Abstract
Silver (Ag) is an established additive for improving the mechanical properties of single grain, (RE)Ba2Cu3O7−δ [(RE)BCO, RE = Sm, Gd and Y] bulk superconductors. The presence of Ag in the (RE)BCO bulk composition, however, typically reduces the melting temperature of the single crystal seed in the top seeded melt growth (TSMG) process, which complicates significantly the controlled nucleation and subsequent epitaxial growth of a single grain, which is essential for high field engineering applications. The reduced reliability of the seeding process in the presence of Ag is particularly acute for the SmBCO system, since the melting temperature of SmBCO is very close to that of the generic NdBCO(MgO) seed. SmBCO has a high superconducting transition temperature, Tc, and exhibits the most pronounced ‘peak’ effect at higher magnetic field of all materials in the family of (RE)BCO bulk superconductors and, therefore, has the greatest potential for use in practical applications (compared to GdBCO and YBCO, in particular). Development of an effective seeding process, therefore, is one of the major challenges of the TSMG process for the growth of large, high quantity single grain superconductors. In this paper, we report a novel technique that involves introducing a buffer layer between the seed crystal and the precursor pellet, primarily to inhibit the diffusion of Ag from the green body to the seed during melt processing in order to prevent the melting of the seed. The success rate of the seeding process using this technique is 100% for relatively small batches of samples. The superconducting properties, critical temperature, Tc, critical current density, Jc and trapped fields, of the single grains fabricated using the buffers are reported and the microstructures in the vicinity of the buffer of single grains fabricated by the modified technique are analysed to understand further the effects of buffers on the growth process of these technologically important materials.
Highlights
Bulk, single grain superconductors in the (RE)BCO family 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]
A buffer layer of composition of 75wt%Sm-123 + 25wt%Sm-211, diameter 5 mm and weight 0.3 g has been identified as an effective aid to the seeding process using a generic NdBCO(MgO) single crystal seed
A success rate of 100% of the seeding process has been achieved using the buffer layer technique, which is an essential requirement for fully growing single grains on a large scale
Summary
The oxide nature of (RE)BCO single grains means they tend to form brittle compounds and exhibit mechanical properties that are characteristic of ceramic materials. This is significant given that the tensile forces associated with a high, trapped magnetic field can be hundreds of tonnes, which makes single grain bulk superconductors extremely susceptible to mechanical failure by fracture initiating from defects, pores and cracks that pre-exist in as-processed single grains. Single grains fabricated by this technique are presented, their microstructures in the vicinity of the buffer are analysed and their superconducting properties, including Tc, Jc and trapped fields, are measured in order to demonstrate the positive effects of the buffers. A number of reasons for the apparent success of the novel seeding process are proposed
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