Minghui Zheng

Jc–B characteristics for bulk single domain YBCO superconductors

Abstract We have studied the dependence of the magnetic critical current densities ( J c ) on the applied field for the samples taken from various positions of a single domain YBCO disk. For most of the measured samples the J c – B curves exhibited the peak effect at field of around 0.8 T, and the highest J c value at 1 T and 77 K was found in the position of about 4.5 mm depth away from the surface of the disk. For various positions the features of J c ( B ) curves were different. It is suggested that the peak effect is attributed to the field induced pinning of weak superconducting regions due to very wide superconducting transition as well as the distributions of the second phase particles. For the bulk sample the J c ( B ) performance is inhomogeneous in the entire body.

Effects of cooling rate on single domain growth and the superconducting properties for YBCO bulk

Abstract A series of YBCO single domain samples were prepared by using the top seeded melt growth method with different cooling rate during the melt processing. For all of the samples, the DC magnetization and the levitation force were measured to determine the effects of cooling rate on the critical transition temperature T c magnetization critical current density J cm and levitation force F . The results indicate the influence of cooling rate on superconducting properties is very strong and the cooling rate has an optimum range. For instance, the rates of 0.9–1.1 °C/h are suitable for the sample with 11 mm in diameter, above which single domain size becomes small and below which the superconducting properties degrade obviously. The similar result has been also observed for the sample with 30 mm in diameter.

Study of oxygenation process during the preparation of single domain YBCO bulk superconductors

Abstract The oxygenation process is an important step during the preparation of YBCO bulk superconductors by the MTG process. A high pressure oxygenation device has been built by our group. High pressure oxygen has been used in annealing. It is found that different oxygenation processes would lead in the different results. In comparison with the oxygenation process by flowing oxygen, the high pressure oxygenation technique has many more advantages. For example the capacity of the oxygen gas can be saved and the efficiency of oxygenation can be raised greatly.

Experimental study on a high-Tc superconducting permanent magnet

Abstract With two φ 26 mm YBCO bulks, a superconducting permanent magnet of 7.09 T with low creep was achieved at 29.8 K. Cracks occurred when the activating field of 10 T was applied, showing that the critical field is limited by material strength. With one YBCO bulk, 4.38 T (4.2 K) and 2.71 T (50 K) was achieved.

The observation and explanation of the spontaneous nucleation behavior in SmBaCuO–Ag solution

LReBaCuO (LRe=Nd, Sm, Gd) single domains possess a superior superconducting performance and have exhibited potential applications in the fields of superconducting bearings and permanent magnets. Therefore, the requirement of a large size for LReBaCuO single domains with high values of TC and JC is desirable. However, it has been found that some unexpected spontaneous nucleation actions always take place, which interrupt the growing process of the single domains. In this paper, the behavior of spontaneous nucleation in SmBaCuO–Ag solution has been investigated systematically. Experimental results reveal that the spontaneous nucleation that frequently takes place during the growing process could be eliminated greatly by reducing the cooling rate to a proper level. Based on the framework of the constitutional super-cooling theory, combined with the results obtained in experiments, we discuss the possible explanations for the spontaneous nucleation behavior in SmBaCuO–Ag solution. An assumption that the distribution of solute concentration CL, as well as the solid–liquid phase transition temperature TL, in SmBaCuO–Ag solution might be of a time-dependent nature is put forward. A graph of temperature versus time has been worked out based on the assumption, which matches the growth process behavior satisfactorily with the experimental results.

Effect of Ultrafine Gd211 on Processing and Superconducting Properties of Gdbacuo Bulk Superconductors

GdBaCuO bulk superconductors from precursors of Gd123 and ultrafine Gd211 powder have been fabricated by melt-textured-growth method in air. The ultrafine Gd211 powder was synthesized by an improved Pechini method. It was found that the lower temperature of synthesizing ultrafine Gd211 powder, the higher peritectic reaction temperature of GdBaCuO bulk superconductors. Jc reaches 4.7×104 A/cm2 at 77 K and self-field for ultrafine Gd211 starting particles with 0.2μm in diameter, and higher compared to larger Gd211 starting particles fabricated by solid state reaction. However, a large secondary peak effect did not appeared at Jc-B curves. ,

Study of the trapped field distributions and the microstructure for YBCO disks

Abstract The trapped field distributions on the surface of MTG YBCO disks were measured by a Hall probe automated scanning system, and exhibit a cone profile for sample A with levitation force up to 12.6 N/cm 2 , whereas four peaks for sample B with 4.4 N/cm 2 . However, c -axis orientations of two samples are all perpendicular to the sample surfaces according to the pole figure. For sample B, the boundaries between the sectors seem to be weak links, and the T c in the sector, which is very low, is higher than that in the boundary. The microstructure reveals that the agglomerates of non-superconductive phases occur in the boundary and the center for sample B, but do not occur in the sectors of sample B and nor in any part of sample A. So, it can be assumed that the agglomerates of non-superconductive phases in the boundary are mainly responsible for the low levitation force and trapped field with four peaks in distribution of sample B.