High activation energy for TAFF and pinning by fine inclusions in melt textured Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7- delta /

In Bridgman melt-textured Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7- delta / (YBCO) the activation energy, U/sub 0/, for thermally activated flux flow (TAFF) has been found to be almost four times higher than in conventional sintered bulk material by measuring rho (T, B) curves. One reason could be very fine inclusions with an average diameter of 35 nm and associated dislocations. An estimation of core pinning by these fine particles yields critical current densities, j/sub c/, at 1 T of 4.8*10/sup 4/ A/cm/sup 2/ and 2*10/sup 5/ A/cm/sup 2/ for 77.3 K and 4.2 K, respectively. This calculated current density is on the same order of magnitude as the j/sub c/,/sub Delta M/ value estimated from M(B) measurements at 4.2 K and 1 T, but at 77.3 K and 1 T the calculated current density is about one order of magnitude higher than the j/sub c/,/sub Delta M/ value. The lower critical current density at 77.3 K could be caused by TAFF or oxygen deficient regions near the small angle grain boundaries, which become normally conducting at high fields and temperatures. >