MICROSTRUCTURE OF DIRECTIONALLY SOLIDIFIED HIGH-CRITICAL-CURRENT YBA2CU3O7-Y2BACUO5 COMPOSITES

The complex microstructure of directionally solidified YBa2Cu3O7-Y2BaCuO5 composites having up to 30 vol % 2:1:1 precipitates with sizes down to 0.1 mum and critical currents above 10(5) A/cm2 at 77 K and zero magnetic field has been investigated through detailed transmission electron microscopy observations. It is shown that samples with a high Y2BaCuO5 concentration display polygonization as a mechanism to relieve stress while only small microcracks occur which are very efficiently stopped by small Y2BaCuO5 precipitates. This crystal polygonization leads to the formation of either sharp or diffuse interfaces within the YBa2Cu3O7 matrix. In the first case low angle grain boundaries occur which have a minor perturbation of the critical currents, while strongly disordered smooth interfaces are observed in other cases which because of their irregular distribution can only have a minor relevance on the flux-pinning mechanism of these superconductors. Stacking faults are found to be a common defect in the YBa2Cu3O7 matrix but they are very inhomogeneously distributed thus preventing to establish a clear correlation between their density and critical currents. The formation mechanism of these stacking faults is discussed on the basis of their observed distribution. Sharp Y2BaCuO5/YBa2Cu3O7 interfaces have been observed at the atomic level, even if a high density of stacking faults and dislocations occur at certain orientations of the interfaces. Owing to the homogeneous distribution of these sharp interfaces we conclude that they constitute the dominant pinning mechanism at low magnetic fields in the YBa2Cu3O7-Y2BaCuO5 composites.