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A persistent current switch (PCS) is used for superconducting applications, such as superconducting magnetic energy storage (SMES) system. The authors have proposed a mechanical switch of YBCO bulk as a mechanical PCS. From the previous research, it turned out that there were improvements of reducing contact resistance and load in the YBCO switch. In this paper, the phenomenon of the YBCO-deposited metal interface was reported. The authors experimentally investigated as one side curvature silver sample (above contactor), other side YBCO sample deposited silver layers (below contactor). Additionally, the contact area of the sample was analyzed with finite element method (FEM). This analysis result may lead to the elucidation of a contact mechanism with the contact surfaces deposited metal.

We investigated processing, microstructure and superconducting properties of Gd123 single grain bulk superconductors with a large amount of BaCeO 3 . The melt-textured bulk samples with compositions of Gd123 : BaCeO 3 : Gd211 = 100 : 40− x : x ( x = 0–40) plus 20 wt.% of Ag 2 O and 0.5 wt.% of Pt were fabricated in Ar–1%O 2 gas. The single grain bulk samples 25 and 33 mm in diameters were obtained with the composition of x = 5–40. It was observed that BaCeO 3 particles of 1–2 μm in diameters and Gd211 particles of 0.5–1 μm in diameters were dispersed in the melt-textured samples. Magnetic trapped fields of the 25 mm-sized samples with x = 10–40 were around 1 T at 77 K and the 33 mm-sized sample with x = 20 was 1.4 T at 77 K.

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 1 mm in diameter was artificially drilled into the center of bulk Gd–Ba–Cu–O 25 mm in diameter and 18 mm 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 4 K and in increasing the trapped field by 25% at 44 K. 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.

We have studied field trapping ability of single grain Gd–Ba–Cu–O bulk superconductor 140 mm 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 1 T at liquid nitrogen temperature. The maximum trapped magnetic field at 77 K and 65 K were 2.30 T and 3.66 T respectively, when a field of 4 T 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.

Gd–Ba–Cu–O bulk superconductors have significant potential for various applications due to the high critical current density and the highly trapped magnetic fields. Recently we have developed a large sized bulk superconductor using Gd210, which is discovered in the microgravity experiment. We investigated the mechanical properties and the cryostability of the Gd–Ba–Cu–O bulk superconductor to determine how to improve toughness and heat conduction of the large sized bulk superconductor. We introduced a stainless ring around the circumference of the bulk to increase the fracture strength of the bulk. Also, we introduced Al wires inserted in the hole along the c -axis of the bulk, and then the sample was subjected to the impregnation by using Bi–Sn–Cd alloy. We measured the trapped magnetic fields and the repulsive forces of the samples. The trapped magnetic field distributions were 1.13–1.36 T. The repulsive forces at 1 mm gap between the sample bulk and the permanent magnet with the surface magnetic induction of 0.37 T were about 70 N at 77 K. We have also measured the temperature dependence of the trapped magnetic field, and confirmed the effect of metal impregnation.

We studied the mechanism of (RE)Ba 2 Cu 3 O y phase formation via (RE) 2 BaO 4 in the case of GdBa 2 Cu 3 O y + 40 mol% Gd-211 combination, using mixture of Gd 2 BaO 4 , BaCuO 2 and CuO. Pellets of this composite were sintered at various temperatures between 700 °C and 1100 °C and held for 1 h in Ar (100%), Ar–1% O 2 , Ar–0.1% O 2 atmospheres, then rapidly quenched in oil. The samples were characterized by XRD, SEM, and EPMA analyses. XRD and EPMA analyses indicated that Gd-123 phase forms at around 900 °C but the temperature depends on the chosen atmosphere. The samples quenched from high temperatures (>1000 °C) showed that final formation of a secondary phase depends on the temperature held before the quench.

We achieved a very low resistance at 0.02 ~ 0.03 µΩ (12 nΩcm2) across the nonsuperconducting joint with an area of 2 × 20 mm2 for YBCO coated conductors produced by pulsed laser deposition (PLD). The joint technique was very simple in which YBCO coated conductors were stuck in a face to face manner with Ag surfaces in contact and heat-treated at 500°C in an oxygen atmosphere. We also confirmed that the good superconducting properties of the YBCO coated conductors were maintained after completion of the series of joint procedures. Furthermore, a low contact resistance of 0.5 nΩcm2 at the interface between the YBCO film and the Ag stabilizing layer was confirmed.

