Melt-textured growth of polycrystalline YBa2Cu3O7- delta with high transport Jc at 77 K.

Abstract

The progress toward major applications of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$-type high-${T}_{c}$ superconductors has been hindered by low critical current densities (${J}_{c}$) and their significant deterioration in weak magnetic fields. The present work demonstrates that these problems can successfully be overcome through proper microstructural control using molten oxide processing. Melt-textured growth of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ from a supercooled melt created an essentially 100% dense structure consisting of locally aligned, long, needle-shaped grains (typically 40--600 \ensuremath{\mu}m in length). The needles appear to have their long axes parallel to the conduction plane (basal plane) of the orthorhombic structure, with a low-angle orientation change between adjacent grains. This new microstructure, which completely replaces the previous granular and random structure of the sintered precursor, exhibits a dramatically higher transport ${J}_{c}$ (7400 A/${\mathrm{cm}}^{2}$ at 77 K) than the typical sintered materials (${J}_{c}$=150--600 A/${\mathrm{cm}}^{2}$). Even more significant is the much reduced field dependence of ${J}_{c}$(\ensuremath{\approxeq}1000 A/${\mathrm{cm}}^{2}$ at H=1 T as compared to \ensuremath{\approxeq}1 A/${\mathrm{cm}}^{2}$ in the sintered structure), indicating that the coupling between grains is much stronger in the new structure. The mechanism responsible for the suppressed weak-link behavior in the melt-textured material is inferred to be the combined effects of the densification, alignment of crystals, and formation of cleaner grain boundaries.