E. Babić

High-transport critical current density above 30 K in pure Fe-clad MgB2 tape

Abstract Fe-clad MgB 2 long tapes have been fabricated using a powder-in-tube technique. An Mg+2B mixture was used as the central conductor core and reacted in situ to form MgB 2 . The tapes were sintered in pure Ar at 800°C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 μm. The Fe-clad tape shows a sharp transition with transition width of ΔT c of 0.2 K and a T c0 of 37.5 K. We have achieved the highest transport critical current reported so far at 1.7×10 4 A/cm 2 for both 29.5 K in 1 T and 33 K in zero applied field. Resistivity temperature dependence and transport critical current were also measured in magnetic fields applied perpendicular and parallel to the tape plane. Not only is the use of an Fe sheath necessary for the successful processing of in situ reacted powder-in-tube MgB 2 , it confers on the finished wire the additional benefit of magnetic screening.

Superconductivity in zirconium-nickel glasses

Abstract The superconducting transition temperatures (Tc) and magnetic susceptibilities of amorphous Zr100−xNix alloys have been measured. Tc decreases linearly with increasing x. The results are compared to those for amorphous ZrPd and ZrCu alloys and discussed in terms of changes in the electron to atom ratio on alloying.

The influence of pinning centres on magnetization and loss in Fe-Ni-B-Si amorphous alloys

Abstract The magnetic hysteresis of as-received Fe 60 Ni 20 B 18 Si 2 and Fe 40 Ni 40 B 18 Si 2 amorphous ribbons has been measured by the induction method, at different values of the maximum magnetization ( M m ) and in a broad frequency range (0.1–10 5 Hz). The variations of the remanent magnetization, coercive field and hysteresis loss with M m at different frequencies are analysed in parallel, in order to explain the nature of processes occurring during the magnetization of the samples. All observed variations can be qualitatively explained by assuming the existence of two types of domain wall pinning centres with different pinning energies. The frequency dependence of the loss at selected M m values also supports this picture. The relevance of these results for the improvement of the soft magnetic properties of amorphous alloys is briefly discussed.

Sugar as an optimal carbon source for the enhanced performance of MgB2 superconductors at high magnetic fields

In this paper we report the results of an extended study of the effect of sugar doping on the structural and electromagnetic properties of MgB2 superconductors. High values of the upper critical field (Bc2) of 36 T and the irreversibility field (Birr) of 27 T have been estimated at the temperature of 5 K in a bulk MgB2 sample with the addition of 10 wt% of sugar. The critical current density (Jc(Ba)) of sugar-doped samples has been significantly improved in the high field region. The value of transport Jc has reached as high as 108 A m−2 at 10 T and 5 K for Fe-sheathed sugar-doped MgB2 wire. The analysis of the pinning mechanism in the samples investigated indicated that dominant vortex pinning occurs on the surface type of pinning defects, such as grain boundaries, dislocations, stacking faults etc, for both pure and doped MgB2. In sugar-doped samples, pinning is governed by numerous crystal lattice defects, which appear in MgB2 grains as a result of crystal lattice distortion caused by carbon substitution for boron and nano-inclusions. The drastically improved superconducting properties of sugar-doped samples are also attributed to the highly homogeneous distribution and enhanced reactivity of this dopant with host Mg and B powders. The results of this work suggest that sugar is the optimal source of carbon for doping MgB2 superconductor, especially for application at high magnetic fields.

Correlation between mechanical, thermal and electronic properties in Zr–Ni, Cu amorphous alloys

We show that mechanical properties (stiffness and hardness) of Zr–Ni, Cu amorphous alloys increase linearly with Ni, Cu content over a wide composition range (22 ≤ x Ni,Cu ≤ 65 at%). This correlates with the observed increase in the Debye temperatures and densities with x and shows that the strength of interatomic bonding increases with x in these alloys. Accordingly, the thermal stability (e.g. the crystallization and glass transition temperatures) of these alloys also increases with x. Since the electronic density of states at the Fermi level decreases linearly with x within the same x-range, a very simple relationship exists between the electronic structure and mechanical and thermal properties. We also deduce the mechanical properties of hypothetic amorphous Zr and briefly discuss the possibility of its preparation.

Low temperature resistivities of FexNi80-xP14B6 metallic glasses

Abstract The resistivities of six FexNi80-xP14B6 alloys have been measured between 1.5 and 50 K. It is found that the resistivity variations both below and above the resistivity minima depend on the transition metal composition. The room temperature coefficients of the resistivity indicate the existence of the magnetic contribution to the resistivity.

Stoner excitations in the strong itinerant amorphous ferromagnets FexNi80−xB18Si2 and Fe80B20

Abstract Accurate measurements of the static magnetization of the amorphous ferromagnets FexNi80−xB18Si2 (x = 15, 20, 40, 60%) and Fe80B20 are reported. The results are analyzed in terms of spin-wave and Stoner excitations, and the latter indicate strong itinerant ferromagnetism with a Stoner gap Δ varying between 20 and 60 K. The other fitting parameters give values of the spin-wave stiffness and a quantity simply related to the saturation magnetization. The well-known anomaly regarding the spin-wave stiffness observed in earlier measurements in thus explained quantitatively.

Some properties of AlSc solid solutions

Measurements have been made of lattice parameters residual electrical resistivities and microhardness values of AlSc alloys containing up to 3.28at%Sc. The change in lattice parameter a with concentration c is found to be (da/dc)/a=0.122+or-0.011 at%-1, in residual resistivity rho 0/c=3.5+or-0.5 mu Omega cm at%-1 and in relative microhardness (d sigma /dc) sigma =1.0+or-0.1 at%-1. Changes in lattice parameter and residual resistivity support the virtual bound state model of Al-3d alloys and changes in microhardness correlate with deviations from Vegards law for these alloys.

Temperature dependent impurity resistivity in A1-based 3-d transition metal alloys

Abstract The impurity resistivity of Al based 3- d transition metal alloys has been investigated at 4.2 and 500 K. While the impurity resistivity at 4.2K plotted as a function of the atomic number of the transition metal shows a single-peaked structure, the resistivity measured at 500 K follows a double peaked curve characteristic of noble metal alloys containing 3 d impurities. These results are compared with the variation of resistivity obtained assuming a temperature dependent phase shift distribution in the expression for temperature dependent resistivity as well as with results of measurements of charge perturbation around the impurity in the same system.

Quantum corrections to conductivity of glassy Zr100−xCux alloys

Abstract The variation of the conductivity of glassy Zr100−xCux alloys (x=50, 58, 67 and 71) in the temperature range 2–300 K is discussed in some detail. It is shown that the conductivity varies as √T for T θ D 3 and T for the lower temperatures which is consistent with the predictions of a weak localization theory. Another strong temperature variation of the conductivity sets at the lowest temperatures which could be either due to interaction effects or due to electron scattering on the unstable ionic configurations. Some support to the latter effects is presented.