Abstract
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.
Highlights
Single grain, (RE)-Ba–Cu–O [(RE)BCO, where RE is a rare earth element or Y], bulk high temperature superconductors, have the ability to trap large magnetic fields that are typically more than an order of magnitude larger than those generated by conventional permanent magnets [1]
The uniformity of the trapped field profile is much greater for the sample grown with the additional liquid-rich phase than for the sample fabricated using conventional Top seeded melt growth (TSMG), as is apparent from the more circular and more uniformly spaced contour lines in figure 6(d)
Sample 2 exhibits much less variation in Jc between subspecimens, and a significantly higher and more uniform distribution of the maximum value of Jc for all the sub-specimens along the c-axis, with the exception of sub-specimen 1tc. This suggests that the provision of additional liquid-rich phase produces a much more uniform distribution of flux pinning centers, and a more uniform distribution of the Y-211 precipitates, and a much more uniform distribution of Y-123 superconducting phase than in the sample grown by the conventional TSMG process
Summary
Single grain, (RE)-Ba–Cu–O [(RE)BCO, where RE is a rare earth element or Y], bulk high temperature superconductors, have the ability to trap large magnetic fields that are typically more than an order of magnitude larger than those generated by conventional permanent magnets [1]. The RE-211 pellet remains solid while the liquid-rich source pellet melts, infiltrating upwards into the preform and reacting with the RE-211 phase to form the RE-123 phase, which will subsequently form the superconducting matrix [18] This technique overcomes directly the issues of RE-211 particle coarsening and inhomogeneity associated with Ostwald ripening and particle pushing and trapping, since the preform does not melt in the IG method. Additional liquid-rich phase was incorporated in the TSMG process for the fabrication of YBCO single grains in this study in an attempt to improve the reliability of complete sample growth and to produce a single grain sample with a highly uniform distribution of RE-211 particles and, more uniform and potentially improved superconducting properties. Conclusions have been drawn on the effect of the provision of additional liquid-rich phase on samples fabricated by the TSMG process
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