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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.

High-temperature superconducting (HTS) bulk materials of RE–Ba–Cu–O (RE: Y or rare-earth elements) exhibit excellent $J_{c}\!-\!B$ 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 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.

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

The GdBaCuO superconducting bulk with stronger pinning characteristics (B T = 1.8 T at 77 K by the field cooled magnetization (FCM)) has been magnetized at T s = 70 - 20 K by the pulsed field magnetization (PFM) techniques; a sequential pulsed-field application (SPA) and a modified multi pulse technique with stepwise cooling (MMPSC). With decreasing T s, the pinning force F p increases especially at the growth sector boundaries (GSBs), and then the nonuniformity of the trapped field profile becomes more and more conspicuous on the bulk surface. At low T s, the SPA technique is not necessarily a suitable technique to enhance the trapped field B T and the total trapped flux Phi T. However, these values can be enhanced by the MMPSC method. The properties of the bulk with stronger pinning characteristics during PFM are discussed.

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 65 mm in diameter by melt-processing under a low oxygen partial pressure. The samples of 33 and 47 mm diameters could trap magnetic fields of 1.4 and 1.7 T at 77 K, respectively. The position dependence of T c in the sample of 33mm diameter is very small throughout the sample. In 65 mm-sized sample, however, T c and J c 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.

For practical machine application of high-temperature superconductor (HTS) bulk magnet, a pulsed field magnetization (PFM) using small-sized magnetization coils is quite essential. HTS bulk with a diameter of 60 mm has been magnetized successfully and has made the tested axial-type motor to be stable operation by using a couple of vortex-type armature coils. In the present study we report the pulsed magnetization of HTS bulk with a diameter of 140 mm. The crystallization of such large bulk tends to be inhomogeneous around the periphery. For the sample of GdBa 2 Cu 3 O y − δ with 140 mm in diameter and 20 mm thickness, new split-type magnetizing coils of 100 and 140 mm in diameter were employed. When the PFM was performed with the coils of 100 mm in diameter, magnetic fluxes penetrate near the center rather than periphery. On the other hand, when we used the coils of 140 mm in diameter, magnetic flux penetrates the whole body, and magnetic flux tends to be immobile in the area. Thus, the PFM with the coils of 100 mm in diameter is effective to magnetization for near the center of HTS bulk. They are also effective to obtain a conical shape trapped flux density distribution. The PFM with the coils of 140 mm in diameter is effective to enhance the total magnetic flux.

In the production of melt-textured RE-Ba-Cu-O bulk superconductors, the precursors are generally prepared by uniaxial and/or cold-isostatic pressing of raw powders. The employment of fine starting powders is effective in fabricating high-performance RE-Ba-Cu-O bulk superconductors. However, cracks often form in the green bodies when fine powders are unaxially pressed. In this work, we have investigated the defects in green bodies during the powder compression process for bulk superconductor fabrication. Fine and large YBCO powders and granulates of binder-added fine powder were uniaxially pressed into tablets (20 mm in diameter) using different pressures from 31 to 125 MPa. When the fine powder was pressed at pressures above 62 MPa, several defects such as lamination, capping and cracks on the parallel in the direction of pressing were found in green bodies. These defects in green bodies partially remained after melt-processing and affected the field-trapping abilities of melt-textured bulk samples. The addition of an organic binder is very effective method for reducing defects in green bodies.

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.

The ab-plane thermal conductivity κ(T) and thermoelectric power S(T) have been investigated for DyBaCuO bulk superconductors for various contents of the Dy2BaCuO5 (Dy211) phase X. The absolute κ(T) value is pretty small compared with that of other REBaCuO bulks (RE = Y and rare earth elements). The S(T) value above the superconducting transition temperature Tc changes from negative to positive and κ(T) slightly decreases with increasing oxygen deficiency in the DyBa2Cu3O7−δ (Dy123) superconducting phase. The point-defect-type phonon scattering due to oxygen deficiency affects the κ(T) value, but is not a main origin for the low κ(T) even for the fully oxidized DyBaCuO bulk. Another type of powerful phonon scattering centre governs the thermal transport properties in the DyBaCuO bulk.

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.

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 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.

