Improvement of Reversible Strain Limit for Critical Current of DI-BSCCO Due to Lamination Technique

The DI (dynamically innovative)-BSCCO-Bi2223 tapes achieved high critical current as well as high modulus of elasticity. Further the reversible strain limit and the corresponding stress for critical current have been remarkably increased by means of lamination technique. During the course of development, their optimized architecture has been designed based on the principle of the rule of mixture for maximizing the force free strain exerted on the superconducting component. The reversible strain/stress limit (A _rev/R _rev) was defined as a strain, at which the critical current recovers to the level of 99% I _co. Selecting several kinds of laminating materials and changing condition of the fabrication, the excellent Cu alloy-3ply tape with I _co of 311 A/cm was realized of which A _rev and R _rev reached 0.42% and 300 MPa, respectively. Further during the theoretical analysis, the increase of reversible strain limit were made clear to be attributed to the increase of thermally induced residual strain as well as the compensation effect of laminated layers against a local fracture mode.

I. Introduction

High critical current DI-BSCCO-Bi2223 tapes have been successfully developed on the basis of controlled over-pressure (CT-OP) technology. Voids included in the Bi2223 filaments were remarkably reduced and volume fraction of non-superconducting phases was reduced. Consequently the high modulus of elasticity and the high critical current have been realized. In order to improve further their performances, the residual stress/strain control as well as high strengthening is important issue to develop tapes with high tolerance against strain/stress. Due to the difference of coefficient of thermal expansion (CTE), a compressive strain is exerted on the superconducting (SC) component. As this compressive strain generates advantage to get the high performance, the high-alloying of the coverage layer and the lamination of metallic foil have been applied. In order to analyse the residual strain/stress effect, various models have been developed [1]–[3]. Some attempts to measure directly the residual strain exerted on the BSCCO component were carried out by diffraction techniques using synchrotron radiation and neutron beam [4]–[7]. However it is still necessary to get quantitative knowledge on the strain/stress effect with electromagnetic property in order to design the further higher performance tapes.