Formation of SrSnO_3 shell-like inclusions in the Bi_2Sr_2CaCu_2O_(8+x) superconductor via chemical reaction

摘要

Superconducting composites Bi-2212-SrSnO_3 have been prepared by reacting between the Hi-Sr-Ca-Cu-O precursor and Sr_2SnO_4 or Ca_2SnO_4 at 800-950 deg C, followed by crystallisation of Bi-2212 from the partial melt at decreasing temperature. The samples have been characterised by powder X-ray diffraction, scanning electron microscopy and magnetic measurements. The materials consist of large Bi-2212 lamellae and complex-shaped fine inclusions of SrSnO_3. The composite obtained using Sr_2SnO_4 contains almost all the Sr_SnO_3 phase in the form of micron-sized spherical shells, which are partly included in Bi-2212 lamellae, partly agglomerated in-between. The shells are perforated, 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 thermal treatment between 800 and 850 deg C. A mechanism of the SrSnO_3 shell 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 >= 30 K in comparison with the undoped Bi-2212 sample prepared using the same experimental procedure.