ROLE OF INTERDOMAIN AND INTERPLATELET BOUNDARIES ON MAGNETIC-FLUX PENETRATION IN MELT GROWN YBCO

The microstructure of a melt processed YBCO sample reveals the presenc e of random domains with aligned platelets separated by microcracks in side the domains, The lower critical held (H-c1) and critical current density (J(c)) of these boundaries will be less than that of the grain s, and their role in determining the bulk properties cannot be neglect ed, We have employed a sensitive lock-ill fiat-band detection techniqu e and recorded the low field sc M-H loops at 33 Hz and 77 K and derive d the flux profiles. The results show an enhancement in H-c1 due to me lt processing. Inclusion of Y2BaCuO5 and Ag enhances the H-c1 further, The flux profiles show distinct slope changes at certain fields which reflect different coupling strengths of weaklinks that become operati ve for flux entry at different applied fields, While the low field flu x profiles are found to reflect the effect of residual weaklinks (like liquid phase and micropores) and domain boundaries, those at higher f ields might reflect the effect of platelet boundaries.