Abstract
Bulk (RE)–Ba–Cu–O ((RE)BCO, where RE stands for rare-earth), single grain superconductors can trap magnetic fields of several tesla at low temperatures and therefore can function potentially as high field magnets. Although top seeded melt growth (TSMG) is an established process for fabricating relatively high quality single grains of (RE)BCO for high field applications, this technique suffers from inherent problems such as sample shrinkage, a large intrinsic porosity and the presence of (RE)2BaCuO5 (RE-211)-free regions in the single grain microstructure. Seeded infiltration and growth (SIG), therefore, has emerged as a practical alternative to TSMG that overcomes many of these problems. Until now, however, the superconducting properties of bulk materials processed by SIG have been inferior to those fabricated using the TSMG technique. In this study, we identify that the inferior properties of SIG processed bulk superconductors are related to the presence of a relatively large Y-211 content (∼41.8%) in the single grain microstructure. Controlling the RE-211 content in SIG bulk samples is particularly challenging because it is difficult to regulate the entry of the liquid phase into the solid RE-211 preform during the infiltration process. In an attempt to solve this issue, we have investigated the effect of careful control of both the infiltration temperature and the quantity of liquid phase powder present in the sample preforms prior to processing. We conclude that careful control of the infiltration temperature is the most promising of these two process variables. Using this knowledge, we have fabricated successfully a YBCO bulk single grain using the SIG process of diameter 25 mm that exhibits a trapped field of 0.69 T at 77 K, which is the largest value reported to date for a sample fabricated by the SIG technique.
Highlights
Further details of the top seeded melt growth (TSMG) sample preparation can be found elsewhere [7]. Photographs of these two, single grain samples and their corresponding microstructures are shown in figure 2. It can be seen from figure 2(a) that Seeded infiltration and growth (SIG) processing overcomes the problem of shrinkage, which is unavoidable with the TSMG technique
The superconducting properties, including trapped field and fielddependent Jc, were measured for each single grain sample at 77 K, with the results shown in figure 3
Both attempts to improve the properties of the samples fabricated by SIG reported in the present work enabled the Y-211 content in the fully YBCO BA-SIG processed single grain to be controlled
Summary
The TSMG approach is well established, it exhibits a number of intrinsic problems that, limit the field generating potential of bulk superconductors These include the considerable shrinkage of the final bulk sample that occurs during TSMG processing, the presence of a large amount of porosity and regions that are free of the RE-211 phase in the fully processed sample microstructure [9, 10]. The Cu-rich liquid phase (comprising of BaCuO2 and CuO) originates from the liquid phase reservoir during the SIG process, which infiltrates into a RE-211 preform, with which it reacts subsequently to form the RE-123 phase At this stage, heterogeneous nucleation of the RE-123 phase is initiated by the seed crystal, prior to its growth into a large, single grain. A single grain YBCO sample of diameter 25 mm diameter fabricated using this approach exhibits improved superconducting properties, including the highest value of trapped field at 77 K in a SIG sample reported to date
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