We have investigated the superconducting properties of (Gd, Y)–Ba–Cu–O bulk superconductors fabricated from precursors of the Gd123–Y211 with small additions of Pt and CeO 2 in a reduced oxygen atmosphere. The employment of ball-milled ultra-fine Y211 powder as a precursor material was very effective in enhancing critical current density ( J c ), since RE211 inclusions in the melt-textured samples could be drastically refined with decreasing the particle size of Y211 starting powder. The maximum J c value reached an extremely high value of 380 kA/cm 2 at 77 K in self-field. However, size reduction of RE211 led to inhomogeneous distribution of RE211 inclusions. The amount of RE211 was extremely small in the vicinity of a seed crystal, when the ultra-fine Y211 powder was employed. The trapped magnetic fields of Ag-added single-grain samples 25 and 33 mm in diameters were 1.5 and 2.0 T at 77 K, respectively.

We measured the contact resistance of a silver block with convex curvature and silver, a melt-textured Y–Ba–Cu–O sample (YBCO), and a silver-deposited YBCO sample (dAg/YBCO) to clarify the main source of the resistance. The study was performed with the development of a persistent current switch in mind. The contact resistance was largest in the Ag/YBCO contact. It decreased by increasing the thickness of the deposited-Ag layer for the Ag/dAg/YBCO contact. However, the smallest resistance of the Ag/dAg/YBCO contact was even larger than that of the Ag/Ag contact. This is probably due to the fact that the Ag/dAg/YBCO contact has two interfaces. The present result suggests that the contact area is dominant in controlling the contact resistance. A simulation study of the contact area with a finite element method (FEM) also supported this idea.

We used Ba–Cu–O substrates to fabricate bulk Nd–Ba–Cu–O superconductors using a top-seeded melt-growth method. There were several advantages for the use of Ba–Cu–O substrate compared to conventional substrate materials such as MgO, ZrO2, Al2O3, RE123 and RE211 (RE = rare earth). The Ba–Cu–O did not react with the precursor and minimized liquid loss. Accordingly, the introduction of large-sized cracks was suppressed. We also found that Tc values were high at the bottom regions, which was ascribed to the beneficial effect of Ba–Cu–O in suppressing Nd/Ba substitution. As a result, we obtained bulk Nd–Ba–Cu–O superconductors that exhibited fairly good field-trapping capabilities, even at the bottom surfaces.

We investigated the fabrication of melt-textured Gd–Ba–Cu–O bulk superconductors with the employment of a Gd2BaO4 (Gd210)–BaCuO2–CuO precursor. This enabled us to synthesize a very large Gd–Ba–Cu–O bulk superconductor, 140 mm in diameter. The trapped field distribution measured at 77 K revealed that the sample had no serious cracks, and the maximum field was 2.3 T. The formation behaviour and depressing mechanism of cracking were studied using sintered Gd210-containing precursors. The Ba–Cu–O component in the precursor melted at around 900 °C. The formation of such a liquid phase enhanced the densification of the precursor by liquid phase sintering. The mechanical strength of the sintered precursor was increased compared to the conventional Gd123–Gd211 precursor. This will be effective in preventing crack formation in the texturing process of large single-grain samples. The superconducting properties of the bulk sample fabricated from Gd210-containing precursor have been discussed.

Pulsed-field magnetization is a useful magnetization technique for high-temperature superconductivity bulk crystals for practical applications. However, the trapped magnetic flux is reduced due to the temperature increase from the mobile flux, and the distribution of the flux density is considerably distorted in the a–b crystallographic axes plane. We present two proposals to improve the trapped magnetic flux and its distribution upon pulsed-field magnetization. Firstly, split-type vortex pulsed copper coils with diameters smaller than that of the bulk disk. Secondly, two HTS bulks are stacked layered with misalignment of their crystal a–b axes to reduce the flux motion in the four-fold growth sectors, since the critical current density Jc in the growth sectors is lower. Combining these techniques, the decayed trapped field distribution was recovered and the total flux was increased up to 30% in comparison with a conventional pulsed-field magnetization.

Superconducting properties of the melt-growth bulk GdBa 2 Cu 3 O 7− δ (Gd123) were studied with the addition of BaO 2 , fine Gd 2 BaCuO 5 (Gd211) and Gd 2 Ba 4 MoCuO z (Gd2411) from the viewpoint of improvement of the critical current density ( J c ) by growth process in air. The addition of BaO 2 with 0.1 in a molar ratio to Gd123 effectively enhanced J c . This is coming from the suppression of the solid solution growth in Gd123 with Ba-rich phase. The addition of the ball-milled fine Gd211 particles enhanced J c to 90,000 A/cm 2 at 77 K under self-field. Single phase Gd2411 was successfully synthesized in air and added into Gd123 system with substitution in part by Gd211 as the second phase. The J c of the samples mixed with Gd2411 reached 84,500 A/cm 2 at 77 K under self-field. We conclude that the present additive materials with a proper amount are very effective to improve the J c value of Gd123 bulk superconducting magnets.

