Critical current enhancement in YBCO–Ag melt-textured composites: influence of microcrack density

A strong increase of the critical current density of YBa 2Cu 3O 7 (YBCO) melt-textured composites has been observed by adding Ag 2O particles in the precursor mixture while the concentration of Y 2BaCuO 5 (Y211) precipitates remains constant. Critical current density enhancement factors as high as 200% at 5 K ( J c ab (5 K, 0 T)∼2.5×10 6 A/cm 2) and 1000% at 77 K and 2 T ( J c ab (77 K, 2 T)∼2.2×10 4 A/cm 2) have been reached in samples with 20 wt.% of Ag 2O. Critical current measurements at 77 K for H∥ ab also confirm the increase of the J c enhancement factor with the magnetic field (∼100% at 0 T and 140% at 2 T). The improvement of the critical currents in the YBCO–Ag melt-textured composites is correlated with the reduction of the density of microcracks parallel to the ab plane, as observed by scanning electron microscopy (SEM). Results are interpreted in terms of a release of the current limiting factors and an improvement of flux pinning properties by preexisting defects.