Table of contents

Superconducting melt-textured bulk (RE)BCO large grain materials are one of the most promising materials for power applications of high temperature superconductivity at the liquid nitrogen temperature range. Industrial applications are expected in high-speed low-loss magnetic bearings for flywheel energy storage devices, high-dynamic high-torque electric reluctance motors, and MAGLEV transportation systems. The material has high magnetic field trapping capability and therefore a new class of high-field superconducting permanent magnets will soon appear. However, there is still the need to improve the magnetic and mechanical material properties, as well as to increase the single domain size.

This special issue contains papers concerning these topics presented at the International Workshop on the Processing and Applications of Superconducting (RE)BCO Large Grain Materials. The workshop was held on the 30 June-2 July 2003 in Jena, Germany, and was organized by the Institut fuer Physikalische Hochtechnologie, Jena. It was the fourth in the series of PASREG workshops after Cambridge, UK (1997), Morioka, Japan (1999), and Seattle, USA (2001). Sixty two contributions were presented at the workshop, 38 oral presentations and 24 poster presentations. This special issue contains 42 papers.

The editors are grateful for the support of many colleagues who reviewed the manuscripts to guarantee their high technical quality. The editors also wish to thank Doris Litzkendorf and Tobias Habisreuther from Institut fuer Physikalische Hochtechnologie, Jena, for their assistance with the organization and handling of the manuscripts. Many thanks to the workshop co-chairman Gernot Krabbes from Leibniz-Institut fuer Festkoerper und Werkstoffforschung, Dresden, for hosting the workshop participants in Dresden. Finally, all attendees wish to acknowledge the efforts of Wolfgang Gawalek, Tobias Habisreuther, Doris Litzkendorf and the Team of Department Magnetics from the Institut fuer Physikalische Hochtechnologie, Jena, for being generous hosts during the workshop.

The International PASREG Board selected the following distinguished researchers as recipients of the 2003 PASREG Award for Excellence to acknowledge their contribution to the development of bulk high-temperature superconductors: Masato Murakami, ISTEC Tokyo; Günter Fuchs, IFW Dresden; Uichiro Mitzutani, Nagoya University; Bernhard Oswald, OSWALD Electric Motors Co. Miltenberg; Anna E Carillo, Teresa Puig and Xavier Obradors, ICMAB Barcelona.

We have previously presented a design for a cosmic microwave background (CMB) polarimeter in which a cryogenically cooled half-wave plate rotates by means of a high-temperature superconducting (HTS) bearing. Here, a prototype bearing, consisting of a commercially available ring-shaped permanent magnet and an array of YBCO bulk HTS material, has been constructed. We measured its coefficient of friction and vibrational property as a function of several parameters, including temperature between 15 and 83 K, rotation frequency between 0.3 and 3.5 Hz, levitation distance between 6 and 10 mm and ambient pressure of  Torr. We concluded that the low rotational drag of the HTS bearing would allow rotations for long periods with minimal input power and negligible wear and tear, thus making this technology suitable for a future satellite mission.

Melt-textured YBCO superconductors are capable of carrying higher current densities than comparable copper conductors, and can therefore be used in electromechanical devices requiring high power densities. The advantage of textured YBCO superconductors is most pronounced in large systems such as electromagnetic launchers for aircraft carriers. In general, an electromagnetic launcher consists of a series of stationary pulsed electromagnets (the primary), which attract and/or repel a carriage carrying one or more magnets (the secondary). Several possibilities exist for the incorporation of HTSs into EM launchers, with the most direct being upgrading the magnets in the secondary to melt-textured YBCO. A system was developed to study propulsive force in a coaxial-type launcher. Results from this study are presented and their implications for launcher development discussed. A second type of launcher was also studied, with a power supply integrated into the launcher primary, so that the primary serves as a superconducting magnetic energy storage system. A method of optimizing energy conversion in a system of this type has been found. The time dependence of the magnetic field in this type of launcher is presented.

To optimize the superconducting properties of Ni-doped YBCO by selective substitution of Ni dopants on Cu(2) plane sites, the effects of annealing treatments in reduced oxygen atmosphere have been studied. The results indicate that the superconducting transition temperature, T_c, of the Ni-doped samples with optimal oxygen content further decreases after heat treatment in reduced oxygen atmosphere. This suggests that substituted Ni at the Cu(1) sites moves to the Cu(2) sites. However, the flux pinning capabilities of the sample optimally doped with Ni after heat treatment in reduced O₂ atmosphere are weaker compared to melt-processed YBCO doped with Li or Zn.

