Fabrication Of The GdBa₂Cu₃O_7-d Bulk Superconductor

Melt-textured GdBa₂Cu₃O_7-d bulk superconductors have significant potential in engineering applications, such as maglev, magnetic bearings, permanent magnets, flywheel energy storage systems and superconducting motors, since they have high critical temperature(Tc), high critical current density(Jc) and strong trapped magnetic fields. GdBa₂Cu₃O_7-d bulk superconductors can be used in stronger magnetic field than that of the YBa₂Cu₃O_7-d bulk superconductors for the secondary peak effect.The effect of temperature on the preparation of the GdBa₂Cu₃O_7-d powder and bulk superconductors were investigated in this thesis. The effect of temperature on the GdBa₂Cu₃O_7-d powders and growth of the GdBa₂Cu₃O_7-d crystal have been carried out. It is found that the crystal growth speed and quality are closely related with the heat treatment process.The solid state reaction method(sintering process) was used for the synthesis of the Gd123 powders and the Gd211 powders, these powders were made from the powder Gd₂O₃, BaCO₃ and CuO in some proportion.The powder purity, size and carbon content were the three very important parameters of the Gd123 powders, which depend on the heat treatment temperature. The purity and particle size increase, but the carbon content decreases, when the sintering temperature rises. The experiment results show that the optimal sintering temperature is about 900℃and dwelling for 24 hours, and it is necessary to repeat the process several times to reduce the carbon content.The effect of Gd211 phase on the quality of GdBCO superconductors is mainly determined by the particle size, so a proper sintering temperature and ball milling process have to be carried out. The optimal temperature is about 900℃to get Gd211 phase with reasonable particle size and purity.Temperature is one of the most important parameters to obtain high quality GdBCO bulk superconductor in Top-seeded melt-texture-growth method (TSMTG). There are 11 parameters in the TSMTG process and the effect of the parameters are not independent each other, for example, the effect of one parameter maybe eliminated by another one. The sample is heated to 900℃and held for 6 hours to improve the density of the sample. The optimal maximum temperature and dwelling time is 1065℃/0.5h and 1060℃/1h, which ensure the sample melting completely and avoiding liquid phase leakage. The slowly cooling temperature is in the region from 1035℃to 1020℃. It is an effective way to avoid random nucleation and the liquid leakage. The sample was annealed at 350～400℃in flowing oxygen for 200 hours.The DTA results show that the effect of Gd211 addition on the peritectic temperature of GdBCO is not important, but the addition of Ag can reduce the melting temperature of GdBCO materails.