Microstructural cause of the peak effect in superconducting Y-Ba-Cu-O melt-grown bulks

Superconducting Y-Ba-Cu-O bulks with J(c) as high as 30 kA/cm(2) (at 77 K a nd 1 T) containing both Y-123 and Y-211 phases were utilized in the present work. With different post-annealings, we obtained samples with different d egrees of peak effect in the J(c) -vs- H characteristics. Using specimens w ith TEM observable areas as wide as 20 x 20 (mu m)(2), large-area high-reso lution transmission-electron-microscopic (LA-HRTEM) observations were made for the samples with different degrees of peak effect. Statistical analyses were made for the size of Y-211 particles and the distance between adjacen t twin boundaries. It is revealed that (1) Y-211 particles including those of very small diameters have nothing to do with the peak effect, while (2) the inter-twin-boundary distance is undoubtully correlated with the degree of peak effect. Further investigation on the twin boundary structure indica tes that lower T-c regions may be formed in the vicinity of twin boundaries to provide fluxons with mesoscopically distributed weak pinning centers.