Distribution of the superconducting critical current density within a Gd–Ba–Cu–O single grain

The magnitude of the maximum trapped magnetic field in a bulk, single-grain superconductor is a key performance figure of merit. This is determined, generally, by the magnitude of the critical current density, J c , and the length scale over which it flows. As with all type-II superconductors, J c is related closely to the microstructure of the superconducting material and, in the case of RE–Ba–Cu–O [(RE)BCO, where RE is a rare-earth element or yttrium] single grains, RE2BaCuO5 (RE-211) inclusions in the superconducting REBa2Cu3O7−δ (RE-123) phase matrix are key microstructural features that act effectively as flux pinning centres. Although the distribution of RE-211 in single-grain bulk superconductors has been studied extensively, the variation of J c within a given sample has been much investigated much less thoroughly. A detailed experimental understanding of the variation of J c in these technologically important materials, therefore, is required given the growing popularity and significance of numerical techniques for modelling the behaviour of type-II bulk superconductors. Here we report a systematic investigation of the correlation between Gd-211 particle density and sample porosity, which are microstructural features, and T c and J c in a Gd–Ba–Cu–O bulk, single grain fabricated using a buffer layer and a supply of additional liquid phase. This was performed by cutting the sample into numerous sub-specimens of approximate dimensions 1.8 × 2.8 × 1.5 mm3. We observe that J c decreases with distance from the seed, although more strongly with distance along the c-axis than along the a–b plane. In contrast to what might be expected given the assumed contribution of RE-211 inclusions to flux pinning, we find no evidence of a clear correlation between the local RE-211 precipitate density and local critical current on a length scale of mm. We observe that the porosity of the sample is a more dominant factor in determining the distribution of J c within a single grain.