Tiny Crystals Surviving above the Melting Temperature and Acting as Growth Nuclei of the High-Tc Superconductor Microstructure

Abstract

Magnetic field texturing of superconducting oxides has shown the possible existence of intrinsic solid nuclei surviving above the melting temperature Tm and governing the solidification. Tiny crystals could survive above Tm and act as growth nuclei with undercooling ratios θ= (T-Tm)/Tm larger than the theoretical value −2/3 if a negative supplementary volume energy −ε v is added in the Gibbs free energy change associated to the formation of a critical cluster. A double layer of opposite charges could create the solid-liquid interface electrostatic -εv. The observed maximum values θ1 and the dimensionless surface energies α1ls calculated for 38 elements assuming that their melts homogeneous, used to determine εv(θ). The εv values at T=Tm were equal to 21.7% of the fusion heat per volume unit. The quantity α2ls 3× Sm was nearly the same for all elements, α2ls being the dimensionless surface energy and Sm the fusion entropy. After melting these tiny crystals around Tm2=1.20Tm, all the undercooling ratios could tend to -2/3. The bidimensional texture of Bi2212, Bi2223 tapes can be induced by these nuclei during crystal growth when the prereacted compounds in the sheath are melted and annealed at a weak overheating temperature smaller than a critical value.