Important role of the interfaces in the high temperature superconductors

Abstract

The composition, structure, orientation and properties of the interfaces play a crucial role in determining the properties of the HTSC materials. The interfaces act as Josephson weak links, and in effect control the J c in random oriented polycrystalline materials. Chemical examination on the grain boundaries of the HTSC shows that the composition even in the clean boundaries deviates from the interior of the grains. Some high angle boundaries show a flux pinning characteristic. Studies on the grain boundary structures reveal that the lattice matches can be described by a coincident site lattice (CSL) model or a constrained CSL model for YBa 2 C 3 O 7−x (YBCO). Most of the grain boundaries in the textured Bi-Pb-Sr-Ca-Cu-O(BPSCCO) are parallel to the basal plane, while they are perpendicular to the basal plane in YBCO. The impressive high critical current densities Jc obtained for the Ag-clad Bi-based superconducting tapes are of great interest for high current and high field applications, and hence has attracted more attention to the grain boundary studies in these materials. A strong correlation between the high Jc and textured platelike grain morphology has suggested the "brickwall" model. Most of the grain boundaries in the textured BSCCO are twist boundaries in (001) with c-axis as rotation axis. Due to the mica-like layered structure, most of the boundaries are in low-angle boundary region. Grain boundary dislocations are commonly observed in these twist boundaries with high density. The presence of such a high density of the dislocations is probably due to the deviation from misorientation between grains in order for the interface to retain the low energy configuration over most of its area. Recently, studies on the transport and magnetisation J c , and the irreversibility behaviour of the textured Ag-clad Bi-based tapes show no evidence of grain boundaries for being the weak links and the overall J c is controlled by the intragrain pinning at high temperatures. Weak link behaviour becomes evident at low temperatures. The interfaces between the HTSC oxides and the silver sheath exhibit unusual mechanical properties that are responsible for the improved strength and flexibility of the Ag/HTSC composites.