Abstract
Reliable seeding of the superconducting (RE)Ba2Cu3O7−δ (RE-123) phase is a critical step in the melt growth of large, single grain, (RE)BaCuO ((RE)BCO) bulk superconductors. Recent improvements to the top seeded melt growth (TSMG) processing technique, which is an established method of fabricating bulk (RE)BCO superconductors, based on the use of a buffer layer between the seed and green body preform, has significantly improved the reliability of the single grain growth process. This technique has been used successfully for the primary TSMG and infiltration melt growth of all compositions within the ((RE)BCO–Ag) family of materials (where RE = Sm, Gd and Y), and in recycling processes. However, the mechanism behind the improved reliability of the melt process is not understood fully and its effect on the superconducting properties of the fully processed single grains is not clear. In this paper, we investigate the effect of the use of a buffer pellet between the seed and green body on the microstructure, critical current, critical temperature and trapped field of the bulk superconductor. We conclude that the introduction of the buffer pellet evolves the melt growth process towards that observed in the technologically challenging hot seeding technique, but has the potential to yield high quality single grain samples but by a commercially viable melt process.
Highlights
Bulk, single grain superconductors in the (RE)–Ba–Cu–O–Ag ((RE)BCO, r4">4].Single grain bulk (RE) = Nd, Sm, Eu, Gd, Y, Yb) family of materials have considerable potential for practical applications due to their ability to trap magnetic fields up to ten times higher than the fields available using conventional permanent magnets [1,2,3]
We examine the changes in microstructure and superconducting properties of (RE)–Ba–Cu–O–(Ag) single grains associated with the use of a buffer layer and, we discuss briefly the mechanisms involved in the growth process
The diagram beneath each photograph indicates schematically the position of the growth sector boundaries. It can be seen by comparing the micrographs for samples 1 and 2 that the use of a buffer limits the extent of the a-growth sector at the centre of the sample, with the size of the a-sector correlating inversely with buffer layer diameter over a range of sample size
Summary
Even if the melting temperature and lattice structure conditions are satisfied, regardless of which type of seed is used (for example either SmBCO, NdBCO or a generic seed [7]) This occurs more frequently when silver is added to the precursor powder to improve the mechanical properties of the fully processed sample [8, 9]. A 25 mm dia.YBCO single grain grown by buffer-aided infiltration growth technique is shown in figure 1(b) These later two processes involve the generation of relatively large quantities of liquid phase during the crystal growth process, which can influence deleteriously the chemical stability of the seed. We examine the changes in microstructure and superconducting properties of (RE)–Ba–Cu–O–(Ag) single grains associated with the use of a buffer layer and, we discuss briefly the mechanisms involved in the growth process
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