L. R. Motowidlo

Transport critical current densities of silver clad Bi‐Pb‐Sr‐Ca‐Cu‐O tapes at liquid helium and hydrogen temperatures

Using improved processing conditions, we have succeeded in increasing the transport critical current density (Jc) of powder‐in‐tube fabricated silver sheathed Bi‐Pb‐Sr‐Ca‐Cu‐O superconducting tapes. Jc values at zero field now exceed 1.6×105 A/cm2 (Ic∼164 A) at 4.2 K and 1.3×105 A/cm2 (Ic∼133 A) at 20.2 K for short tape samples. At 77 K the same samples exhibit values greater than 3.0×104 A/cm2 (Ic∼32 A). They continue to carry 6.5×104 A/cm2 at 4.2 K and 4.8×104 A/cm2 at 20.2 K with applied magnetic fields as high as 20 T. Long lengths of tape were also fabricated and measured by winding the composites into small spiral or pancake coils. The small pancake coils carried 4.8×104 A/cm2 at 4.2 K and 5.2×103 A/cm2 at 77 K, zero field. More recently, 1 m lengths have been measured with Jc’s as high as 1.0×104 A/cm2 at 77 K, zero field. Detailed transport measurements at liquid helium and hydrogen temperatures with applied magnetic fields up to 20 T are reported for short and long lengths of monocore tapes.

Processing high critical current density Bi-2223 wires and tapes

Considerable progress has been made in fabricating (Bi,Pb)2Sr2Ca2Cu3O10 in high-Tc superconductor wires or tapes with high critical current densities that are attractive for electric power and high-field magnet applications. The powder-in-tube technique appears to be useful for making silver-clad Bi-2223 composites. This article discusses the processing and the excellent superconducting properties of the resulting wires and tapes.

Multifilament NbTi with artificial pinning centers: The effect of alloy and pin material on the superconducting properties

Low‐temperature superconductors containing artificial pinning centers (APC) have produced record critical current densities in NbTi at magnetic fields below 4 T and promise further improvements at these and higher fields. Peak current densities are achieved when pinning center spacings are matched to spacings of the flux line lattice at the field of operation. The enhancement of Jc through inclusion of artificial pinning materials is accompanied by reduction in Tc and Hc2 through proximity effects. We find that the choice of the superconducting alloy as well as the pin material has marked effect on both the characteristic pinning force Fp and the critical magnetic field Hc2.

Ferromagnetic artificial pinning centers in superconducting Nb0.36Ti0.64 wires

We used ferromagnetic artificial pinning centers in superconducting NbTi wires to achieve a large critical current density (Jc) in a magnetic field. Four wires were fabricated that contained nanometer‐sized arrays of Ni or Fe pins inside micron‐sized filaments of Nb0.36Ti0.64 alloy. A ferromagnetic pin volume of only 2% Ni produced Jc’s (e.g., 2500 A/mm2 at 5 T, 4.2 K) that were comparable to those of commercial wires that have a pin volume of ∼20% Ti. We conclude that ferromagnetic artificial pins are more effective than nonmagnetic pins for a given volume percent.

Limiting factors for critical current densities in Bi2Sr2Ca2Cu3O10‐Ag composite superconducting tapes at elevated temperatures

Detailed measurements of I‐V curves at 4.2, 65, and 77 K as a function of applied magnetic field for two Bi(2:2:2:3)/Ag tapes, having critical current densities Jc of ∼1 and ∼2×104 A/cm2 (at 77 K and 0 T), revealed that (1) the primary limiting factor for the transport critical current densities in these tapes is the presence of a large fraction of weak and/or nonsuperconducting grain boundaries; (2) the observed dissipative voltages are due, however, mostly to the motion of the magnetic vortices in the grains; and (3) the E‐J curves are well described by an expression, E∼exp[−(J0/J)μ], rather than the commonly used power law, i.e., E∼Jn, where μ, J0, and n are constants.

