Chengshan Li

Enhanced flux pinning in (Bi, Pb)-2223/Ag tapes by slight Pr-doping

(Bi,Pb)2.2Sr2Ca2.2−xPrxCu3Oy silver sheathed tapes with Pr-doping amount of x=0.0, 0.001 and 0.002 have been prepared by powder-in-tube method. X-ray diffraction analyses manifest that the tapes with x=0.001 and 0.002 (doped) have less Pb-rich phase than the tape with x=0.0 (un-doped). Transmission electron microscopy images show that the Pb-rich phase particles of ∼40 nm are embedded in un-doped (Bi,Pb)-2223 grain matrix, while defects induced by Pr-doping with the size smaller than that of the Pb-rich phase particles exist in the doped samples. As compared with the un-doped tape, the magnetic field dependence of critical current densities of the doped tapes are noticeably improved at 77 K, and their irreversibility lines are also shifted to higher temperatures and magnetic fields and activation energies of flux motion increased drastically. The enhanced flux pinning seems to mainly originate from the fine normal-like defects induced by the partial Pr-substitution for Ca.

Effects of polycarbosilane doping on the superconductivity and microstructure of MgB2

The effect of polycarbosilane (PCS) doping on the superconducting properties and microstructure of MgB2 bulk has been studied in this paper. It is found that the lattice parameter a and the superconducting transition temperature decrease with increase of the doping level. However, the critical current density Jc is significantly enhanced and the Jc(B) properties are improved at low temperature. Microstructure analysis indicates that a lot of Mg2Si nanoparticles, including those 5?10?nm in size, and some lattice defects induced by the PCS doping exist inside the PCS doped MgB2 samples, which may be the main reason for the enhancement of the flux pinning property.

Optimization of Bi-2212 high temperature superconductors by potassium substitution

Polycrystalline bulks of Bi2Sr2Ca1−xKxCu2.0O8+δ (Bi-2212) with x = 0, 0.05, 0.10 and 0.15 were fabricated by the spark plasma sintering technique. The influences of K doping on the microstructures, electronic structures, as well as the related superconducting properties, were systematically investigated. The XRD analyses confirmed that K+ ions have successfully substituted into the matrix of Bi-2212, and lead to a systematical change of lattice parameters. Due to the change of thermodynamic properties, bulks with higher density, larger grain size and better texture structures were obtained after doping. Therefore, ac susceptibility measurement revealed the optimization of intergrain connections, which lead to the optimization of both self- and in-field critical current density, Jc of this system. The optimization of microstructures also caused the enhancement of surface pinning. Based on the enhancements of both intergrain connections and flux pinning properties, an obvious improvement of critical current density was obtained with the optimal doping content of K = 0.05. Meanwhile, Bi-2212 single filament tapes with K doping content of 0 and 0.05 were also fabricated by the powder-in-tube process. The XRD patterns also proved the successful doping of K ions in the Bi-2212 matrix. The critical current density Jc, measured by the transport method under the magnetic field from 0 to 20 T at 4.2 K, proved the effectiveness of K doping on the enhancement of flux pinning properties of Bi-2212.

Optimization of fluorine content in TFA-MOD precursor solutions for YBCO film growth

Several low fluorine solutions containing different contents of fluorine were prepared by a chemical process. The fluorine contents in these solutions with respect to the conventional all-trifluoroacetate solution were calculated as 0%, 7.7%, 15.4%, 23.1%, 30.8%, 38.5% and 53.8%. YBa2Cu3O7−x (YBCO) films were deposited on LaAlO3 and CeO2/MgO/Al2O3/Hastelloy substrates using these low fluorine solutions. The phase formation, texture and microstructure of the YBCO films were characterized by x-ray diffraction and scanning electron microscopy. The decomposition mechanism of the low fluorine solution was also discussed. The results indicate that the ratio of F/Ba and the carbon content in decomposed powders could be controlled by adjusting the fluorine content in the precursor solutions. Fluorine had a great influence on the phase transformation, nucleation and growth of YBCO film during the crystallization process. The optimization of fluorine content was in the range of 15.4%–23.1%, which contributed to the good texture, homogeneous microstructure and high J c value of the YBCO crystallized films.

Fabrication and Superconducting Properties of Rod‐In‐Tube Multifilamentary Nb 3 Al Wire With Rapid Heating and Quenching Heat‐Treatment

In this paper, we reported the recent progress in developing practical kilometer-length rod-in-tube (RIT) multifilamentary Nb₃Al superconducting wires at WST, which included the preparation of Cu/Nb-Al precursor wires, design and manufacture of rapid heating and quenching (RHQ) equipment, and RHQ heat-treatment process of Nb₃Al wires. Superconducting properties and microstructure of the RIT Nb₃Al wires have been discussed. By measuring magnetization and transport critical current properties of Nb₃Al wires, it is found that, for all the samples, the onset superconducting transition temperature T_c reaches 16.8-17.3 K, and at 4.2 K and 15 T, the critical current density J_c of 830 A/mm² at the Nb₃Al wires have been achieved. This paper suggests that it is possible to develop the practical Nb₃Al superconducting wires by using the RIT Nb₃Al precursor wires, in combination with RHQ heat-treatment process.

