Bi–Sr–Ca–Cu–O superconducting fibres processed by the laser floating zone techniqueunder different electrical current intensities

Abstract

The effect of the intensity of the direct electrical current on the morphology and propertiesof superconducting BSCCO fibres grown by the electrically assisted laser floatingzone (EALFZ) technique is presented. Stereological analysis and x-ray diffraction(XRD) pole plots reveal textural enhancement and crystal refinement as the mainfeatures when the current intensity increases. Analysis with scanning electronmicroscopy/energy dispersive x-ray spectroscopy also discloses changes in the phasefraction and nature. Improvement of the grain alignment is a result of the intensificationof ionic migration in the solid–melt interface along the fibre axis. This leads toa net increase in the length of primary dendrites and a reduction in dendritethickness and interdendritic spacing. The increase in current intensity also favoursthe vanishing of the 2201 phase and of the residual melt. The development ofthe 2223 phase during isothermal annealing is very pronounced for the EALFZfibres grown under high current intensities due to the refinement of the reactantphases. The current transport capability after isothermal annealing of fibres isimproved with application of a low current intensity (50 mA), reflecting a netchange of the morphological characteristics. However, when a high current intensity(200 mA) is applied the unfavourable orientation of the newly formed 2223 crystalsand the loss of crystallite quality of the 2212 phase due to intergrowth of 2223structure in the original 2212 structure lead to poor superconducting properties.