The Influence Of Defect Interface In Bi-based Superconducting Material On Their Electrical Transport Properties

In this dissertation, the influence of defect interface in Bi-based superconducting material on their electrical transport properties was systematically investigated. In Bi-based superconducting material, defect interface was mainly derived from the inhomogenous structure, which was induced by the partial substitution between the cations in Bi-based intrinsic superconducting structure, and the grain boundaries between the superconducting grains with different crystal orientations. Due to the existence of two types of defect interface, the polycrystalline Bi-based superconducting material in practice behaves granular superconductivity. In order to understand the influence of these defect interface on the electrical transport properties in polycrystalline Bi-based superconducting material better, in this dissertation, initially the electrical transport properties of polycrystalline Bi-2212superconductor with Bi/La homovalent substitution and polycrystalline Bi-2201superconductor with Sr/Ca substitution were systematically investigated. And then for a further understanding the influence of intrinsic inhomogenous structure in Bi-2212superconductor on their electrical transport properties, the electrical transport properties of c-axis epitaxy Bi-2212superconducting film were studies in detail.In order to satisfy the need experimentally, three types of electrical transport testing systems were home-made and established in sequence. Though continuous improvement, an electrical transport testing system with high performance and high stability was eventually established, in which the testing temperature range is between10K and325K, testing time interval is between three hours and four hours, and the data is automatically collected and stored.By a fraction of La homovalent substitution on Bi sites of polycrystalline Bi-2212superconductor, the influence of defect interface induced by Bi/La dopant on the electrical transport properties of Bi-based superconductor was systematically investigated. The results indicate that as La doping content increases, normal-state resistance increases and gradually evolves a truly-metallic state into a disordered-metallic state. Characteristic two-stage transition is obviously observed in the superconducting state of all the samples, and two types of transition temperature decrease with increasing La concentration. The electrical transport model about the variation of granular superconductivity brought by La micro-doped Bi-2212, it can be included from the model that La dopant acts two roles in current path, that is, the increasing amount of (Bi,La)-2212grains with lower Tc induced by structural inhomogeneity and a rapid increase of the grain boundaries between SC grains because the truly SC path has to be prolonged.With Sr/Ca dopant of polycrystalline Bi-2201superconductor in different proportion, electrical transport properties of80K superconductivity induced by Sr/Ca dopant were analyzed in detail. Results reveal that at zero magnetic fields when Ca doping content is not less than0.2, the onset transition temperature of BCSCO sample varies from81K to85K, which proves that80K Bi-2201superconductivity is located in electrical resistance. During the superconducting transition, two-stage transition can be observed except for x=0.3. The electrical transport model about BCSCO granular superconductivity induced by Ca dopant is established. The model can give a good explanation of the electrical transport properties of80K Bi-2201induced by Sr/Ca substitution. On the other hand, the model is a supplement as the established model about granular superconductivity in Chapters three, which seems as the electrical transport model when intragrain superconductivity is enhanced by the local inhomogenous structure with dopant, in turn that it makes the electrical transport model about granular superconductivity of Bi-based doped superconductor complete. The results of electrical resistance in BCSCO at different magnetic fields indicate that when H=500Gs, the influence of defects including structural inhomogeneity and grain boundaries brought by Ca dopant on granular superconductivity reaches a "saturation state" within weak field range.With regard to four types of c-axis epitaxy Bi-2212film with different crystallinity prepared by Pechini method, the influence of inhomogenous ingredient on their electrical transport properties. Results indicate that coherent transition brought by grain boundaries between superconducting grains is eliminated thanks to the layered structure with a good c-axis epitaxy, thus superconducting transition in superconducting state depends on Bi-2212intragrain superconductivity. For all the film, it can indicated form R-T curve and I-V curve at different magnetic fields that when applied magnetic field is greater than lower critical magnetic field, with the increase of magnetic field, both Tc,zero and Jc(T) turn smaller as well as△Tc becomes wider, the variation of electrical transport properties is induced by thermal active flux flow located in defect structure. Comparing with the relation between the structure and electrical transport properties of four film, the better is the crystallinity of Bi-2212film, the better is the electrical property; the rougher is the surface of the film, the bigger normal-state resistance turns. Generally speaking, c-axis epitaxy Bi-2212film was prepared by Pechini method, which owns zero-resistance transition temperature with more than80K, the critical current density of1.6×103A/cm2and the lower critical magnetic field of1000Gs when the temperature is kept at60K.