Table of contents

A new rolling technique, so called `eccentric rolling', has been developed for texture and density improvement in Bi(2223)/Ag tapes made by PIT. It consists in rolling deformation between the outer surface of the inner roller and inner surface of the outer hollow cylinder. This technique allows to deform the BSCCO/Ag composite in a way which is similar to flat rolling using rollers of very large radius or to a `continual' pressing but without any overlapping parts. In comparison to classical rolling with roller of a similar radius, a higher core density and widening of tape have been obtained. The transport critical current densities have been measured for single-filament and 19-filament tapes deformed by the roller presented and a classical roller. Current densities which are two times higher than in the case of classical roller (at T = 77 K, B = 0 T) have been measured for both types of tapes deformed using the new technique presented.

Various aspects of the problem of current transfer in high- superconductors (HTSs) are reviewed. The spatial inhomogeneities of various types are identified as a primary cause of non-uniformity of both normal currents and supercurrents in real samples of HTSs. The role these inhomogeneities play in transport features of the samples is discussed. The case of grain boundaries in polycrystalline samples is elaborated in detail. The local structural and transport properties of isolated grain boundaries are first reviewed and then integrated into the knowledge of global (macroscopic) charge transport. The paper emphasizes the common ingredients characterizing the transport in various forms and families of HTS samples in small magnetic fields. The phenomenon of percolation is identified as the most obvious one and is shown to dominate a large number of observations covered by this report. The experimental results focused on by this report elaborate primarily the problems of critical currents, initial dissipation and current-voltage characteristics, penetration depth, resistive and metal-insulator transition, resistance noise and magneto-optical studies of current paths. Various models for current transfer (disordered bonds, brick wall and railway switch) are also reviewed and discussed.

Several workers have measured the rates of decay of remanent magnetic moments in granular high- superconductors where the temperature-field histories generated trapped flux profiles comprising two concentric regions of countercirculating persistent currents. Exploiting a simple model based on the critical state for bulk specimens and the conservation of flux for the redistribution of vortices, and neglecting , we account for the major features of these observations although in some cases the number of trapped vortices is small. The model makes readily testable predictions of the relative magnitudes of the initial rates of magnetic relaxation for various configurations of flux density profiles.

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.

It was found that the resistance of (YBCO) films could be well controlled by Ni ion implantation. Both resistance and susceptibility measurements showed that the critical temperature of a film was systematically suppressed from its original value to an insulting state with increasing implantation dose. The sheet resistance varies from to at 77 K with dose ranging from to . For doses above , we observed an ion gettering effect after subsequent annealing in which the implanted ions diffuse against the concentration gradient and getter towards the peak concentration region.

Electron-beam-induced voltage contrast (EBIV) and modulated optical reflectivity (MOR) have been used to study the electrical properties of superconducting microwave resonators. The three resonators studied were patterned (YBCO) films on MgO substrates. The YBCO films consisted of a wide central track of 8 mm length separated from the ground plane by . A qualitative correlation was observed between the EBIV and MOR signals observed throughout the central track of the resonator. The EBIV results were taken close to the superconducting transition while the MOR data were obtained at room temperature. Both techniques are dependent on the local electrical properties which in turn are governed to a large extent by the local oxygen content within the superconductor. The correlation between the MOR technique and the more established EBIV technique has shown MOR to be a useful inspection tool for superconducting devices, particularly as MOR is a contactless room-temperature technique.

Measurements have been made of ultrasonic shear wave attenuation and sound velocity in highly textured polycrystalline samples of in magnetic fields up to 9 T and at temperatures down to 4.2 K. Magnetic field dependent attenuation peaks and velocity enhancements were in good agreement with theoretical predictions based on the assumptions of the thermally assisted flux-flow model of the mixed state. Flux-line pinning activation energy obtained from the data showed a weak anisotropy, for field parallel and for field perpendicular to the c-axis. The flux-line lattice was found to be fully pinned at lowest temperatures. Experiments were also performed in low magnetic fields (up to 9 mT), but showed no evidence of reported ultrasonic effects that could be attributed to weak superconductivity in grain boundaries.

