Table of contents

We wish to acknowledge all the participants of the International Symposium on Processing and Critical Current of High-Temperature Superconductors that took place in Wagga Wagga, Australia, from 2 to 4 February 1998. The Organizing Committee was honoured by enthusiastic response and support from all of the participants. 127 papers from 26 countries, including 57 plenary and invited talks, were presented. Wagga Wagga was chosen as the Symposium site in order to provide an opportunity for a fruitful interaction between the international HTS community and researchers attending the adjoining Australian Condensed Matter Physics Conference (ANZIP). The HTS Symposium and the 21st ANZIP shared two sessions on the last day of the Symposium.

Through intensive research and development during the last decade, the HTS community has made significant advances in both the fundamental science and commercial exploitation of HTSs and it was very encouraging to see excellent research results leading to the establishment of a worldwide HTS industry. Widespread technical and commercial success for HTS is hindered by the current levels of electromagnetic performance; in particular, the critical current density at liquid nitrogen temperatures is still too low for successful application in many types of devices. Furthermore, it is commonly believed that understanding and improving material processing is the key to further improvement of the critical current density in HTSs. Therefore, processing and critical current in HTS materials was chosen as the central theme of this Symposium.

The Symposium included three plenary sessions, six oral sessions and five poster sessions, covering the topics of `Transport and magnetic properties', `Flux pinning and dynamics', `Synthesis and novel processing', `Fabrication of wires, tapes, magnets and bulk materials', `Relationship between structure, microstructure and properties' and `Single crystals and losses'. In order to stimulate the discussion, participants were encouraged to present the posters at the beginning of the Symposium and leave them on until early morning of the last day. The proceedings were reviewed to be published in this special issue of Superconductor Science and Technology.

We would like to express our sincere thanks to Mrs B Allen for her outstanding work in organizing the Symposium. We would like to thank the Organizing Committee: Dr M Apperley (MM Ltd), Dr B Buckley (IRL), Dr M P Das (ANU), A/Prof T Finlayson (Monash), A/Prof H K Liu (UoW), Prof I D R MacKinnon (UoQ), Prof G Russell (UNSW), Dr N Savvides (CSIRO) and Dr L Vance (ANSTO) for their well-coordinated work. We would like to acknowledge the financial support and assistance from the Australian Research Council, the University of Wollongong, Metal Manufactures Ltd, Drs L Vance and S Kennedy of the Organizing Committee of the 21st ANZIP Condensed Matter Physics Conference and Charles Sturt University. Finally we would like to express our sincere appreciation to the plenary and invited speakers, session chairs and all the attendees for their participation and contribution to this Symposium. We hope they all enjoyed the Symposium and their visit to Australia.

S X Dou and P N Mikheenko Proceedings Editors

The electrodynamic behaviour of the high-temperature cuprate superconductors is strongly affected by the anisotropy of these layered materials. Their transport, magnetic and pinning properties are determined by the properties of vortices, which can be regarded as stacks of two-dimensional (2D) pancake vortices (with cores localized in the layers, bilayers or trilayers) connected by Josephson vortices (with cores localized in the nonsuperconducting charge-reservoir layers). When the Josephson coupling between the superconducting layers is strong, as in YBCO, the line energy associated with the Josephson vortices is large, the 2D pancake vortices in adjacent layers are strongly coupled and the magnetic anisotropy of the superconductor is relatively weak. When the Josephson coupling is weak, as in BSCCO, the line energy associated with the Josephson vortices is small, the 2D pancake vortices in adjacent layers are weakly coupled and the magnetic properties are highly anisotropic and nearly 2D. The transport properties for currents flowing parallel to the layers are dominated by 2D pancake vortex motion, while those for currents flowing perpendicular to the layers are dominated by Josephson vortex motion. In this paper, I review these effects and discuss a number of consequences for vortex-lattice melting, flux pinning, flux creep and the critical current density.

Several recent developments in powder-in-tube (PIT) processing are presented. A cryogenic deformation process has been developed, involving rolling or pressing the wires and tapes in supercold conditions, such as in liquid nitrogen. Cryogenic deformation has been found to improve the density, grain alignment and Ag-oxide core interface and to increase dislocation density, thereby enhancing and flux pinning. By incorporating Pb into Bi-2212 phase the sintering temperature can be raised above , resulting in a significant reduction of total sintering time from several hundred hours to 100 h. Recently, a new process to eliminate the decomposition and recovery of Bi-2223 during cooling and heating has been developed that further reduces the heat treatment time for Ag/Bi-2223 tapes to 20-30 h, with and Bi-2223 volume fraction in the tapes comparable with those in tapes treated for 120 h. A two-stage annealing procedure in the final thermal cycle has been used to eliminate residual amorphous phase and Bi-2201, which has been identified to be one of the major causes of weak links in PIT tapes. By incorporating hot deformation in a two-step process not only can Bi-2201 be eliminated, but also texture and density are improved, resulting in a very high at 77 K in multifilamentary tapes.

The complex ac susceptibility of superconductors of various shapes in parallel or perpendicular magnetic field can be calculated from an integral equation for the current density inside the superconductor. The superconductor is characterized by a general current-voltage law E(J), e.g. where E is the electric field, the magnetic induction, a prefactor, the critical current density and the creep exponent. For n > 1, the nonlinear ac susceptibility is calculated from the hysteresis loops of the magnetic moment. For the Bean critical state model results. For n = 1, and for any linear complex resistivity caused by thermally activated vortex motion, the linear ac susceptibility may be calculated from an eigenvalue problem. At high frequencies, the of thick discs in axial field crosses over from perpendicular to parallel behaviour since the skin effect forces the magnetic field lines to flow around the disc.

