Normal Metal Matrix Research Articles

Effective electrical and thermal conductivity of multifilament twisted superconductors

The effective electrical and thermal conductivity of composite wire with twisted superconducting filaments embedded into normal metal matrix is calculated using the extension of Bruggeman method. The resistive conductivity of superconducting filaments is described in terms of symmetric tensor, whereas the conductivity of a matrix is assumed to be isotropic and homogeneous. The dependence of the resistive electrical conductivity of superconducting filaments on temperature, magnetic field, and current density is implied to be parametric. The resulting effective conductivity tensor proved to be non-diagonal and symmetric. The non-diagonal transverse–longitudinal components of effective electrical conductivity tensor are responsible for the redistribution of current between filaments. In the limits of high and low electrical conductivity of filaments the transverse effective conductivity tends to that of obtained previously by Carr. The effective thermal conductivity of composite wires is non-diagonal and radius-dependent even for the isotropic and homogeneous thermal conductivities of matrix and filaments.

Correlation between the extraordinary Hall effect and resistivity

We propose to reconsider the correlation between the extraordinary Hall effect and resistivity by using the skew scattering model and Matthiesen's rule to separate contributions of different scattering sources. The model has been experimentally tested for the cases of scattering by magnetic nanoparticles embedded in normal-metal matrix, insulating impurities in magnetic matrix, surface scattering, and temperature-dependent scattering.

Anomalous magnetization of nanoscale ferromagnet/normal-metal systems: Possible evidence of the electronic spin polarization

The magnetization of two nanoscale ferromagnet/normal-metal systems, random mixtures of Ni-Ag and arrays of Co clusters embedded in a Pt matrix, have been studied down to 0.3 K by means of conventional magnetometry and by the extraordinary Hall effect measurements in fields up to 17 T. Unusually high values and an anomalous temperature dependence of the saturated magnetization in the low temperature limit have been found in both systems. We suggest that the phenomenon is evidence of the temperature dependent spin polarization of the conducting electrons in the normal-metal matrix.

Effect of magnetic impurities on electronic transport of heterogeneous ferromagnets

Electronic transport in granular Co-Ag films is studied between subhelium and room temperatures in fields up to 16 T. A number of unusual properties are found including a strong variation of magnetoresistance at low temperatures and sublinear ${(d}^{2}{R/dT}^{2}l0)$ temperature dependence of high field resistance above 50 K. We show that paramagnetic impurities or small magnetic clusters embedded in the normal-metal matrix influence significantly the electronic transport in heterogeneous ferromagnets. The effect is strongly enhanced as compared with paramagnetic impurities in homogeneous normal metals, and is comparable and even superior over the phonon scattering up to room temperatures. This mechanism is argued to be responsible for the peculiar properties of resistance and magnetoresistance of granular ferromagnets.

Comparison of transport and magnetic AC losses in Bi-2223/Ag tapes — the role of superconducting core geometry

AC losses were measured by 4-probe transport method and by external magnetization method in three samples of Bi-2223/Ag tape: a multifilamentary tape with separated filaments, another multifilamentary tape with `bridges' between filaments, and a two-shell tape. The transport losses agreed with those calculated using I c from DC experiment. Magnetization experiments gave indications about the various paths of induced currents. For the tape with well separated filaments the main part of screening current closes inside individual filaments. Additional screening of the whole filamentary zone involves the normal metal matrix, leading to frequency dependent losses. In the case of tape with `bridged' filaments, supercurrents interconnect the filaments into bundles whose screening (and loss) is frequency-independent. Matching the experimental data indicates that a typical bundle was composed of 8 filaments. Magnetic losses of the two-shell tape were explained by a model for magnetization of superconducting wire with elliptical cross-section.

Effect of intensive flux creep on voltage-current curves and thermal stability of high T/sub c/ composite conductors

Basic dissipative and voltage-current curves of high-T/sub c/ conductors with normal metal matrix are obtained directly from the description of thermally activated flux creep. The effect of different criteria for j/sub c/ is discussed, and current sharing between superconducting and normal layers is analyzed. A comparison with the critical-state model is given. Intensive creep is shown to change considerably the real characteristics of high-T/sub c/ composites, increasing their thermal stability. >

Recent developments in superconducting conductors

Industrial development of superconductors concentrates mainly on the two materials NbTi and Nb 3Sn which can be produced in long lengths of many thin filaments in a normal metal matrix. A review of the state of art in manufacturing techniques and in current carrying capacity is given. Recent results in strand design and development are presented. Fusion magnets require large transported currents (30–50 kA) which implies to assemble a given number of wires in the form of a large size cable. Particular emphasis is put on special problems, requirements and trends in the present development of conductors.

