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Electron diffraction spots splitting into three is an unusual phenomenon. Based on the analysis of crystal structure, the splitting is believed due to some neighboring crystal cells grow along a small angle θ, rather than along a parallel direction as in general case. When a crystal grows along two neighboring boundaries with a small angle, the distance of the two boundaries increases as well. When the distance of the two boundaries reaches as large as a cell constant, a new cell will grow in the site, which is called an edge dislocation. According to the geometric relation of the reciprocal space, small angle grain boundaries can be calculated from the corresponding splitting spots. In this work, two small angles θ are determined to be -2.5° and 3.0°, respectively. Obviously, the grain with small angle grain boundaries has a curved surface and the curvature of the surface is determined by the density of the edge dislocation. The corresponding parameters of the edge dislocation are calculated. Both the edge dislocation and the neighboring cells growing along a small angle are a growing mechanism of YBa[sub 2]Cu[sub 3]O[sub y] and this mechanism is responsible for the curved grain surface. [ABSTRACT FROM AUTHOR]

The transport critical current densities, J[sub c], of superconducting (Bi, Pb)[sub 2]Sr[sub 2]Ca[sub 2]Cu[sub 3]O[sub x]/Ag tapes were measured before and after employing a special radiation technique, which leaves the grain boundary properties largely unaffected. We identify two regions separated by a temperature dependent crossover field H[sub gb-p]. In the low field region, J[sub c] is limited by the transport currents across the grain boundaries, which remain unchanged after irradiation. Above H[sub gb-p], J[sub c] is limited by flux pinning. In this field region, the artificial defects optimize flux pinning and enhance J[sub c]. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]