Papers by Author: Ping Xiang Zhang

Abstract: FeSe superconducting bulks were prepared with high energy ball milling (HEBM) aided sintering process, within which process, tetragonal β-FeSe superconducting phase could be formed directly with one step sintering process, and the formation of hexagonal δ-FeSe non-superconducting phase was effectively avoided. The influences of HEBM time on the sintering process of FeSe bulks were systematically investigated. With different HEBM time, the phase composition and morphology of precursor powders changed correspondingly, which thus influenced the final phase composition and superconducting properties of FeSe superconducting bulks. Due to the formation of FeSe bulks with larger tetragonal phase content and higher superconducting transition temperature, HEBM time of 6.0 h was recognized as the optimal parameter. Shorter HEBM time could lead to the insufficient decrease of particle size and low density. While longer HEBM time caused the formation of amorphous hexagonal δ-FeSe, which crystallized during sintering process. Thus no more tetragonal FeSe could be obtained. The FeSe superconducting bulk with the critical temperature T_c(onset) of 8.0 K was obtained with the HEBM time of 6 h, and sintering temperature of 700 ^oC for 12 h.

Abstract: Bi-2212 bulks have been fabricated by spark plasma sintering technique with the Bi-2212 precursor powders synthesized by co-precipitation process. The Al₂O₃ dopants were added into the precursor powders during the sintering process. The lattice parameters of Bi-2212 increased with Al content, suggesting that Al³⁺ ions entering into the lattice as interstitial ions due to the small ion radii. The carrier concentration has been calculated with measured room temperature thermopower values. The change of carrier concentration can be attributed to the changes of both Al and oxygen contents in the lattice. Therefore, by Al doping the samples have been tuned into optimal doping region with maximum critical temperature of 86K. The appearance of Al interstitial ions in the lattice caused certain amount of point defects, which acted as pinning centers. Therefore the flux pinning properties have been enhanced in the bulks, and obvious improvements of critical current density for over 55% have been obtained at 4.2K self-field.

Abstract: A new apparatus is designed and fabricated for rapid heating and quenching (RHQ) process of Nb/Al precursor short wires. As the primary step, Nb/Al precursor with large Nb/Al filaments is manufactured through rod-in-tube (RIT) method to study the RHQ parameters. This apparatus could heat Nb/Al precursor short wire up to about 2000 °C in 10 ms-100 ms and then quench the wire into molten gallium. Results of line scan from SEM-EDS show that Nb₃Al is formed in short wires. Tests show that the zero-resistance temperature of Nb₃Al short wire is about 17.7 K, which also indicates that stoichiometric Nb₃Al is formed. However, the critical current (I_C) is only 1.8 A at 10.2 K. The low I_C can be explained by the low volume ratio of superconducting Nb₃Al and lots of voids along the wire. Further optimization and improvements will be performed.

Abstract: This paper presents our recent activities for the development of low-loss (Bi,Pb)-2223 tapes with interfilamentary resistive barriers. To suppress the side effect on the phase formation in the filaments during sintering, SrZrO3 were selected as barrier materials. Moreover, small amount of Bi-2212 was mixed with SrZrO3 to improve their ductility for cold working. For non-twisted barrier tapes, transport critical current densities Jc at 77 K and self-field were ranged between 18 and 21 kA/cm2 and its uniformity within 4% along a 1 m length. By combining the barrier introduction, reducing the tape width (< 3 mm) and twisting the filaments tightly, coupling frequency fc exceeded 250 Hz even in an AC perpendicular field at 77 K. Transport Jc of the barrier tapes with tightly twisted filaments were in the range of 1214 kA/cm2 at 77 K and self-field. In our knowledge, this is the first result to achieve both Jc > 12 kA/cm2 and fc > 250 Hz simultaneously in an isolated (Bi,Pb)-2223 tape. At 50 mT and 50 Hz, our twisted barrier tapes showed 60-70% lower perpendicular field losses than a conventional 4 mm-width tape with fully coupled filaments.

Abstract: Y123 and Y211 powders were synthesized by solid-state reaction method. YBCO superconductor specimens were prepared by unidirectional solidification technique. The microstructure and the structural characteristics were observed by means of the optical microscope, X-ray diffraction (XRD), scanning electron microscope (SEM). The results showed that the specimen have a highly aligned structure, it was observed obvious laminated structure of Y123 crystal. Crystal growth mechanism of YBCO was investigated. Jackson's factor of Y123 crystal was calculated. Interface form was judged tentatively theoretically. The specimen was rapidly quenched by quick pulling. Solid/liquid interface structure was observed. It was concluded that a-b plane was faceted grow forms and layer growing along c-axial on a-b plane.

Abstract: The effects of BaF2 on thermal behavior and microstructure of melt-processed YBCO superconductors have been investigated. The differential thermal analysis (DTA) results indicated that the peritectic decomposition temperature of YBCO precursor powder was lowered when BaF2 was added (from 1020
to 976
). The peritectic decomposition temperature was lowest at the content of 5 wt.% BaF2, and the variation of melting temperature was not significant above 5 wt.% BaF2. The microstructures of the doped samples have been observed by scanning electron microscopy (SEM). The results showed that the    of BaF2        microstructure  .

Abstract: The powder-in-tube MgB2 wires were prepared by high energy milling of Mg and B powder. The powder was not mechanically alloyed for 2h short milling time. However, the MgB2 grains in wires were very small (20~100nm) and resemble the dimple after post-heat treatment. The clear evidence for transcrystlline fracture was observed. It indicated that the grain connection was greatly improved and the fluxing pinning was significantly enhanced. Another point to view was no intermediate annealing during the whole rolling process. The influence of the post-heat treatment on the transport current density was studied. Despite the lower Tc of about 35K, the transport current density reached to 3×104A/cm2 at 15K and 3.5T for wires sintered at 700°C.

Abstract: Nb3Sn superconductors are widely used in high magnetic field application. Internal tin processed Nb3Sn wires used for ITER coils (at 4.2K, 12T) were heated by two steps, local heat treatment and reaction heat treatments. The superconducting properties of Nb3Sn were investigated as a function of reaction heat treatment (HT) for strands during 625°C~665°C.To study the heat treatment effects on Jcn and n-value of Nb3Sn strands, different HT-parameters, i.e., annealing temperature and times, were applied on the Nb3Sn multifilament strands.

Abstract: Influence of cold rolling total thickness reduction, annealing temperature and annealing time on the textures in powder metallurgic Ni-5at.%W substrate tapes have been quantitatively investigated with the EBSD (Electron Back-Scattered Diffraction) technique. A “copper” type-like texture consisting of {1 2 3}<6 3 –4>+{0 1 –1}<3 –1 –1>+{1 –1 2}<1 –1 –1> has been found in the as-rolled tapes. This texture was much intensive in the tape at 99% thickness reduction than in the tape at 87% thickness reduction. The 99% reduction tape showed a higher fraction and sharper cube texture than the 87% reduction tape after annealing treatment. In addition, annealing at 1000°C was much favourable for the cube texture formation than annealing at 950°C and resulted in a higher cube texture fraction.

Abstract: The Phase formation in the precursory powder, the particle size and the homogenization phase distribution have been investigated in this paper. Size of the powder particle has been reduced by a ball-milling process. The effects of the ball-milling on the characteristic of the powder have been studied by XRD. The modality and distribution of 2212, Ca2CuO3 and CuO in the powder were observed by SEM. Results show that the Ca2CuO3 particle ground was about 200nm in size. The precursory powder with the non-superconducting phases distributed uniformly in the mixture powder was obtained for Bi-2223 long tape.