Shape change during solidification of bulk, single grain Y–Ba–Cu–O samples fabricated by top seeded melt growth

Abstract

The shape of single-grain bulk Y–Ba–Cu–O (YBCO) samples fabricated with and without Ca-doping by top seeded melt growth (TSMG) has been investigated. ‘Standard’ bulk samples of YBCO containing 0.1 wt.% Pt and enriched by 30 mol% Y 2 BaCuO 5 (Y-211) exhibit a tapered upper surface, with a high point at the position of the seed. Bulk TSMG samples fabricated from Y 1.975 Ca 0.025 Ba 2 Cu 3 O 7 precursor powder enriched by 30 mol% Y-211, on the other hand, are characterised by a flat upper surface. We propose a mechanism for the taper formation in standard bulk TSMG samples, based on the edge melt distribution (EMD) effect, which is associated with liquid transport from the slower growing a -axis growth front to the more rapidly growing c -axis growth front at the interface between a - and c -growth sectors. Pressure consequently induced by this difference in growth rates is relaxed by displacing the growing crystal upwards to form a taper from the position of the seed. Y-211 particle pushing in standard samples prevents backflow of liquid to the diffusion layer in the vicinity of the growth front, which maintains this pressure during the growth process. Y-211 particle pushing is less apparent in Ca-doped samples, which accounts for the absence of a tapered geometry in the top surface of this material.