Microstructure and transport properties of YBCO zone melted samples processed in a microwave cavity and infra-red furnace
Résumé
The unidirectional solidification method was used to prepare well textured YBa2Cu3O7−δ (Y123) bars for current lead applications using two different processes. The first one consists in using the Melt Textured Growth method (MTG) in a classical furnace, applying an axial thermal gradient (G) of about 60°C/cm. In the second process, a microwave cavity working at 2.45 GHz is used, allowing to obtain larger G (about 260°C/cm). The different values of G corresponding to the two processes were considered to study the effect of G on the Y123 growth rate, microstructure, crystal orientation and transport properties. Thus, for both processes, the maximum pulling rate which permits a continuous growth of Y123 was determined. Microstructure of samples was studied, using polarised optical microscopy for each process. Moreover, the microstructure was correlated with the transport properties of textured bars, using pulse current with the four-point technique. Although the crystal orientation was not perfect (a–b planes are not aligned with the bar axis), high values of self field Jc (higher than 20 kA/cm2) along several centimetres were measured with a high reproducibility for samples presenting single-grain features independently on the texturing process. However, when the pulling rate increases (R>2 mm/h), the measured Jc values were higher for samples obtained using the microwave melt process than for samples processed under lower thermal gradient. The difference in the term of Jc is well correlated with the different microstructure.