BRIDGMAN GROWTH AND ENHANCED CRITICAL CURRENTS IN TEXTURED YBA2CU3O7 - Y2BACUO5 COMPOSITES

A new method of melt textured growth of monolithic YBa2Cu3O7 superconductors has been developed based on a vertical Bridgman method. Long bar samples have been prepared having typical sizes of 100 x 3 x 2 mm3 and the growth conditions were typically R = 1 mm/h and G = 20-degrees-C/cm. The microstructure of the samples was investigated by means of polarized optical microscopy. Single domain textured ceramics having different concentration and size of Y2BaCuO5 (211) precipitates have been obtained . We show that our method allows preparation of monolithic conductors having a very homogeneous distribution of micronic or submicronic precipitates while the growth front is maintained along several cm. Critical currents of single domain samples have been measured with a SQUID magnetometer and we have found J(c)c values as high as 6 x 10(4) A/cm2 at 77 K. We show also that the enhancement effect of critical currents by 211 precipitates is strongly field dependent , being effective only at low magnetic fields . This field dependent effect encompasses a saturation effect in the displacement of the irreversibility line towards higher fields. The mechanisms leading to the observed complex behavior are analyzed.