H.T. Ren

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.

Fishtail effects in YBa2Cu3Oy crystal prepared by melt-textured growth method

Abstract The magnetization curves of a YBa 2 Cu 3 O y crystal prepared by a melt-textured growth method were measured; the fishtail effects were observed over the temperature range from 30 K to 87 K. The magnetization curves and the volume pinning force exhibit a scaling behavior, indicating that a single mechanism is involved for all the temperatures studied. The fishtail effects can be described by collective-pinning theory very well at high temperature.

Scaling properties of the magnetization curves of the DyBa2Cu3Oy crystal prepared by the melt-textured growth method

Abstract The magnetization curves of the DyBa 2 Cu 3 O y crystal prepared by the melt-textured growth method were measured from 10 to 85 K with the magnetic field parallel to the c -axis. A scaling behavior of the magnetization curves was observed and explained in terms of an extended Bean model.

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.

Scaling law and flux pinning in neutron-irradiated YBa2Cu3Oy crystal prepared by melt-textured growth method

Abstract The magnetization curves of neutron-irradiated YBa 2 Cu 3 O y crystal prepared by melt-textured method were measured from 1.5 K to 85 K for H ∥ C . The volume pinning force can be described by a Fietz-Webb separable variable form, which provides strong evidence that flux pinning is the mechanism which determines J c in this material.

Scaling properties of the magnetization curves of the neutron-irradiated YBa2Cu3Oy crystal prepared by melt-textured growth method

Abstract The magnetization curves of neutron-irradiated YBa 2 Cu 3 O y crystal prepared by melt-textured method were measured from 5 K to 85 K for H ‖ C . The virgin magnetization curves, the complete hysteresis loops and the critical current density all exhibit scaling behavior. A new scaling law for the volume pinning force was observed. The scaling behavior observed was explained in terms of the extended Bean model.

Effect of magnetization process on levitation force between a superconducting disk and a permanent magnet

The levitation forces between a permanent magnet and a coaxial superconducting disk after different magnetization processes are measured. Significant effect of the magnetization process on the levitation force is observed. Theoretical calculations of levitation force based on the equations for current motion with temperature and field-dependent critical current density, and the heat dissipation due to the flux motion are in perfect agreement with the experimental data.

THE EFFECT OF THE NEUTRON IRRADIATION ON THE CRITICAL CURRENT AND THE MICROSTRUCTURE OF THE MELT–TEXTURED GROWTH YBa2Cu3Oy

We have studied the effect of the fast neutron irradiation on the critical current properties and the microstructure of Melt-Textured Growth (MTG) YBa2Cu3O7−δ with fluences of 2.3 × 1016n/cm2, 1.16 × 1017n/cm2, and 5.8 × 1017n/cm2 respectively. The hysteresis loops have been measured at 77 K and 1.5 K in the applied magnetic fields between −6 T and 6 T. The irreversibility lines H(T*) were also demonstrated. The critical magnetization current density Jc is up to 4.7 × 105A/cm2 at 77 K in the field of 0.1 T for the irradiated sample with the fluence of 1.16 × 1017n/cm2. It is tenfold higher than that of the unirradiated sample. This increase of Jc is due to the pinning at irradiation–induced pinning centers. Much higher neutron irradiation with fluence of 5.8 × 1017n/cm2 is harmful to the increase of Jc value because the orthorhombic structure has been transformed into the tetragonal one. The amorphous state and the cellular structures are also observed in the irradiated MTG sample.