Abstract
The current-voltage (I-V) characteristics of melt-textured YBCO superconductors have been studied in single-grain samples as well as samples containing grain boundaries of high current-carrying capability. The latter samples contained one or more grain boundaries of angles in excess of 20-degrees. The measurements were performed at 77 K using both DC and pulsed currents in self-field. In samples possessing critical current densities in the range of 13 000-63 000 A cm-2, the I-V characteristics exhibit a nearly power-law behavior with a negative curvature in a double-logarithmic plot. The power exponent n is found to be correlated to the value of J(c) in these samples. On the other hand, the I-V characteristics for samples containing grain boundaries and possessing critical current densities of approximately 10 000 A cm-2 are predominantly of a linear type. It is concluded that a possible J(c) limiting mechanism for these specimens is flux flow along the grain boundaries due to reduced pinning forces in the grain boundary plane.