Porous superconductors have inherent cooling advantages over their bulk counterparts and, as a result, are emerging as an important class of materials for practical applications. Single-domain Y–Ba–Cu–O (YBCO) foams processed with a pre-defined, open porous structure, for example, have significant potential for use as elements in resistive superconducting fault current limiters. In this case, the interconnected porosity is ideal for producing reinforced composites with improved mechanical and heat conducting properties. In this paper we describe a few simple methods for fabricating large grain YBCO superconductors with various predefined porous structures via an infiltration process from tailored, porous Y₂BaCuO₅ (Y-211) pre-forms manufactured by a variety of techniques, including slurry-coating of standard polyurethane foams to replicate their structure. Foams produced by this method typically have a strut thickness of a few hundred µm and pore sizes ranging from 10 to 100 pores per inch (PPI). Foams with increased strut thickness of up to millimetre dimensions can be produced by embedding organic ball spacers within the Y-211 pre-form followed by a burn-out and sintering process. Single-domain YBCO bulk materials with cellular and pre-defined 3D interconnected porosity may be produced by a similar process using tailored wax structures in Y-211 castings.

The progress of heavy-load HTS bearings depends on improvements in design, material quality and reliable cooling. We have constructed, manufactured and tested a 200 mm HTS journal bearing with a thermally encapsulated YBCO ring. For maximum force the larger gap due to the bearing cryostat (>4 mm) requires adjustment of the magnetic excitation pole distance and the Fe collector shim thickness. HTS material progress is obtained by top-seeded single- or multiple-grain growth which increases the averaged trapped magnetic flux density. Successful YBCO ring growth with radial c axis distribution by seeding the inner ring surface has been performed. The encapsulation ensures a substantially reduced cryogenic effort and stabilizes bearing operation at 78–79 K.

In recent years we have successfully designed, built and tested several reluctance motors with YBCO bulk material incorporated into the rotor, working at 77 K. Our last motor type SRE150 was tested up to 200 kW. The aim of our investigations is the construction of motors with extremely high power density and dynamics. In comparison to conventional motor types the advantage of HTS reluctance motors with respect to size and dynamics could be demonstrated. Some fields of possible future applications will be described. These motors show a significant improvement in performance using high quality HTS bulk elements in the rotor. Until now the motor parameters have been limited by the current density which could be obtained in the bulk material at 77 K and by the geometric dimensions of the segments available. Therefore we expect further improvements in the case of these materials. Since the total motor including stator and rotor is working at low temperature we have to optimize the windings and the magnetic circuit to these operation conditions. A new design of a 200 kW motor in order to achieve increased power density and the theoretical results of our calculations will be shown.

A magnetized c-axis oriented single-domain Sm123 bulk superconductor of 60 mm in diameter was employed in place of a conventional Nd–Fe–B permanent magnet in a planar-type magnetron sputtering apparatus and its sputtering performance was studied by measuring the Ar gas pressure dependence of deposition rate for elemental targets Cu, Ni, Fe, C and Al of different thicknesses. The value of on the surface of the Cu target of 3 mm thickness reached 0.63 T in comparison with 0.05 T for the Nd–Fe–B magnet in conventional magnetron sputtering. The use of such a high value enabled us to carry out sputtering down to the extremely low pressure of 1 × 10⁻³ Pa, which is two orders lower than in a conventional procedure.

Grain boundaries of melt-textured YBa₂Cu₃O_y (YBCO) are preferentially doped with silver by a solid-state diffusion method. Microstructural and local compositional analysis reveals that the Ag exists in the YBCO as individual atoms with their distribution being highly localized within a thin layer near the grain boundaries. Compared with the pristine samples, the Ag-doped YBCO textured polycrystals show a significant improvement in the J_c–H behaviour. The value of J_c at 60 K and 7 T reaches 4.6 × 10⁴ A cm⁻², which is almost three times the value of J_c for the undoped samples. We propose a model in which Ag could partially replace the Cu in CuO₂ planes near the grain boundaries of YBCO, therefore reducing the geometric distortion of Cu–O bonds and making the effective grain boundary thinner.

We report the melt processing of Nd-123 precursor pellets enriched with Nd₄Ba₂Cu₂O₁₀ (Nd-422), Y₂BaCuO₅ (Y-211) and/or nano-sized Y₂O₃. Microstructural analysis reveals that two different second phase particles of distinct acicular and spherical morphologies form during solidification in the samples containing Y. A large fraction of the volume of the sample prepared with excess Y₂O₃ contains 300–500 nm sized, spherical-shaped particles. The width and length of the acicular shaped particles in the samples are <1 µm and , respectively. 90% of second phase particles are of –600 nm size. The transition temperature, T_c, is below the value expected for NdBCO due to the formation of a (Y_1−xNd_x)Ba₂Cu₃O_7−δ solid solution phase. The superconducting properties, T_c and J_c, are homogeneous within (Y, Nd)BCO single grains compared to that of NdBCO due to suppressed Nd substitution in the Ba site. Significantly improved critical current densities derived from magnetic moment measurements are observed at 77 K in this system compared to both YBCO and NdBCO.