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 succeeded in enhancing the critical current density (Jc) of melt-textured Y?Ba?Cu?O superconductors by employing ultra-fine Y211 powder as precursor. The Y211 starting powders were reduced down to 110?nm in diameter with ball-milling. Single-grain Y?Ba?Cu?O samples up?to?33?mm in diameter were fabricated from the precursors containing finely milled Y211 powders. The Y211 inclusions in the melt-textured samples could be refined by decreasing the particle size of the Y211 starting powders. The maximum value of Jc at 77?K was 1.1 ? 105?A?cm?2 in a self-field. However, the size reduction of Y211 led to an inhomogeneous distribution of Y211 particles, in which the volume fraction of the Y211 phase greatly decreased near a seed crystal. An Ag-added bulk sample 33?mm in diameter exhibited a high trapped magnetic field of 1.6?T at 77?K.

We have fabricated melt-textured Ho-Ba-Cu-O bulk superconductors with a composition of HoBa2Cu3Oy(Ho123)+0.4Ho2BaCuO5(Ho211) in air. Several kinds of precursor compounds having Ho211 starting powders with different sizes are prepared by calcinining Ho2O3, BaO2 and CuO at 1173∼1273 K. The diameter of Ho211 secondary particles dispersed in the Ho123 phase is reduced with decreasing the particle size of the Ho211 starting powder, resulting in the improvement in Jc properties. The maximum Jc value is 7.2×108 A/m2 at 77 K in self-field. The Ag added large single-domain bulk of 32 mm in diameter is also fabricated. The trapped-field of 1.2 T has been achieved at 77 K for this bulk sample.

We have prepared (Nd,Sm,Gd)Ba 2 Cu 3 O y “NSG-123” with various contents of (Nd,Sm,Gd) 2 BaCuO 5 “NSG-211” or Gd 2 BaCuO 5 “Gd-211” with the aim of optimizing flux pinning. Microstructure observed by transmission electron microscopy exhibited a high density of RE-rich RE 1+ x Ba 2− x Cu 3 O y (RE-123ss) clusters with their size and dispersion depending on the initially added 211 content. Gd-211 10 mol% distributed as small clusters ranging from 3 to 10 nm in size enhanced pinning at intermediate fields. The magnetization measurements suggested that the optimum oxygen pressure during melt-growth process for this system was around 1% p O 2 . It was confirmed that this material can be grown into single domain more then 40 mm in diameter.

Sub-micron Gd2BaCuO5 (Gd-211) particles were embedded in the ternary (Nd0.33Sm0.33Gd0.33)Ba2Cu3Oy (NSG-123) melt-textured superconductors. To enhance critical current density at low fields, different amounts of the secondary phase were added and refined by addition of 0.5 mol% Pt and 1 mol% CeO2. Optical and scanning electron microscopy indicated sub-micron, mostly Gd-211, particles in the final product. Critical currents in remanent state increased monotonously with 211 concentration but the secondary peak did not nearly change. Critical current density of 100 kA cm−2 at 77 K and the applied field of 2 T was achieved in the NSG-123 sample with 10 mol% Gd-211.

We report on the superconducting properties of melt-textured Dy123 bulk superconductors containing 5 to 40 mol% Dy211. Dy?Ba?Cu?O bulk samples were fabricated with the cold-seeding method in air using a Nd123 seed crystal. The superconducting properties of the bulk samples were strongly affected by the Dy211 content and the oxygen-annealing temperature. In the Jc?B curve of the Dy123 sample with 5 mol% of Dy211, the secondary peak was clearly observed at the applied fields of 0.5?2.0 T. The peak Jc values were 32 000?37 000 A cm?2 at 77 K. The peak position shifted to lower fields with increasing oxygen-annealing temperature, presumably due to an increase in oxygen vacancies. We believe that the peak effect in Dy?Ba?Cu?O is ascribed to oxygen deficiency, since RE/Ba substitution is not present in the system. This idea was supported by the fact that the secondary peak decreased with increasing Dy211 contents and disappeared in the Dy123 sample with 40 mol% of Dy211.

