A new seeding technique for the reliable fabrication of large, SmBCO single grains containing silver using top seeded melt growth

Abstract

Silver (Ag) is an established additive for improving the mechanical properties of single grain, (RE)BCO bulk superconductors (where RE = Sm, Gd and Y). 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 the highest 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 batch samples. The superconducting properties, Tc, Jc and trapped fields, of the single grains fabricated using the buffers are reported and the micro-structures in the vicinity of the buffer of single grains fabricated by the modified technique are analysed.