Nano-pinning centres ( nm) have been successfully introduced into melt-textured YBCO by doping NiO. The doping of NiO was realized by adding NiO nanoparticles to presintered YBCO followed by melt-texture processing. This method can reduce the substitution level of Ni in the YBCO matrix and can create Ni-rich second-phase precipitates with a size around 10 nm. The critical current density of these Ni-doped samples is significantly increased and the J_c–H behaviour is remarkably improved. Our results suggest that NiO-induced nano-pinning centres may have high pinning efficiency at a temperature around 70 K and be one of the main factors for improving the performance of YBCO. Our study reveals that nano-pinning centres are also a feasible solution to solve the pinning degradation at high temperature in YBCO.

We report on a new type of nanometre-scale pinning defect in melt-processed (Nd,Eu,Gd)Ba₂Cu₃O_y 'NEG-123' with a Gd₂BaCuO₅ (Gd-211) addition. A reasonable amount of the Gd-211 particles refined by a long-term ball milling to about 70–200 nm reduced their size during the melt-texturing process down to 20–50 nm. Chemical analyses revealed that these particles contained a significant amount of zirconium. These particles caused J_c enhancement by an order of magnitude not only at 77 K but also at 90 K. As a result, we could levitate a permanent magnet over the (Nd,Eu,Gd)Ba₂Cu₃O_y superconductor cooled by liquid oxygen. This is a path to non-contact pumps of liquid oxygen for different fields of practice.

(Sm,Eu,Gd)Ba₂Cu₃O_7−δ (RE123) single-domain superconductors were grown in 1% O₂ in Ar with a modified hot-seeding melt process. Spatial superconducting variation and a local RE–Ba modulation were suppressed with the control of processing conditions. Pre-annealing at 960 °C for 10 h in Ar prior to melt growth was found to significantly lower the range of RE–Ba substitution and diminish the number of pores and microcracks. Excess BaCuO_2−x was added to induce a Ba-enriched liquid and thereby hinder the nucleation of RE_1+xBa_2−xCu₃O_7−δ solid solution (RE123ss). The proper ratio of BaCuO_2−x to RE123 was around (0.17–0.25):1. The optimal thermal schedule was a fast cooling with a rate of 2 °C h⁻¹ for 10 h followed by a 100 h slow cooling of 0.15 °C h⁻¹. A field of 1 T was trapped in a pellet of 35 mm diameter at 77 K and 2 T, which corresponded to the maximum external flux density of 3 T including the bias applied field.

The evaluation of current distributions in thick superconductors from field profiles near the sample surface is investigated theoretically. A simple model of a cylindrical sample, in which only circular currents are flowing, reduces the inversion to a linear least squares problem, which is analysed by singular value decomposition. Without additional assumptions about the current distribution (e.g., constant current over the sample thickness), the condition of the problem is very bad, leading to unrealistic results. However, any additional assumption strongly influences the solution and thus renders the solutions again questionable. These difficulties are unfortunately inherent to the inverse Biot–Savart problem in thick superconductors and cannot be avoided by any models or algorithms.

The dynamic response of cylindrical and ring-shaped YBaCuO bulk parts to pulsed magnetic fields is calculated by using small sets of finite elements. Some comparisons with experimental results are provided, and they give confidence in the modelling of the superconducting properties. Transient magnetizations as a function of time and space as well as shapes and absolute values of trapped magnetic flux profiles are presented. The influence of the sample geometry is investigated for different millisecond pulsed magnetization processes. Results are reported for different radial thicknesses and heights, different pulse durations, peak magnetic fields and pulse sequences with and without stepwise cooling. Comparisons concerning the achievable trapped magnetic field and flux are made, and implications for the use of high-temperature superconductor bulk parts as cryo-permanent magnets in potential applications like electric machinery are discussed.

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 have succeeded in constructing a superconducting permanent magnet system capable of generating 3.2 T by melt-processed bulk superconductors. The trapped field was measured in the open gap between the magnetic poles settled face to face after the pulsed-field magnetization technique called the 'IMRA' method operated at 38 K.

A novel face-to-face field generator has been composed by a pair of wide magnetic poles installed with seven bulk superconductors arranged in one plane. The magnetic pole is 150 mm in diameter. The performance of the magnetic field by the field cooling method by a 5 T conduction-cooled superconducting solenoid reached 1.4 and 0.9 T at the centre of the vacuum chamber surface and at the centre of the open space between the magnetic poles with a gap of 34 mm.

