Supercond. Sci. Technol. 10 (1997) 693–701. Printed in the UK PII: S0953-2048(97)83216-2 Bending of silver-sheathed (Bi,Pb)-2223 tapes investigated by magneto-optical flux visualization M R Koblischka, T H Johansen and H Bratsberg Department of Physics, University of Oslo, PO Box 1048, Blindern, 0316 Oslo 3, Norway Received 8 April 1997, in final form 27 May 1997 Abstract. The effect of bending on silver-sheathed Bi-2223 tape is investigated by means of magneto-optical (MO) visualization of flux distributions. One single piece of rolled tape is used throughout all experiments starting from as-prepared, and subsequently the tape is bent to 4 mm diameter and then down to 1 mm diameter in 1 mm steps. Flux patterns are visualized using an intact tape, i.e. the visualization is done through the silver sheath after removing the bending strain. It is shown that bending of the tape causes a series of cracks running perpendicular to the tape axis (rolling direction). This crack pattern remains practically unchanged down to 1 mm bending diameter. The flux penetration changes from nearly homogeneous (as-prepared) to entirely crack-governed. Vortices are found to penetrate the tape preferentially along the cracks and from there into the remaining bulk. In bent samples, a very homogeneous flux distribution is observed when the external field is decreased after having reached the full-penetration field. A further decrease of the field forces vortices to leave the sample along the cracks, and trapped vortices remain within the Bi-2223 grains (grain clusters). With each step of bending, the values for the first flux penetration and full-penetration field are decreased. The observations of flux patterns at 1 mm bending diameter clearly demonstrate that intergranular currents are still flowing in the sample. Our observations demonstrate the capability of the MO technique for non-destructive testing of superconducting tapes. 1. Introduction (Pb,Bi) 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) silver-sheathed tape is a unique high-T c system, as the grain alignment in conjunction with a high density supports large intergranular (transport) currents. Additionally, intragranular currents can flow inside each grain. The magnetic moment measured on such samples is then composed of two contributions [1, 2]. In well prepared tapes, the intergranular contribution to the magnetic moment may be much larger than the intragranular one; this being in stark contrast to other non-grain-aligned, polycrystalline high-T c superconductors, like granular YBa 2 Cu 3 O 7−δ . Correspondingly, flux patterns obtained by magneto-optical (MO) imaging in as- prepared (rolled) Bi-2223 tapes are found to be nearly homogeneous, and governed essentially by the large intergranular currents. Weak disturbances of the ideal homogeneous flux distribution are caused by cracks which are not healed during the preparation (typically caused by additional pressing [3]) and by foreign phases and/or silver particles found inside the tape core [4]. Bending of the tapes is an important issue for practical use, e.g. for the preparation of coils and cables. Several authors have shown that bending (and subsequent straightening) of the tapes decreases the intergranular critical current density vastly [3, 5]. It is now an important issue to understand the structural changes which occur during bending and their influence on the transport properties and flux distributions. It is generally accepted that bending of the tapes produces cracks in the superconducting core and breaks the connectivity between the grains. However, the detailed process of flux penetration into bent samples remains an open question. Compared to the usual integral measurement techniques like magnetometry or transport current measurements, the local MO investigations of flux and current distributions may provide a deeper insight. The MO flux visualization techniques [6, 7] are ideally suited for such investigations, as a relatively high spatial resolution can be achieved while investigating an intact tape, i.e. the observations can be performed through the silver sheath of the tape. MO visualization of flux distributions is very sensitive to any kind of structural defects as shown e.g. in [3, 8]. Several experiments have been carried out on Bi-2223 tapes, mainly focusing on the current flow in as-prepared tapes, both mono- and multifilamentary [3, 9]. Furthermore, the flux patterns can be directly linked to the current flow in the samples as shown in [10]. 0953-2048/97/090693+09$19.50 c  1997 IOP Publishing Ltd 693