Xingyi Zhang

Relaxation transition due to different cooling processes in a superconducting levitation system

We present an experimental study of relaxation of vertical and horizontal force components in a high-temperature superconducting levitation system, with different initial cooling process after fixing the levitated body in an expected position statically. In the experiment, the bulk YBaCuO cylinder superconductor and the permanent magnet disk are employed. For a selected levitation height (LH) and a lateral displacement (LD) of the system, the experimental results show that the relaxations of the vertical and horizontal forces are strongly dependent on the initial cooling height (CH). With CH decreasing, the transition of the lateral force from repulsion to attraction is found as well as the changing characteristics with time from decrease to increase. Additionally, when LH is fixed at the CH, the transition phenomenon is also observed in the levitation force behavior and their relaxation under different LDs.

Relaxation properties of magnetic force between a magnet and superconductor in an unsymmetrical levitation system

We present an experimental study of the relaxation of vertical and horizontal force components in an unsymmetrical high-temperature superconducting levitation system, with different initial cooling processes, after fixing the levitated body statically in a given position. It was found that the values of the relaxation measurements of the levitation force and lateral force remained constant or increased with time after vertical and horizontal traverses. The phenomenon has been theoretically described based on the Bean model and the thermally activated flux creep theory. The criterion developed in the present work is considered to be suitable for providing qualitative predictions of the relaxation properties in the levitation force and lateral force.

Current transport of the [001]-tilt low-angle grain boundary in high temperature superconductors

Current transport in grain boundary is one of the crucial factors which can affect the macro-supercurrent characteristics of the high temperature superconductors. van der Laan et al. [Phys. Rev. Lett. 103, 027005 (2009)] presented the strain dependence of the critical current density with a power-law fitting function, which has been verified by many experimental measurements. Here, we present a theoretical analysis of current transport in the [001]-tilt low angle grain boundary according to the strain energy of dislocation. An analytical expression is obtained which has the similar form of the fitting function, and in which the physical characteristics of parameters are distinct, and their values are close to the reports in literature.

Experimental and theoretical investigations on the singularity of the intensity factor of the current in high temperature superconductors

This paper presents experimental and analytical investigations into the distribution of current around a through-edge crack located in an infinitely long cylindrical high temperature superconductor (HTS) on the basis of the Bean model. Current distributions around the crack in the HTS without deformation are obtained by a simple conformal mapping. It is shown that the current is singular at the tip of the crack. An intensity factor of the current is introduced to characterize the singularity, the order of which is equal to ?1. Additionally, an experimental verification according to the trapped field distributions around the crack is carried out, and the experimental results show little difference with the theoretical analysis.

INFLUENCE OF WAITING TIME ON THE LEVITATION FORCE BETWEEN A PERMANENT MAGNET AND A SUPERCONDUCTOR

This paper describes the experimental results of the levitation force of single-grained YBaCuO bulk superconductors preparing by the top-seeded melt-growth method with different waiting time tw below an NdFeB permanent magnet. It was found that waiting time has large effects on the zero-field-cooled (ZFC) and field-cooled (FC) levitation force, and the levitation force shows aging characteristics at the liquid nitrogen temperature.

A direct tensile device to investigate the critical current properties in superconducting tapes

We construct an instrument to study the behavior of the critical current in superconducting tapes as a function of magnetic field and axial tension strain. The apparatus combines a material testing machine made by the non-magnetic stainless steel, which is capable of producing mechanical forces up to 1000 N and magnetic field up to 5 T with a homogeneous range of Φ150 × 110 mm. Moreover, the apparatus allows the automatic measurement of time dependence of voltage (V-t) under different magnetic fields and applied strains, which can be used to investigate the vortex instability and its time effect in the superconducting tapes. As an example, the simultaneous measurements of critical current and voltage relaxation with time at various strains and magnetic fields for the YaBa2Cu3O7-x coated conductors are carried out. Comparisons are made with the earlier reports in literature; the strain and magnetic field dependence of critical current indicate consistent behavior of this instrument.

Self-enhancement of the critical current of YBa2Cu3O7−x coated conductors caused by the axial tension

We have measured the critical current of the YaBa2Cu3O7−x coated conductor under an axial tension. In the case of liquid nitrogen temperature and self-field ambient, a fascinating self-enhancement (about 25%) of the critical current of the specimens after the axial tension and a suitable waiting time were observed in the experiments. A theoretical model based on the waiting time and the tension strain is presented, which quantitatively agrees with the experimental results very well.

Lap Joint Characteristics of the YBCO Coated Conductors Under Axial Tension

In this paper, we present a solder method for the lap joint of the YBa2Cu3O7-x coated conductor (YBCO CC) without removing the metallic stabilizer. Several solder joints with the same overlap of 80 mm were manufactured. In the case of self-field and at liquid nitrogen temperature, the resistances of solder joints are equal to about 10 nΩ. In addition, the resistance is not influenced by the axial tension force and external magnetic field. The critical current degradation caused by the axially applied strain and external magnetic field exhibits consistent behavior of the original YBCO CC. At room temperature, the mechanical property of the specimen with joint is determined by the original tape. Moreover, at 77 K, the tension characteristic of the sample with and without joint exceeds that at room temperature.

Suppression of Magnetic Force Relaxation in a Magnet-High

We have measured that the levitation forces relax at the different order equilibrium positions of a bulk YBaCuO superconductor. In a given symmetrical levitation system, high order equilibrium positions can be obtained by increasing the number of the reversal movements of the magnet; the higher order equilibrium position, the lower the relaxation of the levitation force. This method is confirmed experimentally and theoretically. For the unsymmetrical levitation system, one movement mode of the magnet and/or superconductor can reduce the relaxations of the levitation force and guidance force synchronously while the quantities of the levitation force and guidance force are depressed little.

T_{c}

Artificial muscles transformed by fishing lines or sewing thread, have distinguished advantages, e. g., fast, scalable, nonhysteretic, and long-life, which have been proposed by Haines et al. [Science 343, 868 (2014)]. In this paper, we present a geometrical model to predict the effective Young’s modulus of the basic structure that is twisted by three fishing lines with the same diameter. Moreover, series of experiments are carried out to verify the present model, and it is found the theoretical calculations take good agreements with the experimental results.