We investigated the effect of Y 2 Ba 4 CuMO 10 (M = Zr, Sn and Ce) addition on the superconducting properties of melt-textured Y123 bulk materials. Melt-textured Zr-doped Y123 bulk materials were fabricated from the mixed powders of Y123 and Y 2 Ba 4 CuZrO 10 . As an initial attempt, T c of the sample exhibited a low value of 86 K. However it was improved to be 90.5 K by addition of a small amount of Pt. The dispersion of fine Y 2 Ba 4 CuZrO 10 inclusions was observed in the melt-textured samples. The critical current density was enhanced with increasing the amount of Zr-phase particles. The Y 2 Ba 4 CuMO 10 (M = Sn and Ce) added Y123 bulk superconductors were also fabricated and their microstructures and superconducting properties were investigated. When the Y 2 Ba 4 CuCeO 10 was employed, BaCeO 3 particles were dispersed in the melt-textured bulk samples.

We developed a shielding current distribution model in a bulk superconductor with the aim of estimating the levitation force. First, we directly measured the magnetic field in axial and radial directions above a bulk Y–Ba–Cu–O superconductor levitated by an electromagnet using a three-dimensional Hall sensor. We then calculated the field distribution by assuming two types of the shielding current distributions and compared the results with the measured values. The field distributions based on the model in that the shielding currents are flowing concentrically at the surface were not in good agreement with the measured data. We then assumed that the shielding currents are not uniform and the boundary has the form of n th hyperbola, which could reproduce the measured field distribution reasonably well.

In melt-textured RE-Ba–Cu–O bulk superconductors, an excessive size reduction of RE211 inclusions results in inhomogeneous distribution of RE211 particles. To reduce such inhomogeneity, we attempted to combine granules containing fine RE211 into the melt-textured RE123 materials. First, we prepared Y–Ba–Cu–O granules containing Y123 and ultra-fine Y211 (0.1 μm in size) powders with organic binder. The size of the granules was about 1.0 mm. Next, Dy–Ba–Cu–O powders containing Dy123 and Dy211 (2 μm in size) were prepared. The Y–Ba–Cu–O granules and Dy–Ba–Cu–O powder were compacted together and melt-processed in air. We found that fine RE211 inclusions were uniformly distributed in this sample, which led to improved spatial homogeneity in critical currents.

We measured the temperature dependence of the trapped field and the magnetic levitation force for c -axis-oriented single-grain Gd–Ba–Cu–O bulk samples 48 mm in diameter. Trapped magnetic field of the samples was 2.1–2.2 T at 77 K and increased with decreasing temperature and reached 4.1 T at 70 K, however the sample fractured during the measurements at lower temperatures due to a large electromagnetic force. The reinforcement by a metal ring was effective in improving the mechanical strength. The sample encapsulated in an Al ring could trap a very high magnetic field of 9.0 T at 50 K. In liquid O 2 the Gd–Ba–Cu–O bulk exhibited a trapped field of 0.42 T and a magnetic levitation force about a half value of that in liquid N 2 .

Unmanned space experiments were performed with the aim of growing large grain Gd–Ba–Cu–O superconductors in space. The advantage of the space experiment was the support of a large precursor block by a small seed crystal alone. Three samples were safely returned from space. It was interesting to note that one of the samples was mainly composed of Gd 2 BaO 4 pillar shaped crystals. Detailed characterization of this new phase showed that the phase is very stable under severe conditions and will be a firm framework for the melt growth process. Infiltration of Ba–Cu–O phase into a precursor Gd 2 BaO 4 block enabled us to grow a large grain bulk Gd–Ba–Cu–O 14 cm in diameter.

Nanoscopic surface stripes were observed in high quality SmBa 2 Cu 3 O 7− δ single crystals grown by a top-seeded pulling growth and their roles on the peak effect and enhanced flux pinning were investigated with a SQUID magnetometer. The crystal surface exhibited an ordered modulation with a roughness of 1–2 nm and a wavelength of about 50 nm. The study evidenced that such a periodic array is not dependent on surface processing. The analysis further unveiled that nanoscaled stripes may act as a kind of δ T c -style pinning defects which contribute to an enhanced peak effect.

