Critical Current Density In Bi Research Articles

Improvement of Granular Critical Current Density in Bi: 2212 Superconductors

Improvement of Granular Critical Current Density in Bi: 2212 Superconductors

Transport critical current density of Bi–Sr–Ca–Cu–O/Ag superconductor tapes with addition of magnetic nanopowder γ-Fe2O3

The effect of sintering temperature on the transport properties of Ag-sheathed-(Bi1.6Pb0.4)Sr2Ca2Cu3O10–(γ-Fe2O3)0.01 superconductor tapes prepared by the powder-in-tube technique with sintering time fixed at 50 h has been investigated. The maximum transport critical current density, Jc of 6490 A/cm2 at 77 K, was observed at sintering temperature of 845 °C. A further single intermediate rolling step increases Jc to 9560 A/cm2. Sintering temperature from 830 to 845 °C increases the 2223 phase content and resulted in improved Jc. At 850 °C, the content of 2223 phase decreased resulting in a corresponding decrease in Jc. X-ray diffraction patterns suggest that the 2212 phase reacts with non-superconducting phase such as CaCuO2, (SrCa)2CuO3, CaO, and CuO to form the 2223 phase. Samples without γ-Fe2O3 prepared under the same condition showed a lower Jc with maximum at 1560 A/cm2. Our results show that nanomagnetic γ-Fe2O3 addition improved Jc which supports previous calculations on the possibility of frozen flux superconductor with nanomagnetic addition in this class of materials.

Determination of the critical current density of Bi(2223)/Ag tapes from magnetic measurements: peculiarities and problems in the interpretation

Multifilamentary Bi(2223)/Ag tapes, prepared by the powder-in-tube technique, have been characterized by SQUID magnetometry at temperatures between 4.5 K and 85 K with the magnetic field applied both parallel and perpendicular to the conductor. By analysing measurements for H|| tape we evaluate the current density versus temperature and we compare it with that obtained by transport measurements. In such a way we clearly specify a temperature domain where the current density is limited by the grain boundaries and, on the contrary, where the current density is limited by the intrinsic properties of the material. The temperature dependence of the intragrain current density of the Bi(2223) phase has also been estimated. By analysing measurements for H|| tape the current density anisotropy and problems in the interpretation of the measurements related to the intrinsic grain misalignment are discussed.

Effect of the phase compositions at the final stage of heat treatment on the critical current density in Bi:2223/Ag tapes

The effect of the phase compositions after the final stage of heat treatment on the critical current density in Bi:2223/Ag tapes has been investigated. A high volume fraction of Bi:2223 has been considered the most critical factor in achieving a high ; however, it is extremely difficult to attain pure Bi:2223 phase via routine thermomechanical processing. Although a higher volume fraction of Bi:2223 phase can be attained using a higher processing temperature, it is always found to co-exist with Bi:2201 which resides at Bi:2223 colony boundaries. As these possess a plate-like morphology, the Bi:2201 joins weak links in a conductible chain. Bi:2201 can be eliminated using an annealing step at the end of the last thermal cycle, resulting in a significant improvement in . At annealing temperatures below , Bi:2223 decomposes into Bi:3221 and other cuprates, which degrade further. Magnetization, AC susceptibility and resistivity measurement results confirm that the Bi:2201 and Bi:3221 act as weak links.

Phase formation and critical current density in Bi,Pb(2223) tapes

The various fabrication and reaction processes leading to multifilamentary Bi,Pb(2223) tapes with high transport are described. In air, the Bi,Pb(2223) phase in pressed powders is found to be formed by nucleation, following the complete decomposition of the Bi,Pb(2212) intermediate phase. In this work, the precise formation conditions of the Pb-free phase Bi(2223) are described for the first time. It is shown that the presence of Pb leads to a lowering of the melting temperature from 879 to and of that of the `transient liquid' . The latter is formed after decomposition of the 2-layer compound and is necessary to the formation of the 3-layer compound. Thus Pb is found to have no influence on the formation mechanism of the 3-layer compound, which always starts by a nucleation process. Nucleation processes are also found to occur in the first phase of reaction inside Ag sheathed tapes, intercalation being possible at later stages. The highest values of the critical current density in Bi,Pb(2223) tapes reported at the present day are compared, taking into account the analysed tape lengths. The variation of in mono- and multifilamentary Bi,Pb(2223) tapes as a function of the applied magnetic field at 77 and at 4.2 K is described. A new process including deformation by a four-roll machine is presented, and the distribution of inside various tape configurations with similar critical current densities is analysed: for monofilamentary tapes, a maximum of is found at the borders, the ratio being 2.4, while for conventional multifilamentary tapes the maximum is found at the tape centre (, ratio 1.6). For the new four-rolled tapes, a ratio of only 1.1 was measured, indicating a more homogeneous current distribution. In order to lower the AC losses due to the highly conductive Ag matrix, the new concept of `oxide barriers' is introduced. A barrier of surrounds each filament, thus leading to an enhanced radial resistivity, which in turn reduces AC losses. Finally, the current transport mechanism inside Bi,Pb(2223) tapes and the various current limiting mechanisms are discussed.

