| Y-based superconductors is one of the most promising materials for high temperature superconducting materials, which can be widely used for various applications, so they are concerned all over the world.The single domain GdBCO superconductors can be prepared by the top-seeded melt texture growth process (TSMTG). They have significant potential for practical applications, such as maglev, strong magnetic field of permanent magnets, superconducting flywheel energy storage systems and superconducting motors, etc, since they have high critical temperature(Tc), high critical current density(Jc) and high trapped magnetic fields. In order to further promote the levitation force and critical current density of single domain GdBCO bulks. The effective flux pinning centers have to be introduced into the superconducting matrix by doping methods.In this paper, the precusor powders and the new second-phase nanoparticles Gd2Ba4CuNbOy(GdNb2411) powders were prepared by the traditional solid-state reaction method. Single domain GdBCO bulk superconductors doped with GdNb2411 nanoparticles were prepared by the TSMTG, meanwhile, their growth morphology, magnetic levitation force and microstructure were analyzed systematically. In addition, the effect of different size and layer position of GdNb2411 nanoparticles on the physical properties of the single domain GdBCO bulk superconductors have also been investigated.The initial powders of GdBa2Cu307-δ(Gdl23), Gd2BaCuO5(Gd211), YBa2Cu307-δ(Y123) and Y2BaCuO5(Y211) were prepared by solid state reaction method with three times sintering and four times ball milling process, the sintering temperature is 940℃,930℃,920℃and 910℃respectively. Meanwhile, the phase composition, cystal structure, microstructure, particle size and carbon content of GdNb2411 powders were investigated systematically, then the optimal sintering temperature of the powders were defined to be 1140℃.The effect of GdNb2411 dopant on the growth morphology, magnetic levitation force, microstructure and chemical component at the cystal growth front of the single domain GdBCO bulk superconductors have been investigated. The samples with compositions of Gd123:Gd211: GdNb2411=1:(0.4-x):x. The surface morphologies of the samples are closely related to the doping content x. It is of a smooth surface with four fam sections for the samples when x≤0.06 mol, but it is of a wrinkle surface for the samples when x≥0.08 mol and the single domain area is reducing with the increasing of x. The levitation force of samples increases first and then decreases with the increasing of x, the largest levitation force of 25N is obtained in the sample with x=0.06mol. The microstructure of the samples shows that the density of GdNb2411 particles is increasing with the increasing of x and the particles size in the range of 100nm to 300nm. The pyramidal surface morphologies and the smaller levitation force of the samples may be due to the aggregation of GdNb2411 particles when x>0.06mol. Additionally, the chemical composition at growth front of the samples shows that a large number of Gd211 and GdNb2411 particles are remaining in the liquid phase matrix, the reason for the stopping growth of GdBCO cystal is because of serious deviation from the elements in Gd:Ba:Cu=1:2:3 ratio.The effect of different size and layer position of GdNb2411 nanoparticles on the growth morphology, levitation force and microstructure of the single domain GdBCO bulk superconductors have been investigated. The results shows that the surface of the samples are of four fan sections. The levitation force measurement shows that the levitation force is increasing with the decreasing of particle size and the lowering of the GdNb2411 layer position in the samples. The largest levitation force of 25N and 14N are obtained for the samples with GdNb2411 particle sifted with n=360 mesh seive, and the layer position h=3/4 at the lower part of Gd123 sample. Additionally, the microstructure of the samples shows that the GdNb2411 nanoparticles are uniformly distributed in the Gd123 matrix for all the samples with different GdNb2411 particle size, but the decreasement of the levitation force is because of the reduction of the total GdNb2411/Gd123 interface in the samples. The GdNb2411 particles size is nearly the same, no matter where the GdNb2411 layer placed in the sample, but the levitation force of the samples is dependent on the volume of bulk superconductor above the GdNb2411 layer.Finally, it is found that there are still some problems need to be solved, such as the Gd211 particles agglomeration, voids and macro-cracks in the GdBCO samples, solving the problems will help us to improve the quality GdBCO bulk superconductor. |
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