Fabrication and characterization of internal Sn and bronze-processed Nb3Sn strands for ITER application

Long multifilamentary Nb 3 Sn strands for ITER (International Thermonuclear Experimental Reactor) have been successfully fabricated by internal Sn and bronze processes, respectively. To improve the bonding of Nb and Cu and the diffusion between Sn and Cu for the internal tin process, the traditional RIT (rod-in-tube) and new hot extrusion (HE) processes have been employed to manufacture the Cu/Nb rods. The final internal tin process strand with a unit length of 3000-5000 m comprises a Cu stabilizer, a Cu/Ta barrier and 19 subfilaments. Every subfilament consists of about 280-330 Nb filaments in a Cu matrix surrounding an Sn-Ti core. The bronze process Nb 3 Sn strands with 11581 and 9805 filaments of Nb7.5Ta with an unit length of more than 3 km have been produced, respectively. The Ta or Nb barriers were used to compare the influence of different barrier materials on the fabrication process and superconducting properties of Nb 3 Sn strands. The microstructure details of two kinds of strands before and after heat treatment have been investigated by scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX). The non-Cu J c (12 T, 4.2 K) value of 812 A mm -2 with hysteresis loss of 760 mJ cm -3 for the bronze-process strand and 1069 A mm -2 with hysteresis loss of 956 mJ cm -3 for the internal tin-process strand have been obtained. The influence of microstructure on the transport property and hysteresis loss of strands has been discussed.

Improved superconducting properties in bulk MgB2 prepared by high-energy milling of Mg and B powders

MgB2 bulks were prepared by high-energy milling of Mg and B powders. The correlations between milling times, microstructure and superconducting properties were investigated in MgB2 bulks. The samples were characterized by x-ray diffraction (XRD), energy dispersive spectrometry (EDX) and scanning electron microscopy (SEM), and the magnetization properties were examined with a superconducting quantum interference device (SQUID) magnetometer. The investigations showed that high-energy milling is an effective approach for obtaining fine crystalline (40?100?nm) bulk MgB2 with good grain connectivity and high Jc performance. The critical current density reaches 2.0 ? 106?A?cm?2 at 15?K and 0.59?T, 5.7 ? 105?A?cm?2 at 2?T and 3.0 ? 104?A?cm?2 at 5?T.

Axial tensile and compressive strain effects on the critical current of Bi-2223 superconducting tapes

Abstract The effects of axial strains e a on the critical current of Bi-2223/Ag and Bi-2223/AgMn tapes produced by the powder-in-tube method have been studied at 77 K. The normalized critical current I cn vs axial strain e a curves I cn ( e a ), and its empirical formulae as I cn =1−( e a / a ) b , where a and b are constants, have been obtained. The critical current degrades more rapidly under large axial tensile strains than under axial compressive strain. The irreversible strain limit of 19-filament, 37-filament Bi-2223/Ag and 61-filament Bi-2223/AgMn superconducting tapes are 0.23–51%, 0.30–36% and 0.43–0.54%, respectively. Strain rate has influence on the irreversible strain limit. The whole I cn ( e a ) curve is divided into four ranges: compressive strain regime, first tensile strain regime, second tensile strain regime, third tensile strain regime.

The AC loss of transport current in (Bi, Pb)-2223 superconducting tapes

Abstract The self-field AC loss of the untwisted (Bi,Pb)-2223 tapes made with different matrices, number of filaments, critical currents and process techniques was measured in the frequency range from 20.8 to 363 Hz at 77 K. The losses of different matrix (Bi,Pb)-2223 tapes under different tensile strain were investigated at 50 Hz, 77 K. We find that: (1) in the case of I m / I c I m / I c >0.4, the eddy current loss must be taken into consideration; (2) above the critical current, the loss, which is shown resistive, is independent of current frequency, and differences in processing have a major impact; (3) below the critical current, the losses, P , increase in proportion with I m / I c with slopes less than 3, which is not in accordance with the Bean model; (4) during the elastic stage the self-field AC losses of AgMn and Ag matrix (Bi,Pb)-2223 tapes do not change, while in yielding stage the unstable relations of AC loss vs. axial tensile strain maybe caused by the serrated curve of stress vs. tensile strain relation, and the losses of both AgMn and Ag matrix tapes increase more in the plastic stage.

Investigation of MgB2/Fe wires with different diameters

MgB2/Fe wires with different diameter were fabricated by using the in situ powder-in-tube method. Samples were sintered at 750 °C for 1 h in a flow of high purity argon gas. The phase composition is identical for all the wires. The Jc(B) values of these wires generally increase with decreasing diameter from 1.60 to 1.40 to 1.29 mm, but decrease significantly to the lowest value at a diameter of 1.20 mm. It is revealed that the optimum diameter of 1.29 mm, at which the MgB2/Fe wire possessed the optimal Jc(B) property, was determined by the variations of the green density of the Mg/2B mixture in the core and the relative thickness of the Fe sheath during the mechanical deformation process.