Local Dendrite Development during Seeded Peritectic Solidification of Large YBa₂Cu₃O_7−δ Grains

Abstract

The growth morphology of large YBa 2 Cu 3 O 7؊␦ grains durthat the steady growth state of 123 is driven by high supercooling between the 123 growth front and local 211 particles so ing peritectic solidification has been reported to be responsible for the generation of processing defects, such as that the required Y flux is effectively provided only by the latter. 24 Apart from the extent of the 211 particles which source platelets, and an inhomogeneous distribution of 211 particles, both of which influence significantly the superconduct-Y cations, both models share a common assumption that the 123 growth front propagates steadily in a planar pattern at low ing properties of the fully processed material. The present study demonstrates both experimentally and theoretically growth rates. This is not true when the growth rate of the 123 phase is increased (by increasing the cooling rate of the the formation of local dendrites at macroscopically planar YBa 2 Cu 3 O 7؊␦ growth fronts which propagate along differspecimens, for example), however, in which case planar growth is replaced by cellular and dendritic morphologies. 24,25 ent crystallographic directions and identifies these as key growth features of the peritectic solidification process.The present study is based on the 211 particle solubility/ solidification mechanism described above and demonstrates both experimentally and theoretically the formation and devel-I. Introduction opment of local dendrites as key growth features of unconstrained 123 growth fronts propagating in a macroscopically E NGINEERING applications of large-grain, high-temperature superconducting YBa 2 Cu 3 O 7Ϫ␦ (YBCO) such as flywheel planar pattern. energy storage systems 1 and magnetic bearings 2-7 generally require the material to trap a large magnetic field over a large II. Experimental Procedure sample area at liquid-nitrogen temperatures (77 K). The magniLarge YBCO grains were prepared using a well-established tude of field trapped by a superconductor is directly proporseeded peritectic solidification processing technique 15,26-28 in tional to the product of the critical current density, J c , and the which a grain of nominal stoichiometry Y 1.6 Ba 2.3 Cu 3.3 O 8.5Ϫ␦ length scale over which it flows, which has a maximum value grew freely at 985ЊC under the influence of a SmBa 2 Cu 3 O 7Ϫ␦ of the sample dimensions. 8 Hence, achieving a high J c of at single-crystal seed. The seed was oriented carefully on the least 10 5 A/cm 2 at 77 K 9-15 that flows uniformly around large unprocessed pellet with its c-axis perpendicular to the sample grains 16,17 is a key material's processing objective in the fabricasurface. This yielded an YBCO grain which exhibited rectangution of large-grain YBCO. Accordingly, it is necessary to conlar geometry both in the plane of the seed and in its cross trol the formation of processing defects during the peritectic section. Finally, the specimen was fast cooled to room temperasolidification of this material, which may potentially limit J c . ture at about 600Њ...

Anisotropic growth morphology and platelet formation in large grain Y–Ba–Cu–O grown by seeded peritectic solidification

“…27,35,36 Planar growth stability theory 37,38 and computer simulations of the yttrium concentration distribution around 211 particles 39 have been applied on the length scale of formation of the dendrites observed in the present study (i.e., over dimensions of a few microns). Such a process has been reported by other authors, albeit on a different scale.…”

Growth features and intergranular connectivity of melt processed YBCO

“…In view of the discrete nature of the Y211 particles, a change in their number density or volume proportion in the liquid may subsequently result in a local pile-up or deficiency of Y cations. This will, in turn, result in the development of local growth inhomogeneities [26,27].…”