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Large single grain RE-Ba-Cu-O (RE=Y, Nd) high temperature superconductors have been fabricated using a top-seeded melt growth process. These grains have been prepared using powder produced by two solution-based techniques; spray-drying and evaporative decomposition of solution (EDS). The use of these solution-based methods enables the effective addition of small amounts of dopant (such as Pt and Au) to the precursor powder. The dopant refines the size and distribution of non-superconducting phases in the RE-Ba-Cu-O matrix, which is considered essential for obtaining a high critical current density.

Crystal growth of melt-textured Nd-123 pseudo-crystals was investigated via an isothermal solidification with top-seeding technique under a 1%O/sub 2/ in N/sub 2/ atmosphere. Nonsteady state solidification was observed at low undercooling, in contrast to an almost linear growth at higher undercooling. Similar to processing in air, the substitution of Nd/Ba was found to decrease from the seed position to the edge of the crystal. In addition, the volume fraction of Nd-422 particles decreased in the solid as solidification proceeded. As a result of these microstructural inhomogeneities, the critical temperature and the critical current density varied within the crystal even for samples processed isothermally, despite the narrow solid solution range of the Nd-123 phase under a reduced pO/sub 2/ atmosphere.

Doping the rare earth site in superconducting YBa/sub 2/Cu/sub 3/O/sub x/, (Y-123) with Ca generates additional positive charge carrier density in the Cu-O planes and, at a certain hole concentration, enhances the critical current density and irreversibility field of the material. 3 cm sized large, single grain samples of YBCO doped with various amounts of Ca have been fabricated by a top seeded melt growth technique. Very fine, sub-micron sized secondary phase particles of Y-211, which enhance flux pinning, have been observed in the (Y,Ca)Ba/sub 2/Cu/sub 3/O/sub y/ matrix without the addition of Pt. It is considered that Ca doping within the Y-211 phase is responsible for the refinement of these particles. In this paper we discuss the effect of Ca doping on growth rate, microstructural features and superconducting properties of large YBCO pseudo-crystals.

Large, single grain NdBaCuO (NdBCO) composite samples of NdBa2Cu3O7−δ (Nd-123) containing 15 and 20 mol. % non-superconducting Nd4Ba2Cu2O10 (Nd-422) phase inclusions have been fabricated successfully by a variety of techniques based on top-seeded melt growth under reduced oxygen partial pressure. Specifically, individual grains up to 2 cm in diameter have been grown using (100) oriented MgO seeding, self (NdBCO) seeding at elevated temperature and self-seeding of Ag and Au doped precursor pellets. The latter exhibit a reduced peritectic decomposition temperature compared with the undoped compound. These techniques, which vary in degree of difficulty and hence reliability, yield grains with a range of microstructural homogeneity. This paper describes the general aspects of large NdBCO grain fabrication and presents the results of the different fabrication techniques.

Y-Ba-Cu-O (YBCO) precursor powder composed primarily of YBa2Cu3O7-δ (Y-123) and Y2BaCuO5 (Y-211) containing 0.1 w/w% Pt has been prepared by the evaporative decomposition of solution (EDS) and subsequent calcination for the fabrication of bulk melt-processed ceramics. The decomposition process in the as-produced powder and the formation of the Y-123 and Y-211 phases have been studied in detail using differential thermal analysis and powder x-ray diffractometry. The morphology, microstructure and particle size distribution of the powder has been investigated using scanning electron microscopy, transmission electron microscopy and particle size analysis, and the powder has been found to contain a significant proportion of sub-micrometre-sized particles. Samples fabricated by seeded melt growth using the EDS-prepared powder have been observed to exhibit a finer distribution of the Y-211 phase than is typically obtained in similar samples prepared via a mixed oxide route. This provides direct evidence of the suitability of EDS precursor powder for the fabrication of large-grain, high-Jc melt-processed YBCO monoliths.

Large, single-grain Y–Ba–Cu–O (YBCO) was fabricated via the infiltration of Ba–Cu–O liquid into a precursor body composed of solid, porous Y2BaCuO5 (Y-211) and observed to trap a magnetic field of 0.15 T at 77 K. In this process a NdBCO seed crystal was used to promote heterogeneous nucleation, which allows the fabrication of single-grain YBCO containing a uniform and very fine distribution of Y-211 inclusions in the YBa2Cu3O7−δ(Y-123) matrix without the addition of Pt. These superior microstructural features and significant field trapping ability compared with samples processed by conventional top-seeded melt growth suggest this technique could be a practical alternative for processing large, single-grain superconductors for engineering applications.

