L Kopera

Behaviour of filamentary MgB2 wires subjected to tensile stress at 4.2 K

Different filamentary MgB2 wires have been subjected to tensile stress at 4.2 K. Stress–strain and critical current versus stress and strain characteristics of wires differing by filament architecture, sheath materials, deformation and heat treatment were measured and compared. It was found that the linear increase of critical current due to the pre-compression effect (ranging from 5% up to ≈20%) is affected by thermal expansion and the strength of used metallic sheaths. The values of irreversible strain eirr and stress σirr depend dominantly on the applied outer sheath and its final heat treatment conditions. Consequently, the strain-tolerance of MgB2 wires is influenced by several parameters and it is difficult to see a clear relation between Ic(e) and σ(e) characteristics. The lowest eirr was measured for Monel sheathed wires (0.3–0.6%), medium for GlidCop® sheath (0.48–0.6%), and the highest eirr = 0.6–0.9% were obtained for MgB2 wires reinforced by the stainless steel 316L annealed at temperature between 600 and 800 ° C. The highest eirr = 0.9% and σirr = 900 MPa were measured for the work-hardened steel, which is not considerably softened by the heat treatment at 600 ° C/2.5 h.

Filamentary MgB2 wires twisted before and after heat treatment

Two different filamentary MgB2 wires were subjected to twisting before and also after the final heat treatment (HT). Wires twisted before HT showed only degraded critical currents (Ic) starting from the ratio Lt/dw < 30–40, where Lt is the twist pitch and dw is the wire diameter. Increased Ic (by 8.0–20%) were measured for wires twisted after HT at an Lt/dw ratio of 38–90 depending on the annealing conditions (temperature/time) and also on wire composition. Reduced irreversible strain (eirr) was also measured for wires twisted after HT. Improved Ic and decreased eirr of examined filamentary MgB2 wires are explained by the change of residual strain inside the composite wire caused by torsion stress.

Electromechanical characterization of selected superconductors

This paper describes the design and performance of a new tension test instrument for measuring stress–strain characteristics of a freestanding sample and critical current degradation of superconducting wires and tapes under variable tension and magnetic fields. The performance of the instrument has been tested at liquid nitrogen and liquid helium temperature up to a tensile force of 1000 N. Stress–strain and Ic–strain characteristics of advanced Bi-2223, Y-123 and MgB2 composite superconductors were measured. The results show the best electromechanical properties for the tapes mechanically reinforced by soldered stainless steel strips. MgB2 tape with a Ti-barrier and Monel sheath is less reinforced due to an apparent softening of these materials during the final annealing at 800 °C/30 min.

The design and performance of a Bi-2223/Ag magnet cooled by a single-stage cryocooler

The design and fabrication of a split coil Bi-2223/Ag tape superconducting magnet cooled by a single-stage cryocooler is presented. The magnet consists of six double-wound pancakes impregnated by epoxy resin with inner diameter 50 mm and outer diameter 98 mm. Critical currents of individual pancakes, double pancakes and the whole magnet are measured, calculated and discussed. The magnets performance was tested at liquid helium and nitrogen temperatures as well as at solid nitrogen temperatures and cooled by an AL230 single-stage Gifford-McMahon cycle cryorefrigerator. The measured magnet currents are compared with those estimated by calculations using short sample data.

Electromechanical Properties of Filamentary

This work describes the performance of several MgB2 composite wires when subjected to tensile stress at 4.2 K. Electromechanical properties of these wires are influenced by the composite elements encasing the filament and also by the filaments mechanical strength. A linear increase in critical current by tension up to ≈20% was measured due to compensation of the precompression effect caused by different thermal expansions of composite elements. Final heat treatment influences the electromechanical characteristics of composite wires due to stress recovery and also by interface reactions among the composite elements. Results showed that the highest irreversible strain εirr = 0.9 % was obtained for MgB2 wires reinforced by stainless steel, which has more than doubled εirr in comparison to commercially available MgB2/Nb/Cu/Monel wires with the same filament number.

