V Abächerli

Critical currents versus applied strain for industrial Y-123 coated conductors at various temperatures and magnetic fields up to 19 T

The strain dependence of the critical current in Y-123 tapes has been studied as a function of field and temperature up to strain values of 0.81%. For the particular tape configuration with a Ni?W substrate, the critical current was found to be reversible up to 0.51%. The Ic(?) curves have been measured at 4.2, 50 and 77?K at fields up to 19?T. In the reversible region, the decrease of Ic(?)/Ic(? = 0) with ? was found to be more pronounced at higher fields and temperatures: at 17?T the decrease of Ic/Ico at ?irr was of the order of 9% and 25% at 4.2 and 50?K, respectively. At 77?K and 6?T, the corresponding decrease was 12%. Using the Walters spiral for tapes up to 0.80?m length, the V ?I curves have been measured over three decades. At low electric field (0.01??V?cm?1) the n value is constant up to 0.81% strain, while at higher electric field it decreases with the applied strain.

The influence of Ti doping methods on the high field performance of (Nb,Ta,Ti)/sub 3/Sn multifilamentary wires using Osprey bronze

We have analyzed the influence of the two different Ti doping methods on the superconducting properties of bronze processed (Nb,Ta,Ti)/sub 3/Sn multifilamentary wires with identical configuration, consisting of a Nb7.5wt.%Ta alloy processed to 14641 filaments of 4.5 micrometer size embedded in an Osprey bronze and externally stabilized by Cu. The first doping method introduces NbTi rods into NbTa filaments in a Cu15.4Sn Osprey bronze and has been applied to two wires containing 1 and 2 wt.%Ti in the filaments, respectively. The second method uses a Ti added Cu15.5Sn0.25Ti Osprey bronze, while the filaments of two thereby fabricated wires contain 0 and 0.5wt.%Ti, respectively. A reference wire containing no Ti addition was also prepared. All five wires were manufactured by three hot extrusion steps, cold drawing and several intermediate anneals. Wire samples with round cross section have been prepared and reacted between 600 and 730/spl deg/C. The residual Sn content in the bronze after reaction and the residual niobium ratio of the filaments were determined by means of EDX and SEM analysis, respectively. The grain size of the A15 layer has been compared by FE-SEM and the local composition profile has been observed by systematic quantitative EDX measurements made on a TEM. Non-Cu J/sub c/ values up to 300 Amm/sup -2/ and n values up to 50 at 17 T and 4.2 K reveal clearly the Ti added bronze as more performing.

Transport properties and exponential n-values of Fe/MgB2 tapes with various MgB2 particle sizes

Fe/MgB2 tapes have been prepared starting with pre-reacted binary MgB2 powders. As shown by resistive and inductive measurements, the reduction of particle size to a few microns by ball milling has little influence on Bc2, while the superconducting properties of the individual MgB2 grains are essentially unchanged. Reducing the particle size causes an enhancement of Birr from 14 to 16 T, while Jc has considerably increased at high fields, its slope Jc(B) being reduced. At 4.2 K, values of 5.3 × 104 and 1.2 × 103 A/cm2 were measured at 3.5 and 10 T, respectively, suggesting a dominant role of the conditions at the grain interfaces. A systematic variation of these conditions at the interfaces is undertaken in order to determine the limit of transport properties for Fe/MgB2 tapes. The addition of 5% Mg to MgB2 powder was found to affect neither Jc nor Bc2. For the tapes with the highest Jc values, very high exponential n factors were measured: n=148, 89 and 17 at 3.5, 5 and 10 T, respectively and measurements of critical current versus applied strain have been performed. The mechanism leading to high transport critical current densities of filamentary Fe/MgB2 tapes based on MgB2 particles is discussed.

Specific heat of Nb3Sn wires

Multifilamentary superconducting Nb3Sn wires are widely used for industrial applications. Wires processed by the Bronze Route technique are characterized by a large number of filaments Nb1−xSnx, with 0.18≤x≤0.25, corresponding to a distribution of Tc between<10 and 18 K. This distribution is a critical property of the wires and is important for the optimization of the conductors. However, it is not accessible to conventional techniques, due to percolation and/or magnetic shielding effects. In order to determine the Tc distribution in the sample, we have carried out specific heat measurements of various Nb3Sn multifilamentary wires (including the bronze matrix) in zero field and at 14 T. A deconvolution of the calorimetric data at the superconducting transition by means of a thermodynamical model was used for obtaining the distribution of Tc in the whole wire volume. The measurements were extended to Nb3Sn wires containing Ti additions, and the results were compared. The present calorimetric method is of primary importance for the complete characterization of Nb3Sn wires.