The fabrication of Nd-Ba-Cu-O (NdBCO), which exhibits significant potential for high-field engineering applications, is extremely sensitive to firing atmosphere, undercooling and secondary phase content and presents considerably greater processing challenges than that of other (RE)BCO superconductors. A detailed study of NdBCO grain growth rate has been performed and a schematic process phase diagram constructed in the peritectic solidification region from isothermal growth experiments at various temperatures for several NdBCO compositions. This phase diagram has been used to fabricate large, single grains up to 2 cm in diameter of NdBa2Cu3O7 (Nd-123) containing 15 mol% of non-superconducting Nd4Ba2Cu2O10 (Nd-422) phase inclusions using a top-seeded melt growth technique (TSMG) in reduced oxygen partial pressure. Magnetization measurements of samples cut from a large grain exhibit a pronounced peak effect over a wide temperature and field region and an anisotropic irreversibility line for fields applied parallel (≈9 T at 77 K) and perpendicular (extrapolates to greater than 40 T at 77 K) to the crystallographic c-axis. The temperature dependence of both the measured critical current density, Jc, and that associated with extrinsic pinning in this material has been observed to follow a power law above ≈70 K, suggesting that the density of pinning sites does not vary significantly with temperature.

Large, single grain Nd-Ba-Cu-O (NdBCO) has been successfully fabricated using a top-seeded melt growth technique under reduced oxygen partial pressure. We report a study of the grain growth rate within the temperature range for which heterogeneous nucleation can be achieved in this system and investigate the variation in size, morphology and spatial distribution of Nd-422 phase inclusions in the Nd-123 matrix with Nd-422 phase inclusion concentration. The influence of Nd-422 phase particles on the growth rate, pores and macrocrack formation is also studied.

A large, single-grain Nd–Ba–Cu–O (NdBCO) composite consisting of superconducting NdBa2Cu3O7-δ containing nonsuperconducting Nd4Ba2Cu2O10 phase inclusions was fabricated up to 2 cm in diameter using a top-seeded melt-textured growth technique. A MgO single-crystal seed was used to provide a heterogeneous nucleation site at the center of a presintered pellet heated above its peritectic temperature and cooled continuously in a conventional tube furnace in reduced oxygen partial pressure. This process produces individual grains with the c axis oriented at ≈10° to the seed surface which, from vibrating-sample magnetization measurements, exhibit a pronounced peak effect in their magnetic moment over a wide temperature range (50–90 K) when the supercurrent flows in the a-b planes. A very high irreversibility field (>9 T at 77 K) is also observed in these grains for field applied both perpendicular and parallel to the crystallographic c axis which is significantly greater than that observed in good-quality melt-processed Y–Ba–Cu–O. These results underline the potential of NdBCO for high-field engineering applications.

The magnetization of top seeded-melt processed ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (Nd-123) containing 15 mol % ${\mathrm{Nd}}_{4}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{2}{\mathrm{O}}_{10}$ (Nd-422) has been investigated by vibrating sample magnetometry over a wide temperature $(50\mathrm{K}lTl94\mathrm{K})$ and field $(0l{\ensuremath{\mu}}_{0}Hl12\mathrm{T})$ range. The origin of the peak in the M-H loops and high irreversibility field values observed in this system are discussed on the basis of these data. The normalized pinning force determined from the hysteresis width is observed to scale as a function of reduced magnetic field ${(h=H/H}_{\mathrm{irr}})$ and exhibits a peak at ${h}_{\mathrm{max}}=0.33,$ suggesting that normal defects dominate the pinning mechanism in this material. This is in contrast with other recent reports of ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}},$ which suggest that the peak effect is associated with a local variation in superconducting transition temperature due to the formation of a distributed solid-solution phase in the Nd-123 matrix.

Large-grain melt-processed YBCO has been joined to create nominally `zero-angle' grain boundaries. Similar broadening of the resistive transition in both grains and the artificial boundary is observed in transport measurements up to 7 T. Direct and pulsed currents are used to obtain the high-field voltage-current characteristics which display power-law behaviour. The temperature and field dependence of the critical current in the boundary compares well with that of the grain material. The large critical current densities, above 2000 A cm-2 at 77 K for 4 T c-axis, show great promise for the joining technology to produce large-scale engineering artefacts.

The nucleation and growth of NdBa2Cu3O7–δ (Nd123)–Nd4Ba2Cu2O10 (Nd422) single-grain composites in a controlled 1% O2 in N2 atmosphere were investigated in detail as a function of solidification temperature and Nd422 phase content using a top-seeded melt growth technique. A schematic process phase diagram in the peritectic solidification region of Nd–Ba–Cu–O (NdBCO) was constructed primarily from constant isothermal growth experiments at various temperatures for several compositions and used to fabricate large single-grain material by both isothermal and continuous slow cooling over a limited temperature range. The nucleation at the seed surface and subsequent growth of uniform grains was observed to depend critically on the controlled rate of grain growth and the temperature range over which solidification occurred.

Large grain NdBaCuO has significant potential for application in bulk superconducting ‘permanent magnet’ engineering devices. This study describes the preparation of NdBaCuO precursor powder by spray drying and subsequent calcination for the growth of large grains using a seeded peritectic solidification technique. Precursor powder prepared by this method has been used to fabricate large NdBaCuO grains which exhibit good growth morphologies. Surface roughening associated with a continuous phase transformation that occurs in the precursor bodies prior to and during melt processing has been identified as a source of inhomogeneity in the grain growth morphology. This may be avoided by calcining the precursor powders at high temperature for a short period of time to yield only the desired crystalline and structural phases.