Recent issues in fabrication of high-Tc magnets and long-length multifilament conductors

Abstract After fabricating long lengths (30–100 m) of Ag-clad Bi-2223 tapes by the powder-in-tube process, we co-wound them into pancake-shaped coils by the “wind-and-react” approach. Test magnets were then fabricated by stacking and connecting the coils in series. We then characterized the magnets at the temperatures of liquid helium (4.2 K), liquid neon (27 K), pumped liquid nitrogen (64 K.), and liquid nitrogen (77 K) and in background fields of up to 12 T. A test magnet containing 10 pancake coils generated magnetic fields as high as 2.6 T at 4.2 K. Multifilament conductors of BSCCO-2212 and BSCCO-2223 have also been fabricated in this study. A BSCCO-2212 multifilament conductor was fabricated with both pure silver and dispersion-strengthened silver (Ag-Al 2 O 3 ) as the sheath materials. After final heat treatment, the yield strength of the Ag-Al 2 O 3 matrix was twice that of the pure Ag matrix. At 4.2 K, critical current density of the Ag-Al 2 O 3 multifilament conductor approached 5 × 10 4 A/cm 2 at zero applied field and >2 × 10 4 A/cm 2 at 12 T. This paper addresses certain issues concerning high- T c magnets and long-length multifilament conductors.

THE POWDER-IN-TUBE PROCESSING AND PROPERTIES OF BI-2223

Substantial progress has been made in fabricating long lengths of flexible silver-clad high-critical-temperature superconductor tapes by the powder-in-tube method. Tapes with high critical current density (Jc) that are attractive for electrical power and high-field magnet applications have been produced. At liquid helium (4.2 K) and liquid neon (27 K) temperatures, tapes made by this improved processing method yielded Jc values greater than 100 kA/cm2 at zero field; at liquid nitrogen temperatures (77 K), the Jc values exceeded 40 kA/cm2. This article discusses the processing and properties of short-length tapes and of pancake coils made from long-length tapes via “wind and react.”

Comparative study of Jc‐H characteristics for silver‐sheathed superconducting Bi(2:2:1:2) and Bi(2:2:2:3) tapes

Critical current densities Jc of silver sheathed Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 composite tapes fabricated by a partial melt and a powder‐in‐tube process, respectively, were measured at 4.2, 27, and 64 K as a function of applied magnetic field and the angle between the tape face and the direction of applied magnetic field. These measurements indicate that (1) the fraction of the grain boundaries, which are strongly coupled, are greater in the Bi(2:2:1:2)/Ag tapes than in the Bi(2:2:2:3)/Ag tapes; (2) the alignment of the Bi cuprate platelets with the c‐axis perpendicular to the tape face is significantly greater for the Bi(2:2:1:2) than for the Bi(2:2:2:3)/Ag tapes; and (3) the E‐J curves for both kinds of the tapes are described well by an expression, E∼exp[−(J0/J)μ], where μ and J0 are constants below their transition magnetic fields H*g.

Spatially resolved tunneling spectroscopy of superconducting wires with artificial pinning centers

We used scanning tunneling microscopy and spectroscopy to study with nanometer resolution the spatial variation of superconductivity in the vicinity of the interface between normal and superconductor regions. The samples were novel superconducting wires consisting of ordered arrays of sub-micron diameter normal metal filaments, either Cu or Ni, embedded in a NbTi superconducting matrix. By taking topographic images simultaneously with current–voltage curves, we obtain information about the local quasi-particle density of states on both sides of the interface.

Pinning and vortex lattice structure in NbTi alloy multilayers

We made thin film multilayers of Nb/sub 0.37/Ti/sub 0.63//Nb and Nb/sub 0.37/Ti/sub 0.63//Ti (d/sub NiTi/=14-27 nm and d/sub N/=4-11 nm) to examine geometries and materials relevant to flux pinning in commercial NbTi conductors. Samples were characterized by transport measurements between 4.2 K and T/sub c/, in magnetic fields nearly parallel to the layers, up to 6 T. For some multilayers, pinning forces had a large peak at intermediate fields whose onset occurred near /spl sim/0.2 H/sub c2/. We suggest this peak effect is caused by a change in the vortex lattice structure, driven by the strong intrinsic pinning. We have measured the highest pinning force density (113 GN/m/sup 3/ at 4.2K and 5 T) ever achieved in the NbTi system.