P.J. Kung

Critical-current anisotropy, intergranular coupling, and effective pinning energy in Bi2Sr2CaCu2O8+δ single crystals and Ag sheathed (Bi,Pb)2Sr2Ca2Cu3O10+δ tapes

Abstract Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals and Ag sheathed (Bi,Pb)2Sr2Ca2Cu3O10+δ (Bi-2223) tapes have been characterized by magnetometry and electron microscopy. Dislocations on the (001) planes appear to be the main defect bserved in these samples and are considered to be important for flux pinning. Their distribution is more homogeneous in the Bi-2223 tape as a result of thermomechanical processing. In Bi-2212 single cyrstals, the measurement of magnetic hysteresis reveals a strong anisotropy in the magnetization critical current density (Jcm). For the two cases H | c and H ⊥ c, the measurable Lorentz forces (FL) that limit Jc are found to be perpendicular to the c-axis; one causing vortices to hop in the ab plane and the other to slide along the ab plane. In contrast to this, we have probed the relative strength ratio of the intrinsic pinning to the extrinsic pinning in Bi-2223 tapes when we compare the results of H | c with H ⊥ c. No significant difference in Jcm is observed, but for the same average J value, the effective pinning energy (Ueff) calculated from the time derivative of the magnetization (M) for H ⊥ c is two orders of magnitude higher than that for H | c. For these two superconductors, their Ueffs follow a power-law expression, Ueff(J, T, H) = (Ui/Hu)[1 −(T/Tx)2]1.5(Ji/J)μ, where Ui is the scale of activation energy, n is of the order of 1, Tx is related to the magnetic irreversibiklity temperature (Trrr), and both Ji and μ depend on the flux-bundle size and the value of J/Jc. A two-dimensional (2D) flux-line lattice (FLL) is clearly identified in the Bi-2212 single crystals; however, imperfect texturing in the Bi-2223 tape smears out to some extent the observed J dependence of Ueff. Consequently, a larger anisotropy in the normalized relaxation rate, S = − d ln M/d ln t, and much higher S values are observed in Bi-2212 than in Bi-2223.

Granularity, microstructure, and flux creep in TlBa2Ca2Cu3O9+δ superconducting powder and silver-sheated tapes

Abstract Silver-sheathed TlBa 2 Ca 2 Cu 3 O 9+δ (Tl-1223) tapes, with a transport critical current density, J ct , of 6200 A/cm 2 at 75 K under zero magnetic field, were fabricated by the oxide-powder-in-tube (OPIT) method and characterized using an X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and superconducting quantum interference device (SQUID) magnetometer. The results of the magnetization and SEM study indicate that, in these tapes, individual grains are distributed randomly in orientation and weak links exist. However, strongly linked percolative current paths within the tape persisting in increasing fields, accompanied by strong intrinsic interplanar coupling, sustain a significant J ct at high fields and lead to the plateau in the J ct - H curve. Dislocation networks, which may act as effective pinning centers, are the common features observed in the tapes. To investigate the effects on flux pinning due to thermomechanical processing, magnetic relaxation at 1 and 2 T over 5–50 K was measured. The tape shows slightly lower normalized relaxation rates ( S =-(1/ M o )d M / dln t ) than the cauliflower-like precursor powder. Analyses of the relaxation data obtained from the tape, after incorporating temperature dependence and field scaling, yield an expression for the effective pinning energy U eff ( J , T , H ) = ( U 1 / H 0.54 )[1-( T / 72.5) 2 ] 4 ( J / J i ) μ . This result was compared with the prediction of the collective flux-creep model, which suggests that Tl-1223 has a three-dimension-like (3D-like) vortex lattice. Presumably, a more plate-like powder morphology may result in improved texturing by the OPIT process. Tl 0.5 Bi 0.5 Sr 2 CaCu 2 O 7+δ ((Tl,Bi)-1212) powder with this morphology was therefore synthesized for comparison.

Transport and magnetization currents in BSCCO/Ag tapes

A (Bi,Pb)SrCaCuO-2223/Ag tape has been produced by thermomechanical processing. Transport critical current densities, J/sub c/s, have been measured between H=0 and 7 T and T=7 to 75 K. Magnetization currents have been determined for similar temperatures and fields (0 to 5 T) oriented parallel and orthogonal to the thickness of the tape. Dimensional scaling arguments show that flux penetrates most quickly along the tape thickness direction (for fields parallel to the tape plane and therefore the ab planes of the textured BSCCO grains). This implies that the anisotropy ratio of J/sub c/ and J/sub ab/ is reduced in tapes as compared with single crystals, which is consistent with predictions of the brick wall model. The H dependence of the magnetization currents and effective pinning potentials were also measured for similar field orientations. Fast magnetic relaxation causes an exponential field dependence of transport and magnetization currents. Temperature dependences for H/sub 0/ for transport and magnetization measurements are interpreted in terms of different time scales (or voltage criteria).<<ETX>>

