Distribution of critical current density in large YBa/sub 2/Cu/sub 3/O/sub 7-δ/ grains fabricated using seeded peritectic solidification

Abstract

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...

Synthesis of YBa2Cu3O7−δ and Y2BaCuO5 Nanocrystalline Powders for YBCO Superconductors Using Carbon Nanotube Templates

“…63,64 For example, J c is usually the lowest under the seed, 63,64 and gradually increases away from it. 64 The schematic in Figure 9a shows the four specimen positions that we used to measure J c relative to the seed, cut from the slice (shown in light green) taken from the single grain. The axes relative to the sample orientation are also shown.…”

“…In general, for single grain samples fabricated via TSMG, J c varies across the sample. , For example, J c is usually the lowest under the seed, , and gradually increases away from it . The schematic in Figure a shows the four specimen positions that we used to measure J c relative to the seed, cut from the slice (shown in light green) taken from the single grain.…”

Properties of grain boundaries in bulk, melt processed Y–Ba–Cu–O fabricated using bridge-shaped seeds

“…It is well known that J c varies with position in YBCO, bulk single grains fabricated by TSMG, with the lowest J c occurring typically at the position immediately below the seed and increasing with increasing distance from the seed along both a/b and c axes, as shown in figures 3 and 4 of [29]. Different single grains fabricated in different batches may exhibit, often subtly, different superconducting properties due to the use of different precursor powders and variations in details of the specific TSMG process.…”

Modelling and comparison of trapped fields in (RE)BCO bulk superconductors for activation using pulsed field magnetization

“…This modelling framework allows the presence of the various inhomogeneities that arise during the processing of (RE)BCO bulk superconductors to be considered, including inhomogeneous J c distributions (between top and bottom regions or between the seed and outer edge of the bulk [32,33], for example) and the presence of current-limiting grain boundaries and cracks. It can be used to assist optimisation of processing techniques, such as samples with novel seed arrangements [34][35][36][37], as well as PFM techniques and pulse coil design, for practical bulk superconductor applications.…”