A technique for joining large-grain YBCO has been used to produce bulk-bicrystal grain boundaries of different orientations. The behaviour of boundaries nominally of 0/spl deg/[100], 0/spl deg/[001] and asymmetric 15/spl deg/[100]-tilt misorientation have been investigated using low frequency transport measurements in fields up to 7 T. The boundaries exhibit a metallic normal state and a superconducting transition which broadens with increasing field having an irreversibility line which closely matches that of the adjoining grains. This same qualitative behaviour is seen in all the samples measured for field applied parallel and perpendicular to the c-axis and up to the highest current densities employed (20 Acm/sup -2/). Variation between the different samples due to microstructural differences are discussed. We interpret these results as strong suggestive evidence that this joining technique can be used to produce strongly-coupled large-grains for bulk-scale engineering applications.

We have investigated the superconducting properties of bulk melt-processed YBCO using different experimental techniques. First, single domain materials (i.e. containing no grain boundaries) have been studied. Electrical resistivity, current-voltage characteristics, M-H loops, and A.C. susceptibility have been measured for different field and current directions. The intragranular properties are characteristic of high-quality melt-processed YBCO material, with a critical current density J/sub c/(T=77 K, B=1 T//c)>10/sup 4/ A/cm/sup 2/. The anisotropy ratio J/sub c/(ab)/J/sub c/(c) is found to be close to 3. Both magnetic and transport measurements show that defects in the microstructure do not significantly impede the current flow inside the single domain. These results were compared to those measured on samples containing one "natural" single grain boundary which sometimes appears during the grain growth process. The intergranular properties show a much stronger current and magnetic field dependence than that measured within the grain and these differences are discussed.

Investigations are being carried out into the use of superconductors as fault current limiters. The authors are investigating devices constructed from various high T/sub c/ materials. Of particular interest are limiters constructed from superconducting YBCO thick films on YSZ substrates. In order to predict the limiting characteristics of a fault current limiter and to optimise the design parameters, an accurate numerical simulation has been developed. This model includes measured data in the form of E-J characteristics, thermal properties and R-T data. Data sets for different materials can be selected in order to compare the performance of candidate materials. A major concern during limiting is the way that heat is generated in the superconducting film. The authors' simulation treats the superconductor and substrate as a number of small elements in order to determine the temperature distribution within the film and substrate. Further additions model the heat loss mechanisms to the surrounding environment. This thermal model is included in an overall FCL model, which includes voltage sources and line or load impedances. One can use this model to accurately predict the current-time waveforms achievable with typical limiters, and to easily explore the effect of a change in operating conditions.

A non-weak link joining technique has been developed for YBCO pseudo-crystals fabricated by seeded peritectic solidification based on the formation of a liquid phase which segregates from the platelet boundaries at temperatures above /spl ap/920/spl deg/C. Electrical and magnetic measurements on these boundaries suggest that their irreversibility field can be as high as 7 T at 77 K in fully oxygenated pseudo-crystals joined along their crystallographic ab-planes which is comparable to the irreversibility behaviour of the adjacent YBCO grains.

An investigation is being made into building superconducting machines using YBCO bulks as quasi-permanent magnets. To investigate the performance of the superconductors in motor applications a test rig has been built in which a moving or vibrating magnetic field can be applied to the superconductor. The test rig allows two-dimensional motion of a magnet relative to a superconductor, in order that two-dimensional vibration modes can be investigated. This allows more accurate simulation of the conditions in a motor, and allows values of the absolute forces between magnet and superconductor during the motion to be inferred. The vibration methods used in the paper are suitable for measuring long-term behaviour of the superconductor-magnet interaction, including long-term decay of the trapped field when exposed to oscillations. The shorter tests reported here measure force/torque characteristics, and the stable limits of operation. Beyond these limits, a superconductor will be de-magnetised, resulting in a sudden loss of torque and machine damage.

The temperature dependence and the anisotropy of the electron momentum distribution of high temperature superconductors YBa 2 Cu 3 O 7− δ and Bi 2 Sr 2 CaCu 2 O 8 have been studied using Compton scattering of high energy X-rays from a synchrotron source. Both high resolution (0.16 a.u. of momentum) and low resolution (0.60 a.u.) spectrometers were used. Polycrystalline and single crystal samples were measured below and above the transition temperature. No temperature effect in the Compton profiles was found contrary to an earlier report. The electron momentum anisotropy and its temperature dependence were also measured between the c -axis and the basal plane. The anisotropy in the YBCO Compton profile is consistent with recent band calculations: larger effects are obtained in BSCCO.