Force-displacement characteristics of flux lines were measured using the Campbell method in the vicinity of the peak effect for an overdoped Bi-2212 single crystal. It was revealed that there are two quasi-stable states with different critical current densities, suggesting a phase transition of the first order. It was also found that the interaction distance increased discontinuously around the peak field, while the Labusch parameter showed no anomaly. This shows that the flux-pinning mechanism does not change across the peak field and that the peak effect is caused by the change in the property of flux lines. It is considered that the dimensional crossover of flux lines brings about the first-order transition and such characteristics. The elastic correlation length of flux lines along the c-axis was of the order of and agreed with the theoretical prediction. This shows that the flux lines are not in the complete two-dimensional state even above the peak field.

(YBCO) thin films have been grown on vicinal single crystals. The films show a pronounced anisotropic resistivity and flux pinning properties in the substrate plane. UHV scanning tunnelling microscopy and transmission electron microscopy have been used to investigate the substrate surface, film morphology and the growth induced defect structure. The anisotropy is caused by planar defects generated via self-organization of the YBCO which lead to an exceptionally large critical current density up to at 4.2 K. The results demonstrate the possibility of a controlled enhancement of the critical current by tailoring the growth induced defect structure.

Transport critical current densities in single crystals of the strongly anisotropic high- superconductor were measured in a wide range of temperature, applied field and angles between field direction and c-axis of the crystals, above and below the decoupling line. In the low-field region, below the decoupling line, an analysis of the field and angle dependences of the allows us to assume a dominance of the collective pinning mechanism, which is similar to that found in moderately anisotropic single crystals. We provide scaling analysis of the dependences to probe the vortex topology and to locate the decoupling line on the diagram.

Using AC diamagnetic and transport measurements we have derived a three-dimensional diagram for high-quality Bi2223/Ag tapes. The diagram shows an abrupt decrease of critical current caused by the melting of a vortex glass. The melting line was defined as the contour line for the surface at , where showed the steepest decrease with H and T. This line coincided with the locus of the maximum, at a frequency Hz and an AC field amplitude Oe. The melting line reflects the process of the separation of unit-cell stacks. The most important are the low-number stacks that contain one to three unit cells. These stacks have the lowest Kosterlitz-Thouless transition temperatures and the lowest corresponding vortex melting temperatures. A magnetic field causes the separation of the stacks and reduces in each stack to the melting temperature of the lower stack. At the universal , the vortex melting line is the combination of for different, mainly low-number, stacks.

The biaxial textures created in metals by rolling and annealing make them useful substrates for the growth of long lengths of biaxially textured material. The growth of overlayers such as high-temperature superconductors requires flat substrates with a single, sharp texture. A sharp cube texture is produced in high-purity Ni by rolling and annealing. We report the effect of rolling reduction and annealing conditions on the sharpness of the cube texture, the incidence of other orientations, the grain size and the surface topography. A combination of high reduction and high-temperature annealing in a reducing atmosphere leads to >99% cube texture, with a mosaic of about the rolling direction, about the transverse direction, and about the normal direction.

Texturing of superconductors is essential to achieve high critical current densities. For coatings or coated conductors the symmetry-anisotropy of the substrate has a major impact on the formation of a biaxial texture. The substrate symmetry is broken for example by a rolling texture in the RABiTS process or by anisotropic damage in the IBAD process. However, anisotropy can also be introduced by using multiphase substrates, providing an anisotropic but regular arrangement of the individual phases. Such an arrangement leads to anisotropic growth conditions owing to the different properties of the phases involved. Moreover, nucleation is probably influenced by the aligned boundaries between the substrate phases. As a preliminary result obtained from infiltration of regularly arranged fibres, preferred growth along the fibre directions is observed.

The melt texturing process, which has been under development since 1988, is now well established in producing components such as levitators and permanent magnets which can be used at liquid-nitrogen temperature. Recently, major accomplishments have been achieved which suggest that bulk applications of this process in industry are quite feasible. Studies have been performed to increase the rate of texturing and to achieve high critical currents through high-angle grain boundaries. These new developments, individually or in combination, can make the processing of 123 superconducting materials faster, more efficient, more robust, and also scalable, in order to satisfy industrial needs. In this paper, two of the new developments in melt texturing will be described, and some of the approaches to potential bulk applications will be reviewed.

When uranium (U) is added to Y123 powders, after texturing the deposits containing U are found to be nm. These deposits act as pinning centres, and increase by 90% per 0.8 wt% U. Neutron irradiation further increases . Total increases of by factors of e.g. (at 77 K, 0.25 Tesla), and (at 50 K, 10 Tesla) result. Magnitudes of are (at 77 K, 0.25 T) and (at 50 K, 0.25 T). The U/n process is not yet optimized. Chemistry, microstructure, radiation effects and cost are discussed.

Y123 samples with varying amounts of added Y211, and have been melt processed and quenched from temperatures between and . The microstructures of the quenched samples have been characterized using a combination of x-ray diffractometry, optical microscopy, scanning electron microscopy, microprobe analysis, energy-dispersive x-ray spectroscopy and wavelength-dispersive x-ray spectroscopy. The Ba-Cu-O-rich melt undergoes complex changes as a function of temperature and time. A region of stability of (BC1) and (BC2) exists below in samples of Y123 + 20 mol% Y211. is stabilized by rapid quenching but appears to separate into BC1 and BC2 at lower quenching rates. and additions affect the distribution and volume fractions of the two Ba-Cu-oxide phases.