The intense magnetic fields are investigated for application in various industries such as the magnetic separation technique and so on in the Iwate CREATE (Collaboration of Regional Entities for the Advancement of Technological Excellence) project supported by the Japanese government.

A new Ag trapped particle morphology has been discovered in melt-textured YBa₂Cu₃O₇/Ag composites where the interface energy between particle inclusions and a solid matrix has been enhanced with BaZrO₃ additives. The enhanced pushing effect generates square-like macrosegregation bands where the secondary additives Y₂BaCuO₅, BaZrO₃, and Ag are accumulated. It is shown that elongated Ag particles with a long axis and aspect ratios as high as a can be trapped in the YBa₂Cu₃O₇ matrix free of any other additive with a very anisotropic orientation. It is demonstrated that the elongated Ag particles lie with the long axis parallel to the growth direction in all the growth sectors generated by the top seeding growth. The pushing–trapping theory is used to explain qualitatively the unusual phenomenon of a growth-induced morphological shaping of inclusion particles.

High-temperature superconducting (HTS) fault current limiters (FCLs) can make electric power networks more reliable. For an inductive type FCL we have developed and applied a special design to significantly reduce the impedance in normal operation mode, and to properly adjust the impedance in limitation mode. The FCL can be designed with predictable parameters, and can withstand thousands of activation and recovery cycles. For practical application of HTS FCLs in industrial environments we have investigated the possibilities to increase power rating and reliability. For the purposes mentioned we have examined various FCL topologies: series, parallel and matrix arrangements. In this paper we present the results of experimental studies of the mentioned and practically useful FCL topologies. Tests were performed for series and parallel connections of FCLs to investigate the fault processes in the individual units and the resultant effects of the whole FCL group.

Theoretical and experimental studies have led to an overall design for superconducting magnetic bearings (SMB) that is suitable to meet the requirements of industrial applications. The main benefits are high load capacities, compact dimensions and a 'warm' suspended part of the application. Two applications have been designed with a suspension provided only by SMB; one has already been built and tested successfully.

In this paper we present investigations of the growth-related structure of melt-textured YBCO by means of magnetic measurements. We used the well established trapped field mapping as well as a new technique where the scanning probe consists of a Hall probe and a permanent magnet which magnetizes the sample locally during the scanning process. In order to investigate the properties in the whole sample volume, two samples were cut into six slices parallel to the ab-planes or into two pieces parallel to the c-axis, respectively. The results clearly show the growth sector structure and allow the calculation of the local critical current density. For the ab-growth sector we obtained a critical current density of j_c = 5 × 10⁴ A cm⁻² and for the c-growth sector j_c = 1.5 × 10⁴ A cm⁻² at T = 77 K and  mT.

The two key parameters for levitation performance over a permanent magnet (PM) guideway are the levitation force and the stiffness of the high temperature superconductor (HTS) YBaCuO bulk. Their distributions along a lateral displacement (LD) were measured in zero-field-cooling (ZFC) and in field-cooling (FC) conditions. There are two cases of applied magnetic field for a single HTS bulk: symmetrical or unsymmetrical about the bulk axis. Although the levitation force has a linear relationship with the applied field in the symmetrical case, such a linear relationship disappears once the applied field becomes unsymmetrical. Furthermore, the levitation stiffness is measured in FC, and they still change more drastically than the levitation force in ZFC. In particular, the levitation stiffness has a linear relationship with the associated levitation force, whether the applied field is symmetrical or unsymmetrical. Its lateral distribution at a gap of 20 mm in FC agrees with the levitation force distribution at a gap of 10 mm in ZFC.

Guidance force is calculated when a high-temperature superconductor moves laterally on a permanent magnetic guideway. Both the superconductor and the guideway are infinitely long. The dependence of guidance performance upon both geometrical parameters, such as the shape of the superconductor and the guideway, and intrinsic parameters, such as critical current density of the superconductor and uniform magnetization of the guideway, are studied. The results may be helpful to the design and optimization of the superconducting magnetically levitated train system.

Superconducting (SC) bearings are one of the most promising applications of superconductors, which could contribute significantly to a relevant progress to kinetic energy storage. A key feature of these bearings with a special view on their industrial applications is the coefficient of friction. Losses of different configurations are intensively investigated worldwide. Numerous structures and geometric arrangements have been developed in order to decrease the coefficient of friction and to increase the utilization of the superconductors (SCs) and permanent magnets (PMs) (Mulcahy et al 2001 IEEE Trans. Appl. Supercond.1 1729–32; Sung et al 2002 Cryogenics42 357–62; Filatov and Maslen 2001 IEEE Trans. Magn.37 3913–23)

In this paper we present mostly experimental analysis of multi-layer, multi-pole magnetic configurations. On the basis of the results of these measurements we tried to determine the best configuration regarding the losses, stability and load capability of the bearings.