We have reported previously the successful fabrication of single grain Gd–Ba–Cu–O (GdBCO) bulk superconductors in air by top seeded melt growth (TSMG) using a generic Mg–Nd–Ba–Cu–O (Mg–NdBCO) seed crystal. Here, we report a new processing route that enables the fabrication of bulk GdBCO single grains in air using precursors with higher Ba content than has been achieved in previous studies. In particular, the composition of the superconducting matrix of single grains prepared with high, but varying, levels of Ba is analyzed by EPMA. The degree of substitution Ba for Gd (corresponding to a negative value of x in the composition formulation), in GdBCO is proposed for the first time based on the superconducting, compositional, chemical and structural properties of the Gd-123ss (Gd 1+ x Ba 2− x Cu 3 O 7− δ ) phase matrix observed this study.

The characteristic inhomogeneous distribution of nonsuperconducting Y2BaCuO5 (211) inclusions in melt-processed YBa2Cu3O7−δ (123) grains has generally been attributed to 211 particle pushing by 123 growth fronts during peritectic solidification on the basis of reduced total surface free energy. Analysis of the morphology of the interfaces at the 211–211–123 and 211–211-liquid triple points in seeded melt-processed samples invalidates this assumption for the pure YBCO system and has implications of the mechanism of 211 particle segregation.

A range of experimental techniques have been used to characterize melt-processed YBa 2 Cu 3 O 7− δ samples containing single-grain boundaries. Both natural high-angle boundaries, which sometimes appear during the grain growth process, and artificial low-angle boundaries, obtained by joining two single domains, have been investigated. Electrical resistivity, current–voltage characteristics, magnetic moment measurements and Hall probe mapping techniques have been employed to investigate the boundaries. Results are compared with the properties of single domain material (i.e., containing no grain boundary) for which T c ≈89 K and J c ‖ ab (77 K, 1 T)>10 4 A/cm 2 . Resistance measurements across all the grain boundaries show a stronger dependence on current and magnetic field than that measured within the grains and exhibit a pronounced resistive `tail'. The I – V curves obtained for the high-angle natural grain boundary are sharp and differ from the rounded I – V curves which are characteristic of single-grains. Field mapping measurements used to evaluate the critical current anisotropy are in agreement with magnetisation measurements. The limitations of this technique for investigating boundaries are discussed. It was found that current anisotropy can conceal a weak link between two grains, leading to a false indication of single-grain behaviour. Artificially engineered boundaries are shown to have significant potential for applications in high fields at 77 K.

The potential of high temperature superconductors to generate large magnetic fields and to carry current with low power dissipation at 77 K is particularly attractive for a variety of permanent magnet applications. As a result large grain bulk (RE)-Ba-Cu-O ((RE)BCO) materials have been developed by melt process techniques in an attempt to fabricate practical materials for use in high field devices. This review outlines the current state of the art in this field of processing, including seeding requirements for the controlled fabrication of these materials, the origin of striking growth features such as the formation of a facet plane around the seed, platelet boundaries and (RE)2BaCuO5 (RE-211) inclusions in the seeded melt grown microstructure. An observed variation in critical current density in large grain (RE)BCO samples is accounted for by Sm contamination of the material in the vicinity of the seed and with the development of a non-uniform growth morphology at ≈ 4 mm from the seed position. (RE)Ba2Cu3O7-gd (RE-123) dendrites are observed to form and broaden preferentially within the a/b plane of the lattice in this growth regime. Finally, trapped fields in excess of 3 T have been reported in irradiated U-doped YBCO and (RE)1+xBa2-xCu3Oy (RE = Sm, Nd) materials have been observed to carry transport current in fields of up to 10 T at 77 K. This underlines the potential of bulk (RE)BCO materials for practical permanent magnet type applications.

Investigations are being carried out on the use of superconductors for fault current limiting applications. A number of computer programs are being developed to predict the behavior of different ‘resistive’ fault current limiter designs under a variety of fault conditions. The programs achieve solution by iterative methods based around real measured data rather than theoretical models in order to achieve accuracy at high current densities.

A thermal processing system incorporating four sets of independently controllable heating elements and a cold finger which generates uniform temperature gradients of up to 25 °C cm−1 has been developed and optimized to fabricate large undoped and Pt-containing YBCO pseudo-crystals by seeded peritectic solidification. Evolution of oxygen gas and loss of liquid during peritectic decomposition have been identified as key variables for the fabrication of large, single grain samples. The processing conditions associated with this system have been investigated for varying precursor body heating rate, undercooling, growth rate and sample thickness. Optimum values of these parameters were determined as heating rate: 10 °C h−1, undercooling: ≈ 15 °C and sample thickness

The homogeneity of the superconducting properties of a large grain (2 cm diameter) YBa 2 Cu 3 O 7-δ (YBCO) sample fabricated by SmBa 2 Cu 3 O 7 - δ -seeded melt growth has been investigated in detail and correlated with variations in sample composition and microstructure. Magnetic characterisation of small ( 8 mm 3 ) individual specimens cut from various positions in the bulk sample indicates a significant variation in the superconducting transition temperature, critical current density and irreversibility field within the large YBCO grain. The transition temperature increases smoothly with sample radius to a distance of the order of several millimetres from the seed which correlates well with an observed local Sm contamination of the YBCO matrix. The observed variation of the critical current density and irreversibility field with position is associated primarily with the reduced transition temperature in the vicinity of the seed, a degradation in the microstructure towards the sample edge and a variation in the Y 2 BaCuO 5 inclusion density with sample radius.

