Improving the mechanical properties of Y-Ba-Cu-O single grain bulk superconductors

The overall aim of this work was to improve the mechanical properties of single grain YBCO bulk superconductors without causing significant detrimental effects to the superconducting properties of the material. It is widely acknowledged that large single grains of GdBCO and SmBCO with silver can be batch processed successfully with superior mechanical properties compared to silver-free GdBCO or SmBCO, grown by standard top seeded melt growth (TSMG). However, the successful, reliable growth of large single grains of YBCO-Ag by TSMG is significantly more difficult and has not previously been documented. The ability to alloy the YBCO system is desirable, since this base material has better inherent mechanical properties than GdBCO or SmBCO. This work approached the successful and reliable single grain growth of YBCO-Ag via a number of incremental stages. The first stage investigated the provision of additional liquid-phase to the recycling of multi-grain samples of YBCO, GdBCO-Ag and SmBCO-Ag. The provision of sufficient additional liquid-phase improved both the reliability and uniformity of growth of large single grains. The additional liquid enabled the growth of single grains even when large silver or cerium-rich agglomerates were present in the melt. The provision of additional liquid-phase was then trialled in the primary growth of YBCO by TSMG. This addition resulted in a higher reliability of growth of large single grains. In addition, these single grains had superconducting properties of magnitude comparable to, or higher than, those of YBCO grown by standard TSMG with a greater uniformity. The increase in reliability and in the tolerance to silver-rich agglomerates provided by the provision of additional liquid-phase suggested that this technique may enable the successful growth of single grains of YBCO-Ag. Initially the growth rate of YBCO-Ag was investigated. Large single grains of YBCO-Ag were then successfully and reliably grown by TSMG using heating profiles derived from the growth rate models. A large number of large single grains were grown reliably and their superconducting properties were tested. In addition, this development enabled the mechanical properties of YBCO-Ag to be analysed in detail for the first time.