V. Seshubai

Extensive Nanotwinning: Origin of High Current Density to High Fields in Preform-Optimized Infiltration-Growth-Processed

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 YBa2Cu3O7-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 Jc (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.

\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7 - \delta}

In this paper, we compare YBa2Cu3O7-δ samples fabricated by the infiltration growth process: the chosen samples have microstructures that are considered ideal for supporting high current densities; they have small spherical Y2BaCuO5 precipitates uniformly distributed in their superconductor matrices. However, they differ dramatically in their critical current density .Ic versus H curves at high magnetic fields. It is expected from theory that flux pinning at high magnetic fields would be modified by defects of nanometric dimensions occurring in the superconductor. We have investigated the superconductor matrices of the samples using a nanoindenter, with the objective of probing differences in nanometric volumes within them. The measurements show highly enhanced nanohardness and elastic modulus in the superconductor matrices of samples that support high current densities at high magnetic fields; such an enhancement is absent in a sample whose current density, although starting at a high value at zero magnetic field, falls rapidly with increasing field. The enhanced nanohardness in samples showing enhanced current density at high magnetic fields has been attributed to nanometer-sized defects associated with twinning present in the superconductor matrix.

Superconductor

Doping of non-interacting nanoparticles in YBCO superconductors in order to get fine microstructure defects has been studied. Y-211 inclusion, itself has large potential to create structural defects without affecting Y-123 matrix phase. Nanoparticles of grain refining agent like CeO2/Pt/PtO2 are interesting dopants, especially CeO2 for being cheaper than Pt. POIG process, as is one of the advance processing techniques to get uniform distribution of fine Y-211 particles of the order of 1-2 μm, throughout the YBCO composites. Finer Y-211 particles can drastically increase the pinning centers. Introduction of fine sized CeO2 as grain refining agent to reduce the size of Y-211 particles would create additional pinning centers. In the present work, the effect of CeO2 on Y-211size and distribution in POIG processed YBCO composite was studied. It was observed that with increase in CeO2 content Y-211 particle size decreases below 1μm and even less than 0.5μm for 10 weight percent CeO2 doping. The mechanism for refinement appears to be the formation of BaCeO3 on the surface of Y-211 during liquid phase infiltration that leads to splitting of Y-211 into smaller particles.