P Kováč

The role of MgO content in ex situ MgB2 wires

An experimental study of the effect of MgO content in the MgB2 powder used for ex situ made composite wires was carried out. Two single-core MgB2/Fe/Cu wires were made using commercial MgB2 powders from Alfa Aesar containing different fraction of MgO. Critical temperature and critical currents of as-deformed and heat-treated wires were measured. The differences between the wires are discussed and correlated with the MgO content. It was found that by increasing the amount of MgO, the inter-grain connectivity worsens, but well distributed and low size MgO particles improve flux pinning.

Influence of external magnetic fields on critical currents of solenoids wound with anisotropic HTS tapes : theoretical analysis

Theoretical analysis of the critical currents in solenoids wound with anisotropic superconductors was performed. The results of the numerical calculations indicate that applying the external magnetic field to the cylindrical coils which are made of anisotropic HTS tapes may lead to some increase in their critical current. Taking into account the angle dependence of the of short Bi(2223)Ag tape samples measured at 77 K we have shown how the external homogeneous magnetic field parallel to the axis of cylindrical symmetry influences the value of the coils critical current. We also present the analysis of the influence of the external magnetic field on the change in position of weak turns in the winding. It is shown that determination of the magnet critical current represents mathematically the solution of a non-linear equation. The methods as well as the computer procedure enabling the evaluation of the coils expected critical current and the location of the position of weak places in the winding are described. Further, the expressions that can be applied to the calculation of critical currents of the magnets, made of superconducting wires and tapes, whose characteristics are isotropic, were derived. The possibility of obtaining analytical solutions for simplified linear and for the general case of is also discussed.

Compensation of the radial magnetic field component of solenoids wound with anisotropic Bi(2223)Ag tape

The critical current of a solenoid made of Bi(2223)Ag tape with a high anisotropy in the critical current - magnetic field characteristic at 77 K is determined in particular by the radial component of the magnetic field, which is large close to the coil flanges. The original idea of utilizing supplementary (e.g. copper) windings to compensate partially the undesired radial magnetic field component of the high-temperature superconducting (HTS) coil is described. The results of extensive numerical calculations on the model coil indicate that an increase of the critical current of more than 50% can be expected.

Improvement of the current carrying capability of ex situ MgB2 wires by normal particle additions

A wide variety of powders, including Nb, Ti, Zr, Hf and W, metal oxides like Nb2O5, Ti2O5, V2O5 and other compounds like SiC, SrCO3 and boron nitride, have all been used at the 10 wt% level as additions into commercial MgB2 powder for the fabrication of single-core, ex situ wires using groove and two-axial rolling deformation and final heat treatment at 950 °C /0.5 h in Ar. Transport current measurements have shown that metal particle addition leads to an especially significant improvement in current carrying capacity. The presence of normal particles influences both the resistivity and also the thermal conductivity of the MgB2 core, and the improved internal stability may be responsible for the increased critical current values. This observation may be very important for future development of practical MgB2 composite superconducting wires.

Current profiles and ac losses of a superconducting strip with an elliptic cross-section in a perpendicular magnetic field

We study the case of a hard type II superconductor in the form of a strip with an elliptic cross-section when it is placed in a transverse magnetic field. We approach the problem in two steps, both based on the critical-state model. First, we calculate numerically the penetrated current profiles that ensure complete shielding in the interior, without assuming an a priori form for the profiles. In the second step we introduce an analytical approximation that asumes that the current profiles are ellipses. Expressions linking the sample magnetization to the applied field are derived covering the whole range of applied fields. The theoretical predictions are tested by comparison with the experimental data for the imaginary part of ac susceptibility.

Relation between critical current and exponent n in Bi(2223)/Ag tapes

Transport critical currents at and exponents n of the power law characteristics of single-filament Bi(2223)/Ag tapes were measured at 77 K in the self-field and in external magnetic fields up to 1.2 T. The effects of tape orientation in a transverse magnetic field from to (related to the basic state at -axis) were investigated as well. The values of the transport critical current and exponent n as well their mutual relation reflect the quality of the Josephson-linked current network connectivity at ( is the cross-over field at which the weak-link currents cease to contribute to the transport current) and the ability of pinning centres to pin flux lines in the whole magnetic field region. The weak-link connectivity and the pinning potential are directly related to the structural state of the samples resulting from their different thermomechanical treatments. The better the structural quality, the higher the ratio. This ratio seems to reflect the effective number of intergrain connections and pinning centres.

In situ investigations of phase transformations in Fe-sheathed MgB2?wires

The phase evolution inside Fe-sheathed wires containing precursor powders consisting of a mixture of Mg and B has been studied in situ by means of x-ray diffraction with hard synchrotron radiation (90 keV). Mg was found to disappear progressively during the heating stage. At 500 °C, the intensity of the Mg diffraction lines is reduced by about 20%. This effect is partly attributable to MgO formation. The MgB2 phase was detected from 575 °C. Fe2B was forming at the interface between the sheath and the ceramic core at sintering temperatures of 780 and 700 °C, but not at 650 °C. The formation rate of this phase is strongly dependent on the heat treatment temperature. Its presence can be readily detected as soon as the average interface reaction thickness exceeds 150–200 nm.

Transport currents of two-axially rolled and post-annealed MgB2/Fe wires at 4.2 K

Single- and four-core MgB2 wires have been made by the powder-in-tube (PIT) method using a commercial MgB2 powder (Alfa Aesar) and two-axial rolling deformation in an Fe sheath. The deformed wires were subjected to annealing at temperatures from 950 ?C up to 1100 ?C for 30 minutes in argon. The interface layer thickness of 10?25.4 ?m caused by inter-diffusion and reaction between the MgB2 core and the Fe sheath has been observed in all annealed wires. Transport currents of as-deformed and post-annealed wires were measured at the temperature 4.2 K and external magnetic field B = 0?5.5 T. The presence of a ferromagnetic sheath in MgB2 composite wire influences the Ic(B) characteristics leading to Ic hysteresis and Ic anisotropy, which is more pronounced for those that have not been annealed (weakly linked grains) in comparison to well-connected MgB2 grains after recrystallization. A comparison of the overall current densities versus the external magnetic field of NbTi, Nb3Sn and Bi-2223 superconductors with four-core MgB2 wire is also presented.

Partitioning of transport AC loss in a superconducting tape into magnetic and resistive components

Superconducting wire carrying transport alternating current with amplitude around its critical current exhibits two main loss mechanisms: besides the irreversible pinning of magnetic flux leading to magnetic losses that have prevailed at low currents, resistive loss appears due to nonzero resistivity in the overcritical regime. We tested the hypothesis that the total loss is an algebraic sum of both these loss components. Resistive losses were derived from the measured DC current-voltage curve, and magnetic losses established by a contactless method. We found that, in the case of a multifilamentary Bi-2223/Ag tape, the sum of two terms was in fair agreement with the total transport loss measured in the traditional way.

Field distribution effect on the performances of coils wound with Ag/Bi-2223 tape

A small coil (35 mm bore) was made by winding a superconducting Ag/Bi-2223 multifilamentary tape. The coil, composed of eight pancakes, showed reduced performance in the pancakes placed at the sides with respect to central ones. This effect was attributed to the radial field at the coil ends. In order to explain this behaviour, a general method for predicting the critical current of a coil, starting from basic information on short samples, was developed. The method was used to understand the possible advantages from a reduction of radial field in the coil through a suitable iron yoke for flux return. The measured performance (in terms of critical current) of the coil with iron improved by 10%. The central field at 77 K was 1100 Gauss and 6000 Gauss at 4.2 K.