Relevance of Nanosized Defects to Hardness and Current Density of YBCO Superconducting Composites

In this paper, we compare YBa
₂
Cu
₃
O
_7-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 Y
₂
BaCuO
₅
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.