Filament Microstructure and Superconducting Properties of ${\rm MgB}_{2}$ Wires With Thin Filaments

Nineteen-filament MgB 2 /Ti/Cu/SS composites have been made by an in situ approach using only drawing deformation to a filament size of 16.6-58 μm. Circular cables containing 133 filaments were assembled from 1 + 6 strands of diameter 0.25 mm and 0.375 mm. All wires and cables were heat treated at 800 • C/0.5 h in pure Ar atmosphere. The critical current densities J c of the different wire sizes were measured and EDX element mapping was done for some of them. It was found that the critical current density decreases with reduced filament size, especially below the filament diameter of 27 μm due to copper penetration through the thin Ti barrier (<10 μm) and consequent contamination of MgB 2 . In spite of Cu contamination, J c > 10 4 A cm −2 was measured at 4.2 K and field 5.5 T for the thinnest 16.6 μm filaments published up to now.

Section: Filament Sizementioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

Fine-filamentaryin situMgB₂wires

Kòváč¹,

Hušek²,

Rosová³

et al. 2010

“…The introduction of new MgB 2 superconductors in 2001 has stimulated interest in many areas of investigation for practical use. Although numerous configurations of MgB 2 composite superconductors have been introduced over the past few years [1][2][3], the production technology is still in a developmental stage. Utilization of these superconductors cannot intensify without overall characterization of their electromechanical performance, which is affected by the manufacturing process.…”

Section: Introductionmentioning

confidence: 99%

Calculated and measured normal state resistivity of 19-filament MgB₂/Ti/Cu/stainless steel wire

Kopera¹,

Kòváč²,

Hušek³

2012

This work describes the resistivity evolution of filamentary MgB 2 /Ti/Cu/stainless steel wire under the influence of solid state interaction between individual elements of the composite. The experimental samples were heat treated at temperatures of 650-850 • C and the properties of the interface layers have been estimated on the basis of solid state growth of intermetallic compounds formed by diffusion. The corresponding composite resistivity along the longitudinal direction was calculated using a model represented by an equivalent parallel circuit of n resistors. The temperature dependence of the electrical resistivity was measured for the experimental sample, a specially prepared Cu/Ti couple, and all individual metallic components used in the sample, using a cryogen-free characterization system. Very good agreement between the calculated ρ(T) dependences and the experimentally measured resistivity of 19-filament MgB 2 /Ti/Cu/stainless steel wire has been obtained.

“…In particular, the issue regarding the achievement of a large number of very fine filaments (10-30 µm) seems not to have a simple solution. The in situ way has been already employed to realize multifilamentary wires with a number of filaments up to 61 [15] and a filament size down to 40 µm [7,16]. However, a J C degradation has been observed with increasing the number of filaments and in particular the granulometry of the starting precursors seems to be the bottleneck in achieving homogeneous fine filaments.…”

Section: Introductionmentioning

confidence: 99%

Study of the MgB₂grain size role inex situmultifilamentary wires with thin filaments

Malagoli¹,

Braccini²,

Bernini³

et al. 2010

The MgB 2 superconductor has already demonstrated its applicative potential, in particular for DC applications such as MRI magnets, thanks to the low costs of the raw materials and to its simple production process. However further efforts have still to be made in order to broaden its employment also towards the AC applications such as SFCL, motors, transformers. The main issues are related to the reduction of the AC losses. Some of these can be faced by obtaining multifilamentary conductors with a large number of very fine filaments and, in this context, the powders granulometry can play a crucial role. We have prepared MgBB 2 starting powders with different granulometries and by the exsitu P.I.T method we have realized multifilamentary wires with a number of filaments up to 361 and an average size of each filament lowered down to 30 μm. In particular we have studied the relationship between grain and filament size in terms of transport properties and show that the optimization of this ratio is possible in order to obtain suitable conductors for AC industrial applications. IntroductionNowadays the MgB 2 -based conductors fabricated by the Powder-In-Tube (PIT) technique can be considered not only as promising superconductors but also as an actual industrial product. In parallel to a broad research addressed to improve the critical current density (J C ) behaviour, a number of groups have focused their efforts on developing multifilamentary conductors -being more interesting than a monocore from an applicative point of view -following the ex-situ as well as the in-situ route [1]. On the one hand the research has been focused on the pinning issue and several doping procedures have been developed in order to improve the behaviour in magnetic field, most of them regarding the in-situ [2-5] even though works using the ex-situ do not lack [6]. On the other hand, the clear applicative prospect has prompted the development of techniques to produce multifilamentary wires [7][8][9][10] or cables [11,12]. After few years since the discovery of superconductivity in MgB 2 , long multifilamentary strands -of the order of kmhave been fabricated by Hyper Tech Research using the in-situ [13] and Columbus Superconductors using the ex-situ [9] way. The stabilized multifilamentary tape produced by Columbus Superconductors has even been successfully employed to realize a low field magnet [14] operating at 20 K for a new superconducting M.R.I. machine that is at present marketed. However in order to make MgB 2 useful not only for DC but also for AC applications further conductor development and optimization are still needed, in particular to reduce the AC losses caused by magnetic hysteresis in the MgB 2 core, filaments coupling and eddy currents flowing through the metallic matrix. In this context the research works should be focused on multifilamentary strands with a large number of very fine filaments, twisted filaments and nonmagnetic and high resistivity sheath. In particular the issue regarding the achievement of a large...