The fabrication of high-quality thin films of high-temperature superconductors (HTS) of the type and (Bi-2212) by pulsed laser deposition (PLD) is discussed. The surface morphology, lattice parameters, electrical properties, transition temperature and the deposition rate of PLD films depend strongly on the laser fluence and spot size, the type and pressure of the ambient atmosphere, etc. The strong non-equilibrium conditions in PLD allow new metastable compounds to be synthesized. Films with a step-like morphology have been fabricated on off c-axis oriented [001] and MgO substrates. Such films permit us to investigate the strongly anisotropic properties of HTS that are not available as single crystals. Among these properties are the electrical conductivity and its enhancement under large-area illumination, the Seebeck effect and the thermopower.

Systematic measurements of the critical current versus temperature over a range between 10 K ad and of the energy barrier for vortex motion versus current density were performed for ten YBCO thin films made by different deposition methods in various laboratories. A new technique, which uses ring-shaped samples and a scanning Hall probe, has been developed to study thin film properties. The results imply a universal behaviour of and for all YBCO films independent of thin growth conditions, substrates, film thickness and the magnitude of or . We suggest that this behaviour is governed by an intrinsic property of YBCO, the Josephson nanostructures in the a-b planes.

Development of high-temperature superconductor technology will make possible the design and fabrication of smaller, lighter and more efficient power devices such as motors, generators, transformers, transmission cables and fault-current limiters. A prototype fault-current limiter, a 200 hp motor and a 50 m long transmission cable have already been demonstrated using Ag-clad Bi-2223 superconductor tapes. We have recently enhanced the transport current properties of long lengths of multifilament Ag-clad Bi-2223 tapes through increased packing density of precursor powder, improved mechanical deformation, optimization of conductor design and adjusted cooling rate. These improved processing parameters had a pronounced effect on the transport critical current of the superconducting tapes. Our improvements are briefly discussed and their implications are assessed in this paper.

Some long-length Bi(Pb)-2223 tapes produced using the PIT technique and multilayered structure technique have been developed for an industrial material for which the requirement is the class at 77.3 K in long-length wires. For this purpose, we have achieved recent progress in performance, such as reproducibility of the class at 77.3 K and low magnetic fields for long-length tapes having sound filaments, overall critical current density of in a low silver:superconductor ratio tape, allowable strength of more than 100 MPa using silver alloy sheath and low AC loss of less than in kA class conductors using twisted wires.

We fabricated a strengthened silver-sheathed multifilamentary tape coil with parallel co-winding Ag-0.5wt%Mg tape between turns and an additional Cu-50wt%Ag tape on the outer coil surface. The reinforced double-pancake coil with a wide bore 280 mm in diameter was investigated in a large hoop stress state up to 240 MPa at an external magnetic field of 10 T and 4.2 K. It is found that good transport properties without degradation for the test coil are revealed under a successfully reduced strain of less than 0.2% at 240 MPa. Moreover, it is pointed out that thermal stability for the silver-sheathed tape is closely related to the current sharing characteristics as a result of cryogenic stabilization with heat transfer from the metal surface to liquid helium.

An overview of the fabrication and electromagnetic properties of high-temperature conductors processed by the powder-in-tube (PIT) technique with reference to texture development and critical anisotropy data is presented. Special emphasis is given to the optimization of the physicochemical and electromagnetic parameters of the multifilamentary and single-filament conductors with superconducting cores of Bi-2223, Tl-1223 and Y-123 superconducting phases. The influence of the multifilamentary and single-filament structures on texture development is discussed. Also, the importance of the local disturbances of the grain alignment and microdefects for the current distribution across and in the plane of the whole conductor is analysed. A comparative study of the critical current anisotropy with field direction in low magnetic fields of Tl-1223 and Bi-2223 conductors manufactured by the PIT technique is presented. For Tl-1223 PIT conductors the anisotropy coefficient shows a very pronounced minimum, followed by a monotonic reduction of anisotropy with the increase of the magnetic field. This is explained in terms of poor grain alignment with weak intergranular superconducting coupling which cause 3D current percolation and also by the demagnetizing effect of the grains and the ceramic core in the PIT Tl-1223 tapes.

Bi-2223 multifilamentary tapes with Ag and Ag alloy (AgMn, AgPdAu) sheaths have been fabricated using the OPIT technique. From thermal analysis and XRD investigations one can conclude that the formation rate and microstructure of the Bi-2223 phase is strongly influenced by the occurrence of a liquid phase during the precursor transformation. The parameters of the thermomechanical treatment were optimized such that values of (77 K, 0 T) could be achieved for tapes of a length up to 550 m, independent of the tape length. On Ag alloy sheathed tapes values of were measured. The value of the tensile strength, which can be applied to the tapes without degradation of was doubled by substitution of Ag alloys for pure Ag sheath material.

Results on the enhancement of critical current densities in melt-textured bulk (Y-123) superconductors, which were doped with and exposed to thermal neutron irradiation to create fission tracks, are presented. These tracks, which all originate from U-containing clusters of a stable UPtYBaO compound, form isotropic `starlike' defects with diameters of approximately 10 nm and lengths of . However, if we consider projections onto the symmetry plane of the flux line lattice and allow for a certain tilt modulus , their pinning-effective size is mostly (90%) between 10 and 20 nm, i.e. only twice as large as the characteristic defect structure (amorphous collision cascades with diameters of nm) introduced into undoped superconductors by fast neutron irradiation. We report on studies of and the irreversibility lines in superconductors containing various amounts (0.3-0.7 wt%) of uranium and various amounts of the fissionable isotope . In the best case (0.3 wt% U, thermal neutrons), we find (77 K) to be around (0 T) and (5 T). Control experiments on samples containing (nominally) only and exposure to thermal or to thermal plus fast neutrons confirm the above flux pinning considerations. As reported before for undoped melt-textured Y-123, the irreversibility lines remain practically unchanged due to the presence of strongly pinning Y-211 precipitates.

