Thermally and mechanically induced residual strain and strain tolerance of critical currentin stainless steel-laminated Bi2223/Ag/Ag alloy composite superconductors

Abstract

It is known that the tensile strain tolerance of the critical current of Bi2223 compositesuperconductors is improved when the compressive residual strain of Bi2223 filaments inthe current transport direction (sample length direction) is enhanced. In the present work,the thermally and mechanically induced residual strain accumulation process of a stainlesssteel-laminated Bi2223/Ag/Ag alloy superconducting composite tape fabricated atAmerican Superconductor Corporation was studied. A calculation procedure is presentedfor a description of the strain change of the constituents (Bi2223, Ag, Ag alloy and stainlesssteel) during cooling and heating, and during lamination, followed by the stressrelaxation. As the input values, the mechanical property values of the constituents andthe residual strain of Bi2223 filaments in the laminated composite tape at roomtemperature, measured with x-rays, were used. Such an approach revealed the changeof residual strain of each constituent in the thermal and mechanical process inthe laminated composite tape. Then, from a comparison of the residual strainaccumulation in the laminated tape with that in the insert tape, the effects ofthe stainless steel lamination on the enhancement of the compressive residualstrain of Bi2223 filaments and the improvement of the tensile strain toleranceand strain tolerance window of critical current at 77 K of the composite tape arediscussed.