Energy storage by a flywheel system has recently been developed as a promising application of high temperature superconducting bulks. Rotation loss reduction is necessary for an efficient operation of the superconducting magnetic bearings (SMB), which is the most essential component of the system. Typical radial type SMB consists of a superconductor stator and a permanent magnet rotor that is sustained by a magnetic force deduced from high flux pinning of the bulk superconductors. If the stator is a simply segmented circular array of Y–Ba–Cu–O bulks, inhomogeneity of the trapped magnetic field is unavoidable, which induces eddy currents in the metallic component in the rotor. Such a rotation loss is one of the major problems for a practical use of SMB. We will present the relationship between the inhomogeneous magnetic field and the rotation loss of SMB and give a general principle for reduction of rotation loss in SMB.

We investigated magnetic homogeneities of Gd–Ba–Cu–O bulk superconductors by means of a conventional scanning Hall probe (SHP) method and a recently developed magneto-scan method. In the SHP method, we measured the trapped flux density distributions of the superconductors by scanning a Hall sensor above the sample surface. On the other hand, in the magneto-scan method, we scanned a Hall probe sensor together with a permanent magnet, which allowed us to detect the field shielding behavior on the sample surface without applying a strong external field. It was found that the magneto-scan method was more sensitive than the SHP method for the detection of local structures in the flux density distributions.

We have measured contact resistances between two bulk YBCO superconductor blocks with the application to a persistent current switch (PCS) in mind. In order to reduce a contact resistance, we deposited indium and silver on the sample surfaces. The resistance was reduced by increasing the thickness of the deposited metal layers, but it saturated when the thickness reached a certain level. The saturation thickness was much smaller in indium than silver. Such a difference is understandable by considering the hardness of these two metals. The resistance was also reduced by increasing the mechanical load. Overloading however caused the coupling of metal layers, resulting in the peeling off of the deposited layers when the switch was opened.

Nanoscale secondary phase Gd 2 BaCuO 5 “Gd-211” particles less than 60 nm in size, added to mixed ternary LREBa 2 Cu 3 O y [LRE = (Nd 0.33 Eu 0.33 Gd 0.33 ), (Sm 0.33 Eu 0.33 Gd 0.33 ), (Nd 0.33 Sm 0.33 Gd 0.33 )] “NEG-123, SEG-123, NSG-123” oxide powders and melt-processed in Ar–1%pO 2 gas mixture, significantly improve electromagnetic performance at low magnetic fields. As confirmed by transmission electron microscopy and dynamic force microscopy, the nanoparticles in the LRE-123 matrix not only survive the melt-texturing process but, thanks to their contamination by Zr coming from ball milling process, these particles in majority transform into a new type of even smaller defects. Further improvement in flux pinning was reached when both 123 and 211 powders were thoroughly ball milled before the melt-growth process to the similar small size. As a result, J c values of 260 kA/cm 2 and 55 kA/cm 2 were observed in remnant state at 77 K and 90 K, respectively.

Bending tests have been commonly carried out to investigate the mechanical properties of melt-processed rare earth based bulk superconductors. Tensile tests by using small specimen, however, are preferable to evaluate the detailed distribution of the mechanical properties and the intrinsic elastic modulus because no stress distributions exist in the cross-section. In this study, the tensile test method at low temperature by using specimens with the dimensions of 3 × 3 × 4 mm from Y123 and Gd123 bulks was examined. They were glued to Al alloy rods at 400 K by using epoxy resin. Tests were carried out at liquid nitrogen temperature (LNT) by using the immersion type jig. Although the bending strength in the direction perpendicular to the c-axis of the bulks at LNT is higher than that at room temperature (RT), the tensile strength at LNT was lower than that at RT. Many of specimens fractured near the interface between the specimen and the Al alloy rod at LNT. According to the finite element method analysis, it was shown that there was a peak thermal stress in the loading direction near the interface and it was significantly higher at LNT than that at RT. It was also shown that the replacement of the Al alloy rod to Ti rod of which the coefficient of thermal expansion is close to that of bulks significantly increased the tensile strength.

Several practical applications of melt-textured bulk superconductors require the complex-shaped products such as curved, ring-shaped, and drilled blocks rather than simple shaped pellets. In general, complex-shaped bulk products are prepared by machining melt-grown massive blocks. However, melt-textured bulks are often damaged when they are mechanically machined. With the aim of reducing such damages, we have investigated the preparation of the complex-shaped bulk superconductors by previously machining the precursors compacted with organic binders. The mixed powders of Y123 and Y211 were pressed into pellets, for which the organic binder was added are machined into complex-shaped bodies. After removal of the organic components, melt-processing was performed in air, which led to a reproducible fabrication of complex-shaped bulk products. The superconducting properties were not degraded with this process.

This review reports on two new nanometer scale pinning media in melt-textured LREBa 2 Cu 3 O y superconductors and the ways of their creation. Microstructure analysis by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM), capable of recognizing nanometer size pinning defects, show their shapes and dispersion and identify the key structural features responsible for the improved superconducting performance. The two new pinning agents extend the use of LREBa 2 Cu 3 O y composites towards high magnetic fields and high temperatures.

