Materials Science Forum Vols. 546-549

Abstract: The effects of BaF2 on thermal behavior and microstructure of melt-processed YBCO superconductors have been investigated. The differential thermal analysis (DTA) results indicated that the peritectic decomposition temperature of YBCO precursor powder was lowered when BaF2 was added (from 1020
to 976
). The peritectic decomposition temperature was lowest at the content of 5 wt.% BaF2, and the variation of melting temperature was not significant above 5 wt.% BaF2. The microstructures of the doped samples have been observed by scanning electron microscopy (SEM). The results showed that the    of BaF2        microstructure  .

Abstract: On the requirement of high temperature superconducting (HTS) Maglev transportation application, levitation capability of bulk YBa2Cu3O7-x on the HTS Maglev test vehicle has been investigated experimentally by pulsed field magnetization (PFM) and field cooling (FC). The result showed that YBa2Cu3O7-x sample magnetized by the pulsed magnetic field behaves like a magnet in the other external field, and the magnetization direction of bulk determines its levitation force and lateral force mode (magnitude and direction) over NdFeB permanent magnetic guideway (PMG). On the other hand, static field cooling condition is with an electromagnetic field realized by Field Control Electromagnets Workbench. Two important forces of the same bulk were measured and compared with the PMG when the bulk had the same max trapped field by FC and PFM methods, and the results show the difference effects by these two magnetization methods.

Abstract: Y123 and Y211 powders were synthesized by solid-state reaction method. YBCO superconductor specimens were prepared by unidirectional solidification technique. The microstructure and the structural characteristics were observed by means of the optical microscope, X-ray diffraction (XRD), scanning electron microscope (SEM). The results showed that the specimen have a highly aligned structure, it was observed obvious laminated structure of Y123 crystal. Crystal growth mechanism of YBCO was investigated. Jackson's factor of Y123 crystal was calculated. Interface form was judged tentatively theoretically. The specimen was rapidly quenched by quick pulling. Solid/liquid interface structure was observed. It was concluded that a-b plane was faceted grow forms and layer growing along c-axial on a-b plane.

Abstract: The unidirectional solidification technology by a zone melting method was performed to obtain the large single domain YBCO. The interface morphology and chemical composition at the growth front of the YBCO crystal were investigated in order to make clear the growth characteristic of the YBCO crystal during melting growth by unidirectional solidification. It was found that YBCO crystal would cease growing when yttrium was depleted in the liquid phase at the YBCO crystal growth front. For maintaining the continuous growth of YBCO crystal, compositions of Y, Ba and Cu in raw samples have to be adjusted so as to make yttrium rich in the liquid phase at the YBCO crystal growth front during the melting growth process. It is very useful for the study on the mechanism of the YBCO crystal growth.

Abstract: The relationship of the resistivity versus synthesizing temperature of sol-gel YBa2Cu3O7-x samples, with Tc
91K, was studied while synthesizing in flowing oxygen atmosphere. A set of high temperature  -T curves were obtained for the whole process. After four rounds of synthesizing, the resistivity of the sample was =1.00×10-3cm at room temperature. The -T curve of the last round also showed that the orthorhombic to tetragonal phase transformation of the sample occurred around 600oC that is lower than the YBa2Cu3O7-x sample prepared in ambient atmosphere. Other measurements such as X-ray diffraction, SEM measurement, low temperature R-T and M-T measurement were also taken to get more information of these samples.

Abstract: The effect of magnet moving speed on the relaxation of the maximum levitation force of YBCO bulk has been investigated. The experiment was carried out between a cylinder permanent magnet and a single-domain YBCO bulk, and the levitation force was measured using a self-made levitation force measurement system. It is found that the levitation force decreases with time after the gap distance between the magnet and YBCO bulk reduced to 2 mm and kept for the test. It is also found that the levitation force and its relaxation rate is different for different moving speed between the magnet and the YBCO bulk, and the higher the moving speed, the greater the relaxation rate of levitation force. However, the largest levitation force is obtained at an optimal speed, the higher the levitation force, and the faster the relaxation of levitation force.

Abstract: The effect of the moving speed between a permanent magnet and a superconductor on the levitation force of single-domain YBCO bulk has been studied at liquid nitrogen temperature. It is found that the levitation force is much different with different moving speed, the maximum levitation force and the stiffness of the force-distance curve are increasing first and then decreasing with the increasing of the moving speed and, there is a reasonable speed to get the maximum levitation force. It is also found that the maximum attractive force is decreasing with the increasing of the moving speed.

Abstract: This paper describes the details on synthesizing nanocrystalline Gd2BaCuO5 (Gd211) using low-temperature combustion synthesis(LCS). The morphology and structure of the powders are characterized by field emission scanning electron microscopy (FESEM) and XRD. The size of obtained powders by this method calcined at 800
C is about 100nm, which is much smaller than the powders synthesizing by conventional solid state route. Besides, the Gd211 powders synthesizing by LCS have a low calcination temperature with a high-purity. The microstructure of melt-textured samples is also observed by FESEM, the result shows that the size of Gd211 particles trapped in GdBa2Cu3O7 (Gd123) matrix could be refined by using ultrafine Gd211 powder as a precursor. The refined Gd211 particles with a good distribution resulted in the improvement of Jc values in this system.

Abstract: The analysis of magnetic field in high temperature bulk superconductors is presented in this paper. The macroscopic field equations were constructed on the basis of the critical state model and levitation forces ware analyzed between permanent magnets (PM) and HTSC using analytical method. The levitation force between permanent magnets (PM) and HTSC calculation were obtained by using Lorentz force equation [1]. The analytical results show that the presented method is comparatively accurate by comparing the measured levitation forces and the calculation ones. Some relations between B(r, t) and levitation force’s hysteresis are investigated.

Abstract: Positron annihilation technology and simulation analyses are used to investigate the doped YBCO. The results suggest that Co ionic cluster effect is stronger than Al in YBCO. The cluster effect not only causes the local electron density to reach the saturation, but also suppresses the superconductivity significantly.