The characteristic platelet-like structure of large grain superconducting Y–Ba–Cu–O fabricated using peritectic solidification techniques has been documented widely as a key microstructural feature of this material. The platelet formation mechanism is investigated via a detailed comparison of the difference in morphology of YBa2Cu3O7–δ (123) growth fronts propagating along different lattice directions. The development of YBa2Cu3O7–δ dendrites between the growth front and local Y2BaCuO5 (211) particles is observed to be a key feature of the growth mechanism along all directions. Dendrites broaden rapidly for growth fronts propagating along the c-axis due to the enhanced growth rate of Y–Ba–Cu–O in the a-b plane to yield a uniform, regular growth morphology. Dendrite broadening is inhibited for grain growth along the a-b directions, however, due to the slower growth rate along the c-axis, which yields an irregular extended growth front. Growth along the a/b direction commonly results in the formation of regions consisting of 123 dendrites which may connect 211 particles. Continued solidification of the 123 phase in such regions results in the development of platelet structures perpendicular to the crystallographic c-axis in the YBa2Cu3O7–δ phase matrix which may impede the flow of current through the grain in the superconducting state.

Accurate and detailed image analysis of the (Y-211) particle distribution in large-grain Pt-doped and undoped fabricated by seeded peritectic solidification has been performed using scanning electron microscopy. The size distribution of Y-211 particles along the a or b and c directions of doped and undoped grains has been measured and observed to approximate well to a log-normal distribution. The mean Y-211 particle size, which is independent of growth direction, is approximately and in the doped and undoped grains, respectively. The particle size distribution is significantly narrower for the Pt-containing material, however, corresponding to a much greater particle size uniformity. Such a uniform dispersion of fine Y-211 particles is desirable since it both enhances the critical current density of melt-processed YBCO and reduces liquid loss during peritectic solidification.

High values of critical current density, Jc, in large-grain superconducting melt-processed YBa2Cu3O7-δ (YBCO, Y-123) have been reported to correlate closely with the distribution of Y2BaCuO5 (Y-211) particles in the Y-123 phase matrix. Extensive image analysis of the homogeneity of the Y-211 particle distribution in a large-grain sample has been performed on high-resolution, secondary scanning electron micrographs of seeded melt-processed YBCO. The variation of key parameters, such as area fraction, number density and size along the a/b axis of the Y-123 grain has been investigated in detail as a function of distance from the centre of the seed. Both area fraction and number density of Y-211 particles are observed to increase continuously with distance, whereas the mean and standard deviation of the size distribution decrease slightly towards the grain boundary. This suggests that the increase in area fraction can be attributed to an increase in number density of the Y-211 particle distribution along the a/b axis of the specimen. The implications for Y-211 particle pushing, ripening and coalescence are discussed in the light of these results. © 1998 Chapman & Hall

The availability of high quality SmBa2Cu3O6.5 seeds of known orientation is essential for the fabrication of a large grain YBa2Cu3O7−δ (YBCO) superconductor via a seeded peritectic solidification technique. The suitable seed must have a close lattice parameter match to YBCO and a relatively high melting temperature. We report a melt texturing process for the fabrication of SmBa2Cu3O7−δ (Sm-123) seeds using SrTiO3 to control grain nucleation. The physical and structural properties of the seeds were confirmed by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Seeds prepared in this study were used to grow large single grain YBCO superconductors of up to 2 cm in diameter.

Melt-textured YBCO samples with embedded nanosize particles of Y 2 Ba 4 CuNbO x (Nb-2411) have been studied by means of electron backscatter diffraction (EBSD). The microstructure of these samples shows some distinct and unusual features, not observed in melt-textured bulk samples of other compositions. The resulting texture of the superconducting YBCO matrix is found to be excellent. Y 2 BaCuO 5 (Y-211) particles, which are also present in these samples, exhibit no preferred orientation, as is the case generally in melt-textured YBCO samples. The embedded Y 2 Ba 4 CuNbO x particles, in contrast, exhibit only one orientation within the YBCO matrix. Furthermore, we find in the bulk pellets distinct areas with a high second phase particle concentration, adjacent to regions without secondary phase particles.

The quality and preparation of Sm–Ba–Cu–O (SmBCO) seeds, the level of Pt-doping, and the temperature distribution in the thermal processing system have been investigated in detail and identified as key processing factors in the fabrication of large melt processed Pt dope Y–Ba–Cu–O pseudocrystals. Optimization of these process variables and features has been performed to enable controlled processing of these materials up to several centimeters in diameter. Inhomogeneities and second phases present in the YBCO-Pt pseudocrystal microstructure have been analyzed and correlated with a variation of superconducting properties within the bulk of the sample. These observations have clear implications for the controlled processing of large YBCO-Pt pseudocrystals for electromagnetic applications.

