Theoretical Inverstigation On Critical Properties、AC Loss And Mechanical Behavior Of Superconductors

Superconductors often operate under high electromagnetic fields, the critical properties of which is subjected to the effect of magnetic field、transport current and applied load. They will generate AC loss when acted as power transmission cables. Flux-pinning may induce mechanical deformation even the destruction of superconductors when there exsit crack. To inverstigate the critical properties 、 AC loss and mechanical behavior are the fundamental subjects of superconducting theory and technology. On one hand, this thesis focused on the study fracture behavior、magneto-elastic behavior and AC loss from the macroscopic perspective. On the other hand, the critical properties and flux flow instability in the ideal type-II superconductors were analyzed from microscopic view.First of all, according to critical state model and theory of flux penetration into thin strips, numerically studied the fracture behavior in superconducting strip with longitudinal cracks. Applying the virtual crack closure technique, the maximal stress intensity factor appears at the field descent, from which the field-dependent model is larger than that from the Bean model. Due to the dependence of current density on the flux density, the results for the field-dependent model are much more accurate than those for the Bean model. Taking into account the effect of ferromagnetic substrate on the electromagnetic response of superconductor, a theoretical study of magneto-elastic behavior in superconductor/ferromagnetic(SC/FM) bilayer strip is presented. The theoretical results coincide well with the numerical results.Secondly, as a model of second-generation superconduting power transimission cable, applying conformal mapping, anaylatic inverstigation on AC loss in a curved SC/FM strip and round SC/FM cable are conducted. Moreover, the expressions of AC loss in low currents or field are presented. The influence of the field amplitude、width and number of curved strip on the AC loss in round cable is discussed.Finally, based on the Ginzburg-Landau theory of deformed superconductors, the effect of strain on upper and lower critical magnetic field in the ideal type-II supercondutors. The upper critical field decreases with increasing strain, which coincides with the experimental results. Furthermore, we predicted the lower critical magnetic field increases as the strain increases. The influence of coupling parameters and Ginzburg-Landau parameter ? on the lower critical magnetic field was also analyzed. Combined with the two-fluid model, the relationship of strain and critical temperature was propoesd. The critical temperature decrease with increasing strain, and the tensile and compressive strain effect is asymetric, which agrees well with experiments in the case of small strain. Electric-field-induced flux flow instability and vortex dynamics in mesoscopic superconducting strip with artifical defects is inverstigated within the time-dependent Ginzburg-Landau theory. The nucleation of phase-slip lines(PSLs) and its effect on the current-voltage characteristics is numerically analyzed. We demonstrated that the titled angle、length and interval distance of narrow slits have a great impact on the vortex dynamics. Therefore the guidance of PSLs and a rich variety of vortex dynamical scenarios can be reached via adjusting the size and position of defects and the value of applied field.