Influence of filament spirality on current-voltage characteristics of twisted multifilamentary NbTi superconductors

It is shown experimentally, that the current-voltage characteristics of a twisted NbTi multifilamentary superconductor placed in a perpendicular magnetic field differ from those of untwisted superconductors. It was explained by the current transfer across the normal metal matrix due to the inhomogeneous electric field distribution along each filament. Electric field inhomogeneities along each filament arise as a result of varying the angle between the external magnetic field and filament axis. Several model conductors were prepared to investigate the role of the normal metal matrix resistivity and the geometrical arrangement of filaments. A two-dimensional theoretical model explaining the measured characteristics is also presented.

Composite conductors based on the superconducting compounds La-Sr-Cu-O and Y-Ba-Cu-O

The authors report that long single- and multicore composites with circular and rectangular cross sections, containing in a normal-metal matrix superconducting phases with the composition La-Sr-Cu-O and Y-Ba-Cu-O, were prepared by methods of mechanical-thermal treatment. These compounds were prepared by synthesis from the starting components in the temperature range up to 1000/sup 0/C. The critical temperature of the superconducting phases was found to be at the level of the published data. Because of the brittleness of the superconducting compounds a method of deformation in plastic and hard shells, providing a favorable stressed state of the ceramic component of the composite, was employed.

Non-uniform distribution of normal zone in composite superconductors with contact resistance

A region of resistive domains is found in composite superconductors with thermal and electrical contact resistance between the normal metal matrix and the superconductor. The V-I characteristic of a sample with resistive domains is found. It is shown that the existence of resistive domains leads to the emergence of hysteresis upon decay and recovery of superconductivity in the presence of transport current. It is shown that the degradation of composite superconductors with contact resistance may be attributed to the formation of resistive domains.

Effects of heat treatment and doping with Zr on the superconducting critical current densities of multifilamentary Nb3Sn wires

Various wire samples consisting of 19-Nb3Sn filaments in a normal metal matrix were made by heat treating composite wires of 19-Nb, Nb-0.5 wt% Zr, Nb-0.75 wt% Zr, and Nb-2.5 wt% Zr filaments embedded in a Cu-10 wt% Sn matrix. The Nb-0.75 wt% Zr rod contained the precipitate ZrO2, while the Zr in the remaining Nb rods was in solid solution. The over-all size of the composite was 2.5×10−2 cm and each filament was approximately 2.5×10−3 cm in diameter. The critical current density Jc at 4.2 °K was measured at 40 and 100 kG for a variety of heat treatments. It was found that the critical current density was a strong function of the thickness of the Nb3Sn layers formed during heat treatment. This dependence of Jc on the thickness d could be approximately represented by the expression Jc(d)≃J0 exp (−nd), where J0 is the extrapolated zero thickness critical current density. J0 was found to be approximately 2.4×106 A/cm2 at 40 kG for the Nb3Sn formed from pure Nb, 1.2×106 A/cm2 for Nb doped with solid-solution additions of Zr, and 9.5×105 A/cm2 for that formed from Nb doped with ZrO2. The exponent n falls in the range 0.52–0.67 (μm)−1 for pure Nb3Sn depending on heat treatment temperature, 0.154–0.287 (μm)−1 for Nb doped with solid-solution additions of Zr, and only 0.11 (μm)−1 for the Nb3Sn composite wire containing ZrO2. The magnetic field dependence of Jc for some of the composites was also measured to 150 kG.

Theoretical Behavior of Twisted Multicore Superconducting Wire in a Time-Varying Uniform Magnetic Field

A simplified model is used to study the response to a slowly changing uniform magnetic field of a wire consisting of fine filaments of superconductor embedded in a normal metal matrix, the entire wire being twisted about its axis. Expressions for the critical twist pitch, the power loss in the normal metal, and the time constant are obtained.