Flux pinning mechanism for Nd-Ba-Cu-O superconductor

Unlike YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (Y123) which is a line compound for the cations, the Nd-Ba-Cu-O superconductor forms a solid solution Nd/sub 1+x/Ba/sub 2-x/Cu/sub 3/O/sub 7-/spl delta// (Nd123ss) between Nd and Ba. The presence of such a solid solution tends to cause a depression of the superconducting transition temperature (T/sub c/) and a broad transition. Processing in a low oxygen partial pressure (Po/sub 2/) can produce a high T/sub c/ and enhanced critical current density (J/sub c/). However, there are controversies on flux pinning mechanisms for this material, In this paper, we investigated the upper solubility limit of Nd123ss in industrial argon and critically reviewed these flux pinning mechanisms on the basis of a phase transformation theory. We believe that precipitation mechanism based on the change of the solubility limits of Nd123ss with Po/sub 2/ offers a consistent explanation to the enhanced flux pinning in the system. This may provide guidelines for further improvement in superconducting properties of bulk 123 materials.