Spin-down measurements were performed in a vacuum chamber (10⁻⁴ mbar), in order to measure the coefficient of friction, and to verify the stability of the superconducting bearings over a high speed range. For the measurements a special small brushless permanent magnet DC motor with an ironless stator was developed and used. A maximum speed of 11 000 rpm was successfully reached.

Nano-sized Sm₂BaCuO₅ ('Sm211') and Nd₄Ba₂Cu₂O₁₀ ('Nd422') non-superconducting particles were prepared by a typical sol–gel process of mixing poly-acrylic acid (PAA) with metal ion solutions. Small amounts (1 to 1 × 10⁻² wt%) of nano-sized Sm211 and Nd422 powders were mixed homogeneously with the precursors of Sm123+25 wt% Sm211 followed by the melt-texture processing in air. It is observed that superconducting properties of SmBCO with nano-Sm211 or nano-Nd422 additives were greatly enhanced and a clear secondary peak effect was observed. The scale rule analysis shows that the δT_c pinning mechanism may be responsible for the J_c enhancement at high field. A systematic study of the relationship of J_c(H,T) with different amounts of nm-Sm211 and/or Nd422 additions was performed to determine the optimum nano-particle doping contents and their pinning mechanisms respectively.

Several research efforts on improving the performance of melt-processed bulk superconductors are reported. First, we extended our study on the Nd–Eu–Gd (NEG) system by exploring the effect of ball milling the secondary phase powders of the precursor, and observed an increase in the critical current density (J_c) as well as in the trapped field. We also studied the effect of changing the ratio of the rare earth elements of the secondary phase powders of the NEG system. The samples prepared with a larger amount of Gd were found to trap a larger magnetic field without fracturing. On the other hand, the samples with a nominally Nd-rich composition showed a somewhat lower critical temperature. We also studied the effect of doping Zn into DyBCO, and found a favourable influence in terms of J_c and trapped field. Calculations of the magnetic flux density above a field-trapped superconductor indicated that increasing the domain size is very effective to improve the performance of the superconductor even if the larger sample cannot be prepared with the same quality as a smaller one. Therefore, we studied the melt-processing of SmBCO superconductors that are 60 mm in diameter. It was crucially important to release a sufficient amount of oxygen from the melt for the preparation of a homogeneous sample, and it is demonstrated that the magnetic flux usable in applications significantly increased on enlarging the domain size.

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.

The angular dependence of the peak effect was investigated in melt-processed (Sm,Eu,Gd)Ba₂Cu₃O_7−δ crystals by rotating the angle, θ, between the applied fields and the ab-plane. The peak field, at which the magnetic moment shows an unusual maximum, shifted to a higher field, associated with a systematical decrease of the peak amplitude with θ inclining to zero. For , the peak effect displayed the highest peak field and the minimum peak height. Meanwhile, out-of-plane ordering of compositional modulation was unveiled by transmission electron microscopy. Such a three-dimensional distribution of chemical fluctuation could account for the angular dependence of the peak effect.

An infiltration and growth process is here used as an alternative to the classical top-seeded melt-textured growth process for the production of Dy-123 single-domains with finely dispersed small size Dy-211 particles. The starting materials are the 211-particles and a barium and copper rich liquid phase precursor. The infiltration and growth process allows for controlling both the spatial and size distribution of the 211-particles in the final superconducting 123-single-domain. The main parameters (set-ups, maximum processing temperature with respect to the peritectic temperature, nature of reactant, porosity of the 211-preform) of the infiltration and growth process are discussed. Moreover, different processes of chimie douce are shown in order to produce Dy-211 particles with controlled shape and size, particles that can be used as precursors for the infiltration and growth process.

Microstructural changes during the oxygenation of YBCO bulks were studied. It was shown that a lower temperature of oxygenation leads to the formation of a denser structure of a/b- and c-macrocracks and causes faster and more homogeneous oxygenation of the sample. The opening of created macrocracks is the way in which the macroscopic stresses induced by macroscopic 211 particle concentration inhomogeneity are released. This is very important, because it prevents the formation of fatal c-macrocracks, which divide the sample into more domains, during cooling from oxygenation temperature or during sample performance. Oxygenation with a multistage programme causes the oxygen concentration difference between the oxygenated layer and the tetragonal matrix to be smaller, and consequently fewer macrocracks are formed. This leads to the prolongation of oxygenation times for full oxygenation and to the insufficient release of macroscopic stresses. 211 low concentration regions and pores also enhance the oxygenation rate of YBCO bulks.

