M.H. Apperley

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.

Microstructure and flux pinning in superconducting Bi-Pb-Sr-Ca-Cu-O wires

Abstract The critical current density ( J c ) of Ag-clad Bi-Pb-Sr-Ca-Cu-O wire has been measured to be 1.2×10 4 A/cm 2 at 77 K in zero field. The high J c is attributed to a combination of elimination of the poisoning effect of Ag on superconductivity, grain alignment, and enhancement of flux pinning. J c - H dependence was significantly improved in the Ag-clad tape, which has a J c of 1.0×10 3 A /cm 2 at 77 K and 4000 Oe, while the J c of the sintered pellet drops two orders of magnitude at only 100 Oe. A pronounced anisotropy in J c under high magnetic field is attributed to the grain alignment. Planar defects, such as heavy stacking faults parallel to the a - b plane in the rolled tape, are considered to be effective pinning centres.

Optimization of processing to improve critical current density of Ag/Bi-2223 tapes

Several recent developments in powder-in-tube (PIT) processing are presented. A cryogenic deformation process has been developed, involving rolling or pressing the wires and tapes in supercold conditions, such as in liquid nitrogen. Cryogenic deformation has been found to improve the density, grain alignment and Ag-oxide core interface and to increase dislocation density, thereby enhancing and flux pinning. By incorporating Pb into Bi-2212 phase the sintering temperature can be raised above , resulting in a significant reduction of total sintering time from several hundred hours to 100 h. Recently, a new process to eliminate the decomposition and recovery of Bi-2223 during cooling and heating has been developed that further reduces the heat treatment time for Ag/Bi-2223 tapes to 20-30 h, with and Bi-2223 volume fraction in the tapes comparable with those in tapes treated for 120 h. A two-stage annealing procedure in the final thermal cycle has been used to eliminate residual amorphous phase and Bi-2201, which has been identified to be one of the major causes of weak links in PIT tapes. By incorporating hot deformation in a two-step process not only can Bi-2201 be eliminated, but also texture and density are improved, resulting in a very high at 77 K in multifilamentary tapes.

Phase and texture formation in Bi-2212/Ag tapes processed in oxygen

Bi-2212 phase formation was studied in silver-clad tapes processed by melt-texturing in an oxygen atmosphere. Stoichiometric Bi-2212 was found to undergo a peritectic decomposition, which starts at , into a Bi-rich liquid and two alkaline-earth cuprate solid phases: and . This phase assemblage remained unchanged up to , where the solid phase crystals were found to have a random orientation. Some Ag was found to dissolve into the liquid, and concentrate around the 14:24 crystals. On cooling, at , the Bi-2212 crystals randomly nucleated from the liquid. At , the Bi-2212 crystallite size was in the range of 0.1 to 0.4 . The phase assemblage consisted of Bi-2212 as the major phase, and Cu-free and trace 14:24 crystals as the minor phases. The 14:24 crystals were highly oriented relative to the silver interface. As the minor phases diminished, the texture of Bi-2212 crystals, having the c axes perpendicular to the silver-core interface, improved. These observations suggest that Bi-2212 crystals nucleate from the liquid on the surface of the 14:24 phase as well as at the silver-core interface, and the dissolved silver in the liquid may play an important role.

Crystallite alignment of YBa2Cu3O7-x through texture growth

Critical current densities of superconducting YBa2Cu3O7-x can be increased by several orders of magnitude by sintering processes that encourage textured growth of microcrystals from the melt. Alignment parallel to the a-b planes maximises the conduction within grains, reduces grain boundary resistivity, facilitates oxygen diffusion through vacancies in the a-b planes, and eliminates microcracking resulting from anisotropic thermal expansion. Measured critical current densities in excess of 2400 A cm-2 at 77 K are limited not by the specimen but by ohmic contact resistance.

Fabrication of Ag-sheathed Bi-superconducting tapes and coils

Ag-clad (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+z/(2223) multifilament tapes up to 46 meters have been fabricated using powder-in-tube technique. A I/sub c/ of 15 A at 77 K over the entire length has been achieved with a fluctuation along the length less than 10%. Coils up to 306 turns with a I/sub c/ of 45.6 A at 4.2 K were produced through reaction and winding procedure using multifilament tapes. J/sub c/ versus phase formation, degree of texture and grain growth of Ag-sheathed Bi-based superconducting tapes have been investigated during thermomechanical processes. The initial modest increase in J/sub c/ is attributable to the increase of 2223 phase and grain alignment, while in second stage grain growth is mainly responsible for a rapid increase of J/sub c/. Prolonged annealing in the third stage results in recrystallisation of the large grains, and hence a decrease in J/sub c/. A I/sub c/ of 8 A at 77 K and 67 A at 4 K and zero field has been achieved for Ag-clad Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+z/(2212) tape using a partial melting and atmosphere-controlled process.<<ETX>>

On using tape to make a toroidal coil

High- superconducting (HTS) multifilamentary tape, was used to prepare an HTS toroidal coil. Magnetic field behaviour of the HTS coil is studied for this toroidal configuration. The magnetic field distribution of the HTS coil is investigated for use at 77 K and 4.2 K with the anisotropic HTS wire having a strong magnetic field dependent critical current. Since the magnetic field component along the HTS wire c-axis is significantly reduced in the form of a toroid, it is suitable to make a high-ampere-turn or high-inductance coil with the present HTS tape having an anisotropic transport property.