Gdba <sub> 2 </ Sub> Cu <sub> 3 </ Sub> O <sub> 7-�� </ Sub> High-temperature Superconductors Microstructure And Critical Current Density Study

In the viewpoint of practical application, element substitution is a simple approach to introduce the flux pinning centers. For GdBa₂Cu₃O_7-δ (Gd123) superconductor, the Gd_1+yBa_2-yCu₃O_7-δ solid solution due to the substitution of Ba²⁺ by Gd³⁺ can act as strong flux pinning centers and are highly effective in increasing the flux pinning capability. And consequently Gd123 superconductor exhibits higher trapped fields and higher critical current density compared with those of YBa₂Cu₃O_7-δ superconductor. Moreover, the preparation process for Gd123 superconductor is simpler than that for YBa₂Cu₃O_7-δ superconductor. So Gd123 superconductor is very attractive for various industrial applications.In addition, neutron irradiation is a controlled way for the introduction of flux pinning centers and provides a feasible route to investigate the flux pinning mechanisms in superconductor. In this dissertation, firstly, we have grown Gd123 single domain bulks by a powder melt process (PMP). Secondly we investigated their microstructure, flux pinning properties and critical current density with the aim of optimizing the applied properties. Finally, we studied the effect of neutron irradiation on microstructure and critical current density of Gd123 superconductor.In chapter one, two methods, element substitution and neutron irradiation, for the enhancement of critical current density were introduced. The microstructure of the solid solution by RE (RE = Nd, Eu, Sm, Gd) substitution for Ba in the REBa₂Cu₃O_7-δ superconductor and the flux pinning characteristic of the REBa₂Cu₃O_7-δ superconductor were summarized. The main interactions between neutron and the atoms of the high T_c superconductor (HTS) and the effects of the neutron irradiation on the microstructure and the critical current density of the HTS were also introduced.In chapter two, a series of the nominal Gd_1+2xBa_2+xCu_3+xO_7-δ (x = 0, 0.1, 0.2, 0.3, 0.4) single domain superconductors were synthesized by PMP. The microstructures of the single domains were observed and the effects of the Gd211 addition on the Gd_1+y,Ba_2-yCu₃O_7-δ solid solutions were investigated. The steady reduction of c of single domain bulks with increasing x means that the occupation of Gd³⁺ ions in Ba²⁺ sites dose not reach the maximum amount even Gd211 addition high up to 40 mol%, i.e. x = 0.4.In chapter three, the superconducting properties of the nominal Gd_1+2x,Ba_2+xCu_3+xO_7-δ (x = 0, 0.1, 0.2, 0.3, 0.4) single domain bulks which consist of Gd123, Gd₂BaCuO₅ (Gd211) and Gd_1+y,Ba_2-yCu₃O_7-δ, have been investigated through magnetization and resistivity measurements. The Gd211 particles in the bulk sample can pin simultaneously several vortices and usually are added to increase pinning in low magnetic fields. However, the Gd_1+y,Ba_2-yCu₃O_7-δ solid solution due to the substitution of Ba²⁺ by Gd³⁺ can act as strong flux pinning centers and are highly effective in increasing the flux pinning capability in high magnetic fields. With increasing Gd211 content x, the trapped magnetic field reaches a maximum for the sample x = 0.2. The second peak in the critical current density J_c vs magnetic field B curve shifts toward high magnetic regime for the sample x = 0.2 compared with that for the sample x = 0, but decreases to low magnetic field for the samples x > 0.2. Two superconducting transitions in resistivityρ_ab indicate that the low-T_c clusters, i.e. Gd_1+y,Ba_2-yCu₃O_7-δ solid solutions, intersperse in GdBa₂Cu₃O_7-δ superconducting matrix. TheδT_c-pinning resulting from the compositional fluctuations should be responsible for the enhancement of J_c and trapped field for the sample x = 0.2.In chapter four, the effects of neutron irradiation on the microstructure and critical current density of the nominal Gd_1.4,Ba_2.2Cu₃O_7-δ and Gd_1.8,Ba_2.4Cu₃O_7-δ single domain superconductors were investigated. It is found that the more serious irradiation damage was produced by the interactions of the neutrons and the Gd atoms in Gd_1.8,Ba_2.4Cu_3.4O_7-δ sample than in Gd_1.4,Ba_2.2Cu_3.2O_7-δ. For the nominal Gd_1.4,Ba_2.2Cu_3.2O_7-δ superconductor, the fishtail can not reappear after annealing the irradiated sample due to the large irradiation damage and the dissolution of Gd123 (containing Gd_1+y,Ba_2-yCu₃O_7-δ solid solutions) by irradiation. In chapter five, the effects of neutron irradiation on the microstructure and critical current density of the nominal GdBa₂Cu₃O_7-δ single domain superconductor were studied. It is found that the fishtail in magnetization curve was repressed in the irradiated sample. However, the fishtail was enhanced in annealed irradiated sample and the J_c value was higher than that of nonirradiated sample.