Essentially single-phase Hg-1223 and Cu-1223:P samples in wide hole-doping ranges were successfully synthesized by a high-pressure (5 GPa) technique utilizing internal and/or external oxidizing agents and by subsequent post-annealings performed in a thermobalance. The hole-doping level of the as-synthesized samples was found to be in the overdoped side for both of the phases by thermoelectric power measurements. values as low as 107 K and 67 K were obtained for the overdoped Hg-1223 and Cu-1223:P samples, respectively. The values of and were calculated from the measured M versus H data employing, respectively, the Bean model and two different criteria: (at ) and (as ). It was found that the heavier the doped hole concentration was, the more improved were the versus characteristics for both the Hg-1223 and the Cu-1223:P phases. This tendency was the same for versus characteristics calculated using either criterion. The improvement with increasing hole-doping level was especially profound in the case of Cu-1223:P: the versus characteristics of a Cu-1223:P sample with K, probably being close to the optimally doped state, were as poor as that reported for Bi-2212, while an overdoped Cu-1223:P sample with K was superior to an overdoped Hg-1223 sample with K. The mechanisms as well as the usefulness of the observed phenomena are discussed.

Mechanical deformation and subsequent thermal treatment have been widely used in BSCCO superconductor processing. The purposes of this treatment include introducing texture microstructure, obtaining desired phases and healing the microcracks. It has been shown that most supercurrent flows through the thin layer of superconductor next to the silver sheath. It was proposed that this was related to the good grain alignment in this area. However, the effects of mechanical deformation on the formation and distribution of the texture microstructure in BSCCO superconductors have not been fully understood. The present paper reports a detailed study of the relationships between the mechanical deformation and texture distribution in the BSCCO tapes. Results indicate that the texture distribution behaviours in Bi2223 and Bi2212 are different. Mechanical deformation produces a stronger texture and deeper texture distribution in Bi2223 than in Bi2212. Based on the experimental results, the formation mechanisms of the texture microstructure during mechanical deformation are discussed.

The high-resolution magneto-optical technique has been used to visualize the magnetic flux distribution in Bi(Pb)-2223/Ag multifilamentary tapes. Topographies of the magnetic flux will be presented for a seven-element x-array filamentary distribution which has both exposed and silver-sheathed filaments. This study was over the temperature range 40-80 K for applied magnetic fields in the range 0-560 G and direct currents up to 60 A. Analysis of the visual patterns will be presented and in the case of the x-array compared with the work of Mawatari (1996 B 54 13 215) and Müller (1997 Physica C 289 123).

A technique has been developed to investigate the magnetic and electrical properties of individual layers of filaments in three-layer, close-packed, 27-filament tapes. This has enabled the improvements to individual filaments resulting from various processing conditions to be examined. The magnetic properties of each individual layer of filaments suggest that the second layer of filaments using the current processing conditions is the best textured and aligned, with at 5 K of over twice that of the outer layer.

High-resolution O K edge x-ray absorption near-edge structure spectra of the high- cuprates of (Tl, Pb-1212), (Hg, Pb-1212) and (Cd, Pb-1212) were measured by a bulk-sensitive total x-ray fluorescence yield technique using synchrotron radiation. Near the O 1s edge, the pre-edge peak at is ascribed to the transitions to O 2p holes located in the planes. The intensity of this pre-edge peak increases with increasing doping level of into the sites in Tl, Pb-1212, Hg, Pb-1212 and Cd, Pb-1212. The results can provide insight into the understanding of hole distribution and the role of holes in superconductivity in 1212-type cuprates which may give a direction to the search for new high- materials.

The relatively large and non-logarithmic magnetic relaxation of high-temperature superconductors has prompted a re-examination of the simple Kim-Anderson flux creep model. Proposed modifications of the pinning potential have included (i) energy-depth distributions, (ii) spatial distributions and (iii) non-linear current dependences. One can avoid making a priori assumptions regarding the current dependence of the pinning energy, the U-J curve, by using the Arrhenius rate equation, , as the starting point. This permits the U-J curve to be constructed from magnetic relaxation and hysteresis data. The methodology of this process of construction is presented and analysed. This analysis is applied to magnetic measurements performed on a bulk, polycrystalline sample of .

Large crystals have been grown by a melt-processing technique from material with the starting composition that had been doped with , Pt and . The crystals have been investigated using a number of different techniques and for the undoped and those doped with Y/Nd(211) and/or Pt there is no evidence for substituting on sites. Crystals doped with show a second peak on the magnetization curve but in all cases the highest critical current densities measured with was and at 0 and 1 T, respectively. To enhance at high magnetic fields further doping materials and changed melt-processing conditions are being studied to bring about either substitution and/or a homogeneous distribution of very small Y/Nd(211) particles.

Magnetic hysteresis loops (M-H) between 5 and 50 K were measured on (Bi-2212) crystals grown by the spiral or layer-by-layer growth mechanism using KCl flux and self-flux with a large temperature gradient growth technique. The spiral-grown crystals with a large density of spiral steps showed a strong fishtail effect with at 1000-2000 Oe between 20 and 50 K, for both high- (86 K) and low- (76 K, oxygen underdoping) samples. For the layer-by-layer-grown crystals with an extremely smooth surface and annealed in oxygen-nitrogen, a weak fishtail effect with at 300 Oe was observed between 20 and 40 K. The fishtail effect disappeared when the spirals were removed from the crystal surface, whereas the fishtail effect for the layer-by-layer-grown crystals was mainly controlled by oxygen content. The peak effect is fully reversible in the layer-by-layer-grown crystals by a proper annealing in oxygen and in nitrogen. From this comparison we conclude that the peak effect in Bi-2212 is caused by either surface pinning or oxygen vacancies for spiral- and layer-by-layer-grown crystals. Furthermore, the TEM study helps to show that the dislocation networks are not responsible for the fishtail effect. In the layer-by-layer-grown crystals, the presence of oxygen vacancies is a necessary but not a sufficient condition for the fishtail peak effect, but the networking of these vacancies may play a dominant role.