The target of an unmanned space experiment is the fabrication of large grain Gd–Ba–Cu–O superconductors 12 cm in diameter in microgravity environment. The advantage of the space experiment is the support of a huge sample by a seed crystal alone. For this experiment, the cold seeding (CS) method must be employed to meet the requirements of the space program. Although the CS method is easy to apply, it has been difficult to grow large grain samples because of a small temperature window for a stable crystal growth. We thus studied the heat schedules for the space experiment using a prototype ground experiment furnace, which is similar to the furnaces launched into space. We could find the melt growth conditions that enabled us to successfully grow Gd–Ba–Cu–O bulk superconductors 10 cm in diameter even by the CS method.

We have studied the effect of preloading on the relaxation of the levitation force of bulk Y–Ba–Cu–O superconductors. The levitation force was measured by applying the external field with a superconducting solenoid. We set the standard magnetic induction ( B st ) to be 1.2 T supposing that the bulk Y–Ba–Cu–O disk experiences this field in operation. Preloading was applied such that the external field was first ramped to 1.1 B st and reduced to B st . We confirmed that the preloading was effective in reducing the relaxation of the levitation force, although the effective levitation force was largely depressed compared to that without preloading. The employment of an extra step to lower the field below B st before reaching B st was effective in improving the effective levitation force without sacrificing the ability to reduce the relaxation rate. This is qualitatively understood in terms of the field distribution inside the disk based on the critical state model.

Pulsed field magnetization (PFM) is a practical method for magnetizing bulk superconductors, but the amount of trapped magnetic flux is usually smaller than that magnetized by the field cooling (FC) method due to the heat generation by the flux motion during pulsed field application. In this study, we magnetized a Y–Ba–Cu–O bulk superconductor grown from a seed which was placed at deviating positions from the centre of the bulk to investigate the effect of inhomogeneous penetration of the flux into the bulk superconductor during the PFM. The maximum trapped field by the PFM was almost the same value as by the FC method. This suggests that the inhomogeneity of the bulk superconductor affects the trapped field on the PFM. The magnetic flux can penetrate into the centre of the bulk even if the applied field is small, which contributes to reducing the extra heat generation due to the flux motion.

We have investigated the superconducting properties of (Gd, Y)-Ba-Cu-O bulk superconductors fabricated from precursors of Gd123-Y211 in a reduced oxygen atmosphere. The employment of ball-milled ultra-fine Y211 powder as a precursor material was very effective in enhancing critical current density (J/sub c/), since RE211 inclusions in the melt-textured samples could be drastically refined with decreasing the particle size of Y211 starting powder. The maximum J/sub c/ value reached an extremely high value of 380 kA/cm/sup 2/ at 77 K in self-field. However, size reduction of RE211 led to inhomogeneous distribution of RE211 inclusions. The trapped magnetic fields of Ag-added single-grain samples 25 mm and 33 mm in diameters were 1.5 T and 2.0 T at 77 K, respectively.

We studied spatial inhomogeneities of Gd-Ba-Cu-O bulk superconductors by using two different types of Hall probe scanning methods. One is a conventional scanning Hall probe (SHP) method, which can measure the trapped flux density distribution of superconductors after magnetized by external fields. The other is a recently developed magneto-scan method, which scans a Hall probe together with a permanent magnet and can detect shielding behavior of the sample surface without applying an external field. In the SHP method, the flux density distributions obtained by the field cooling (FC) scan were compared with those in the zero field cooling (ZFC) scan. It was found that the ZFC scan can acquire much more information on the sample inhomogeneity than the FC scan. The magneto-scan method was also adopted to the characterization of the same sample. It was found that this method was more sensitive than the SHP method, and allowed us to clearly detect some degradations in the vicinity of the seed crystal in the sample surface.

We investigated the effect of BaCeO3 addition on the superconducting properties of melt-textured YBa2Cu3O7-δ (Y123) bulk materials. Bulk superconductors with various compositions of Y123 + xBaCeO3 (x = 0.05-0.5) were fabricated by melt-processing in air. BaCeO3 particles of 1-2 μm in diameter were dispersed in the Y123 matrix phase. The addition of a small amount of Pt was effective for improving the Tc and Jc values. The Jc in the low field region increased as the amount of BaCeO3 for the sample was increased with x = 0.05-0.4. This indicates that BaCeO3 can function as effective artificial pinning center like Y2BaCuO5 particles in conventional melt-textured Y-Ba-Cu-O bulk materials.

