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A major limitation to the widespread application of Y–Ba–Cu–O (YBCO) bulk superconductors is the relative complexity and low yield of the top seeded melt growth (TSMG) process, by which these materials are commonly fabricated. It has been demonstrated in previous work on the recycling of samples in which the primary growth had failed, that the provision of an additional liquid-rich phase to replenish liquid lost during the failed growth process leads to the reliable growth of relatively high quality recycled samples. In this paper we describe the adaptation of the liquid phase enrichment technique to the primary TSMG fabrication process. We further describe the observed differences between the microstructure and superconducting properties of samples grown with additional liquid-rich phase and control samples grown using a conventional TSMG process. We observe that the introduction of the additional liquid-rich phase leads to the formation of a higher concentration of Y species at the growth front, which leads, in turn, to a more uniform composition at the growth front. Importantly, the increased uniformity at the growth front leads directly to an increased homogeneity in the distribution of the Y-211 inclusions in the superconducting Y-123 phase matrix and to a more uniform Y-123 phase itself. Overall, the provision of an additional liquid-rich phase improves significantly both the reliability of grain growth through the sample thickness and the magnitude and homogeneity of the superconducting properties of these samples compared to those fabricated by a conventional TSMG process.
The widespread use of ceramic (RE)Ba2Cu3O7 bulk superconductors (RE-123), where RE=Y, Gd or Sm, is generally hindered by their poor mechanical properties. While a large number of techniques can be used to improve the mechanical properties of conventional ceramic materials, many of these are incompatible with the growth of single grain, bulk RE-123 superconductors using the top seeded melt growth (TSMG) process. Complications arise due to the need to minimize the degradation of the superconducting properties and produce a single-grained sample. Nonetheless, the addition of Ag to RE-Ba-Cu-O [(RE)BCO] precursor powder has been demonstrated to be effective in improving the mechanical properties of single grain bulk superconductors fabricated by TSMG without deleterious effects on performance. Although large single grains of GdBa2Cu3O7-Ag and SmBa2Cu3O7-Ag have been successfully and reliably grown, it has proven more difficult to grow large single grains of YBa2Cu3O7-Ag. We recently demonstrated the growth of single grain YBa2Cu3O7-Ag bulk superconductors that exhibit relatively good superconducting properties. In this work, we report the flexural stress at a number of locations within YBa2Cu3O7 single grains grown by TSMG with and without additional liquid phase and with silver addition. In addition, we have compared the distribution of the failure stress with the pore and silver distribution.
The use of RE-Ba-Cu-O [(RE)BCO] bulk superconductors, where RE=Y, Gd, Sm, in practical applications is, at least in part, limited by their mechanical properties and brittle nature, in particular. Alloying these materials with silver, however, produces a significant improvement in strength without any detrimental impact on their superconducting properties. Unfortunately, the top seeded melt growth technique, used routinely to process bulk (RE)BCO superconductors in the form of large, single grains required for practical applications, is complex and has a large number of inter-related variables, so the addition of silver increases the complexity of the growth process even further. This can make successful growth of this system extremely challenging. Here we report measurements of the growth rate of YBCO-Ag fabricated using a new growth technique consisting of continuous cooling and isothermal hold process. The resulting data form the basis of a model that has been used to derive suitable heating profiles for the successful single grain growth of YBCO-Ag bulk superconductors of up to 26 mm in diameter. The microstructure and distribution of silver within these samples have been studied in detail. The maximum trapped field at the top surface of the bulk YBCO-Ag samples has been found to be comparable to that of standard YBCO processed without Ag. The YBCO-Ag samples also exhibit a much more uniform trapped field profile compared to that of YBCO.
The widespread use of single grain RE-Ba-Cu-O [(RE)BCO] bulk superconductors, where RE is typically Sm, Gd, or Y, is, in part, limited by the relatively high costs of precursor powders and the low success rate of the manufacturing process. Both these problems can be addressed by recycling primaryprocessed grains in which the initial growth process has failed in some way. Key to the use of recycled grains in practical applications is an assurance that their properties and performance are not inferior to those of primary grown grains. In this work, we describe the differences between the growth process, microstructure, and properties of primary and recycled (RE)BCO single grains. We observe that the mechanism of growth is the same for both primary and recycled single grain samples in all three RE-based systems investigated. In the recycling process additional liquid-rich phase powder is provided beneath a failed sample, whereby this liquid phase infiltrates upwards and contributes a sufficient concentration of additional RE species at the growth front to enable samples to grow relatively easily in the form of single grains by producing a more uniform composition at the growth front, which leads directly to an increased tolerance to the presence of Ag and Ce-rich agglomerates. Importantly, we observe that the recycled samples have a much more uniform composition, and therefore exhibit more uniform superconducting properties, than single grain samples fabricated by a primary grown process.
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