Extensive Nanotwinning: Origin of High Current Density to High Fields in Preform-Optimized Infiltration-Growth-Processed $\hbox {YBa}_{2}\hbox {Cu}_{3}\hbox {O}_{7 - \delta }$

The origin of high current densities to very high magnetic fields (better than 10³ A · cm^-2 to 6.5 T at 77 K) in YBa₂Cu₃O_7-δ superconductor fabricated by the preform-optimized infiltration-growth process (POIGP) is investigated. The main techniques used in the paper are field-emission scanning electron microscopy and transmission electron microscopy (TEM). An electron-backscattered-diffraction study of the samples is also carried out. A comparison between the microstructures of the optimized sample with the nonoptimized ones show that extensive twinning on a nanometer scale with crossing twins occurring near the optimally separated Y₂BaCuO₅ precipitates can be the origin of the observed high J_c (H) in the POIGP sample. The TEM study reveals the presence of very fine defects starting from the twin boundaries. The observed defect spacing and densities account for the uniformly high current densities observed to high fields.