High quality large grain high T/sub c/ superconducting ceramics offer enormous potential as "permanent" magnets and in magnetic screening applications at 77 K. This requires sample dimensions /spl sim/cm with uniform high critical current densities of the order 10/sup 5/ A/cm/sup 2/ in applied magnetic fields of 1T. We report a study of the magnetic characterisation of a typical large YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// grain, prepared by seeded peritectic solidification, and correlate the magnetically determined critical current density, J/sub c/, with microstructural features from different regions of the bulk sample. From this data we extract the temperature, field and positional dependence of the critical current density of the samples and the irreversibility line. We find that whilst the bulk sample exhibits a good J/sub c/ of order 10/sup 4/ A/cm/sup 2/ (77 K, 1T), the local J/sub c/ is strongly correlated with the sample microstructure towards the edge of the sample and more severely at the centre of the sample by the presence of SmBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// seed crystal.

The potential use of YBa/sub 2/Cu/sub 3/O/sub 7/ as an active component in a magnetic bearing is being investigated. Although the load bearing capacity is high and increases with the square of the magnetic field trapped, the stiffness is low. Both the stiffness and the levitation height are a function of the loading history of the bearing. The authors have been investigating the effects of dynamic loading such as single large excursions from steady state loads and cyclically applied loads such as vibrations. Since a superconducting bearing has little inherent damping, cyclic loads applied at or near its natural frequency can have catastrophic effects. The information being gathered by the authors will be used to enable these effects to be mitigated in the bearing design process.

We report an investigation of the processing of large-grain Y - Ba - Cu - O (YBCO) containing 0.1, 0.2 and 0.5 wt% platinum (Pt) doping using seeded peritectic solidification in a vertical tube furnace. In this study differential thermal analysis of the Pt-doped YBCO powders and careful temperature calibration of the furnace were performed in order to determine optimum values of the thermal process parameters for each composition. The seeded melt process was observed to be extremely sensitive to the melting temperature, peritectic solidification temperature and cooling rate during grain growth, and failure to optimize these parameters can result in various undesirable effects such as incomplete melting, the nucleation of multiple grains and the formation of local regions of impurity concentration within the sample. Scanning electron microscopy and vibrating sample magnetometry were employed to investigate the microstructure and current-carrying capabilities of the samples. From these, 0.1 wt% Pt doping was determined to be sufficient for the fabrication of melt-processed samples containing fine, sub-micron-sized Y-211 particles which exhibited high critical current densities. No improvement in sample properties was observed for higher levels of Pt doping.

Melt processed HTSC bulk samples usually show a high inhomogeneity. These inhomogeneities influence application-relevant properties such as the levitation force or the trapped field. In this contribution a technique is presented which allows investigation of these inhomogeneous properties. The measurements are performed by scanning the sample surface with a small coil system and detecting the first and third harmonic of the inductive response. The critical current density j/sub c/ is calculated from the measured signal using a modified critical state model. J/sub c/-distributions yielded by this technique are shown.

The growth morphology of large YBa2Cu3O7−δ grains during peritectic solidification has been reported to be responsible for the generation of processing defects, such as platelets, and an inhomogeneous distribution of 211 particles, both of which influence significantly the superconducting properties of the fully processed material. The present study demonstrates both experimentally and theoretically the formation of local dendrites at macroscopically planar YBa2Cu3O7−δ growth fronts which propagate along different crystallographic directions and identifies these as key growth features of the peritectic solidification process.

A summary is given of the various techniques used to prepare melt processed YBCO. The use of these materials as permanent magnets is discussed. Since nearly all applications involve demagnetizing fields of some kind, these are compared with the equivalent ferromagnetic material. Superconductors behave in the opposite way to ferromagnets in that a reversed field can increase the magnetization, while a field parallel to the magnetization quickly reduces it to zero. Then ways of obtaining maximum repulsion and attraction without demagnetizing the sample are described. A summary of the forces and torques available is given. In particular, it is shown that YBCO can provide extremely high current densities in rotating machines.

The properties (superconducting and non-superconducting) of YBa2Cu3O7-δ bulk ceramics (YBCO) are strongly dependent on the oxygen deficiency, δ. Unfortunately, whether sintering or melt-processing is used to form the material, the final oxygen content is usually far from that which gives the optimum superconducting properties (δ≲0.1). Because of this, an additional oxygenation stage has to be carried out in which the material is heated and held at some elevated temperature (∼400–500 °C) for a significant time (often > 100 h) to allow oxygen to diffuse into the sample. Other diffusion-based processes (e.g. sintering) are known to be substantially enhanced in the presence of a microwave field, and it is reasonable to expect that the diffusion of oxygen into YBCO may also benefit from the application of such technology. The oxygenation of melt-processed YBCO using conventional and microwave-assisted heating has been compared. The diffusion coefficient at 400 and at 450 °C is shown to be enhanced by about 30% through the application of a high-frequency microwave field. Because the oxygenation time is inversely proportional to the diffusion coefficient, this represents a significant reduction in process time. The equilibrium value of the oxygen content is not affected by the application of a microwave field, and therefore the sample temperature must be the same for both microwave-assisted and conventional processing. Consequently, the observed enhancement is an example of a genuine non-thermal microwave effect.

