Controlling the Critical Current Density under Magnetic Field of Melt Processed Bulk High Temperature Superconductors

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The change of the critical current density (J_c) as well as the trapped field of melt-processed bulk superconductors brought about by an element substitution, oxygen annealing, changing the composition of the starting materials and so on were systematically studied. First, we optimized the starting composition of the Dy-system. Then, CeO_2 was added, which decreased the loss of the melt during the processing, resulting in a more than 10% increase in the trapped field. The addition of ZnO to this composition increased J_c between 0 and 3 T, and the trapped field increased for 20%. The optimum content of ZnO for increasing J_c and the trapped field was determined to be about 0.025 wt.% independent of the sample size and regardless whether CeO_2 is added or not. When ZnO was added to the Sm-system, the trapped field increased by a factor of 1.5. It was elucidated that the increase in J_c with Zn doping is particularly large in the region beneath the seed crystal where the Sm211 phase is d … More eficient, and this brought about the increase of the trapped field. We have also succeeded for the first time the preparation of single-domain bulk samples of the ternary Nd-Eu-Gd system with a diameter equal to or larger than 30 mm. The trapped field of these samples was larger when the ratio of Gd in the 211 phase of the starting materials was smaller, but the material strength increased when the content of Gd was large. Consequently, the magnetic flux density that the sample could trap without being cracked at low temperature increased. Similarly, the trapped field at the liquid nitrogen temperature of the ternary Sm-Eu-Gd system was large when the content of Gd of the 211 phase was 50%. The diffusivity of oxygen in a melt-processed superconductor was found to be between single-and poly-crystalline samples, and decreases when Ag is added because the microstructure is denser. It was important to perform the oxygen annealing at low temperature for the Sm-system in order to obtain a sample with large J_c and trapped field. However, the diffusivity decreases at lower temperature, and therefore, we studied the effect of high pressure oxygen annealing under 10 atm. Based on that result, we proposed a two-step annealing process, and confirmed its effectiveness. We also prepared samples of the Sm-system with adding Y_2Ba_4CuZrO_y, which was reported to have a small size in melt-processed superconductors, and found that the addition of this material compensates the Sm211 phase that is deficient under the seed crystal, and the trapped field increased with a very small content of about 1mol%. Less