A key requirement for technological application is to have superconductors with high critical current density at practical operating temperatures and magnetic fields. The critical current density is strongly related to underlying properties of high superconductors, such as laying, anisotropy and other intrinsic material structures. The thallium-based superconductors attracted much attention at an early stage mainly due to their high superconducting transitions. Our single crystal studies on show that these materials appear to be a better choice for achieving higher critical current density because of a stronger interlayer coupling between superconducting layers.

The powder-in-tube route for fabricating long lengths of multifilament Ag-sheathed Bi-2223 tape can be used to manufacture tapes with a variety of filamentary configurations. The design of the filament array within a tape will influence the electrical properties including the ac loss which becomes a significant consideration when designing a closed cooling system. The effect of periodically arranged filaments on the transport ac losses of Bi-2223 tapes has been investigated and the results compared with the Norris equations.

/silver composites were fabricated by drawing a silver tube packed with precursor powders into round wire and deforming the round wire into flat tapes by longitudinal rolling, transverse rolling and uniaxial pressing respectively. The resultant tapes were observed by optical microscopy to examine the superconductor core/silver interface. Short pieces were cut from the tapes and heat-treated by a thermomechanical process consisting of alternate sintering and intermediate mechanical deformation. Intermediate deformation was carried out for each tape using the method by which the tape was formed. The effect of different deformations on the microstructure and transport property of the final tapes was investigated. It was found that deformation method affected the core/silver interface of tapes significantly. Pressing produced a wavier core/silver interface (sausaging) and more cracks than longitudinal and transverse rolling. As for critical current density, pressing yielded the highest value, due to the higher density and better grain alignment in the pressed tapes than in the longitudinal and transverse rolled ones.

The effect of the phase compositions after the final stage of heat treatment on the critical current density in Bi:2223/Ag tapes has been investigated. A high volume fraction of Bi:2223 has been considered the most critical factor in achieving a high ; however, it is extremely difficult to attain pure Bi:2223 phase via routine thermomechanical processing. Although a higher volume fraction of Bi:2223 phase can be attained using a higher processing temperature, it is always found to co-exist with Bi:2201 which resides at Bi:2223 colony boundaries. As these possess a plate-like morphology, the Bi:2201 joins weak links in a conductible chain. Bi:2201 can be eliminated using an annealing step at the end of the last thermal cycle, resulting in a significant improvement in . At annealing temperatures below , Bi:2223 decomposes into Bi:3221 and other cuprates, which degrade further. Magnetization, AC susceptibility and resistivity measurement results confirm that the Bi:2201 and Bi:3221 act as weak links.

For Bi-2223 Ag-sheathed tapes, pressing conditions between sintering were optimized. It was found that with an increase in the pressure from to , of the tape was increased. To elucidate whether Li doping increases the of the tape as reported in the bulk sample, Li doping was carried out. It was found that doping of 0.1-0.3 at.% Li increased by 10%, which is consistent with the bulk data. As a result of -B measurement at a temperature of 4.2 K, it was found that of the 0.3 at.% Li-doped tape under a magnetic flux density of 10 T was 20% higher than that of the non-doped tapes.

A small amount of was added to the in silver-sheathed (Bi2223/Ag) tapes. Our results showed that addition can accelerate the formation of the high- phase ( K), enhance flux pinning strength and decrease weak links. The transport properties of the tapes, therefore, were improved by addition.

The effect of intermediate rolling on the final critical current density of Ag-sheathed Bi-2223 tapes has been investigated. The optimization of intermediate rolling was found to be dependent on the sintering temperature and time. XRD analysis indicated that the volume fraction of 2212 after the initial sintering was a good guide to estimate the optimum intermediate rolling reduction.

Magnetic separation techniques have been widely used, and the separation efficiency is related to the strength of the magnetic field applied and the field gradient to trap the magnetic particles. Conventional low- superconducting wire has been used to increase the field and to reduce normal electromagnetic winding resistive loss, and now the Bi-2223/Ag HTS wire provides a new opportunity for higher magnetic field and efficiency for use in a high-gradient magnetic separator as the magnetic field winding. Magnetic separation techniques are reviewed with regard to their magnetic fields, and the magnetic field winding required for this application is analysed with the consideration of using the HTS wire.

High- superconducting (HTS) multifilamentary tape, was used to prepare an HTS toroidal coil. Magnetic field behaviour of the HTS coil is studied for this toroidal configuration. The magnetic field distribution of the HTS coil is investigated for use at 77 K and 4.2 K with the anisotropic HTS wire having a strong magnetic field dependent critical current. Since the magnetic field component along the HTS wire c-axis is significantly reduced in the form of a toroid, it is suitable to make a high-ampere-turn or high-inductance coil with the present HTS tape having an anisotropic transport property.

Nano-MgO particles have been introduced into silver-sheathed tapes fabricated by the rod-in-tube process. It is found that the transport properties of the nano-MgO particle doped sample are noticeably improved compared with those of the undoped sample. The irreversibility line of the MgO-doped sample shifts to higher temperatures and magnetic fields, as compared with the undoped sample. Therefore the introduction of nano-MgO particles can effectively enhance the flux pinning.

Bi-2223 precursor powders with different stoichiometries and synthesized using different heat treatments were compared with powders obtained from external sources in powder-in-tube tapes. In the final microstructure, it was found that the Bi-2223 grain size varied considerably between tapes made from different powders. The critical current density was found to increase with increasing grain size. Among the factors found to determine the Bi-2223 grain size were powder stoichiometry and Bi-2212 recrystallization rate.