The microstructure of large grain melt-processed YBa2Cu3O7−δ containing 10 molar% excess Y2BaCuO5 prepared and oxygenated under atmospheric pressure has been investigated by transmission electron microscopy (TEM) and optical microscopy. These materials always contain parallel structural and microscopic platelet-like features in the crystallographic a-b plane of a few microns spacing which have been variously described as grain boundaries or microcracks. We have observed such features, which clearly influence the flow of current in melt-processed YBCO, to consist of copper deficient, impurity phase material which can be either amorphous or crystalline in nature. A variety of defects have been observed by high-resolution electron microscopy (HREM) in the vicinity of these platelet boundaries, including double and triple CuO layer stacking faults, which may constitute effective flux pinning sites.

Large grain melt processed Y-Ba-Cu-O (YBCO) samples have been prepared by seeded and unseeded growth techniques. The current carrying ability within individual grains and across grain boundaries has been investigated and correlated with features in the microstructure of samples fabricated by both techniques. The development of an inhomogeneous, cell-like growth microstructure in seeded samples at distances ≈4 mm from the seed is related to a saturation in inclusion density and volume proportion of Y2BaCuO5 (Y211) particles. A significant decrease in critical current density is associated with this change over a wide temperature range. The resistance of high angle c-axis grain boundaries is observed to depend critically on the magnitude of the injection current whereas the intra-granular resistance is not influenced significantly by this variable. Jc of a grain boundary fabricated by unseeded melt growth is estimated to be less than 100 A cm−2 at 77 K in zero applied field, which is more than two orders of magnitude lower than the intragranular Jc. Field screening measurements suggest that low angle grain boundaries do not form weak links between grains in modest magnetic fields and hence do not present a significant barrier to current flow.

We report the results of dc magnetization measurements of Bi2Sr2CaCu2O81d single crystals as a function of temperature. The evolution of the hysteresis loops with temperature is examined and the irreversibility line extracted. These loops can be scaled onto each other from 4.2 to 20 K, while the lower field part of the loops for the virgin curves continue to scale up to Tc . At intermediate temperatures, the scaling is broken by the appearance of the second peak or ‘‘arrowhead’’ which appears at around 20 K but gradually disappears above about 30 K. While the behavior is dominated by significant bulk pinning at low temperatures, the hysteresis and irreversibility fields at high temperatures are considerably modified by geometrical and surface barrier effects. We identify the onset, with increasing field, of bulk pinning and significant critical current from the emergence of the ‘‘arrowhead’’ feature from the background surface barrier-determined magnetization curve. Scaling in the low-temperature regime, and extrapolation, enables the irreversibility line at 0 K to be estimated at about 110 T. We propose a phase diagram up to high magnetic fields over a wide range of temperature. @S0163-1829~96!03521-7#

The ability to process large grain, uniform high temperature superconducting ceramics that exhibit high critical current densities at 77 K is essential if the enormous potential of these materials for a range of permanent magnet-type applications is to be realized. We report a study of the fabrication of large grain YBa2Cu3O7−δ by seeded peritectic solidification in which key processing parameters such as the peritectic melting process, the seed-YBCO reaction, and the YBCO solidification kinetics are investigated in detail. Evolution of the sample microstructure during various stages of the growth process, in particular, has been studied extensively. The superconducting properties of specimens cut from different regions of large grain samples have been measured using vibrating sample magnetometry, and the results correlated with the microstructure of the materials.

YBa2Cu3O7−δ precursor powder suitable for the fabrication of melt-processed ceramic has been prepared by spray drying and subsequent calcination. The dehydration and decomposition processes in the nitrate-based spray-dried powder and the formation processes of the YBa2Cu3O7−δ and Y2BaCuO5 phases have been studied in detail using thermal analysis and powder x-ray diffractometry. The morphology, microstructure, and size distribution of the particles in the calcined powder have been investigated using high resolution scanning electron microscopy and particle size analysis and found to contain a significant proportion of submicron sized particles of Y2BaCuO5. Melt-processed samples prepared using the spray-dried powder have been observed to exhibit a significantly finer distribution of the 211 phase than is typically obtained in similar specimens prepared via a mixed oxide route. This provides direct evidence of the suitability of spray-dried precursor powder for the fabrication of large grain, high Jc melt-processed YBCO ceramics.