We fabricated large-grain c-axis-oriented Gd–Ba–Cu–O bulk superconductors with very fine 211 particles and studied their field-trapping properties. Single domain Gd–Ba–Cu–O/Ag bulk samples with different molar ratios of Gd-123:Gd-211 = 10:x (x = 2 to 6) were melt-textured under controlled oxygen partial pressure of 1.0% by employing Gd-211 starting powders with a particle size of 1.0 μm. The maximum trapped magnetic field of 2.0 T was recorded at 77 K in the bulk 48 mm in diameter with the composition of x = 5. It is notable that the trapped field in the space of two Gd–Ba–Cu–O bulks reached an extremely high value of 3.3 T at 77 K. Furthermore, the field-trapping performance of the bulk sample was improved by applying the ultra-fine 211 powders of a size of 0.2 μm. The single-bulk Gd–Ba–Cu–O 50 mm in diameter exhibited a trapped field of 2.6 T at 77 K.

The critical current density and trapped field of large grain RE123 bulk superconductors have drastically increased in the last few years. High-quality bulk superconducting magnets can trap a very high magnetic field of 3T at 77K and above 15T at 20K. The recent progress in bulk superconducting materials has led to many applications such as superconducting flywheel systems, current leads, superconducting motors, magnetic separation systems, magnetron sputtering machines. In this paper, we review recent research and development of the processing, critical current density, trapped field and mechanical properties of melt-textured bulk superconducting materials.

We investigated the superconducting properties of melt-textured Ho123 and Er123 bulk superconductors containing 5 to 40mol% Ho211 and Er211, respectively. The samples were fabricated with the cold-seeding method in air using a Nd123 bulk seed crystal. The superconducting properties of the Ho-Ba-Cu-O samples were strongly affected by the Ho211 content and oxygen annealing temperature. In the Jc-B curves of the Ho123 samples with low Ho211 contents, a secondary peak effect was clearly observed. The peak position shifted to lower fields as oxygen annealing temperature increased, presumably due to an increase in oxygen vacancies. A secondary peak effect was also found in the Er123 sample with 5mol% of Er211. We believe that the peak effect in these materials is ascribed to oxygen deficiency, since RE/Ba substitution is not present in either system.

We studied the effect of the combined addition of Pt and CeO2 on the dispersion of secondary phase Gd2BaCuO5 (Gd-211) particles in melt-processed (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy (NEG-123) by varying the CeO2 content from 0 to 3%. Scanning electron microscopy revealed that submicrometre-sized Gd-211 particles were distributed uniformly throughout the NEG-123 matrix for additions of up to 2 mol% CeO2. The critical current density (Jc) increased with increasing CeO2 content, particularly at low magnetic fields. The zero-field Jc value reached its maximum of 100 kA cm-2 at 77 K for 1 mol% CeO2. An improvement in low-field Jc was attributed to the refinement of Gd-211 particles.

Bulk Gd–Ba–Cu–O superconductors were melt processed in air. The Gd 2 BaCuO 5 (Gd211) particles dispersed in the melt-textured bulk could be refined by employing fine Gd211 starting powders. Since T c of the bulk samples was lowered with decreasing the size of Gd211 particle, J c was not improved. However, we found that a depression of T c was prevented by Ag addition, leading to higher J c values. The J c of Gd–Ba–Cu–O/Ag with Gd211 particles about 0.7 μ m in size exhibited 60 000 A/cm 2 at 77 K and self-field.

The relationship between the particle size of Gd211 powder in the precursor and the particle size of Gd211 inclusions in Gd-Ba-Cu-O bulk has been investigated. Gd211 starting powders with various diameters were prepared by the calcination of Gd2O3, BaO2 and CuO powders at different temperatures between 800 and 1000 °C. The particle size of Gd211 in the melt-grown bulk was proportional to the particle size of the initial Gd211 powder. In conclusion, the employment of fine Gd211 powder led to a size reduction of 211 particles in the bulk, while largely enhancing the Jc values in low magnetic fields. A large Gd-Ba-Cu-O/Ag bulk sample, 32 mm in diameter, could also be fabricated by the hot-seeding method. The maximum trapped field value revealed 1.5 T at 77 K.