Power engineering applications of HTSC tapes are often restricted by AC losses. Both direct and indirect methods have been used to measure AC losses in short samples. This paper reports on the development of a direct calorimetric method for AC loss measurement techniques for short tapes based on measurements of slight temperature increases of the sample. This method has the advantage of measuring total losses in the HTSC sample. The test rig discussed here consists of a calorimeter and associated data collection and analysis equipment. The calorimeter uses a sensitive temperature sensor and an ohmic heater, needed for the calibration of the system, surrounded by a thermal insulator. A platinum sensor is used which works at 77 K and may be used in a magnetic field. Sensors and controlled variables are interfaced to the controlling computer via IEEE interfaces. The power loss of a sample carrying alternating current has been measured with a high accuracy using this experimental set-up. The sample is a seven-filament BSCCO-2223 tape. In addition, the effect of the frequency and amplitude of the signal has easily and quickly been analysed.

A novel high current lead, utilizing silver-sheathed Bi-2223 tapes, has been developed and tested, which allows addition or removal of individual, vapour cooled tapes, thus adjusting the current capacity and optimizing the heat leak per current. Minimum heat leak is obtained by tapering the lead, using more tapes at the high temperature end. To date, the leads have been tested using applied currents up to 900 A DC. However, they are designed for a current capacity of several kA.

Bi-2212 phase formation was studied in silver-clad tapes processed by melt-texturing in an oxygen atmosphere. Stoichiometric Bi-2212 was found to undergo a peritectic decomposition, which starts at , into a Bi-rich liquid and two alkaline-earth cuprate solid phases: and . This phase assemblage remained unchanged up to , where the solid phase crystals were found to have a random orientation. Some Ag was found to dissolve into the liquid, and concentrate around the 14:24 crystals. On cooling, at , the Bi-2212 crystals randomly nucleated from the liquid. At , the Bi-2212 crystallite size was in the range of 0.1 to 0.4 . The phase assemblage consisted of Bi-2212 as the major phase, and Cu-free and trace 14:24 crystals as the minor phases. The 14:24 crystals were highly oriented relative to the silver interface. As the minor phases diminished, the texture of Bi-2212 crystals, having the c axes perpendicular to the silver-core interface, improved. These observations suggest that Bi-2212 crystals nucleate from the liquid on the surface of the 14:24 phase as well as at the silver-core interface, and the dissolved silver in the liquid may play an important role.

The influence of mechanical deformation on critical current has been studied, in order to assess the effect of sample handling on the current carrying capabilities of Bi-2223/Ag tapes. Transport critical current was measured at 77 K using a criterion. The effect of progressive bending of the tapes on grain connectivity and flux pinning were investigated by measuring against H. It was observed that bending can affect weak and strong links to a different degree. Damaged samples have a low , but often have a large critical current which is carried through strongly linked paths, compared to carried through weak links. The influence of tensile deformation on was also studied. was found not to degrade until the deformation of the tape approached its elastic limit.

Ag-sheathed Bi-2223 multifilamentary tapes were prepared with a hot-pressing technique to study its effect on grain connectivity. The critical current density after hot-pressing was significantly enhanced with the maximum increase reached more than double that before. Hot pressing raised the from to for 81-filament tape. However, the critical current density in an applied magnetic field was not improved by hot-pressing. The in a magnetic field of 1 T decreased from 33% of for the original tape to 21% of for the hot-pressed tapes before and after hot pressing. The fraction of strong links was improved from 64% to 73% for the 81-filament tape as a result of hot pressing. Microstructural analysis showed that hot pressing chiefly improved grain connectivity, increased core density, recovered microcracks and reduced secondary phase impurities.

Strong flux pinning is essential in high- superconductors for high current applications. A number of methods are in use to increase the critical current of MTG-grown ReBCO material where Re stands for rare earth or Y. Further improvement requires additional research into the nature of flux pinning in high- superconductors. We present effective activation energies derived from thermopower measurements on high-purity YBCO crystals grown in yttria crucibles. These measurements show that there is a larger activation energy for quasiparticles travelling parallel to the c-axis compared with those travelling through the a-b-plane. In addition, there is a maximum in the effective activation energy for both orientations near the oxygen content , which coincides with the optimal doping level. For underdoped and overdoped crystals the effective activation energy is reduced.

During magnetoresistance measurement of Bi-2212 single crystals, we found the resistance in the c-axis direction exhibited a huge magnetic enhancement (H perpendicular to the a-b-plane) while the onset transition temperature along the c-axis showed a large depression when the magnetic fields were over a certain value (H >180 Oe). No such phenomena were observed in small applied magnetic fields and in a-b-plane magnetoresistance measurements. By changing different probes (eight probes being attached to the sample, each side having four probes) for the measurement of the magnetoresistance along the c-axis with H = 2000 Oe, we found that the resistance along the c-axis in the temperature region was only related to the measuring dimensions and onset remained unchanged. Our results provided evidence that both Lorentz-force-independent fluctuation-induced dissipation and Lorentz-force-dependent vortex motion dissipation exist in the mixed state.

DC magnetization of a large high-quality Bi-2212 single crystal with a size of has been investigated in magnetic fields up to 7 T. The anisotropy of the single crystal has been studied by using a SQUID magnetometer equipped with a sample rotation system having an angle adjustment accuracy of when the magnetic field was applied at different angles between to with respect to the crystallographic a-axis of the crystal. The irreversibility line for magnetic field applied along the a-axis of the crystal was found to be located in much higher fields than those predicted by an anisotropic scaling approach. The results indicate that the anisotropy parameter for this material may be as large as 1300.

