Non-uniform stresses in thin high temperature superconducting films under electromagnetic force: General models of curvature-stress relations and experimental results

Abstract

Strong flux pinning in high temperature superconducting (HTS) film has significantly boosted its capability of carrying high currents; however, it, in turn, induces huge electromagnetic force in the HTS film, and hence film stress evaluation under electromagnetic force is indispensable for assessing their reliability. In this work, first, an analytical model is presented for evaluating the electromagnetic-force-induced stresses in the thin HTS film that is placed in a vertical magnetic field. In this model, we relate curvatures to film stresses through electromagnetic body force, whose mechanism is very different from common film stress sources like misfit strain or thermo mismatch. The new model shows that the film stresses depend not only on the “local” curvatures at a same point of the substrate but also on the “nonlocal” curvatures of other points. Next, a coherent gradient sensor system for cryogenic measurement is implemented to monitor the stress states of the HTS film during the processes of magnetization and demagnetization. Finally, full fields of hoop stress, radial stress of the YBa2Cu3O7 − x film, and shear stress at interface between the film and the (00l) SrTiO3 substrate subjected to various magnetic fields are obtained, and all the stresses manifest irreversible behavior, which are first experimentally found in the thin HTS film-substrate system.