Influence of Fatigue and Bending Strain on Critical Currents of Niobium Superconducting Flexible Cables Containing Ti and Cu Interfacial Layers

Niobium is a viable material for thin-film superconducting flexible microwave cables. To aid in the design of superconducting flexible cables using Nb, it is important to evaluate not only the superconductor electrical performance, but also mechanical reliability performance since these cables should be reasonably robust when flexed. In this paper, we performed fatigue and bending tests on Nb-only and Ti/Nb/Cu multilayer signal lines on flexible Kapton substrates and measured the change in critical current (I c ) of these wires. From the fatigue tests, I c , degradation of Nb-only cables occurred at a lower number of cycles than the Ti/Nb/Cu cables. After 250 fatigue cycles, Ti/Nb/Cu wires with the thickest Ti adhesion layer and Cu capping layer exhibited the lowest I c degradation of ~1.2% and 0% in tensile and compressive cases, respectively. From bending tests, where the sample was held in an intentionally curved configuration during testing, I c degradation of the Nb-only cables was more severe than that of the Ti/Nb/Cu cables during tensile bending and the I c of Ti/Nb/Cu cables was minimally affected during compressive bending. These results demonstrate that a Ti adhesion layer and Cu capping layer provide reliability enhancement for superconducting Nb flex cables fabricated on Kapton.