Uniaxial strain dependence of the critical current of DI-BSCCO tapes

Abstract

In order to explain the effect of uniaxial strain on the critical current of DI-BSCCO-Bi2223 tapes, we employed a springboard sample holder that can smoothly and continuously apply both tensile and compressive strains to tape samples. Over a narrow tensile strain region, the critical current in the tapes decreased linearly with increasing strain and returned reversibly with decreasing strain. When compressive strain was applied, the critical current first increased and then reached a weak maximum. Thereafter, it decreased monotonically with further increases in compressive strain. At room temperature, the local strain exerted on BSCCO filaments was measured by means of a quantum beam diffraction technique. Over the whole tensile strain region up to 0.2% and the small compressive strain range, the local strain changed linearly with applied strain. When the compressive strain was applied beyond the relaxation strain, the local strain (measured by diffraction) versus the applied strain (measured using a strain gauge) deviated from linearity, which is characteristic of strain relaxation and the onset of BSCCO filament fracture. Thus, the strain at the maximum critical current corresponds to a crossover point in strain, above which the critical current decreased linearly and reversibly with increasing applied strain, and below which the critical current decreased due to the BSCCO filament fracture. In this paper, we clearly characterize the reversible range terminated by both compressive and tensile strains, in which filaments do not fracture. Our analysis of the compressive regime beyond the relaxation strain suggests that although BSCCO filament fracture is the primary factor that leads to a decrease in critical current, the critical current in those regions of filaments that are not fractured increases linearly and reversibly with decreasing applied strain at compressive strains well beyond the reversible region for the tape. List of symbolsA a Applied strain A acr Compressive relaxation strain A aff Force free strain A atr Tensile relaxation strain A i Intrinsic strain (A i = Aa − Ap)

Drastic Improvement in Mechanical Properties of DI-BSCCO Wire With Novel Lamination Material

“…Besides, pre-tension technique leads to extra compressive strains on the filaments. It has been reported that I c performances tends to drop more rapidly in the compressive strain than in the tensile strain on Type HT-CA wires [13], Type H, and Type HT-SS wires [14]. As seen in Fig.…”

Mechanism for the uniaxial strain dependence of the critical current in practical REBCO tapes

“…However there is also a non-negligible bending strain because of the finite thickness of tape. However, as discussed previously [11], the contribution from the bending strain component was not considered in the present study, because its influence is quite limited. Because the strain gauge is further from the neutral axis of the springboard than the REBCO layer, caused by the thickness of the Cu lamination, the thickness of the glue for the strain gauge and the half-thickness of the strain gauge itself, the measured strain has been corrected by multiplying a factor k. The procedure to evaluate the factor k is explained in Appendix A.…”

“…For practical REBCO and BSCCO wires, a maximum in the critical current has also been observed in the strain dependence. In studies to date, Ap tended to be inconsistent with Aff [10,11]. It is the aim of this work to understand better the true nature of the critical current maximum and its dependence on strain by explicitly examining directly, the change of critical current as a function of local strain exerted on the SC component.…”

A feasibility study of high-strength Bi-2223 conductor for high-field solenoids

“…Moreover, to achieve the high current densities that are required to allow for sufficient margin for adding reinforcement to the mechanically relatively weak conductor, the reaction needs to take place at 25 to 100 bar in an oxygen-inert gas mixture [13, 26] (figure 1). There is presently discrepancy in the literature on whether densification of the Bi-2212 by overpressure reaction improves the allowable longitudinal strain limit beyond the 0.3 to 0.4% that is commonly observed in Ag/Ag-alloy matrix Bi-2212 and Bi-2223 wires and tapes [27, 28, 29, 30, 31, 32, 33, 34, 35]: Substrate-based experiments suggest no improvement and a 0.3% – 0.4% strain limit, whereas tensile tests suggest a 0.6% strain limit [36, 37, 38, 39]. That Bi-2212 is mechanically-weaker than both REBCO and high-strength Bi-2223 (below) is clear and there are recent attempts to diffusion-bond high-strength materials to aspected Bi-2212 wires to improve the allowable tensile stress from 120 to 150 MPa [39, 40] to more than 400 MPa [40].…”