Strain Effects on the Critical Current Density in Superconducting NbN Tapes Prepared by Reactive Sputtering

Abstract

The strain effect on the critical current density, J_c, of NbN tapes has been investigated in 4.2K and 9T. Superconducting films of NbN have been prepared on metal tapes of Hastelloy B and 304 Stainless steel by reactive sputtering in an argon-nitrogen atmosphere. The strain dependences of J_c are influenced by the substrate material and NbN film thickness. The NbN tapes with Hastelloy B substrate maintains constant values of J_c up to strain values of above 1%. However, those with 304 Stainless steel substrate show a gradual decrease in J_c from 0.3 to 0.9% and exhibit a marked decrease in the range of above -0.9%. The value of the irreversible strain limit ε_irrev. where each NbN tape is permanently damaged, is in good agreement with the ratio between the proportional limit and Young's modulus in an investigated stress-strain curve. The maximum value of ε_irrev. reaches -1.4% and has been obtained in the tape consisting of the Hastelloy B substrate and a 3, 000Å NbN film. In tapes thickly coated with NbN, which have been measured up to above their ε_irrev. values, some cracks are found and films of NbN are frequently peeling off their substrates.