Direct atomic scale imaging of grain boundaries and defects in Bi-2223/Ag high-T-c superconducting tapes

Grain boundaries and defects in (Bi,Pb)(2)Sr2Ca2Cu3O10+delta(Bi-2223)/Ag co mposite tapes are investigated by the atomic resolution Z-contrast imaging technique in the scanning transmission electron microscope. One of the most prevalent types of boundary between superconductor grain colonies is the c -axis twist boundary that is located in the middle of the double BiO layers . Although no amorphous layers are found to exist at these boundaries, loca l phase variations at/near the boundaries are frequently observed. Another typical grain boundary is the small-angle asymmetrical tilt boundary (ATB), which is parallel to the basal planes of one grain. At the small-angle ATE , different types of phases with various numbers of CuO2 layers in their un it cell are formed periodically along the boundary plane. This means that t his type of the boundary also occurs in the middle of the BiO double layers . These local phase variations exist in close proximity to the Ag interface and may give rise to weak link behavior. Additionally, many dislocations a nd stacking faults are observed within the filaments. Although these defect s may provide a benefit in the pinning of magnetic flux, they also give ris e to local variations of the superconducting properties. (C) 2001 Elsevier Science B.V. All rights reserved.