((RE)_0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_y bulk superconductors (RE =  Sm, Nd) were textured by the top seeded melt textured growth method in a reduced oxygen atmosphere, so-called oxygen controlled melt growth (OCMG). Large single domains of diameter 16 mm were successfully grown. Their microstructure and their magnetic and transport properties were investigated and compared. The resistive transition was found around 88 K, which is not the optimal T_c of 95 K that could be obtained on a small re-oxygenated sample cut out from the bulk. Transport measurements were also performed on these samples at various temperature from 77 to 88 K. In the case of Nd_0.33Eu_0.33Gd_0.33Ba₂Cu₃O_y (NEG) composition, the angular dependence of the critical current densities along the (a,b) planes as a function of magnetic field was also measured.

Transport critical current densities higher than 50 000 A cm⁻² are reached at 77 K and up to 2 T, corresponding to the nominal critical currents of 90 A injected through sections less than 0.25 mm² reproducibly. This confirms the high quality of the single domain obtained with a well controlled process, evidencing that mixing of rare earth is a very promising composition for transport and high-field applications.

The formation of superconducting junctions between MT-YBCO using TmBa₂Cu₃O_7−δ powder as a solder has been studied. The method proposed excludes the step of a very slow cooling (at a rate of several degrees per hour) during seam formation. The heating and cooling rate for joining parts produced from single-domain material without visible cracks (macrocracks) can be rather high (500–1000 K h⁻¹) and a holding time at the highest temperature (1010 °C) of several minutes (0.05 h) is enough to form a reliable junction. Reasonable rates of heating and cooling are however around 100 K h⁻¹ if crack propagation is to be avoided in joined blocks used for practical application. Modelling experiments on rings and studies of the ring properties by vibrating sample magnetometer (VSM), field mapping with a Hall probe and magneto-optical microscopy have shown that superconducting properties of the junction were not lower than that of the joined material (j_c of about 30 kA cm⁻² was observed in zero field at 77 K) and that the proposed process of joining did not adversely affect the properties of the material. The structure of the resulting junction was in good agreement with the structure of MT-YBCO.

By means of automated electron backscatter diffraction (EBSD) analysis, we studied the local orientations of embedded Y₂BaCuO₅(211) particles within melt-textured YBa₂Cu₃O_x (YBCO) samples. On both components, we obtained high-quality Kikuchi patterns, thus allowing the automated mapping of the crystal orientations and a two-phase analysis of the samples. Investigations were performed on a variety of melt-textured YBCO samples. In melt-textured YBCO with (001) orientation, we find that the embedded 211 particles do not have any preferred orientation. The EBSD maps also reveal that at certain orientations of the 211 particles the YBCO growth is not altered. From the obtained EBSD mappings, we can conclude that the formation of small 211 particles will not disturb the YBCO matrix growth, whereas the presence of large 211 particles causes severe changes in the YBCO growth, leading to the formation of subgrains. The EBSD results are presented in the form of local orientation maps and local pole figures.

Scanning Hall probe measurements were made on melt-textured Y-123 monoliths under sequentially increasing activation fields. The remnant field profiles are compared to measurements of the local response of the superconductor to a small magnetic field induced by a permanent magnet ('magnetoscan'). Optical microscopy is employed to relate the flux patterns with the microstructure of the sample.

The interface melting process of melt-textured Y Ba₂Cu₃O₇ induced by Ag foils has been investigated in view of developing a large area welding technology. The influence of the processing methodology and the initial microstructure of the melt-textured ceramics has been considered. High temperature quench experiments have shown that the interface molten region can become inhomogeneous due to microstructural features of the initial ceramics such as the pre-existing cracks. It is found that while thick Ag foils () can lead to severe liquid Ag migration, the artificial welds performed with thinner foils () in non-oxygenated melt textured Y Ba₂Cu₃O₇ samples free of cracks are of high quality in large areas. High critical currents through welded interfaces have been demonstrated by means of Hall probe magnetic imaging and Biot–Savart inversion calculations.

