt texture growth. Above the critical temperature Tc, the κab decreases quite moderately as increasing temperature or is almost independent of the temperature. Below the Tc, the well-known enhancement is observed but is relatively small. The κc(T) shows the small absolute values and we found that the anisotropy of the thermal conductivity (κab/κc) for Er–Ba–Cu–O bulks is of about 2–6.

We employed Ba-Cu-O substrates for fabrication of bulk Y-Ba-Cu-O superconductors in the top-seeded melt-growth method. There were several advantages for a use of a Ba-Cu-O substrate compared to conventional substrate materials such as MgO, ZrO2, Al2O3, RE123 and RE211 (RE = rare earth). The Ba-Cu-O substrate avoided crystallization from a substrate, suppressed a liquid loss and then scarcely reacted with a precursor. Furthermore, the introduction of large-sized cracks into a grown bulk greatly was suppressed by propagating along the interface between a grown bulk and a substrate. We could obtain bulk Y-Ba-Cu-O superconductors with uniform trapped magnetic filed distributions and equal maximum trapped filed values on both top and bottom surfaces, which indicates that the field-trapping capabilities were homogenized along the c-axes of the bulks fabricated on the Ba-Cu-O substrates.