Mechanical Fragility and Tri-Dimensional Crack Structure in NdBaCuO Top Seeded and Multi-Seeded Melt-Textured Samples

Abstract

In large TS-MTG Nd123 single-domain samples, the main lifetime limiting problem is the appearing of macroscopic fissures in the bulk, that evolve into cracks and lead to sample destruction. To analyze this phenomenon we grew single-seeded pellets and multi-seeded bars, and characterized them by XRD, PLOM, SEM and EDS. We measured inductively their superconducting properties Tc and Jc. Fissures appear to belong to micro-fissures families, parallel to the crystallographic planes. We correlate their origin to local tetra-ortho structural transition, due to spontaneous oxygenation during growth process, and their evolution toward cracks to elastic relaxation of Nd422-rich quenched liquid deposits present in the growing front concavities. MS-TSG bars growth dynamics appear to differ from pellets one. In fact, with appropriate inter-seed spacing, the absence of quenched liquid deposits near the boundaries joining adjacent domains was observed. This fact reduces mechanical fragility perpendicularly to boundaries, but does not affect the fragility along the same. Further studies are in progress on this aspect.

Reduced twinning efficiency and tri-dimensional crack structure in melt-textured NdBa2Cu3O7−δbulk samples fragmentation process

“…Reduction of the fragility of larger samples can be achieved by using a low R dh . A possible solution could be multi-seeded MTG [43,44], by which the same thermal treatment is used to obtain, in principle, indefinitely long bars, and show an increased mechanical stability with respect to similar size pellets [8]. Finally, several other different procedures, already used for other RE123 compounds, might be considered and adapted to Nd123 TSG pellets, such as sample impregnation by epoxy resin [45] or oxygenation under mechanical pressure [46].…”

“…Naked-eye observation of as-grown d1 pellets show a single Nd123 domain with sharp flat edges, surrounded by sandy looking 'quenched liquid' deposits, mainly constituted by Nd422 particles loosely bound by unreacted BaCuO 2 and CuO (figure 1(a)). These deposits detach spontaneously from Nd123 domains, leaving clean domain edges [8].…”

“…Whereas d1 pellet analysis provides us insights on the first stages of the fragmentation processes, more details on the further development of cracks may be obtained by studying bigger pellets (d3 and d5), and by analysing the fragments resulting from their fragmentation processes. Indeed, in d3 and d5 pellets cracks are generated in large numbers, and their progressive development leads to the almost complete destruction of the samples [8]. This process occurs spontaneously, even in non-oxygenated, non-polished and most carefully stored samples, and completes within a few weeks.…”

“…A successful technique to obtain bulk oriented RE123 samples is top-seeded melt-textured growth (TSG) [1][2][3]. The difficulty of scaling experimental parameters for large grain sample growth is an important limitation of this technique [4], but a more serious limitation for applications is represented by the mechanical fragility of the samples [5][6][7][8], for which various post-growth treatments have been proposed [9,10].…”

Thermal characterization of GdSr2RuCu2Oy-based mixtures in the GdSr2RuO6–CuO pseudobinary system

“…12 In effect, for MTG the appropriate growth temperature window lies between the heterogeneous nucleation undercooling temperature (resolidification temperature in the presence of a seed) and the homogeneous nucleation undercooling temperature (resolidification temperature in absence of any seed). 16 The MTGs, up to now, have been then, probably, performed at too high temperatures. More measurements of the heterogeneous nucleation undercooling temperature, adding a seed in the mixture, will be necessary to better clarify this point.…”