Formation of SrSnO3 shell-like inclusions in the Bi2Sr2CaCu2O8+x superconductor via chemical reaction

Superconducting composites Bi-2212-SrSnO3 have been prepared by reacting be tween the Bi-Sr-Ca-Cu-O precursor and Sr2SnO4 or Ca2SnO4 at 800-950 degrees C, followed by crystallisation of Bi-2212 from the partial melt at decreasi ng temperature. The samples have been characterised by powder X-ray diffrac tion, scanning electron microscopy and magnetic measurements. The materials consist of large Bi-2212 lamellae and complex-shaped fine inclusions of Sr SnO3. The composite obtained using Sr2SnO4 contains almost all the SrSnO3 p hase in the form of micron-sized spherical shells, which are partly include d in Bi-2212 lamellae, partly agglomerated in-between. The shells are perfo rated, thus allowing the Bi-2212 crystals to grow through them. It has been found that the shell-like grains form at an early stage of the precursor t hermal treatment between 800 and 850 degreesC. A mechanism of the SrSnO3 sh ell formation is proposed. The composites exhibit T-c in the range of 82-87 K and reveal up to five times better magnetic flux pinning at T greater th an or equal to 30 K in comparison with the undoped Bi-2212 sample prepared using the same experimental procedure. (C) 2001 Editions scientifiques et m edicales Elsevier SAS. All rights reserved.