Effect of Sb doping on critical current density in Bi (Pb)-Sr-Ca- Cu-O high- Tc superconductors

In this paper we report our results on the effect of Sb-doping on the superconducting properties of the compound Bi 1.6- x Sb x Pb 0.4Sr 2Ca 2.2Cu 3.5O y for x=0.0, 0.1 and 0.2. Through a series of experiments it is shown that the small addition of Sb ( x=0.1) increases the 2223 phase fraction and enhances T c as well as intergrain J c, provided that the sintering parameters are carefully optimized. Thus, for example, the composition Bi 1.5Sb 0.1Pb 0.4Sr 2Ca 2.2Cu 3.5O y sintered at 830°C for 12 h has a T c0 (zero resistivity) = 108 K, a drop in the χ'- T plot at 108 K and in intergrain J c=695 A/cm 2 (as estimated on the basis of Clem's model). Any lowering of sintering temperature below 830°C or prolonging of sintering at 830°C leads to a degradation in the superconducting parameters. Similarly, a reduction in T c and J c is observed if Sb concentration is increased to 0.2. The sintering parameters (temperature and duration) thus appear to be critical for a given composition.

Strength and critical current density of Bi(Pb)SrCaCuO and YBaCuO in silver-sheathed superconducting tapes

The critical current density at 77 K at 0 T and the tensile strength at room temperature of high T c Bi(Pb)SrCaCuO and YBaCuO oxides in silver-sheathed tapes prepared by the powder-in-tube method were investigated. The main results can be summarized as follows. (1) The critical current density and the strength of both oxides increased with decreasing thickness. (2) The strength of the Bi(Pb)SrCaCuO oxide was 120 MPa at most, being comparable to that of the YBaCuO oxide, but far lower than that of Nb 3Sn compound. (3) When the oxides were weak, the higher the strength, the higher was the current density in both YBaCuO and Bi(Pb)SrCaCuO tapes. The textured structure in the Bi(Pb)SrCaCuO tape with thin oxide raised current density effectively, while it raised the strength only slightly.

Effect of Extrusion on the Critical Current Density of Bi1.4Pb0.6Sr2Ca2Cu3Ox Superconducting Wires

The effect of extrusion on improving the critical current density (Jc) of Bi1.4Pb0.6Sr2Ca2Cu3Ox superconducting wires is investigated. Calcined powders (Bi1.4Pb0.6Sr2Ca2Cu3Ox) are first mixed with a forming aid—a thermoplastic polymer (polyethylene)—for workability, and then extruded, using a capillary rheometer, to form wires 2 mm in diameter. The Jc value, measured by the four‐probe method in liquid nitrogen at 77 K, is improved substantially by the following process: (1) the superconducting precursors are extruded at high viscosity with a forming aid, to align the platelike particles unidirectionally; (2) the forming aid alone is carefully burned out, without destroying the extruded configuration; and (3) the extruded wires are annealed at 850°C in air for more than 96 h.

Temperature-dependent critical current density of Bi(Pb)-Sr-Ca-Cu-O tapes in fields up to 20 tesla

The transport critical current density (Jc) of silver-sheathed polycrystalline Bi(Pb)-Sr-Ca-Cu-O (BPSCCO) tapes has been measured at approximately 10 K intervals from 4.2 K up to 77 K in fields of 1-20 T. The tapes were produced by swaging, drawing and multiple final pressing and sintering steps, which developed a preferred orientation of the basal planes of the predominantly 2223 BPSCCO phase within the tape Jc values greater than 104 A cm-2 at 4.2 K were obtained, and these decreased smoothly as the field and temperature were increased. No evidence of any catastrophic change in Jc(H) was seen at any temperature, thus suggesting that flux line lattice melting is not controlling the Jc of these tapes.