The RE/Ba substitution has a significant effect on the growth, microstructure and flux pinning properties of melt-processed Gd 1+ x Ba 2− x Cu 3 O 7− δ superconductors. We prepared a series of Gd 1+ x Ba 2− x Cu 3 O 7− δ , (Gd123 solid solutions) with x = −0.1, 0, 0.05, 0.1, 0.15 and 0.2. The single phase of Gd123 solid solutions was confirmed with both X-ray diffraction and DTA analysis for x = 0 to 0.1 as-grown samples, quenched in air. DTA/TG analysis in air showed that the thermal stability decreases with increasing x . After the post-annealing in flowing argon gas at 940 °C for 10 h (ArPA), the Gd123 solid solutions with x = 0.1, 0.15 and 0.2 were decomposed into Gd 2 BaCuO 5 and Gd123 solid solutions with low substitution levels and minor liquid phases. The critical transition temperatures ( T c ’s) of Gd123 solid solutions with the ArPA show dramatical increase compared to the samples without ArPA. The Gd123 solid solutions annealed in O 2 at 400 °C show a narrower superconducting transition width than those treated at 350 °C. These results contribute to improve the properties of the melt-processed bulk superconductors.

We measured the magnetic field distribution and the levitation force of a bulk Sm–Ba–Cu–O superconductor as it was moved vertically in magnetic fields generated from a Bi–Sr–Ca–Cu–O superconducting magnet. The levitation forces strongly depended on a radial component of the external magnetic field. We measured magnetic fields on the surface and the interior of the bulk superconductor using a three-dimensional Hall sensor device. The field distribution could be understood by assuming that the shielding currents are flowing concentrically at the periphery as predicted by the critical state model. We also found that the flux lines were not straight but slightly curved due to a cylindrical geometry of the samples.

We fabricated a single-domain Gd?Ba?Cu?O bulk superconductor 65?mm in diameter and studied the microstructure, superconducting and field-trapping properties. Melt-processing was performed under a controlled oxygen partial pressure of 1.0% using a precursor containing Gd123 and Gd211 powders in a molar ratio of 2:1, with 0.5?wt% of Pt and 20?wt% of Ag2O added. The distribution of Ag and Gd211 particles was almost homogeneous. The addition of Ag was very effective in reducing the amount of cracking in the sample. The maximum trapped magnetic field recorded was 3.05?T at 77?K. We also measured the trapped field between two Gd?Ba?Cu?O bulk samples in order to minimize the demagnetizing effect and found that the trapped field reached 4.3?T at 77?K.

Single-domain GdBa2Cu3O7−δ (Gd123) bulk superconductors were fabricated in air and studied with different BaO2 contents and Gd2BaCuO7−δ (Gd211) initial particle sizes. The experiments showed that BaO2 addition effectively suppressed the Gd/Ba substitution and therefore enhanced superconducting properties such as Jc and Birr. An optimum addition amount is Gd123:BaO2 = 1:0.1 molar. By employing fine Gd211, Jc at low magnetic fields is further enhanced. At B = 0 and 77 K, Jc reached 86 000 A cm−2, which is a record high Jc value for melt-processed Gd123 bulk samples in air.

Irreversible fields Birr of melt-processed (Sm,Eu,Gd)Ba2Cu3O7−δ bulk superconductors with varying (Sm,Eu,Gd)2BaCuO5 concentrations have been measured with pulsed magnetic fields up to 28 T in a temperature range from 55 to 85 K. The temperature dependence of Birr was studied by taking account of the effect of heat generation. In comparison with that including 5 mol% (Sm,Eu,Gd)2BaCuO5 phase the samples with 40 mol% (Sm,Eu,Gd)2BaCuO5 inclusion displayed a lower Birr value for temperatures above 75 K, while Birr was enhanced below 75 K. TEM observation unveiled a sharp contrast between a uniform nano-scaled compositional modulation in compositions with 40% (Sm,Eu,Gd)2BaCuO5 inclusion and an inhomogeneous distribution with very low (Sm,Eu,Gd)2BaCuO5 concentration. The present result could thus be understood in the context of different defect states induced by locally compositional fluctuation.

The monotonic and the repeated bending tests on the Y123 single grain bulk superconductor sample were carried out. The average Young's modulus at liquid nitrogen temperature (LNT), 148 GPa, was higher than that at room temperature (RT), 132 GPa. The average bending strength at LNT, 90 MPa, was also higher than that at RT, 74 MPa. The Weibull coefficient for the bending strength at RT, 17.6, was significantly larger than that at LNT, 6.1. The specimen cut from the bottom region of the bulk superconductor showed slightly higher value of the Young's modulus and lower value of the bending strength. The fatigue endurance limit for the repeated pulsating loading up to 5 × 104 cycles at LNT, 70 MPa, was about 80% of the bending strength.

