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

Abstract

The effect of size, morphology and crystallinity of seed crystals on the nucleation andgrowth 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 seedcrystal of square planar or irregular geometry leads to point nucleation of theYBa2Cu3O7−δ (Y-123) phase and a subsequent square planarc-sector growth habit. The use of seeds of triangular and round geometries, however, results in adeviation from this growth habit symmetry. The use of large surface area seed crystals of dimensions20 mm × 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 bygrain growth via the usual peritectic solidification in the incongruent melt. Secondly, alarge area seed reduces the processing time of large single grains compared to that achievedby TSMG with small area seeds. The present study is particularly relevant to thefabrication of single-grain samples with controlled growth sectors, reduced processing timesand the multiple seeding of large area bulk materials containing good grain boundaries.