Abstract
Single-grain, (RE)–Ba–Cu–O [(RE)BCO)] bulk high-temperature superconductors have significant potential for application as trapped field magnets in a range of engineering devices. However, it is not trivial to fabricate single grains of (RE)BCO due to the complexity of the growth process, especially when the sample diameter exceeds 25 mm. In particular, difficulties associated with the seed crystal can lead to poor grain growth or to complete growth failure. We have employed an optimized buffer technique, which was determined by optimizing targeted critical parameters of the buffer pellet, including the choice of the buffer pellet composition and its aspect ratio, for the reliable fabrication of large, single grains of (RE)BCO. Potential candidates for the buffer pellet composition have been identified to yield successful grain growth and good superconducting properties. The optimum aspect ratio of the buffer pellet was also determined as part of this study. The optimized buffer pellet capped with the seed crystal has been demonstrated to work effectively as an efficient seed crystal and to aid significantly the growth of the Y-123 phase. We show that this optimized buffer technique ameliorates problems associated with both interfacial stress (commonly occurring at the seed/sample interface) and problems of grain contamination. We have fabricated a 40.8 mm diameter single-grain bulk superconductor and more than 25 single-grain YBCO samples with diameters in the range 25–35 mm by a significantly improved top-seeded melt growth process.
Highlights
The top-seeded melt growth (TSMG) process is a wellestablished route for the fabrication of bulk Y−Ba−Cu−O (YBCO) superconductors that can carry large critical current densities and generate large trapped magnetic fields.[1−4] It is well-known that these materials exhibit a critical current density, Jc, as large as 104 to 105 A/cm[2] within the grain, even at liquid nitrogen temperatures, 77 K.5,6
We investigate the TSMG process using a buffer layer as follows: (i) To determine if the buffer pellet really aids the growth of the Y-123 phase; (ii) to determine an effective composition for the buffer pellet; (iii) to investigate the aspect ratio of the buffer pellet on the growth process; (iv) to determine whether the buffer pellet can negate the effects of any small defects present in the seed crystal
Bulk superconductors using the TSMG process. This involved investigating systematically different buffer pellet arrangements to serve as effective nucleation and growth sites for the Y-123 phase in bulk YBCO
Summary
The top-seeded melt growth (TSMG) process is a wellestablished route for the fabrication of bulk Y−Ba−Cu−O (YBCO) superconductors that can carry large critical current densities and generate large trapped magnetic fields.[1−4] It is well-known that these materials exhibit a critical current density, Jc, as large as 104 to 105 A/cm[2] within the grain, even at liquid nitrogen temperatures, 77 K.5,6. This, again, is an involved task since the growth of the superconducting YBa2Cu3O7−x (Y-123) phase is relatively complicated and depends on various growth parameters.[6,13] Y-123 melts incongruently when heated above its peritectic temperature, Tp (1005 °C in air), and decomposes into a solid Y2BaCuO5 (Y-211) phase and a barium-rich liquid phase comprising BaCuO2 and CuO These phases recombine to form Y-123 on subsequent cooling of the material below Tp. These phases recombine to form Y-123 on subsequent cooling of the material below Tp This process often generates multiple grain nucleation sites, resulting in the formation of a multigrained YBCO microstructure. The growth process becomes more complicated still when large sized bulk samples are to be fabricated (for example, samples of diameter >25 mm) and is influenced critically by the following factors:
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