We have studied the microstructure and critical current density ( J c ) of Gd–Ba–Cu–O and Y–Ba–Cu–O bulk superconductors. The size of RE 2 BaCuO 5 (RE211) particles trapped in REBa 2 Cu 3 O y (RE123) matrix could be refined by employing ultra-fine RE211 powder as a precursor, which led to high J c values exceeding 1 × 10 5 A/cm 2 at 77 K in self-field for Gd–Ba–Cu–O. In the case of Y–Ba–Cu–O, however, the size reduction of Y211 led to inhomogeneous distribution of Y211 particles, in that the volume fraction of Y211 phase largely decreased near a seed crystal. We also studied superconducting properties of (Gd,Y)–Ba–Cu–O bulk material fabricated from precursor of Gd123-Y211. The employment of finely-milled Y211 powders resulted in a dramatic enhancement of J c values in this system. The maximum J c value for (Gd,Y)–Ba–Cu–O reached 2.5 × 10 5 A/cm 2 at 77 K.

Combination of extremely fine Gd-211 particles and the LRE/Ba solid solution pinning disorder in a melt processed (Nd 0.33 Eu 0.33 Gd 0.33 )Ba 2 Cu 3 O y , followed by an optimal oxygenation resulted in outstanding electromagnetic performance of the material. Transmission electron microscopy with energy dispersive X-ray analysis identified nanometer scale Zr-rich NEG–Ba–Cu–O particles. These were evidently responsible for a substantial increase of the critical current density at 77 K and the rise by an order of magnitude at and above 90 K. Efficient field trapping ability at 90.2 K enabled new NEG-123 magnet to be for the first time employed in levitation at liquid oxygen .

Magnetization measurement in pulsed high magnetic fields up to 28 T in the range of temperature from 55 to 85 K have been performed for (Nd 0.33 Eu 0.38 Gd 0.28 )Ba 2 Cu 3 O y (NEG123) having very high irreversibility fields B irr We have evaluated temperature dependence of B irr by taking account of heat generation. Critical current density in pulsed high magnetic fields was somewhat overestimated in comparison with that in fields, but values of B irr nearly coincide. B irr did not strongly depend on the content of Gd 2 BaCuO 5 (Gd211) secondary phase between 2 and 10 mol%. The temperature dependence of B irr can be expressed as B irr (T)=80.2−0.878 T (K).

Pulsed-field magnetization was studied for field-free cooled high-temperature superconductor (HTS) bulk cylindrical disks of melt-textured Gd–Ba–Cu–O samples at the liquid nitrogen temperature. A bulk sample was inserted in between disks of vortex-type pulsed-field copper coil immersed in the liquid nitrogen. The flux was trapped in the centre of the sample surface under the smaller pulsed peak field than the magnetization with a conventional solenoid coil. With intensifying the pulsed-field, the trapped flux density for the maximum peak remanent value in the field cooling process increases monotonously to the liquid nitrogen temperature. In the samples with strong pinning force, which shows large remanent flux on field cooling, the deviation from the conical profile of trapped field distribution was observed. This is attributed to the transient flux motion, which possibly drives temperature increase resulting in the decrease of the trapped field in the growth sector. However, the subsequent single pulsed-field remarkably compensates the formation of a well-dressed conical field density profile. Employing a couple of vortex-type coils enables us to magnetize the HTS bulk cryo-magnets effectively with reduced electric energy per pulsed-current for the magnetization with a pulsed-field solenoid. The present magnetization geometry is acceptable for application of the HTS bulk to the rotor magnet magnetized with an armature in the synchronous rotating machines.

We have performed tensile tests at room temperature for Y–Ba–Cu–O bulk superconductors with artificial grain boundaries, which were fabricated by a joining technique using Ag added Y–Ba–Cu–O solder. The tensile strength of the joint was lower than that of the mother block Y–Ba–Cu–O. Microstructural analyses revealed that a residual liquid phase was observed in the vicinity of the interface between the mother block and the solder in the ac -plane. Cross-sectional observation for the samples after the tensile tests showed that the fracture occurred along the residual liquid phase. Hence, the residual liquid phase was responsible for the lower tensile strength.

We fabricated c-axis oriented large grain (Sm,Eu,Gd)Ba2Cu3O7−δ bulk superconductors with BaO2 addition under 1% O2 partial pressure in Ar. The experiment showed that BaO2 addition significantly reduced a RE–Ba substitution, slightly hindered the coarsening of (Sm,Eu,Gd)2BaCuO5 inclusion, and improved the superconducting properties. An optimum adding amount was around 0.06–0.18 per mol RE123. At 77 K, the critical current density reached 83,000 A/cm2 at zero field.

