Reversible effect of strain on transport critical current inBi2Sr2CaCu2O8 + x superconducting wires: a modified descriptive strain model**Contribution of NIST, an agency of the US government, not subjected to copyright.

Abstract

A reversible strain effect on transport critical currentIc wasfound in Bi2Sr2CaCu2O8 + x (Bi-2212) high-temperature superconducting round wires.Ic showed unambiguous reversibility at 4 K and 16 T up to an irreversible strain limit of about0.3 % in longitudinal tension, prompting hope that the Bi-2212 conductor has thepotential to sustain mechanical strains generated in high-field magnets. However,Ic was not reversible under longitudinal compression and buckling of Bi-2212 graincolonies was identified as the main reason. A two-component model was proposed,which suggests the presence of mechanically weak and strong Bi-2212 componentswithin the wire filaments. Porosity embedded in the weak component renders itstructurally unsupported and, therefore, makes it prone to cracking under strainε.Ic(ε) is irreversible in tension if the weak component contributes to the transport critical currentbut becomes reversible once connectivity of the weak component is broken through strainincrease or cycling. A modified descriptive strain model was also developed, which illustratesthe effect of strain in the Bi-2212 conductor and supersedes the existing descriptive model.Unlike the latter, the new model suggests that higher pre-compressive strains should improveIc if buckling of Bi-2212 grains does not occur, and should result in a widerIc(ε) plateau in the applied tensile regime without degradation of the initialIc. The new model postulates that a reversible strain effect should exist even in the appliedcompressive strain regime if buckling of Bi-2212 grains could be prevented throughelimination of porosity and mechanical reinforcement of the wire.