We studied the effects of Pt and CeO2 addition on the growth of Nd4Ba2Cu2O10 particles using three different starting materials: Nd123-Nd422, Nd422-BaO2-CuO and Nd2O3-BaO2-CuO, all having the same nominal composition of Nd:Ba:Cu = 1.8:2.4:3.4. It was found that both Pt and CeO2 had beneficial effects on the size reduction of Nd422, although CeO2 addition was more effective. Furthermore, it was also clarified that combined addition of Pt and CeO2 was the most effective in refining Nd422 particles and thus increasing the JC values at 77 K.

Superconducting properties of melt-textured (Gd, Nd)-Ba-Cu-O have been studied. Bulk samples were fabricated from precursors of Gd123-Nd422 with Ag2O, Pt and CeO2 in air. Ba-rich Nd422ss (Nd3.6Ba2.4Cu1.8O10-δ) was also used as a starting material. The onset Tc for the bulk sample with stoichiometric Nd422 was only about 80 K. The employment of Ba-rich Nd422ss instead of Nd422 enabled the formation of the high-Tc phase even by melt-texturing in air. Lattice parameters for the 123 phase suggested that the sample with stoichiometric Nd422 formed a LRE1+xBa2-xCu3Oy-type low-Tc solid solution, while the LRE-Ba substitution was suppressed in the sample with Ba-rich Nd422ss. The Jc-B curve for the sample with Ba-rich Nd422ss exhibited a secondary peak effect in about 1 T at 77 K, and the Jc value was larger than that of Gd-Ba-Cu-O bulk prepared in air.

We have studied mechanical properties of melt-processed single-grain Sm-Ba-Cu-O (Sml23+Sm211) bulks with and without silver doping. Tensile stress is induced by thermal stress during heat treatment, and also induced by applying magnetic fields. It is found that the silver addition was effective in decreasing the thermal stress and increasing the tensile strength of the Sm-Ba-Cu-O bulk. The tensile strength of the silver doped sample was 37.4MPa, which is about one and half times larger than that of the undoped sample.

Single-grained bulk superconductors of Gd-Ba-Cu-O 32mm in diameter were grown along the c-axis by melt processing in air. In addition to the stoichiometric 123 composition, the Ba-rich starting compositions of Gd/Ba ratio were also investigated. For comparison, a sample was fabricated in a reduced oxygen atmosphere. In contrast to Sm123 and Nd123, the properties of Gd123 bulk superconductors were not degraded even after fabrication in air. Gd0.95Ba2.05Cu3Oy crystal grown along the c-axis in air showed a Tc of about 93K and Jc of about 3×108(A/m2) at 77K and low applied field. The trapped magnetic field of this sample exhibited about 0.6T at 77K.

Sm123 precursors with various RE211 (RE=Y, Gd and Sm) and Nd422 were melt-textured in a reduced oxygen atmosphere. We have studied the relationship between the size of the 211 phase and critical current density (Jc). When Y211 and Gd211 were added to a starting material, the size of the 211 phase in the bulk sample was remarkably reduced. As a result, the Jc value of these samples exceeded that of Sm-Ba-Cu-O bulk with Sm211 particles. A large c-axis-oriented bulk sample with a 32mm diameter was also fabricated from Sm123-Gd211 precursor with 20wt% Ag2O addition. The trapped magnetic field of this bulk exhibited 1.0T at 77K. This value was higher than that of an Ag doped Sm-Ba-Cu-O bulk of similar size.

We systematically investigated the peritectic decomposition temperatures (Tp) of (Sm1-xGdx) Ba2Cu3Oy solid solutions and superconducting properties (Tc and Jc-B) of binary mixed (Sm1-xGdx)-Ba-Cu-O bulk superconductors melt-processed in a 1%O2-Ar atmosphere. The Tp of (Sm1-xGdx) 123 linearly decreases from 1, 023 to 1, 003°C with increasing x (i.e., the amount of Gd substitution on Sm). The cooling rate of x=0 and 1 needed to fabricate a single-domain bulk is higher than that of x=0.25, 0.50 and 0.75. Although the Tc (93.5-94K) of melt-textured (Sm1-xGdx) BCO is independent of x and the Pt dopant, Jc values increase with Pt doping since the (Sm1-xGdx)2Ba1Cu1Oy is refined.

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.

