An experimental and computational study of strain sensitivity in superconducting Nb3Sn

The superconducting properties of Nb3Sn, a material that is commonly used in high-field magnet applications, are strongly reduced when the material is deformed. The sensitivity to strain is problematic in high-field magnet applications where thermal contraction differences between materials and Lorentz forces during operation may result in a significant reduction in the performance of the Nb3Sn conductor and thus of the application itself. In order to understand the strain sensitivity, an experimental and a computational investigation are combined into a comprehensive and validated model that explains the strain sensitivity of superconducting Nb3Sn.\ud Binary intermetallic A15 Nb-Sn thin films with various compositions were fabricated and characterized in terms of composition and morphology. The resistivity and critical current density of these thin films, as well as the resistivity of bulk samples, were measured as a function of temperature, magnetic field, longitudinal strain, and transverse strain. From the measurements, the strain dependence of the superconducting properties was parameterized.\ud The validity of the previously published MAG description of the temperature, magnetic field, and strain dependent critical current density Jc is demonstrated, which relates the strain sensitivity of the Jc of Nb3Sn to the strain dependent critical temperature Tc and upper critical magnetic field Hc2. Two other commonly used descriptions are found consistent with this description despite being presented in a different form. \ud A computational model is presented which combines ab-initio calculations with microscopic theory, and the results are shown to reproduce the experimentally observed, normal-state-resistivity-dependent martensitic transformation, critical temperature, and upper critical magnetic field. It is shown that the relatively large strain sensitivity of Tc and Hc2 is a result of a strain-induced distortion of the niobium chains, which is referred to as sublattice distortion. This calculation result is experimentally validated with measurements of the normal state resistivity, Tc, and Hc2 as a function of strain on Nb-Sn thin films, bulk samples, and high-Jc conductors. The lower strain sensitivity of Nb3Al and the much lower strain sensitivity of bcc Nb and Nb-Ti are explained in terms of a weaker and a non-existent sublattice distortion, respectively