The dependence of growth rate on isothermal undercooling and composition has been investigated for Nd-Ba-Cu-O single grains containing various amounts of nonsuperconducting Nd4Ba2Cu2O10 (Nd-422) phase inclusions and fabricated under a 1% O2 in N2 atmosphere using a top seeded melt growth technique. The growth rate along the crystallographic c-direction is observed to exceed that along the a/b direction at all undercooling temperatures and exhibits a maximum for a Nd-422 content of ∼10 mol%. The samples have been examined by optical microscopy and the results interpreted within an established planar solidification model.

The influence of the physical and chemical properties of precursor green bodies on the properties of fully melt-processed YBa2Cu3O7−δ ceramic (YBCO) have been investigated. Cold isostatic pressing has been found to allow better control of the size and distribution of Y2BaCuO5 phase inclusions in the melt-processed ceramic than a combination of die pressing and sintering. Astudy of the loss of liquid from YBCO during partial melting has revealed that the total percentage weight loss is sensitive to both the heating rate and proportion of excess 211 phase and is maximum at a temperature corresponding to the peak of the differential thermal analysis partial melting endotherm. Densification and expansion processes in the sample, which compete during melting, have been investigated in detail. The temperature where the expansion rate is maximum has been found to coincide with the maximum rate of oxygen desorption by the sample. The expansion process terminates on completion of partial melting whereas the densification process, which is dominated by the volume proportion of liquid, continues but at a reduced rate. The use of dense green bodies and an optimum heat-treatment process has been found to be essential for the fabrication of large-grain melt-processed YBCO if a fine distribution of 211 phase inclusions and the homogeneity and shape of the sample are to be retained.

A scanning Hall probe has been used to map the distributions of magnetic field in melt-processed YBa 2 Cu 3 O 7−δ (YBCO) discs prepared by seeded and controlled molten-zone techniques. Both shielded and trapped fields were studied as a function of applied magnetic field for each speciment. A clear four-fold symmetry has been observed in the field distribution of the seeded melt-processed sample, suggesting that there are two planes in the disc which exhibit a weak response to an applied field. Similar magnetic properties were observed for the molten-zone processed sample and attributed to the presence of cracks in the specimen. It was found that the applied field required to saturate each sample was much higher than the maximum observed trapped field, in contradiction to the Bean model for a slab geometry. This effect, which is probably due to the plate geometry of the specimens and the variation of critical current density with magnetic field, has implications for practical applications of bulk melt-processed YBCO.

The potential use of YBa/sub 2/Cu/sub 3/O/sub 7/ as an active component in a magnetic bearing is being investigated. Measurements are being made of the load bearing capacity and related stiffnesses in comparison to predictions from the critical state model. Although the load bearing capacity is high and increases with the square of the magnetic field trapped, the stiffness is low. We report on a novel design concept to overcome this problem which uses superconducting bulk materials. In bulk materials where the magnetic field is produced by persistent currents trapped in the superconductor this option is not available. However the available force is a function of the flux density and the area over which it is applied. Thus even if the flux density remains constant then changes in available force may be made by changing the active area of the bearing i.e. increasing the area of overlap between the magnetic components. Using this method active control may be achieved using bulk materials and the inherently low stiffness of the materials is no longer a problem. >

YBaCuO precursor powder specifically for the fabrication of melt processed ceramic has been prepared by spray drying and subsequent calcination. The formation processes of YBa/sub 2/Cu/sub 3/O/sub 7/(-/spl delta/) and Y/sub 2/BaCuO/sub 5/ have been studied in detail using thermal analysis and powder X-ray diffractometry. The morphology, microstructure and size distribution of the particles in the calcined powder have been investigated using high resolution SEM and particle size analysis and found to contain a significant proportion of sub-micron sized particles of Y/sub 2/BaCuO/sub 5/. Melt processed samples prepared using spray dried powder have been observed to exhibit a fine distribution of 211 particles which is desirable for high J/sub c/ applications. >

Magnetic properties of melt-processed YBa2Cu3O7- delta thick films have been measured and correlated with features in the microstructure at 4.2 and 77 K for film thicknesses between 50 and 140 mu m. A pronounced peak has been observed in both the measured volume magnetization and calculated length scale over which current flows at a film thickness of approximately 53 mu m and 4.2 K in fields of up to 10 T. An intra 'hub-spoke' (H-S) type grain current dominates the volume magnetization at this film thickness. Measurements at 77 K, on the other hand, exhibit a peak at 80 mu m, the magnitude of which varies significantly with applied field. This correlates well with observed increased connectivity between individual H-S grains and may be accounted for by the flow of inter H-S grain current. The H-S grains transform to a more granular microstructure for films greater than approximately 100 mu m thick which is characterized by the presence of smaller diameter current-carrying loops. This is observed as a decrease in the volume magnetization at 4.2 K and a levelling off of this parameter at 77 K with increasing film thickness. Further evidence for the presence of intra H-S and inter H-S grain critical current densities at 4.2 K in films up to a thickness of 80 mu m has been observed from length-scale analysis as a function of the difference between maximum and applied magnetic field. A qualitative model for the volume magnetization of the films at 4.2 K in terms of individual contributions from intra H-S grain, inter H-S grain and granular Jc components is proposed.