K.H. Song

The interaction of Ag with Bi-Pb-Sr-Ca-Cu-O superconductor

Abstract Bi-Pb-Sr-Ca-Cu-O superconductor compounds have been doped with up to 30 wt% Ag, sintered under variable oxygen partial pressure, and characterised in terms of the electrical and crystallographic behaviour. In contrast to previous reports that claim that Ag is the only metal non-poisoning to the superconductivity of Bi-Sr-Ca-Cu-O (BSCCO), it has been found that Ag additions to Bi-Pb-Sr-Ca-Cu-O depress T c and J c drastically and cause a large decrease in lattice parameters when samples are treated in air or pure oxygen. However, the lattice parameters, T c and J c remain unaffected by Ag additions when samples are heat treated in 0.030–0.067 atm oxygen. It is clear that the Ag reacts with and destabilises the superconducting phase when the samples are treated in air or pure oxygen while, when the samples are heat treated in low oxygen partial pressures, the Ag remains as an isolated inert metal phase that improves the weak links between the grains. This discovery clearly shows the feasibility of Ag-clad superconductor wire. For Ag-clad superconductor tape of 0.1 mm 2 cross sectional area heat treated in air, J c was measured to be 54 A/cm 2 . The same specimen sintered in 0.067 atm oxygen showed that the J c increased to 2078 A/cm 2 .

Superconductivity in a Ag‐doped Bi‐Pb‐Sr‐Ca‐Cu‐O system

For the Bi‐Pb‐Sr‐Ca‐Cu‐O (BPSCCO) system, a consistent set of data for Tc, x‐ray diffraction, and scanning electron microscopy shows that Ag additions strongly react with BPSCCO to destabilize the 110 K superconducting phase, resulting in a strong depression in Tc and lattice parameters when samples are treated in air or pure oxygen. This is in contrast to the behavior for the Y‐Ba‐Cu‐O and Bi‐Sr‐Ca‐Cu‐O systems, which do not show such degradation. However, Ag additions show no effects on Tc and lattice parameters when samples are treated under low oxygen pressure. The formation of a low‐melting eutectic liquid with Ag2O‐PbO‐CuO solid solution affects the composition of the superconducting phase and degrades superconductivity, while the eutectic may be suppressed by reducing the oxygen partial pressure.

Critical current density in superconducting Bi-Pb-Sr-Ca-Cu-O wires and coils

The critical current density (Jc) of Ag-clad Bi-Pb-Sr-Ca-Cu-O wire has been measured to be 11900 A cm-2 at 77 K in a zero field. A coil of diameter 35 mm made from the wire had a Jc of 1913 A cm-2 at 77 K over the full length (one metre) of the coil. The high Jc is attributed to a combination of elimination of the poisoning effect of Ag on superconductivity through optimal heat treatment, crystal alignment through cold rolling, and enhancement of flux pinning through chemical doping and processing of high-purity powder by freeze drying. The critical current followed a power law of (1-T/Tc)3/2 in the temperature range near Tc, indicating that the weak link structure acts as a proximity effect junction.

Improvement of critical current density in the Bi-Pb-Sr-Ca-Cu-O system through hot isostatic pressing

Abstract The effects of hot isostatic pressing (HIPing) on densification and the superconducting properties of the Bi-Pb-Sr-Ca-O system were investigated. A relative density up to 95% and a critical current density greater than 1100 A/cm 2 at 77 K were achieved through HIPing at 650°C for 2 h under 200 MPa argon. Under these conditions, the product gave a value of J c four times that without HIPing. T o was unaffected by HIPing for samples encapsulated with a combination of glass and silver tubes. However, T o was suppressed from 103 K to 86 K for samples encapsulated with stainless steel and silver tubes while J c increased from 260 A/ cm 2 to 1086 A/cm 2 during HIPing, indicating that the weak links were significantly improved. A new minor phase, having a composition of Bi/Pb/Sr/Ca/Cu=0.58/2.8/3.0/2.1/1.1, was observed in HIPed samples but its effect on J c is not clear.

Magnetic properties of Bi-Pb-Sr-Ca-Cu-O fabricated by various techniques

Abstract AC susceptibility measurements and critical current densities for Bi-based superconductors fabricated by normally sintering powder in the tube technique, hot-pressing and hot isostatic pressing, are presented and discussed. It is argued that the J c of the normally sintered sample is limited by intergrain weak links, while the J c of the mechanically deformed samples is determined primarily by flux pinning. It is shown that the pinning force density is significantly increased through mechanical deformation. For Ag-sheathed tapes and hot-pressed samples, the J c drops 80% when the magnetic field is increased from ambient earth field to 0.5 T, whereas that for the normally sintered samples decreases by two orders in a magnetic field of only 0.01 T. Dislocations may act as effective pinning centres for flux lines in HIPed samples. Uniaxial pressing produces an excellent grain alignment, which allows the crystalline anisotropy to be utilized to strengthen flux pinning. The planar defects along a - b direction may induce internal Josephson weak links leading to a reduction of the J c in the c direction, but not in a - b direction. In the a - b direction, it is believed that the planar defects provide effective pinning. Utilisation of both planar defects and crystalline anisotropy may provide a valuable route towards the goal of high critical current densities.