The paper refers on advances in fabrication of mixed LREBa2Cu3Oy (LRE: Nd, Sm, Eu, Gd; combinations NEG-123, NSG-123, and SEG-123) composites developed for bulk applications at liquid oxygen temperature. In order to enhance flux pinning over 90 K, we introduced to the 123 systems a ball-milled Gd2BaCuO5 powder in various average sizes ranging from 200 to

We investigated the microstructure and mechanical strength of Ag-added DyBa2Cu3Oy (Dy123) bulk superconductors with various Dy2BaCuO5 (Dy211) contents. Single-grain Dy123 bulk samples 32 mm in diameter with the addition of 5-40 mol% of Dy211 and 10 wt.% of Ag2O were fabricated in air. The sample with 5 mol% Dy211 contained many macro-cracks in the ab-plane. The amount of cracks decreased with increasing Dy211 content. Three-point bending tests were performed at room temperature to measure the mechanical properties. The average bending strength of the sample with 5 mol% Dy211 was 73 MPa. The strength was improved to 95 MPa with an addition of 40 mol% Dy211.

In order to investigate the compressive mechanical characteristics of Sm123 single-grain bulk superconductors fabricated by TSMG method, compressive tests in the direction parallel and perpendicular to the c-axis have been carried out by using small specimens cut from the bulk samples. Stress-strain curves in the direction perpendicular to the c-axis were almost linear. The Young's moduli measured were 155-165 GPa. In the case of loading parallel to the c-axis, the curves were exponential in the low stress region. As the stress increased, they tended to a linear relation. The apparent Young's modulus in this region was 75-110 GPa. These characteristics are mainly ascribed to the closure behavior of pre-existing micro-cracks perpendicular to the c-axis. The average fracture strength in the direction of c-axis, 350-400 MPa, was larger than that perpendicular to it, 235-275 MPa. Specimens in the former were generally broken into great many small pieces while those in the latter were broken into some parts by lateral cracking. The compressive Young's modulus and the fracture strength for each direction were higher than those evaluated by the tensile loading, respectively. The effects of specimen length on the compressive mechanical properties were also discussed.

We fabricated large c-axis-oriented single-grain (Sm,Eu,Gd)Ba2Cu3O7−δ bulk superconductors with BaO2 addition under 1% O2 partial pressure in Ar. The experiment showed that BaO2 addition significantly reduced RE–Ba substitution, slightly hindered the coarsening of (Sm,Eu,Gd)2BaCuO5 inclusion, and improved the superconducting properties. An optimum addition amount was around 0.1–0.18 mol−1 RE123. At 77 K, the critical current density reached 83 000 A cm−2 in zero field. A disc 3.5 cm in diameter trapped a field of 1.27 T at 77 K.

High quality (Sm,Eu,Gd)Ba2Cu3O7−δ single domain superconductors were grown in air with BaCuO2−x addition. The flux pinning was investigated with varying temperatures of Ar post-annealing and the Eu/Gd ratio in the initial composition. The amplitude of the peak effect displayed a peak when annealed at 925 °C in Ar while the irreversible field Birr keeping unchanged. Eu enrichment dramatically enhanced Birr and the peak field for the samples annealed at 925 °C in Ar. At 77 K Birr was higher than 9 T in (Sm0.33Eu0.44Gd0.22)Ba2Cu3O7−δ. Structural characterization showed that the average size of (Sm,Eu,Gd)2BaCuO5 inclusions slightly decreased with increasing the Gd concentration. The scaling analysis and field dependence of superconducting transition evidenced a predominant δTc style pinning induced by local compositional fluctuation. The evolution of flux pinning was thus attributed to variable compositional fluctuation.

Two Y–Ba–Cu–O bulk superconductors were joined by using Ag-added Y–Ba–Cu–O as a joining reagent. Two different Y–Ba–Cu–O blocks were prepared individually and subjected to the joining process. Here the joined surfaces were aligned parallel to the (1 1 0) plane. A Y–Ba–Cu–O/Ag plate was prepared via a solid state reaction. The plate was sandwiched by two Y–Ba–Cu–O blocks and heated to a temperature between the decomposition temperatures of Y–Ba–Cu–O and Y–Ba–Cu–O/Ag, and further annealed in flowing oxygen. The trapped field distribution of the joint using a Y–Ba–Cu–O/Ag solder with Y211 powders that were not refined by ball milling showed a small dip, corresponding to a decrease of the maximum trapped field values by 15%, while a single peak with no dip was obtained at the joint using a Y–Ba–Cu–O/Ag solder with finely refined Y211.