Pingxiang Zhang

Effect of heat-treatment temperatures on density and porosity in MgB2 superconductor

Abstract The density, porosity, and microstructures of MgB 2 samples are very important factors for transition critical current density. The effect of heat-treatment temperatures on density and porosity in MgB 2 superconductors has been investigated. The open porosity increases with increasing heat-treatment temperatures, but close porosity decreases. The calculated theory densities from the lattice parameters of the measured samples are 2.6–2.63 g/cm 3 . The average measured total porosity (including open and close porosity) is about 50%.

Fabrication and superconducting properties of MgB2 composite wiresby the PIT method

Dense MgB2/Cu wires with Ta as a buffer layer were successfully fabricated by the powder-in-tube (PIT) method. The microstructure was investigated by optical microscopy. Magnetization measurements were carried out by using a superconducting quantum interference device (SQUID) magnetometer at magnetic fields up to 7 T from 5 K to 35 K. The transition temperature of the MgB2 wire is around 38.4 K and the irreversibility field is 6.6 T at 5 K. The critical current density as high as 105 A cm−2 (5 K, self-field) and 104 A cm−2 (20 K, 1 T) has been obtained. The results suggest that the powder-in-tube (PIT) process is promising in preparing high-quality MgB2 wires.

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.

The microstructures and superconducting properties of MgB2 bulks prepared by a high-energy milling method

We succeeded in the synthesis of high-Jc MgB2 bulks via high energy ball-milling of elemental Mg and B powder at ambient temperatures. Alternatively to longtime mechanical alloying technique, the mixed powder was ball-milled for only 1h and the completed reaction is achieved by subsequent annealing. The correlations among synthesis parameters, microstructures and superconducting properties in MgB2 bulks were investigated. Samples were characterized by XRD and SEM, and the magnetization properties were examined in a SQUID magnetometer. The highJc, approximately 1.7× 10 A/cm(15K, 0.59T), and improved flux pinning were attribute to a large number of grain boundaries provided by small grain size.

The characterization of levitation force and attractive force of single-domain YBCO bulk under different field cooling process

Abstract The interaction forces were measured at 77 K by changing the field cooling distance Z fc between a YBCO bulk and a permanent magnet. It is found that the maximum levitation force ( F mlf ), maximum attractive force ( F maf ) and the gap distance ( Z maf ) corresponding to F maf can be well described by exponential laws as a function of Z fc , which allow us to predict these values according to Z fc . It is also found that the distance between the Z 0fa (gap distance corresponding to the zero force) and Z maf in the ascending process is a constant value, which can be well interpreted by the constant reduction rate of the axial component magnetic flux density along the axis line of the magnet within the constant distance. These results are very interesting to physical research and helpful for the practical designing and applications.

The effect of excess Y2O3 addition on the levitation force of melt processed YBCO bulk superconductors

Abstract The YBCO superconductors with Y2O3 addition were prepared by a modified melt textured process and the effects of excess Y2O3 addition on the levitation force of melt textured YBCO have been investigated. The results indicate that the melt temperature of YBCO samples decreases drastically with the increasing excess Y2O3, whereas the solid state reaction temperature increases slightly as the Y2O3 addition rises. The maximum levitation forces (MLF) are much different for samples with different Y2O3 addition. It is found that the optimum Y2O3 excess in YBCO for the (strong MLF) is about 10 wt.%. Combined with the microstructure and the starting composition, the results are discussed in detail.

The effect of magnet size on the levitation force and attractive force of single-domain YBCO bulk superconductors

The levitation forces between a single-domain YBCO bulk and several magnets of different sizes have been measured at 77 K to investigate the effect of the magnet size on the levitation force. It is found that the levitation force reaches a largest (peak) value when the size of the magnet approaches that of the superconductor when the other conditions are fixed. The absolute maximum attractive force (in the field-cooled state) increases with the increasing of the magnet size, and is saturated when the magnet size approaches that of the superconductor. The maximum attractive force in the field-cooled (FC) state is much higher than that of the maximum attractive force in the zero field-cooled (ZFC) state. The results indicate that the effects of magnetic field distribution on the levitation force have to be considered during the designing and manufacturing of superconducting devices.

Enhanced the flux pinning in Bi-2223_Ag by induced Cr-ion defects

Abstract (Bi,Pb) 2.2 Sr 2 Ca 2.2 Cu 3− x Cr x O y silver sheathed tapes with x =0.0 (un-doped), 0.0005, 0.001, 0.002 and 0.004 (Cr-doped) have been investigated. X-ray diffraction analyses and transmission electron microscopy observation manifest that partial Cr substitution for Cu may introduce a lot of fine Cr-ion defects into the samples. The different performances of critical current densities (at 77 K) under applied magnetic fields, the irreversibility lines and activation energies of flux motion indicate that their flux pinning has been enhanced by induced Cr-ion defects obviously. Their pinning forces have also been analyzed, and the results imply that the flux pinning in Cr-doped tapes enhanced mainly originate from the fine normal-like Cr-ion defects.

Refinement mechanism of CeO2 addition on Y2BaCuO5 particles in PMP YBCO bulks

Abstract YBCO bulk samples with a nominal composition of YBa 2 Cu 3 O 6+ x +15 wt.% Y 2 BaCuO 5 +0.5 wt.% CeO 2 have been prepared by the powder melting process technique and the microstructure has been investigated by using scanning electron microscopy and transmission electron microscopy. An obvious refinement effect resulting in Y 2 BaCuO 5 particles with a mean diameter about 500 nm in textured YBCO bulks has been achieved by CeO 2 addition. About one-fourth of the Y 2 BaCuO 5 particles were observed to contain at least one nanometric core which was proved to be Y 2 O 3 by selected area diffraction. In addition, a BaCeO 3 phase with a square shape was found. Our experimental results provide further evidence for the heterogeneous nucleation mechanism of Y 2 BaCuO 5 particles on Y 2 O 3 previously generated through a reaction between CeO 2 and precursors, which was first proposed by Pinol et al. [4] for the melt texture growth process.

The effect of different field cooling processes on the levitation force and attractive force of single-domain YBa2Cu3O7-x bulk

The effects of different field cooling processes on the levitation force and attractive force of single-domain YBa2Cu3O7−x (YBCO) bulk have been investigated. The forces were measured at 77 K by changing the field cooling distance Zfc between the YBCO bulk and a permanent magnet. We found that the stiffness between the YBCO and magnet increases with the decrease of Zfc; the maximum levitation force (Fmlf) of the sample decreases from 74 N to 48 N with a decrease of the Zfc from 7 cm to 0.7 cm; but the maximum attractive force (Fmaf) increases from 2.96 N to 30.6 N with a decrease of Zfc from 7 cm to 0.1 mm. Both of them obey the first decreasing exponential law. It is also found that the distance Zmaf, corresponding to Fmaf, decreases in an exponential law with the decrease of Zfc. The Fmlf, Fmaf and Zmaf are all saturated in the range of Zfc from 3 to 4 cm. All the relations between Fmlf, Fmaf and Zfc are closely related with the axial magnetic field distribution of the magnet, which allow us to easily predict the values between the Fmlf, Fmaf and Zfc, which is very helpful for practical designing and for applications.