Superconducting properties of Au/Bi-Pb-Sr-Ca-Cu-O composites

The superconducting properties of the Au-doped Bi-Pb-Sr-Ca-Cu-O (BPSCCO) system have been investigated. It was found that while silver severely degraded the superconductivity of BPSCCO samples heat treated in oxygen and air, gold exhibited nonpoisoning behaviour under the same conditions. Ag acts to form a Ag2O-PbO-CuO liquid eutectic and destabilises the superconducting phase when the samples were treated in oxygen or air, while Au remains an isolated phase in the composites. This result contradicts similar work in which Au was found to degrade the superconductivity of the Bi-Sr-Ca-Cu-O (BSCCO) system, while Ag was found to be nonpoisoning to BSCCO when samples were treated in oxygen and air.

Effect of silver addition on superconductivity in the Bi1.6Pb0.4Sr1.6Ca2Cu3O10−y system

Silver/superconductor composites containing 0 to 80 vol% silver have been prepared and their properties determined. Optimum heat treatment at sintering temperatures (∼ 800° C) under low oxygen pressures produces material with high critical current density and improves physical properties. Magnetic susceptibility measurements have been found to be consistent with resistivity results. In order to retain a single high-Tc phase with increasing silver content, decreased oxygen partial pressures are required. Using the normal-state resistivity of these composites, a percolation threshold at a silver volume fraction of ∼43% was observed, while zero resistivity measurements show that a continuous superconducting network can be obtained with up to ∼80 vol% silver. The critical current density of 21 vol% silver-doped samples was found to be 1520 A cm−2 at 77.3 K, compared to 260 A cm−2 for an undoped sample.

Magnetic susceptibility and electrical transport properties of hot deformed silver/(Bi,Pb)2Sr2Ca2Cu3O10°x composites

Abstract Bi 1.6 Pb 0.4 Sr 1.6 Ca 2 Cu 3 O 10+x and Bi 1.8 Pb 0.4 Sr 2 Ca 2.2 Cu 3 O 10+x were hot pressed at 750°, 800°, and 840°C at 50 MPa for 2 h in air, followed by post-annealing at 840°C for 50 h in air. Sinter forging of the former composition with 5, 10, and 15 wt% Ag added was performed at 800°C at 50 MPa for 2 h under 0.067 atm O 2 (N 2 balance), followed by post-annealing at 840°C for 50 or 100 h under 0.067 atm O 2 . The samples were studied using resistivity-temperature (T c ), critical current density (J c ), and a.c. magnetic susceptibility (χ′,χ″) measurements. For the sinter forged samples, the temperature of zero resistance (T o ) decreased with increasing Ag levels. However, after post-annealing for 100 h, the T o rose to 100 K irrespective of Ag levels. For the hot pressed samples, improvements in all of the properties were observed. The improvements may be attributed to the combination of improved densification and alignment.

Superconductor/Silver Composites and Tapes Based on Bi-Pb-Sr-Ca-Cu-O

High-J c , bulk composites, containing 10–80 vol%Ag, and silver-sheathed tapes, 100 µm thick, were fabricated using the (Bi1.6Pb0.4)Sr2Ca2Cu3O10−y compound. A process of pressing and rolling with intermediate sintering was used to promote grain alignment (up to 90% texture) and improve flux pinning. AC susceptibility measurements on the composites show improvement of flux pinning with 20–30 vol%Ag, giving maximum J c values of 1600 A cm−2 at 77 K. The resistivity of composites decreased in the range 10−3–10−6 ohm cm with increasing silver content. J c in tapes depended on phase purity and the degree of c-axis texture. The best tapes had J c = 6.5 × 103 A cm−2 (77 K) in zero field, decreasing with field to ~103 A cm−2 for H⊥c-axis and to ~30 A cm−2 for H||c-axis at 0.4 T. The temperature dependence of J c was fitted by the flux creep model using pinning energy U(0) ≃ 90 meV.

Chemistry of Bismuth-Based High-Tc Superconductors

Abstract The oxidation states of Bi, Pb, and Cu in the Bi-Pb-Sr-Ca-Cu-O (BPSCCO) system have been determined by a combination of volumetric measurement technique and iodometric titration. It was found that, in contrast to previous reports, the concentration of the Cu3+ ions decreased with increasing Pb content, and Cu3+ ions were absent in samples of Bi1.6Pb0.4Sr1.6Ca2Cu3O9.8, while a Tc at 108 K and a Jc of greater than 12, 000 A/cm2 at 77 K were observed. In Pb-doped materials, Bi appears to be trivalent while Pb was determined to be mixed-valence Pb4+ /Pb2+. It is suggested that superconductivity in (BPSCCO) may result from a dynamic transfer of holes from Bi/Pb-O layers towards Cu-O2 planes.