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

The complex microstructure of directionally solidified YBa2Cu3O7-Y2BaC uO5 composites having up to 30 vol % 2:1:1 precipitates with sizes dow n 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 ele ctron microscopy observations. It is shown that samples with a high Y2 BaCuO5 concentration display polygonization as a mechanism to relieve stress while only small microcracks occur which are very efficiently s topped by small Y2BaCuO5 precipitates. This crystal polygonization lea ds to the formation of either sharp or diffuse interfaces within the Y Ba2Cu3O7 matrix. In the first case low angle grain boundaries occur wh ich have a minor perturbation of the critical currents, while strongly disordered smooth interfaces are observed in other cases which becaus e of their irregular distribution can only have a minor relevance on t he flux-pinning mechanism of these superconductors. Stacking faults ar e found to be a common defect in the YBa2Cu3O7 matrix but they are ver y inhomogeneously distributed thus preventing to establish a clear cor relation between their density and critical currents. The formation me chanism of these stacking faults is discussed on the basis of their ob served distribution. Sharp Y2BaCuO5/YBa2Cu3O7 interfaces have been obs erved at the atomic level, even if a high density of stacking faults a nd 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 f ields in the YBa2Cu3O7-Y2BaCuO5 composites.