The microstructure of YBa2Cu3Ox (YBCO)-coated conductors depends strongly on the deposition method and thickness of the YBCO layer. This letter shows how the clear qualitative difference in grain connectivity between vacuum-deposited and solution-grown layers has direct consequences for the spatial distribution of the critical current density (Jc). Pulsed-laser-deposited YBCO conductors usually have a columnar grain structure that results in a two-dimensional current network, as demonstrated with magneto-optical imaging. Consequently, their transport Jc varies considerably on length scales from 50μm up to 5mm, with current suppression occurring even at defects that run parallel to the macroscopic current. In contrast, the thicker YBCO coatings in metalorganic-deposited samples have a layered structure, leading to a three-dimensional current path. Magneto-optically, this is deduced from sample-wide shielding currents, while transport experiments reveal much smaller spatial variations in Jc. These results are encouraging for the further development of nonvacuum produced YBCO-coated conductors, since such three-dimensional systems are inherently more “forgiving” of local defects.

We present a relatively simple experimental method to correlate the grain structure of YBa2Cu3Ox coated conductors with spatial variations of the critical current density on a macroscopic scale. Variations of the current density on a micrometre scale are visualized with magneto-optical imaging, using the flux trapped by colonies of grains to quantify the degree of connectivity. Integrating these trapped flux profiles over larger distances yields direct information of the critical current distribution on a millimetre scale, provided that contributions from negative return flux in colony boundaries are properly eliminated. Flux polarity is determined using the wavelength dependence of the Verdet constant. The validity of this analysis is demonstrated by comparing the results with direct transport data on the lateral current distribution, measured with the magnetic knife technique. The combination of both experiments shows that the critical current density is suppressed over several millimetres of conductor length at areas where a large number of high-angle grain boundaries or defects are present.

Using the tape-in-rectangular tube (TIRT) process, we have made multi-core Bi-2223/Ag tapes with various numbers of filaments (10-162), and with different filament architectures and orientations. We have measured the angular dependence of the transport current of the tape samples with 'parallel' and 'perpendicular' filaments. The transversal I-c distribution obtained by spatially resolved transport measurements ('magnetic knife') illustrates that the filament quality of the TIRT tapes is better at the tape edges than in the centre. The tapes were stressed by two types of tensioning set-up (a short straight sample and a U-shaped spring) and by bending at 77 K. The I-c degradation shows different behaviour for parallel and perpendicular filaments, which is attributed to the difference in filament density and crack propagation.

Transversal and longitudinal homogeneity of Bi-2223/Ag tapes made by a tape-in-rectangular tube process with high filament aspect ratio was studied. Spatially resolved transport measurements (“magnetic knife”), Hall probe measurements and local V–I measurements were used to obtain the Ic distribution across and/or along the tape sample. The Ic distribution in the width of the filaments shows a minimum in the filaments centre and a maximum is measured near the filament edges. A periodical variation of longitudinal Ic values is observed by local V–I and by Hall probe measurements. Transversal and longitudinal filaments cross-sections were obtained and compared to transport critical currents.

Multi-core Bi-2223/Ag tapes with various number of filaments (21–162), different filament architecture and their changing orientation to the tape plane have been made by the tape-in-rectangular tube (TIRT) process. The transport current properties of TIRT tape samples with “parallel” and “perpendicular” filaments have been measured. The transversal Ic distribution obtained by spatially resolved transport measurements (“magnetic knife”) measurement illustrates that filament quality of TIRT tapes is better at the tape edges as in its centre. The Ic degradation due to bending shows a different behaviour for parallel and perpendicular filaments which is attributed to the difference in filament density and crack propagation.