Microstructure and transport properties of YBCO zone melted samples processed in a microwave cavity and infra-red furnace

The unidirectional solidification method was used to prepare well textured YBa2Cu3O7-delta (Y123) bars for current lead applications using two differe nt processes. The first one consists in using the Melt Textured Growth meth od (MTG) in a classical furnace, applying an axial thermal gradient (G) of about 60 degrees C/cm. In the second process, a microwave cavity working at 2.45 GHz is used, allowing to obtain larger G (about 260 degrees C/cm). Th e different values of G corresponding to the two processes were considered to study the effect of G on the Y123 growth rate, microstructure, crystal o rientation and transport properties. Thus, for both processes, the maximum pulling rate which permits a continuous growth of Y123 was determined, Micr ostructure of samples was studied, using polarised optical microscopy for e ach process. Moreover, the microstructure was correlated with the transport properties of textured bars, using pulse current with the four-point techn ique. Although the crystal orientation was not perfect (a-b planes are not aligned with the bar axis), high values of self field J(c) (higher than 20 kA/cm(2)) along several centimetres were measured with a high reproducibili ty for samples presenting single-grain features independently on the textur ing process. However, when the pulling rate increases (R > 2 mm/h), the mea sured J(c) values were higher for samples obtained using the microwave melt process than for samples processed under lower thermal gradient. The diffe rence in the term of J(c) is well correlated with the different microstruct ure. (C) 1999 Elsevier Science B.V. All rights reserved.