Subgrain boundary structure in melt-texturedRBa2Cu3O7(R=Y,Nd):Limitation of critical currents versus flux pinning

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The microstructure of subgrain boundaries occurring in single-domain $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ $(R=\mathrm{Y}$ and Nd) melt-textured composites has been studied by transmission electron microscopy. It is found that subgrain boundaries (SGB's) have a strong tendency to develop parallel to the (100), (010), and {110} planes, while the form of dislocation networks is controlled by the properties of constituting dislocations. In ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7},$ presenting one unique glide plane, (001), boundaries stabilized on {110} planes are accommodated by dislocations with lines running along the c axis. The occurrence of such an unusual line direction is discussed in terms of the line energy anisotropy on the (001) plane. On the other hand, in ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7},$ with (100), (010), and {110} glide planes in addition to (001), c-axis oriented dislocations are also found stabilized on (100)/(010)-faced boundaries. For arbitrary SGB configurations, the form of dislocation networks can be parametrized using the generalized Frank's formula, allowing the calculation of dislocation densities. Besides the underlying dislocation networks, SGB's may develop mesostructures such as faceting and stepped interfaces accommodating the deviation from low-index planes. The way these defects may affect the transport critical currents is discussed on the basis of a simple geometrical model. Since the form of dislocation networks is governed by intrinsic materials parameters, the present results can be extended to other large-scale materials for which a strong incidence of low-angle grain boundary microstructure is anticipated.