Two different types of as-grown (Bi-2212) single crystals grown by a directional solidification and floating zone methods have been annealed in air and 1 atm of flowing oxygen at for 72 h followed by different cooling processes. Experimental results show that there are no change in composition or phase segregation caused by the annealing. Annealing of the single crystals grown by the directional solidification method increases the superconducting transition temperature from 70 K (as grown) to 83 K (annealed), with the superconducting transition width (5-95% of complete superconducting transition) decreasing from 36 K to 12 K. Annealing of the single crystals grown by the floating zone method depresses from 91 K (as grown) to 83 K (annealed), with increasing from 4 K to 7 K. Further, the annealing of the as-grown crystals results in increase of the superconducting volume fraction. The results show that oxygen content and distribution in the crystals play a key role in the superconductivity of these crystals. The quality of the as-grown crystals also affects the results of the annealing experiments.

The structure of dense (99%) high-pressure (2-5 GPa)-high-temperature treated single-phase and melt-textured YBaCuO and NdBaCuO ceramics has been investigated. High-pressure-high-temperature (HP-HT) treatment under 5 GPa in contact with annealed zirconia allows us to prevent oxygen losses from Nd123 and Y123 structures even when heated up to 1100 and , respectively, and thus, keep the superconductive characteristics. For MT-YBaCuO ceramics, a more than twofold decrease in the anisotropy of critical current density occurs because of doubling critical current density in the direction of the c-axis of Y123 crystallites (in the perpendicular direction it remains almost unchanged) may be explained by the increase in dislocation density in the (001) plane (from up to ) and by narrowing of twin domains (from 150 down to 20 nm). For MT-NdBaCuO, narrowing of spacing between the twins has been observed in the polarized optical microscope.

The effect of Fe on the crystal growth of was studied by a floating zone method. The result shows that the planar solid-liquid growth interface breaks down into a cellular growth interface with increasing Fe substitution in the feed rods of . The size of the single crystals decreases with the increase in Fe substitution in the feed rods. When Fe substitution content y is 0.06 in a feed rod, the Bi-2212 single crystals in an as-grown bar are too small to be visible. The maximum thickness along the c-axis of the pure Bi-2212 single crystals is as large as 2 mm. The solubility limit of Fe in Bi-2212 crystals is 0.025. The Fe substitution in Bi-2212 crystals depress sharply not only the superconductivity transition temperature but also the homogeneity of the superconducting phase in the crystals.

The paper presents magnetoresistivity measurements in the vortex liquid phase of YBaCuO single crystals with different angles between unidirected twins and current direction in magnetic fields up to 12 T. Unidirected twin planes are the cause of anisotropic pinning of vortices. In contrast with the measurements of the usual longitudinal magnetoresistance of the isotropic sample which is even with respect to reversal of the direction of the magnetic field, we have observed an additional, odd with respect to reversal of the magnetic field, contribution to the longitudinal resistivity which is due to the presence of unidirected twins in the YBaCuO crystals. A theoretical discussion of the effect is presented.

Micro-Raman spectroscopy is well suited for microstructural analysis of (YBCO) superconductors. Of particular interest is the oxygen stoichiometry, 7-x, on which many superconducting properties depend. It is common to use the frequency of the Raman spectral peak near to obtain a quantitative measure of 7-x. Another peak exists near . The variation of the position of this peak appears to be minimal as 7-x changes. However, this peak is asymmetric for orthorhombic YBCO and symmetric for tetragonal YBCO. We have observed a variation in the asymmetry of this peak with 7-x, in Raman spectra obtained from single grains in a sample of polycrystalline YBCO. We propose the use of this peak's asymmetry as a quantitative means to measure oxygen stoichiometry.

Recent ultrasonic attenuation experiments on Bi2212, Y123 and Nd123 have indicated temperature-dependent structural changes in these materials. A three-stage, three-region model has been developed which provides an explanation of the processes involved in the changes. The model allows activation energies associated with the changes to be determined.

Two mechanisms, magnetic pair breaking and localization of charge carriers, are generally used in the attempt to explain the suppression of superconductivity in . The application of both is unsatisfactory, and this makes the absence of superconductivity in one of the most long-standing problems of HTSCs. We give a new insight into this problem by considering principally a new mechanism of suppression based on the Pr-O resonance. This resonance can be described as the oscillation of the O ion in the plane near its equilibrium position, and this oscillation arises when the distance between O and Pr becomes close to the critical one (2.38 Å) for the realization of the vibronic state. The vibronic state unites the electronic and nuclear modes into a motion with a broad spectrum. In the vibronic state, the 4f configuration of Pr ceases to be isolated, and its magnetic moment spreads on the whole Pr-O complex. Analysis of structural data obtained by neutron diffraction and x-ray absorption fine structure measurements shows that, during oxygen oscillation, Pr resonates between the states of different valence with a period of less than s. Because this is less than the lifetime of a Cooper pair, the competition between the magnetism of the vibronic state and superconductivity makes the creation of a Cooper pair energetically unfavourable.

Various milling systems consisting of agate and polypropylene grinding containers, agate and YSZ balls, and dry and wet milling were used in planetary ball-milling and YSZ balls and YSZ container were used in wet and dry attrition milling. The differently milled powders were then evaluated by measurements of particle size, surface area, porosity, size distribution and chemical analysis of the Si, Zr and C contents. The results show that dry milling is much more efficient for particle size reduction in planetary milling than wet milling, whereas wet milling and dry milling gave quite similar results in attrition milling. Meanwhile contamination was found in powder milled with an agate container with agate balls. Some C contamination from the polypropylene container was detected after milling, but negligible Zr from YSZ balls and C from the grinding carrier (hexane). It was found that after 1 h milling in the planetary mill fracture mechanisms transform from the elastic to the plastic region. Therefore, further milling is not very effective. It was also shown that the Bi2212 phase decomposes into several non-superconducting oxides such as , CuO and a main amorphous phase after extensive dry milling.

