The effect of size, morphology and crystallinity of seed crystals on the nucleation and growth of Y-Ba-Cu-O single-grain superconductors

The effect of size, morphology and crystallinity of seed crystals on the nucleation and growth of melt processed Y-Ba-Cu-O (YBCO) during top seeded melt growth (TSMG) has been investigated. Conventional seeding of bulk YBCO with a small seed crystal of square planar or irregular geometry leads to point nucleation of the YBa2Cu3O7-delta (Y-123) phase and a subsequent square planar c-sector growth habit. The use of seeds of triangular and round geometries, however, results in a deviation from this growth habit symmetry. The use of large surface area seed crystals of dimensions 20 mm x 10 mm, on the other hand, influences the growth of bulk YBCO in two significant ways. Firstly, rapid epitaxial nucleation of the Y-123 phase occurs beneath the seed crystal, followed by grain growth via the usual peritectic solidification in the incongruent melt. Secondly, a large area seed reduces the processing time of large single grains compared to that achieved by TSMG with small area seeds. The present study is particularly relevant to the fabrication of single-grain samples with controlled growth sectors, reduced processing times and the multiple seeding of large area bulk materials containing good grain boundaries.