Crystal structure determination has been accomplished for (K + 0.3 , Cs + 0.7 )-β-ferrite with β-alumina structure to investigate the mixed alkali effect in ionic conduction. According to the structure of conduction plane, BR sites were occupied by cesium ions and mO sites were occupied by potassium ions. It was concluded that blocking effect of cesium ions in BR sites dominate the mixed alkali effect in this ferrite.

We have studied the flux pinning in melt-textured (Nd0.33Sm0.33Gd0.33)Ba2Cu3Oy NSG-123 superconductors with various numbers of Gd2BaCuO5 (Gd-211) particles. Transmission electron microscopy (TEM) showed that submicron Gd-211 particles are uniformly distributed in the superconductive matrix. Dark-field TEM observations further showed that a high density of RE rich RE1+xBa2−xCu3Oy (RE-123ss) clusters 3–10 nm in size were distributed in the NSG-123 matrix. A strongly developed fishtail was observed in the magnetization hysteresis loops of all the samples. A critical current density of 100 kA/cm2 (77 K) was achieved at the secondary peak field of 2 T for the H∥ c axis in the NSG-123 sample with 10 mol % Gd-211. Large grain NSG-123 pellets with 30 mol % Gd-211 and 20 wt % AgO2, 30 mm in diameter and 15 mm in height, exhibited a single-cone profile with a peak value of 1.2 T at 77 K. A higher trapped-field value of 1.5 T was recorded at 2 T, reflecting the secondary peak effect.

The thin films of K + -β- and β″-ferrites have been prepared by dipping-pyrolysis process using potassium tert-butoxide and iron naphthenate. The films exhibited a significant decrease in impedance with an increase in relative humidity. In addition, the magnitude in impedance changes at 25°C was two or three orders and increased with a rise in firing temperature from 600° to 1000°C; that is, with the progress of the crystallization of β and β″ phases. The thin films with the composition of K 2 O·6.1Fe 2 O 3 showed the reversible changes in impedance with the humidity changes. The 95% response time to the humidity change from 43% to 80% RH was 10 s for the film with 1 μm thick obtained at 1000°C. The response time was remarkably shortened in the form of thin film.

The crystal structure of Cd stabilized K + -β″-ferrite was determined by the single crystal X-ray diffraction method. The K + ions occupied only BR sites in the conduction plane, which was different from the alkali distribution in Na + - or K + -β″-alumina. K + content in this crystal is 1.33 per unit plane cell, which is less than the alkali content in other β″-alumina type compounds − 1.7. The K + distribution was explained on the basis of this low K + content. In addition, the conduction slab in K + -β″-ferrite was thicker than that in K + -β″-alumina. This seems to be related to the large activation energy in ionic conduction of this ferrite.

A new hexaferrite (ideal composition: KBaFe 23 O 36 ) with mixed β-alumina and magnetoplumbite structure was synthesized by the solid-state reaction of the mixture 0.45K 2 CO 3 · 0.55BaCO 3 · 6Fe 2 O 3 at 1300 to 1350°C. The alternate stacking structure of half cells of K + -β-ferrite and Ba hexaferrite was proposed based on the X-ray powder diffraction pattern of this ferrite. A similar compound was also found in the Rb 2 OBaOFe 2 O 3 system. The saturation magnetization of the KBa ferrite was very small (1.6 emu/g at 25°C) compared to that of the Ba hexaferrite. The electronic conductivity of the KBa ferrite was 0.3 S · cm −1 at 300°C, which was too high to measure the ionic conductivity based on the β-alumina structure, but the electronic conduction was reduced by MgO doping. The ionic conductivity of Mgdoped KBa ferrite (0.45K 2 O · 0.55BaO · 5.7Fe 2 O 3 · 0.3MgO) was 6 × 10 −3 S · cm −1 at 300°C, comparable to that of K + -β-ferrite.

Thin films of (K+, Na+)-β-ferrites were prepared by the partial replacement of K+ ion in K+-β-ferrite with Na+ ion to obtain a humidity sensing element having almost linear characteristic in terms of log-impedance vs. relative humidity, with humidity-sensitive characteristic controlled by changing the K+/Na+ proportion. Replacing 4mol% of K+ ion with Na+ ion yielded the thin films showing almost linear characteristic. The impedance of the film changed by more than two orders of magnitude with an increase in relative humidity. For a change in relative humidity from 43% to 80%, 95% of the total change was completed within a response time of only 10s. For the reverse change, the response time was less than 15s. These values were faster than expected. Further, the element had excellent durability in terms of humidity-sensitive characteristic. This suggests that thin films with an K:Na alkali ratio 0.96:0.04 is applicable to humidity sensors in terms of linearity in log-impedance change, magnitude of impedance change, response time and durability.