Dense ceramics textured by sinter-forging exhibit superconducting properties among the best for polycrystalline bulk 2223 materials. Investigations showed that the deformation during the sinter-forging process is mainly responsible for the grain alignment of the ceramics. A sharp texture can only be obtained after a strong deformation from the starting cold-pressed powder to the final thin forged disc. To obtain thick ceramics, several thin and highly identically textured discs were linked together by an additional short sinter-forging step. Microscopic observations showed that the interface between the discs disappeared after the hot treatment. Transport critical current measurements performed at 77 K through bars of different sizes confirm that, through an equal texture, the critical current density () is nearly constant with the shape of the bar section: thin or thick, narrow or wide. A ceramic composed of many stacked and sinter-forged discs allowed resistivity and to be measured along the sinter-forging axis. Low anisotropic ratios confirmed the strong link between the thin discs and a very homogeneous whole ceramic. This shape processing of bulk 2223 ceramics is thus attractive for the fabrication of current leads and limiters with high capabilities.

In this work the production of Bi-Pb-Sr-Ca-Cu-O powder precursors by three different chemical route techniques, namely freeze-drying, spray-drying and thermal co-decomposition methods, plus the two-powder oxide route method, were investigated experimentally. The most reactive precursor made by the chemical route was made by the freeze-drying method. The reactivity of the powders was related to the particle size and stoichiometric changes during heat treatment. Sintering for 60 h at was sufficient to form nearly pure single-phase (91%) Bi-2223 when the freeze-drying method was employed, whereas the maximum amount of Bi-2223 phase produced by the thermal co-decomposition method was 77% for 83 h sintering at in air, and the maximum amount of Bi-2223 phase produced by spray drying was 55% for 150 h sintering at in air. The two-powder oxide route was found to be intermediate in efficiency among the other techniques used here, i.e. it is worse than freeze drying and thermal co-decomposition but better than spray drying.

Broad double peaks of the critical current density were observed in an untwinned Y-123 single crystal prepared by a pulling technique. Force-displacement characteristics of flux lines were measured using the Campbell method in the vicinity of the peak fields. It was found that the interaction distance decreased monotonically in a wide range of the field, while the increasing rate of the Labusch parameter changed appreciably at the magnetic field at which the critical current density started to increase. These results are completely different from those in the case of Bi-2212. This fact seems to suggest that the peak effect is ascribed to the change in the flux-pinning mechanism but not to the change in the property of flux lines. Field-induced pinning by weakly superconducting regions seems to be a candidate for the responsible pinning mechanism. However, the characteristic field at which the critical current density starts to increase increases with elevating temperature. Hence, this temperature dependence cannot be explained by such a simple mechanism. The synchronization mechanism known in metallic superconductors may also work in Y-123.

Current voltage characteristics of microbridges with different thicknesses 20 - 110 nm were measured. The resistivity-current density characteristics within the temperature range of , can be fitted by the thermally activated flux creep model with a linear current density dependence of the activation energy U(J). The vortex activation energy within this temperature range has a value of 33-59 meV whereas the hopping frequency was found to be in the range of .

We have studied magnetothermal instabilities, giant flux jumps, both theoretically and experimentally. Magnetostriction and magnetization hysteresis loops with flux jumps were calculated over a wide range of experimental parameters employing two critical-state models: the Kim-Anderson model and the exponential model. The influence of the magnetic history on the flux jumps' magnetostriction and magnetization were investigated for the LaSrCuO single crystal. The shape of the unstable region of the critical state in the temperature-magnetic field plane was constructed from calculations and experimental results. In the Kim-Anderson model H-T diagrams of flux instabilities agree well with experimental results on HTSC single-crystal LaSrCuO and experimental results on LTSC niobium.

The irreversible behaviour of the pinning-induced magnetostriction of a cylindrical superconductor placed in a parallel magnetic field, , is analysed. An exact solution of the magneto-elastic problem is found and we present formulas for the deformation, , and the stress distributions, and , valid for any critical-state model . For the Bean model, explicit results are derived for the various stages during a field cycle. A simple method to infer from measurements of versus is pointed out. When decreases, the stresses become tensile, and the probability of generating cracks in the material is discussed.

We consider the implications of the two-pocket Fermi surface for macroscopic quantum phenomena in cuprates. Superconductivity in this system can be described in terms of two coupled condensates. It results in a collective excitation corresponding to the relative phase oscillation - a phason. The energy of the phason is smaller than the maximum gap on the Fermi surface. We discuss the possibility of searching for this collective excitation in the dynamic resistance of a superconducting interference device (SQUID).

Oxygenation kinetics of superconductor with silver (0%, 10% and 30% Ag by weight) have been studied by using thermogravimetric analysis at annealing temperatures ranging from to in air. The results show that both the silver addition and annealing temperature greatly influence the oxygenation processes of the superconductor. The amounts of the oxygen loaded into Y123-Ag compounds increase with decreasing temperature. The silver additions also increase the saturation oxygen contents, increase the maximum oxygenation rates and slow down the accelerations and deceleration of the oxygenation rates. It is believed that there are two types of diffusion that affect the oxygenation kinetics: oxygen transfer through grain boundaries and pores and oxygen diffusion through Y123 lattice. Added silver changed the microstructures of Y123 specimens by segregating itself onto the grain boundary areas and therefore affecting the oxygen transfer processes and oxygenation kinetics.