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

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.