Characteristics of TlBa2Ca2Cu3O9+δ powder as-synthesized and after grinding

Crystal structure and superconductivity of the Tl--Ba--Ca--Cu--O powder prepared by a solid-state reaction were studied. The results of x-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses indicate that the powder of a major single 1223 phase was synthesized with a cauliflower-like morphology; by grinding, the powder was partially transformed to an amorphous phase. The measurement of magnetic hysteresis was also performed in the temperature range of 7--75 K up to 5 T from which the weak field-dependent behavior of critical current density was observed. The as-synthesized powder, with [ital T][sub [ital c]]=110 K measured from magnetometer and susceptometer, is considered to consist of weak-linked regions. The results obtained from comparing the as-synthesized and the ground powder imply that in the Tl-1223 system, the Ag-sheathed tapes fabricated by the powder-in-tube process may be benefited by forming other intermediate phases with plate-like morphologies to give better densification or grain alignment.

Transport and magnetization critical densities in TlBa/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tapes

The powder-in-tube process was used to produce silver-sheathed tapes of TlBa/sub 2/Ca/sub 2/Cu/sub 3/O/sub 8+x/ (Tl-1223). The powder was produced by thalliating a precursor powder mixture to produce the Tl-1223 phase and then heating to drive off excess Tl and reach the Tl-2223 stoichiometry. The tapes were rolled and pressed, each step followed by a 3-h sintering. The 200- mu m-thick tapes show little sign of texturing; however, the critical current shows a small ( approximately 50%) dependence on the direction of the applied magnetic field. Both transport and magnetization measurements indicate relatively strong pinning at high temperatures. The 75 K self-field critical current density is 62 MA/m/sup 2/. Transport measurements reveal the presence of weak links at all temperatures, but with a relatively weak field dependence above approximately=0.1 T.<<ETX>>

Magnetic hysteresis and flux creep of YBa/sub 2/Cu/sub 3/O/sub x/ grown by the melt-powder-melt-growth (MPMG) process

Magnetic hysteresis and flux creep of melt-powder-melt-growth (MPMG) YBa/sub 2/Cu/sub 3/O/sub x/ containing nominal 0, 25, and 40 mole% Y/sub 2/BaCuO/sub 5/

Flux Creep of Melt Processed YBCO

Flux creep measurements are very informative in understanding the pinning characteristics of type II superconductors. In many creep experiments of high temperature superconductors, logarithmic time decay of critical current or magnetization is observed. This seems to fit a very simple flux creep model based on thermally assisted flux motion. However, the pinning energies obtained from simple Arrhenius plots do not fit the theoretical model that the pinning energy increases with decreasing temperature. This mismatch is ascribed to the fact that the effective pinning barrier is not linear to the current. In this paper, we measured flux creep of three melt processed YBCO with different Jc values under various conditions and found that the effective pinning barrier (Ueff) is not proportional to 1-(J/Jc). It has also been found that if we draw entire Ueff-J curves, the results of flux creep measurements could clearly be explained in terms of flux pinning and were consistent with the idea that the pinning energy is larger in the samples with larger Jc values.

Phase evolution and superconductivity emergence in repetitively annealed Bi-Sr-Ca-Cu-O thin films

Abstract Bi(Pb)-Sr-Ca-Cu-O superconducting thin films on MgO (100) substrates have been made by laser deposition in combination with repetitive post-annealing treatments. The films of nominal composition (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ were deposited at 400°C and then heated in air at a rate of 75–80°C/min, held at 845–847°C for 10 min, quenched to 600°C at a rate of 80°C/ min, and slowly cooled at a rate of 3.3°C/min to room temperature. The Bi 2 Sr 2 CaCu 2 O 8+δ (2212) phase was found to crystallize first in these as-deposited amorphous films, whereas the (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ (2223) phase was grown in large quantities by repeating the annealing cycle. With this protocol, the 2223 phase is developed more effectively than that observed in the long period of annealing, and the formation of pinholes on the surface is significantly reduced. Surface morphologies examined by scanning electron microscopy (SEM) are improved for films repetitively annealed under reduced pressure conditions. The results of magnetic susceptibility measurements on these films correspond well with the phase behavior seen by X-ray diffractometry (XRD) analyses. The transport critical current densities, T c , of the film are 9.0×10 5 and 5.5×10 4 A/cm 2 at 5 and 40 K, respectively. The magnetization critical current densities derived from M ( H ) data at 6 K are two orders of magnitude higher in the orientation of H ⊥ c -axis than of H ‖ c -axis. As a preliminary study, XRD line-broadening analysis was performed to investigate the effects of lattice strain and crystallite size in the set of films that were annealed at various temperatures for 3–6 h. The microstrain values are in the range of 10 -4 -10 -3 , and the crystallite sizes in films obtained for the 2212 and 2223 phases depend on annealing time, temperature, and cooling rate. Microstrain associated with the 2223 phase is more sensitive to annealing parameters. These results suggest that XRD line-broadening analysis may be a valuable method to study the growth mechanism and material properties of the repetitively annealed films.