The mechanism of frictional motion and its effects at 4.2 K in superconducting magnet winding models

Frictional sliding occurs on both a microscopic and a macroscopic scale. Sliding on a microscopic scale appears as discrete events called microslips. Microslips are inherent in all sliding events and are quite different from macroscopic instabilities such as stick-slips. It is thought that the training effect observed in quench current data from a superconducting braid may be caused by microslips.The mechanisms of sliding motion and its effects at 4.2 K were studied in detail for a number of metal/insulator pairs that model superconducting magnet windings; the results impact the performance of superconducting magnets. Organic surface coating materials are generally effective in eliminating macroscopic instabilities. Instrumentation used in these experiments includes a high-resolution extensometer and an acoustic emission sensor, both with sensitivities capable of detecting microslips (～ 1 μm).     

The use of nanophase powders for high Jc superconductor tape production

Advances in processing of superconductors has demonstrated that ceramic powders with exact composition, super-homogeneity, and extremely fine particle size can be prepared by chemical processing to achieve high density and uniform microstructure. The chemical processing routes have great advantage in preparing powder with high reactivity thus promoting solid state reactions required to produce ‘purer’ superconducting phase. We have studied extensively the coprecipitation technique for the preparation of superconducting ceramic powders. Chemical solution methods have been used in order to model several superconducting systems and to predict optimum conditions for obtaining exact composition. Several improvements on the control of precipitation reaction have been implemented in order to obtain powders of few nanometers in size. This has been done by carrying out the reaction in the bulk of a solution or in microemlusion systems. The corecipitated precursor powder has been used for production of 20− 30 meter long Ag-clad tapes. These tapes require considerably reduced processing times. By controlled heat treatment conditions, these tapes have been processed to carry transport critical current density, J_c <4 × 10⁴ A/cm² at 77K in zero applied field.     

History effects in low-field magnetoresistance of BPSSCO polycrystals

Low-field (H<40 G) magnetoresistance measurements have been made on Bi_1·6Pb_0·4Sr₂Ca₂Cu₃O₁₀ polycrystals at several temperatures between 80 and 105 K. Considerable hysteresis in ρ(H) is found in a zero-field-cooled sample when the applied field is increased from 0 to a maximum value and then lowered back to 0 at all temperatures. The observation of hysteresis is taken as an evidence for field trapping in the grains. We show that the hysteresis in ρ(H) occurs for applied fields much lower than that at whichdρ(H)/dH exhibits a discontinuity. In addition, we find that when the applied magnetic field (H _a) is lowered from a maximum field, the effective intergranular field,H _eff, becomes zero forH _c>0, which gives rise to a minimum in ρ(H).     

Factors determining the magnetic field generated by a solenoid made with a superconductor having critical current anisotropy

The magnetic field generation in a simple solenoid is reconsidered for the case where the magnetic field is generated by a superconductor with anisotropy in its critical current density. In this case the influence of the radial magnetic field at the solenoid ends on the weak direction of the conductor has to be taken into account. Instead of the usual load line which stems from the maximum axial field at the inner turns, two load lines must be considered: one as usual, and the second one representing the radial field at the coil end. The maximum field generated by the solenoid is determined by which load line meets its respective j_c-H curve first. For tapes of (Bi,Pb)₂Sr₂Ca₂Cu₃O_x it is the radial field at the solenoid ends which determines the central field which can be generated by the solenoid. This is also the case for most other anisotropic superconductors even with a moderate j_c anisotropy of, for example, two. Insert coils in a background field can significantly raise the maximum central field as the ratio between axial and radial fields is different. This gain for a magnet made from Bi(2223) tapes is of the order of 30% (at T= 77 K). Some alternatives for maximum field generation using anisotropic tapes are discussed.     

高温超导中多个正电子捕获位及其对正电子湮没参数的影响

从正电子陷阱物理图象出发，引入正电子在陷阱中湮没的竞争机制，提出了一个新的陷阱湮没模型，并采用此模型分析了高温超导材料中正电子在陷阱中湮没的特征。     

Phase separation in hydrogen-loaded 123 and 124 compounds observed by Cu NMR and NQR

The transition from superconducting to antiferromagnetic behavior in RE Ba₂Cu₃O₇H_x, RE=Y, Gd, has been studied by Cu NMR and NQR. In the Gd compounds the spectra of Cu(1) and Cu(2) sites could be separated by the strong relaxation induced by the magnetic moments of Gd on the neighboring Cu(2) sites. Already at smallx of about 0.4, antiferromagnetic Cu(2) sites are revealed by an AF-NMR spectrum. At the samex we find a strong broadening of the normal NQR spectrum and a new sharp line at 27 MHz for⁶³Cu, indicating a new EFG of the Cu(1) sites in the AF phase. The transformation to the AF state is complete at aboutx=2.0. Annealing of the doped material at 350 °C leads to the normal oxygen-depleted 123 compounds, proving that the cation lattice is not distorted even at highx. As in the case of the pure YBCO, no static magnetic moments exist at the Cu(1) sites of the hydrogen-containing phase. Hydrogen addition is found to produce a similar antiferromagnetic spectrum in 124 material.     

应用高温超导线(带)的研制现状与进展

提供了当前高温超导线的最新发展,第一代高温超导材料的机电性能参数和热物性参数,以及4种美国超导公司的商业产品和中国超导带材的基本参数.论述了第二代高温超导线的研究进展.从高温超导电力应用的观点,展望高温超导电力技术应用前景,文中的工程与实验数据对高温超导线(带)材的应用可供参考.     

Design of TE01Δ Test Probe for Measuring the Microwave Surface Resistance of HTS Thin Film

A new TE01 test probe with proper transmission factor is fabricated for the measurement of surface resistance of high temperature superconductor (HTS) thin film. Coupling holes instead of coupling loops are used in the probe for its easier machining and relatively low loss. Two 6 mm3 mm8 mm dielectric waveguides, one side of them is coated by silver, are used for coupling. The measurement result of S21 agrees well with the simulation because the size of the probe can be rigidly controlled by machine. The microwave surface resistance of four YBCO/MgO films are measured at 77 K and 12 GHz and scaled to 10 GHz according to the f2 rule. The average surface resistance of four HTS thin films is 0.38 m, the standard deviation and relative standard deviation of one single HTS thin film are 0.009 m and 2.4%, respectively.     

Experimental and theoretical approaches for electrical,magnetic, micromechanical,and structural characterization of BSCCO ceramic superconductors

This study investigates how the preparation and re-pelletization of BSCCO ceramic superconductors affects their structural, magnetic, electrical, and mechanical properties. Samples were prepared using conventional methods including the dry solid state (SS) reaction and wet ammonium nitrate (AN) precipitation, with three variations prepared for each technique. The fabricated samples were then characterized with X-ray powder diffraction (XRD) and scanning electron microscopy. The density, resistance vs. temperature characteristics, AC susceptibility vs. temperature characteristics, and magnetic hysteresis properties were measured. In addition, Vicker's microhardness was measured and revealed that all six samples exhibit the reverse indentation size effect (RISE). Microhardness modeling was also conducted. Calculations with Meyer's law, the Hays and Kendall model, and the proportional sample resistance model indicate that the samples are far from the plateau region, whereas the indentation-induced cracking model was consistent with the experimental results. The elastic modulus, Young's modulus, yield strength, and brittleness index were also calculated for each sample.