Preparation Of Superconducting Material GdBCO Block Approach Improvement

Since the discovery of high-temperature superconductor in1986, a lot of works have been done on the fabrication technique and physical properties of superconducting bulks, tapes, and films. In the REBCO bulk superconductors family, GdBCO bulk is of relatively higher critical temperature (Tc), larger critical current density and higher trapped magnetic flux, therefore, it has significant potential for practical applications, such as maglev, magnetic levitation bearing, a permanent magnets of strong magnetic field, superconducting flywheel energy storage systems and superconducting motor, etc.The high temperature superconductor (HTS) bulk is coming into the practical application, therefore, the simple and fabrication method, low cost, and high working efficiency technology are the key problems to be solved.In this paper, BaCuO2(011), Gd2O3and CuO were used as the initial powders. The011powder was prepared by the solid-state reaction method, sintered at the temperature of910℃for24hours by three times, and ball milled before and after each sintering.In this paper, the Gd2O3,011and CuO powders were mixed in accordance of GdBa2Cu307with Gd2BaCu05ratio of1:x (x=0.2,0.4,0.6,0.8,1.0) as the precursor powder for the fabrication of GdBCO bulk superconductors. The precursor powders were uniaxially pressed into pellets by a steel mould of20mm, the pellets were put into a self-designed furnace with a NdBCO crystal seed on the top surface of the pellets with ab-plane parallel to the surface. Then the samples were heat treated for the crystal growth of single domain GdBCO bulk superconductors by the top-seeded melt-textured growth (TSMTG) process. All the samples are of single domain GdBCO bulk morphology, but with some slightly difference; when x=0.2,0.4, the central area of the bulk superconductors show a slightly concave shape; when x=0.6,0.8,1.0, the surface of the superconducting bulk is in a flat morphology. The levitation force of the sample increases first and decrease then with the increase of x. and when x=0.4, reaches the largest value of about28.53N. So the optimal ratio is x=0.4for high quality the GdBCO bulk superconductor. In this method, only BaCuO2powder must be prepared, this is much easy to work with than that of the conventional TSMTG method in which GdBa2Cu3O7and Gd2BaCuO5powders must be prepared. The new method can also be used for fabrication of the other REBCO materials, greatly improving the working efficiency, and reduce the cost. The microstructure of the GdBCO bulk superconductors were analyzed by scanning electron microscopy (SEM), the result indicate that the second phase particles can be divide into two types, one is of larger size, corresponding to Gd211particles, the average size is about10μm; the other one is of nano-scale, the average size is about300nm.The trapped magnetic flux of a single domain GdBCO bulk superconductor has been investigated by pulsed field magnetization. In the study by changing the pulsed magnetic field, it is found that the trapped field of the GdBCO bulk increases first and decreases then with the increasing of the applied field. The trapped field of the sample split into a multiple domain when the pulsed magnetic field reaches a certain value. This result is closely related with the crystal structure of the GdBCO bulk.Single domain GdBCO bulks have also been fabricated by the top-seeded infiltration and growth (TSIG) method, the mixture of (Gd2O3:BaCuO2=1:1) powder is used as a solid phase, the mixture of (Gd2O3:BaCuO2:CuO=1:10:6) powder is used as the liquid phases, during the TSIG process. The levitation force of the samples increases from ION to35.75N with the increasing of the liquid phases.In addition, we have developed a new method to prepare complex-seed for the fabrication of single domain GdBCO bulk superconductors. The new complex-seeds were prepared by inserting a small GdBCO bulk between the NbBCO seeds and the GdBCO bulk by TSMTG process. It is easy to produce this kind of complex-seed in small batch process. This new seed can not only control the crystal growth orientation of the GdBCO bulk superconductor, but also can improve the quality and stability of single domain GdBCO bulks during the TSMTG process. It is also found that single domain GdBCO bulks can be successfully prepared by this method if we use a NbBCO seed with some defects. This is a very important method for the fabrication of single domain GdBCO bulks.