As described in the previous chapters, experiments following the introduction of the London theory revealed a few serious problems which could not be explained by the theory. Examples are transition to the normal state when the magnetic field is applied parallel to the surface of a superconducting film and the dependence of the penetration depth on the addition of impurities. The latter problem was resolved by Pippard through the introduction of the coherence length as described in the previous chapter. A serious problem which remained unresolved was the surface energy at the boundary of the superconducting and normal phases in the intermediate state of Type 1 superconductors. The stability of the intermediate state observed in experiments requires the boundary energy to be positive, which can not be explained by the London theory. In the next few chapters, the phenomological theory by Ginzburg and Landau, which not only resolved these problems but also led to the prediction of Type 2 superconductors, will be described.
The Bi-system bulk superconductor is expected to be applied to a current lead that has large capacity because of its low thermal conductivity. However, it is a ceramic material and mechanically brittle, so it must be handled very carefully to prevent damaging. Moreover, damage by electromagnetic force becomes an important problem when the conductor is applied to a large energy system. Therefore, the mechanical properties of the Bi-system bulk superconductor must be improved. We have examined the effect of adding short fiber to the Bi1.85Pb0.35Sr1.90Ca2.03Cu3.05Oy (BPSCCO) bulk and studied the possibility of BPSCCO bulk fiber reinforcement. The critical current density of the short fiber-added BPSCCO was lower than that of the BPSCCO bulk because some compounds were created by reaction between the short fiber and the BPSCCO matrix. The interface between the fiber and the BPSCCO matrix was not coherent, so the mechanical property of the short fiber-added BPSCCO was inferior to BPSCCO bulk. Although the bonding force on the fiber/matrix interface is weak, long fibers give a wider contact area between the fiber and BPSCCO matrix. In this study, the influence of the contact area on mechanical properties was investigated using a long-fiber ceramic. Al2O3 long fiber-added BPSCCO (Al2O3 long fiber/BPSCCO) samples were fabricated. The Al2O3 long fibers were arranged unidirectionally in the BPSCCO matrix. The superconductivity and mechanical properties of these samples were examined. The critical current density measurement at 77K showed inferior superconductivity of the Al2O3 long fiber/BPSCCO sample to the BPSCCO bulk. It is considered that the compounds created by the reaction between the Al2O3 long fiber and the BPSCCO matrix degraded the superconductivity of the sample. A room-temperature, three-point bending test of the Al2O3 long fiber/BPSCCO sample sintered at 1, 078K for 90ks showed that a higher volume fraction of the Al2O3 long fiber resulted in lower bending strength and higher stiffness. It was clarified that an increment in the contact area between the fiber and matrix increased the stiffness of the sample, but the bending strength was still lower because of the weak interfacial contact and concentration of stress on the matrix side of the Al2O3 long fiber/BPSCCO-matrix interface. Therefore, to realize the fiber reinforcement of BPSCCO bulk, it is recognized that improvements in interfacial contact must be achieved.
High-Tc superconductor (HTS) is expected to be applied to various fields because of its high critical temperature. However, the HTS is a ceramic and naturally brittle, and this disadvantage is an important issue for its applications. Therefore, the mechanical properties of the HTS must be improved to enable practical use in addition to progress in the material's superconductivity. We have studied the effect of adding ceramic fiber to Bi1.85Pb0.35Sr1.90Ca2.05Cu3.05Oy (BPSCCO) bulk. The critical current density of the ceramic fiber-added BPSCCO was lower than that of BPSCCO bulk because of the degradation of superconductivity by some compounds created during the reaction between the ceramic fiber and BPSCCO matrix. The interface between the ceramic fiber and BPSCCO matrix was not coherent, so the mechanical properties of the ceramic fiber added-BPSCCO were inferior to BPSCCO bulk. In this study, Ag wires were added into BPSCCO unidirectionally, and the samples (Ag-wire/BPSCCO) were examined. The interface condition between the Ag wire and BPSCCO matrix was better than the case of combining ceramic fiber and BPSCCO matrix. The bending strength of the Ag-wire/BPSCCO with an Ag wire weight of 15% was 72-75MPa, and the BPSCCO bulk showed a bending strength of 63-68MPa. It is recognized that Ag wire acts as a reinforcement by creating a suitable interfacial contact. Moreover, it was noticed that the Ag-wire/BPSCCO did not fracture separately and a superconductive current was able to pass through it at 77K even though a crack was induced by the three-point bending test. The Ag wire seems to increase resistance against crack propagation. The critical current densities of the BPSCCO bulk and Ag-wire/BPSCCO (30 wt% of Ag) were 141A/cm2 and 234-236A/cm2, respectively. It is considered that because the orientation of the BPSCCO grains is enhanced on the Ag wire surface, the superconductivity of the BPSCCO is improved by the addition of Ag wire.