On-going, round robin tests of the trapped field at the top and bottom surfaces of two carbon fibre reinforced polymer (CFRP)-clad, epoxy resin-reinforced large grain samples of Sm–Ba–Cu–O fabricated at ISTEC, Japan, by top-seeded melt growth (TSMG) have been performed by four European laboratories over a period of 16 months. Flux profiles were measured by scanning Hall probe by the participating groups and were found to agree to within approximately 7% over the measurement period when the effects of incomplete flux penetration in some of the tests were taken into account. Overall the measurements suggest that the trapped flux profile is most sensitive to changes in magnetizing field, experimental geometry and the sample–Hall probe separation. As a result, variations in these parameters should be minimized during flux mapping. In general a magnetizing field of times the maximum trapped field for field cooled (FC) samples is required to ensure complete magnetization of a homogeneous cylindrical sample with an aspect ratio of , corresponding to the geometry of the samples measured here. Finally, 20 min relaxation time following magnetization was observed to be sufficient to yield consistent measurement of the peak trapped field within the error of the measurement, which was typically less than 4%. Other than the recommended magnetizing field of 3 T, which should be determined by aspect ratio, temperature and maximum trapped field rather than be predefined, the experimental conditions were confirmed by this interim report generally to lie within the guidelines described in a draft International Electrotechnical Commission (IEC) Technical Committee 90 (TC90) standardization document on flux mapping. On the basis of the present preliminary results the proposed 5% spread in measured trapped field and a 20 min relaxation period do not appear to be unreasonable criteria for trapped field measurements performed at different laboratories.

An experiment was performed to test the effect of different uranium compounds on the properties of chemical pinning centres. UO₂, UO₃, and UO₄·2H₂O wereadmixed to Y 123+Pt, and textured. Tests of J_c via measurements of trapped field (B_trap) indicate a clear dependence of B_trap on the U compound admixed to create the pinning centres. In all three cases there is a monotonic increase in B_trap as the mass (M_U) of U is increased. However, the magnitude of the increase in B_trap depends on the admixed U compound. The highest increase in B_trap is measured in samples doped with UO₄·2H₂O, and the lowest is obtained in samples doped with UO₂. Microstructure studies indicate that the composition of the U-rich pinning deposits is the same in all three cases, i.e. all are the previously identified (U_0.6Pt_0.4)Y Ba₂O₆ compound. The primary difference among the three types of samples is that the size of the U–Pt–Y–Ba–O pinning deposits depends on the admixed U compound. While all are in the nanometre domain, the diameter of these deposits was markedly larger in UO₂ doped samples than in UO₃ doped samples, and smallest in UO₄·2H₂O doped samples. Because some form of poisoning limits the amount of U that can be added to create pinning centres, to  wt%, smaller deposits result in a greater number of pinning centres. We conclude that UO₄·2H₂O is more effective than either UO₃ or UO₂ in the formation of U–Pt–Y–Ba–O pinning centres because of diminished pinning centre size, and consequent increase in pinning centre density.

Transmission electron microscopy observations of melt-textured YBa₂Cu₃O₇–Y₂BaCuO₅ composites deformed under uniaxial stress and a superposed gaseous confining pressure reveal the activation of a novel YBa₂Cu₄O₈-type stacking fault source. This mechanism operates as a partial Frank Read source leading to small diameter (100–300 nm) loops. J_c^ab(T) dependences reveal a temperature dependent enhancement which is maximum at low temperatures, i.e., 45% at 5 K. A fit of J_c^ab(T) to a model that considers two contributions to J_c, namely correlated disorder and weak pinning, reveals that only the weak pinning contribution associated with the newly generated in-plane dislocation substructure is enhanced after deformation.

Columnar pinning centres provide the largest pinning potential, U_pin, but not the greatest J_c or pinnable field, B_pin. Characteristics of ion-generated columnar defects which limit J_c and B_pin are discussed, including reduction of the percolation path, and the need for a larger number of columns of damage, for pinning, than are usually estimated. It is concluded that columnar pinning centres limit B_pin to less than 4 T, and also severely reduce J_c. The goal of maximizing U_pin, via columnar centres, appears to have obscured a more rewarding approach and resulted in neglect of a large regime of ion interactions. Evidence is reviewed that multiple in-line damage (MILD), described herein, can provide orders of magnitude higher J_c and B_pin, despite providing lower U_pin. The MILD pinning centre morphology is discussed, and it is estimated that for present-day large grain high T_c superconductors, a J_c value of is obtainable at 77 K, even when crystal plane alignment and weak links are not improved. In addition, the pinned field is increased by over an order of magnitude. An experiment is proposed to confirm these calculations, directly compare MILD pinning to continuous columnar pinning, and determine the optimum MILD structure. Applications of MILD pinning are discussed.

For an efficient preparation of SmBCO seeds in high quantity and quality we investigated two different ways to manufacture Sm123 grains. Sm123 bulks were grown by the top-seeded melt-textured growth method. Using MgO seeds single-grained Sm123 samples with a diameter of 18 mm and a thickness of 10 mm were obtained. Disc-shaped, large-sized SmBCO grains were prepared along the a–b cleavage plane of the bulks. Subsequently, the material was carefully separated into a large quantity of about 2 × 2 × 2 mm³ sized seeds with well-known crystallographic orientation. The results imply the feasibility of a production of larger SmBCO bulks in a batch process. In a second approach, Sm123 bars were melt-processed by a modified Bridgman method in a vertical furnace. Long bar samples were prepared with typical sizes of 7 mm in diameter and 140 mm in length. The sintered bars were pulled up across a hot zone of a maximum temperature of 1090 °C and a longitudinal thermal gradient of 15 K cm⁻¹. For the translation of the samples different constant pulling rates from 0.3 to 5.5 mm h⁻¹ were tested. The microstructure of the samples was investigated by means of polarized optical microscopy.

