Tensile damage and its influence on the critical current of Bi2223/Ag superconducting composite tape

Abstract

We have studied the tensile behaviour of Bi2223 superconducting composite tapes at room temperature, and the influence of the tensile damages introduced at room temperature on the critical current Ic and the n values at 77 K. In the measurement of the Ic and n values, the overall composite with a gauge length 60 mm was divided into six elements with a gauge length of 10 mm in order to find the correlation of the Ic and n values of the overall composite to those of the local elements which constitute the composite. From the measured stress–strain curve of the composite and the calculated residual strain of the Bi2223 filaments, the intrinsic fracture strain of Bi2223 filaments was estimated to be 0.09–0.12%. When the applied strain was lower than the onset strain of the filament damage, the original Ic and n values were retained both in the overall composite and the elements. In this situation, while the overall voltage at the transition from superconductivity to normal conductivity of the composite was the sum of the voltages of the constituent elements, among all elements the overall voltage was affected more by the element with the lower Ic (higher voltage). The damage of the filaments arose first locally, resulting in a reduction of the Ic and n values in the corresponding local element, even though the other elements retained the original Ic and n values. In this situation, the voltage of the overall composite stemmed dominantly from that of the firstly damaged weakest element, and the overall Ic and n values were almost determined by the values of such an element. After the local element was fully damaged, the damage arose also in other elements, resulting in segmentation of the filaments. Thus, the Ic and n values were reduced in all elements. The correlation of Ic between the overall composite and the elements could be described comprehensively for non-damaged and damaged states from the voltage–current relation.

Relation of Shunting Current at Cracked Part to Critical Current and <i>n</i>-Value in Multifilamentary Bi2223 Composite Tape

“…1, have been observed. 1,2) The extent of cracking varies along the tape length. Accordingly, when the sections S1 to S6 schematically shown in Fig.…”

“…The formation of the collective cracks observed in the sample used in this work has been observed also in other fabrication route Bi2223 tape. 1) It is also noted that the collective cracks are formed not only in Bi2223 tape but also in brittle filamentsductile matrix (stabilizer) composite tape such as Nb 3 Sn multi-filamentary conductor. 6) When V (voltage)I (current) curves are measured for many sections ( Fig.…”

“…In such a situation, the collective cracks in each section play a dominant role in determination of VI curve. Due to the highly oriented microstructure of Bi2223 filaments with c-plane parallel to tensile direction, 1) not only the transverse but also the longitudinal cracking occurs through the cleavage of c-plane Fig. 1 Schematic representation of (a) a Bi2223 tape sample constituting of sections (S1 to S6) and (b) cracking process and its difference among sections.…”

Preliminary investigation of Ic homogeneity along the longitudinal direction of YBCO coated conductor tape under tensile loading

“…This behaviour is similar to the case under torsion strain [17] but different with the case under hard bending strain [16]. Further, this is different with the case in Bi-2223 tapes wherein the damage of the Bi-2223 filaments arose first locally in the weakest subsection, resulting in the reduction of the I c only in that subsection, even though the other subsections almost retained their original I c values [19].…”

“…Interestingly, it was observed that the value of the overall I c was similar with the damaged subsection. At higher strains, and after almost all filaments have been damaged in the weakest subsection, damage then propagated to neighboring filaments causing I c and n-value reduction [19,20].…”