Superconducting properties of natural and artificial grain boundaries in bulk melt-textured YBCO

Abstract

A range of experimental techniques have been used to characterize melt-processed YBa2Cu3O7-d samples containing single-grain boundaries. Both natural high-angle boundaries, which sometimes appear during the grain growth process, and artificial low-angle boundaries, obtained by joining two single domains, have been investigated. Electrical resistivity, current-voltage characteristics, magnetic moment measurements and Hall probe mapping techniques have been employed to investigate the boundaries. Results are compared with the properties of single domain material (i.e., containing no grain boundary) for which T-c approximate to 89 K and J(c)parallel to ab (77 K, 1 T) > 10^4 A/cm(2). Resistance measurements across all the grain boundaries show a stronger dependence on current and magnetic field than that measured within the grains and exhibit a pronounced resistive 'tail'. The I-V curves obtained for the high-angle natural grain boundary are sharp and differ from the rounded I-V curves which are characteristic of single-grains. Field mapping measurements used to evaluate the critical current anisotropy are in agreement with magnetisation measurements. The limitations of this technique for investigating boundaries are discussed. It was found that current anisotropy can conceal a weak link between two grains, leading to a false indication of single-grain behaviour. Artificially engineered boundaries are shown to have significant potential for applications in high fields at 77 K. (C) 1998 Elsevier Science B.V. All rights reserved