SUPERCONDUCTING PROPERTIES OF NATURAL AND ARTIFICIAL GRAIN-BOUNDARIESIN BULK MELT-TEXTURED YBCO

A range of experimental techniques have been used to characterize melt -processed YBa2Cu3O7-delta samples containing single-grain boundaries. Both natural high-angle boundaries, which sometimes appear during the grain growth process, and artificial low-angle boundaries, obtained b y joining two single domains, have been investigated. Electrical resis tivity, current-voltage characteristics, magnetic moment measurements and Hall probe mapping techniques have been employed to investigate th e boundaries. Results are compared with the properties of single domai n material (i.e., containing no grain boundary) for which T-c approxim ate to 89 K and J(c)parallel to ab (77 K, 1 T) > 10(4) A/cm(2). Resist ance measurements across all the grain boundaries show a stronger depe ndence on current and magnetic field than that measured within the gra ins and exhibit a pronounced resistive 'tail'. The I-V curves obtained for the high-angle natural grain boundary are sharp and differ from t he 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 foun d 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 appl ications in high fields at 77 K. (C) 1998 Elsevier Science B.V. All ri ghts reserved.