We investigated the effect of Dy₂BaCuO₅ (Dy-211) addition to DyBa₂Cu₃O_x (Dy-123) bulk on superconducting properties. The composite was fabricated by a top seeding method in a temperature gradient in air. The single-domain Dy-123 pellets were about 18 mm in diameter and 5 mm in thickness. The maximum remnant J_c value and the maximum trapped field at 77.3 K were 30 kA cm⁻² and 0.23 T, respectively, for the molar ratio Dy-123/Dy-211 of 1/0.3. In the samples with molar ratios 1/0.1, 1/0.2 and 1/0.3, the number of dispersed Dy-211 particles increased with increasing Dy-211 content. In the sample with the molar ratio 1/0.4, a coarsening of Dy-211 particles was observed.

In this paper we present measurements of the thermal expansion of melt-textured YBaCuO in the temperature range 30–300 K by means of optical fibre sensors. The sample, which had a size of 38 × 38 × 18 mm³, was prepared by our standard melt-texturing process using SmBaCuO seeds. One fibre containing three Bragg gratings which act as strain sensors was glued to the sample surface with two sensors parallel to the ab-plane and one sensor parallel to the c-axis. The sample was cooled down to a minimum temperature of 30 K in a vacuum chamber using a closed cycle refrigerator. In the temperature range we used, the thermal expansion coefficients are in the range of (3–9) × 10⁻⁶ K⁻¹ (ab-direction) and (5–13) × 10⁻⁶ K⁻¹ (c-direction).

Results on an established batch process preparing melt-textured YBCO in high quality and quantity will be reported. We used a standard composition Y_1.5Ba₂Cu₃O_7−X+1 wt% CeO₂ without further doping to fabricate single-domain YBCO monoliths in different sizes and shapes (cylindrical, quadratic) as well as rectangular multi-seeded YBCO monoliths. Up to 2–3 kg melt-textured YBCO blocks are grown reproducibly in one box furnace run. Top seeding by self-made SmBCO was improved and rationalized. Optimization of oxygen annealing treatment leads to macro-crack free YBCO monoliths. Each YBCO monolith was characterized by integral levitation force and field mapping. In a single-domain quadratic monolith with an edge length of 38 mm a maximum induction of 1.44 T at 77 K and a distance of 0.5 mm was frozen. The reproducibility of the batch process is guaranteed. Mean maximum induction from 1.1 to 1.2 T at 77 K per batch was reached. A trapped magnetic field of 2.5 T was achieved between two single-domain monoliths in a gap of 1.5 mm at 77 K.

Depending on the application function, elements with different sizes, designs and more or less complex geometry are constructed in several working steps by cutting, machining, bonding and passivation. Selected function elements were checked with field mapping at 77 K. Results of our function elements in HTSC reluctance motors using single-domain material are shown. We will report on a fly-wheel system and a system to levitate persons.

This paper presents an algorithm to simulate the electromagnetic behaviour of devices containing high temperature superconductors in axially symmetric problems. The numerical method is built within the finite element method. The electromagnetic properties of HTSCs are described through the critical-state model. Measurements of the axial force between a permanent magnet and a melt-textured YBCO puck are obtained in order to validate the method. This simple system is modelled so that the proposed method obtains the current distribution and electromagnetic fields in the HTSC. The forces in the interaction between the magnet and the HTSC puck can then be calculated. A comparison between experimental and simulation results shows good matching. The simplification of using the critical-state model and ignoring flux creep in this type of configuration is also explored.

Since the first successful running of the people-carrying high temperature superconducting (HTS) Maglev test vehicle on 31 December 2000, about 27 000 people have taken it, and the accumulated running distance is about 400 km. The levitation force of the onboard HTS equipment is measured periodically, and new experimental results measured on 5 March 2003 show that the performance of the onboard HTS Maglev equipment is almost the same as that of two years ago. Experimental results indicate that the long-term stability of the HTS Maglev vehicle is good. This further proves the feasibility of the HTS Maglev vehicle for practical transportation. It is worth mentioning that all the results are measured at a low speed; however, investigations of the dynamic performance of the HTS Maglev vehicle at high speed are necessary for practical application. Research on the dynamic performance of the HTS Maglev vehicle is ongoing.