Co-containing γ-Fe2O3 is not prepared by solid state reaction of γ-Fe2O3 and CoO at high temperature but is prepared by wet method, because γ-Fe2O3 transforms into α-Fe2O3 above 400-500°C. However, γ-Fe2O3 is stabilized by the addition of potassium. To be reported here is a finding that the solid state reaction of acicular γ-Fe2O3 and Co(NO3)2⋅6H2O can be accomplished by the addition of K2CO3. When a mixture of γ-Fe2O3:Co(NO3)2⋅6H2O:K2CO3=5:1:0.2(molar ratio)was heated at 500-600°C, the coercivity of the product was above 1000 Oe. In addition, the magnetic property was also improved in the squareness ratio compared with the original γ-Fe2O3. The particle morphology of the obtained Co-containing γ-Fe2O3 was maintained acicular even after the heat treatment.

The samples consisting of SmBa2Cu3Oy (Sm123) precursors with RE2BaCuO5(RE211, RE=Y, Gd and Sm) or Nd4Ba2Cu2O10 (Nd422) were melt-textured in a reduced oxygen atmosphere. We have studied the relationship between the size of RE211 particles and the critical current density (J c ). When Y211 and Gd211 were added to a starting material, the size of RE211 phase particles in the bulk sample was remarkably reduced. As a result, J c values of these samples were larger than those of Sml23/Sm211 sample. The large c-axis oriented bulk sample 32mm in diameter was successfully fabricated from Sml23-Gd211precursor with 20wt% Ag2O addition. The trapped magnetic field of this bulk reached l.0T at 77K.

We have measured the temperature dependence of the in-plane thermal conductivity, κab(T), for both Er–Ba–Cu–O and Ho–Ba–Cu–O superconducting bulks. The absolute values of κab(T) for both bulks are smaller than that of Y–Ba–Cu–O bulk, but are larger than that of Dy–Ba–Cu–O bulk, which is known to show an anomalous small κab. κab(T) for Ho–Ba–Cu–O shows a positive slope (dκab/dT>0) in the normal state, similarly to that for Dy–Ba–Cu–O. On the other hand, a slightly negative slope (dκab/dT

We employed Ba–Cu–O substrates for fabrication of bulk Y–Ba–Cu–O superconductors using the top-seeded melt-growth method. There were several advantages of using a Ba–Cu–O substrate as compared to conventional substrate materials such as MgO, ZrO2, Al2O3, RE123 (RE = rare earth) and RE211. The Ba–Cu–O substrate avoided crystallization from the substrate, suppressed liquid loss and scarcely reacted with the precursor. Furthermore, the introduction of large-sized cracks into a grown bulk was greatly suppressed by propagation along the interface between the grown bulk and the substrate. We were able to obtain bulk Y–Ba–Cu–O superconductors with uniform trapped magnetic field distributions and equal maximum trapped field 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.

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.

Pulsed-field magnetization (PFM) was applied to a Gd-Ba-Cu-O high-temperature superconductor (HTS) bulk of 140 mm diameter by using a split-type pulsed current armature coil. Two kinds of split-armature coils with 100 mm and 140 mm diameter were tested. The PFM with the coil of 100 mm diameter is effective to obtain a conical shape of the trapped flux-density distribution. Another coil of 140 mm diameter contributes to enhance the total magnetic flux. We fabricated a single vortex coil which plays a role as 2, those are 100 and 140 mm in one of them, that is a controlled magnetic density distribution coil (CMDC). The CMDC is composed of a vortex-type coil with 100 mm diameter surrounded by additional winding forming an air core solenoid with an outer diameter of 140 mm. We obtained a trapped flux-density distribution with conical shape together with the maximum trapped flux-density up to 0.6 T which is equivalent with 30 % of the value in the field cool magnetization (FCM) and total flux density with 3.1 mWb which is 40 % of the result of the FCM.