The dependence of transport critical-current density J c on axial tensile strain was measured at 76 K and self-magnetic field for YBa 2 Cu 3 O 7Ϫ␦ ͑YBCO͒ coatings on buffered, deformation-textured substrates of pure Ni, Ni-5-at. %-W, and Ni-10-at. %-Cr-2-at. %-W. Expectations have been that the strain tolerance of these composites would be limited by the relatively low yield strains of the deformation-textured substrates, typically less than 0.2%. However, results show that the irreversible degradation of J c () occurs at a strain equal to about twice the yield strain of the substrate. Therefore, YBCO/Ni-alloy composites may satisfy axial-strain performance requirements for electric devices, including the most demanding applications, motors and generators in which a strain tolerance exceeding 0.25% is needed. Furthermore, the YBCO/Ni-5-at. %-W conductors showed a reversible strain effect, which may be induced by a reversible strain-field broadening around mismatch dislocations at the grain boundaries. This effect may contribute to the unexpectedly large usable strain range of these conductors.

A reversible strain effect on transport critical current I c was found in Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) high-temperature superconducting round wires. I c showed unambiguous reversibility at 4 K and 16 T up to an irreversible strain limit of about 0.3 % in longitudinal tension, prompting hope that the Bi-2212 conductor has the potential to sustain mechanical strains generated in high-field magnets. However, I c was not reversible under longitudinal compression and buckling of Bi-2212 grain colonies was identified as the main reason. A two-component model was proposed, which suggests the presence of mechanically weak and strong Bi-2212 components within the wire filaments. Porosity embedded in the weak component renders it structurally unsupported and, therefore, makes it prone to cracking under strain ε. I c (ε) is irreversible in tension if the weak component contributes to the transport critical current but becomes reversible once connectivity of the weak component is broken through strain increase or cycling. A modified descriptive strain model was also developed, which illustrates the 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 improve I c if buckling of Bi-2212 grains does not occur, and should result in a wider I c (ε) plateau in the applied tensile regime without degradation of the initial I c. The new model postulates that a reversible strain effect should exist even in the applied compressive strain regime if buckling of Bi-2212 grains could be prevented through elimination of porosity and mechanical reinforcement of the wire.

A probe for investigating the eects of temperature, strain, and magnetic eld on transport critical currents in superconducting wires and tapes.', Review of scientic instruments., 71 (12). pp. 4521-4530.

No abstract

The recently discovered reversible strain effect in Y–Ba–Cu–O (YBCO) coated conductors contrasts with the general understanding that the effect of strain on the critical-current density Jc in practical high-temperature superconductors is determined only by crack formation in the ceramic component. Instead of having a constant Jc as a function of strain before an irreversible drop when cracks form in the superconductor, Jc in YBCO coated conductors can decrease or increase reversibly with strain over a significant strain range up to an irreversible strain limit. This reversible effect is present in samples fabricated either with rolling-assisted biaxially textured Ni–W substrates or with ion-beam-assisted deposition on Hastalloy substrates. The reversibility of Jc with strain is observed for thin as well as thick YBCO films, and at two very different temperatures (76 and 4 K). The reversible effect is dependent on temperature and magnetic field, thus indicating its intrinsic nature. We also report an enhancement of the irreversible strain limit εirr where the reversible strain effect ends and YBCO cracking starts. The value of εirr increases from about 0.4% to more than 0.5% when YBCO coated conductors are fabricated with an additional Cu protection layer.