The effect of additive on the high- superconducting materials of the Bi-Pb-Sr-Ca-Cu-O system (2223 phase) has been studied for samples prepared by the sintering reaction technique under , ambient and Ar-7.7% atmospheres. The d.c. electrical resistivity and critical current density and the a.c. magnetic susceptibility were measured by means of the standard four-point probe technique and the inductance method, respectively. It was found that the addition of a small amount of enhances slightly both the critical transition temperature and the current density and decreases the magnetic shielding effect.

Three kinds of screen-printed tapes with a single rectangular, square and circular superconductor core have been fabricated, and the trapped magnetic field distributions after removal of external field perpendicular to a tape surface have been investigated using scanning Hall sensor magnetometry with an active area of . Numerical calculations carried out by modelling current loops explain well the measured magnetic profile for the rectangular tape and the analyses yield a critical current density of that nearly agrees with the value of at 77 K in the self-field found from transport measurements. This establishes a basis for the validity of the calculation procedure for the magnetic profiles and opens a new way to characterize the sample and to determine the value in place of transport measurements. The same calculations give a nice agreement with the experimental profiles for the square and circular tapes, where the conventional dc four-probe method does not work, and the analyses determine the value to be for the former and for the latter in place of the transport data.

We report on the temperature and field dependences of the resistance of single-crystal (NdBCO) in a fields up to 7 T. The dissipation is found to be of the thermal activation type in the presence of defects due to Nd-Ba substitution sites. Transport measurements using the six-terminal configuration together with out-of-plane voltage reveal the vortex velocity correlation along the c-direction. Such a correlation in the vortex state is generally explained as due to the twin planes acting as correlated disorder. The voltage-current behaviour predicts nearly a vortex-glass phase at low temperature. The irreversibility field in was found to be higher in the twinned crystal in comparison with the detwinned one. A small amount of vortex channelling was observed in the angle-dependent magnetization when the field was applied parallel to the c-axis of the crystal. Our magnetic relaxation both in fields and in remnant magnetization shows three distinct regions and a dominant plateau in region II from 30 to 60 K. The existence of this plateau can be explained well using collective pinning theory. The relaxation process is found to be influenced by the shape of the hysteresis loop. The results have been analysed taking into account other relevant theories of the time decay of nonequilibrium magnetization. The magnetic phase diagram from ac susceptibility experiments suggests the validity of the vortex-glass phase transition from the field-temperature scaling. The pinning centres having a size of several nm to several tens of nm are viewed by STM and are primarily responsible for high-field pinning. The pinning ability of Nd-Ba substitution sites is significantly enhanced by the presence of twin boundaries, especially at high field.

Melt processing of (Sm-123) is done by infiltrating liquid phases into an Sm-211 preform and allowing the growth of Sm-123 to take place. The melt-textured samples are found to be free of macrodefects, and the microstructure contains uniformly distributed fine-sized Sm-211 particles. The behaviour of the critical current density of the samples in high fields is reported here.

Electronic devices based on the high-temperature superconductor (YBCO) may deteriorate in the operating environment under the influence of moisture, atmospheric carbon dioxide and frequent thermal cycling. To avoid these impairments a passivation layer was developed which is deposited by polymerization of the silicon organic compound hexamethyldisilasane (HMDS-N) in a plasma enhanced chemical vapour deposition (PECVD) process. The 100 nm thin films exhibit excellent passivation properties which is confirmed by electrical transport measurements of passivated wide and 100 nm high YBCO lines and step edge Josephson junctions exposed to humidity and . No degradation in the critical parameters such as the critical current , the normal resistance and the flux-voltage modulation depth is observed on step edge dc-SQUIDs owing to the deposition of the passivation layer. Atomic force microscope (AFM) measurements show a smooth morphology of these layers without pinholes and an excellent coverage of the YBCO even at the edges of lines. Furthermore, the passivation layer can be used as an isolation and scratch protection between SQUIDs and thin-film flux transformers in flip-chip magnetometers.