\hbox{MgB}_{2}

Critical current densities and mechanical resistance of MgB2 wires made by the rectangular wire-in-tube technique (RWIT) have been studied. Wires prepared from different precursor powders and variable sheath materials are compared. The best electrical performance (104 A cm−2 at 11.3 T) was measured for the wire with mechanically alloyed powder doped by SiC. While the critical current densities, Jc, at 4.2 K are considerably influenced by the powder used, the differences at 20 K are much smaller. Flattened wires show different levels of critical current anisotropy influenced by the precursor powder used. Stress–strain characteristics and critical current degradation are strongly affected by the applied metallic materials and also by the filaments strength. The highest irreversible strain eirr = 0.55% was measured for Ti/Cu/Monel sheathed wire with filaments from mechanically alloyed powder.

Wires

An experimental study of Bi(2223) - Ag tapes undergoing bending and straightening at 300 and 77 K has been performed. The special test equipment allows us to measure changes of the critical current in a short element of the tape during continual decreasing or increasing of the bending diameter at both temperatures mentioned above. Comparison of 300 and 77 K results shows the effect of the Ag sheath strength on the bending sensitivity of Bi(2223) - Ag composite tape. Remarkable lowering of critical current degradation by bending at 77 K was observed. This offers the possibility of winding small-diameter coils with Bi(2223) - Ag tape pre-cooled by liquid nitrogen.

Progress in electrical and mechanical properties of rectangular MgB2 wires

Magneto-optical (MO) imaging, Hall probe measurements and the magnetic knife method were used for sample quality studies of multi-core Bi-2223/Ag tape with a columnar 60-filament structure made by the TIRT (tape in rectangular tube) process. While the magneto-optical imaging was made at 25 K and without transport current, Hall probe and magnetic knife measurements were performed for the sample carrying a direct current at 77 K. The MO image recorded at 25 K and external field 30 mT has shown non-uniform flux distribution in the three filament columns. 2D field profiles were analysed by the finite element method assuming ideal filament homogeneity. Bz(x) was computed analytically with an assumption that current density flows straight through the cross-section of the filament columns. The best agreement of computed profiles with measured ones is achieved for equal currents flowing in all filament zones. According to the magnetic knife measurements, current density is lowest in the centre column, but in this method the magnetic self-field is hardly influencing the current distribution. A better understanding of such experimental differences is obtained with 3D modelling. Three different cases of meandering transport current were studied computationally in order to explain the measured results. Finally, we discuss the advantages and drawbacks of the methods used and present how they can together produce the most complex information.

Bending of Bi(2223) - Ag tapes at 77 and 300 K

In the superconducting applications, the wires are exposed to time-varying magnetic field when the current changes. This generates losses which can be minimized by reducing filament size, twisting the wire, and increasing the transverse resistivity. However, the high losses of magnesium diboride wires often arise from magnetic sheath materials, and therefore, this work presents new type of wires with nonmagnetic matrix and multi-filamentary structure. The results of AC loss measurements, in external sinusoidal magnetic field, are presented. Two MgB2 samples were measured both in two temperature ranges, as two different set-ups were used, one with fixed LHe bath temperature 4.2 K. Second one enabled operation temperatures from 23 K up to the critical temperature of 39 K. Amplitude of magnetic field of the former set-up was up to 0.8 T and frequency range was from 0.1 to 1.4 Hz. In the latter one, the maximum amplitude was 28 mT, and the frequencies were 72 and 144 Hz. The results evidenced that the superconducting filaments were uncoupled and the measurements agreed with theoretical models based on this assumption. In practice, the uncoupling was modeled so that the net current in each filament was set to zero.

Comparison and analysis of Hall probe scanning, magneto-optical imaging and magnetic knife measurements of Bi-2223/Ag tape

Nineteen-filament MgB2/Ti/Cu/SS composites have been made by an in situ approach using only drawing deformation to a filament size of 16.6?58??m. 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 Jc 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??m due to copper penetration through the thin Ti barrier ( 104?A?cm ? 2 was measured at 4.2?K and field 5.5?T for the thinnest 16.6??m filaments published up to now.