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

The origin of high current densities to very high magnetic fields (better than
$10^{3} \ \hbox{A}\cdot\hbox{cm}^{-2}$
to 6.5 T at 77 K) in
$\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7 - \delta}$
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
$\hbox{Y}_{2} \hbox{BaCuO}_{5}$
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.