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.
High quality large grain high T, superconducting ceramics offer enormous potential as 'permanent' magnets and in maginetic screening applications at 77K. This requires sample dimensions -cm with uniform high critical current densities of the order IO5 A/cmZ in applied magnetic fields of 1T. We report a study of the magnetic characterisation of a typical large YBazCusO7.6 grain, prepared by seeded peritectic solidification, and correlate the magnetically determined critical current density, Jc, with microstuctural 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 Jc of order IO4 A/cm2 ('77K, IT), the local Jc 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 SmBazCu~O7.6 seed crystal. I . INTRODUCTIONThe irecent development of a variety of demonstrator devices that incorporate large grain, high temperature superconducting YBazCu307.6 (YBCO), such as magnetic bearings [1]-[5] and flywheel energy storage systems 651, [6] underlines the potential of this material for a range of permanent magnet-type engineering applications. These materials, which consist of a YBazCu3O7 6 (123) phase matrix with Y2BaCuOS (21 1) inclusions, are fabricated by a variety of melt processes based on peritectic solidification which occurs at around 1015°C in the YBCO system [7]. The flux trapping ability of YBCO, which generally forms the basis of these applications, depends fundamentally on the critical (current density, J,., of the material and the length scale over which it flows [8]. The presence of grain boundariies and intra grain domains can limit J, significantly and are clearly undesirable features of bulk YBCO. A main processing aim, therefore is to develop a large grain material with a high J, which flows over the entire grain dimension. Several microstructural features are commonly observed in bulk 'quasi' single crystalline YBCO material as the dimensions of the bulk sample exceed that of the seed crystal. 'These are (1) non-superconducting 21 1 inclusions in the 123 matrix, (2) platelets with thickness of about ten microns separating the bulk of the large grain along the YBCO &planes and (3) a cellular structure of the 123 matrix i n the plane perpendicular to the average c-axis of the 123 matrix. Manuscript received August 27, 1996. WL would like to acknowledge the support of the Croucher Foundation, EA Technology and British Nuclear Fuels Ltd. CDD would like to acknowledge MagneTek for the provision of a CASE Studentship and EPSRC and Fitzwilliam College for conference financial support.If SmBazCu307.~ seeds are used for the solidification processing of the YBCO grains, in contrast to SrTi03 seeds [7], Sm contamination of the 123 matrix occurs at positions close to the seed, resulting from the partial dissolution of the S...
Large, single grain NdBa2Cu3O7−δ (Nd123)–Nd4Ba2Cu2O10 (Nd422) composites have been fabricated up to 2 cm in diameter in a controlled 1% O2 in N2 atmosphere using a top-seeded melt growth technique. The irreversibility field as a function of temperature has been obtained from magnetic hysteresis loops for magnetic fields applied parallel and perpendicular to the c axis of small specimens (≈2×2×3 mm3) cut from individual grains. These specimens are observed to exhibit a pronounced peak effect for current flowing in the a–b plane and a five-fold anisotropy in the irreversibility line over a wide temperature range for fields applied along the major crystallographic axes. An exceptionally high irreversibility field (>12 T at 89 K) is observed in this material for field applied perpendicular to the c axis, which extrapolates to well over 40 T at 77 K.
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