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

Abstract

Grain boundaries and defects in (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+ δ (Bi-2223)/Ag composite 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, local 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 ATB, different types of phases with various numbers of CuO 2 layers in their unit cell are formed periodically along the boundary plane. This means that this 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 and stacking faults are observed within the filaments. Although these defects may provide a benefit in the pinning of magnetic flux, they also give rise to local variations of the superconducting properties.