Critical current versus strain measurement up to 21T and 1000A of long length superconducting wires and tapes

A device has been developed to measure the critical current of long length superconductors under uniaxial strain up to 1000A and up to 21T. The latter is based on a modified Walters spring (WASP) where zero applied strain can be precisely controlled. The length of the investigated conductor is on the order of 1m with a typical gauge length between the voltage taps of about 0.5m. This facilitates the measurement of critical currents at an electric field criterion as low as 0.01μV∕cm, which is particularly important for superconductors to be used in magnets which are operated in the persistent mode. The operation of the WASP was simulated by finite element calculations indicating that the radial stress (contact pressure) of the superconductor is less than 5% of the axial stress. The performance of the probe is demonstrated for several Nb3Sn conductors with round and rectangular cross section, respectively, as well as for a Bi-2223 tape.

Critical current of a Nb3Sn bronze route conductor under uniaxial tensile and transverse compressive stress

A complete set of critical current data for a Nb3Sn bronze wire under axial and transverse loads has been measured as a function of magnetic field in the range between 12 and 19 T. For this purpose a new probe was developed for the application of a transverse compressive force with a gauge length of 120 mm. The device is based on a one-turn Walters spring, thus circumventing the problem of current transfer. The probe allows the application of voltage criteria of 0.01 µV cm−1 or less, allowing a detailed study of the n values. As expected, the conductor is more affected by transverse compressive stress than by axial tensile stress. The conductor already starts to degrade at small transverse stresses and the rate of degradation increases with the magnetic field. On the wires measured so far, unloading from 160 MPa yields a recovery of the critical current to 87% of its initial value. The obtained results encourage further investigations of differently processed Nb3Sn superconductors (internal tin, powder in tube) or others (e.g. Nb3Al) and may be helpful for modelling.

Asymmetric behaviour of Jc(ε) in Nb3Sn wires and correlation with the stress induced elastic tetragonal distortion

The effect of uniaxial strain on the critical current of 0.8 m long Nb3Sn wires up to 21 T is studied by the modified Walters spring (WASP). For Nb3Sn wires, prepared by both the bronze route and the internal Sn diffusion process, the critical current density as a function of the uniaxial strain e is found to exhibit an asymmetric behaviour on both sides of the strain em, where Jc reaches its maximum. Revisiting earlier x-ray and neutron diffraction measurements on bronze route processed wires between 10 and 600 K, it is shown that the asymmetric behaviour of Jc(e) on both sides of the strain value em is connected to individual variations of the stress-induced tetragonal lattice parameters a and c. The present measurements of Jc versus strain for Nb3Sn wires show stronger strain dependence for wires prepared by the internal Sn diffusion method with respect to those obtained by the bronze route. The reasons for this difference are attributed to the individual details of the filament configuration in both types of wire, for example the different Sn distributions inside the filaments and the very different filament sizes, 4 and 80 µm, respectively.

Transport critical current densities and n factors in mono- and multifilamentary MgB/sub 2//Fe tapes and wires using fine powders

Mono- and multifilamentary MgB/sub 2//Fe tapes and wires with high transport critical current densities have been prepared using the powder-in-tube (PIT) process. The fabrication details are described. The effect of powder grain sizes and recrystallization temperature on j/sub c/ has been investigated. At 25 K and 1 T, j/sub c/ values close to 10/sup 5/ A/cm/sup 2/ were measured, while j/sub c/ of 10/sup 6/ A/cm/sup 2/ were extrapolated for 4.2 K/0T in our monofilamentary tape. MgB/sub 2//Fe tapes exhibit high exponential n factors for the resistive transition: n/spl ap/80 and 40 were found at 5 T and 7 T, respectively. The highest transport j/sub c/ values obtained so far in MgB/sub 2//Fe wires with 7 filaments were 1.1/spl times/10/sup 5/ A/cm/sup 2/ at 4.2 K and in a field of E2 T, which is still lower than for monofilamentary tapes. The function F/sub p//spl prop/b/sup p//spl middot/(1-b)/sup q/ has been established over the whole field range, and exhibits a maximum at F/sub p//spl cong/0.18. Improved deformation and recovering processing is expected to lead to higher j/sub c/ values.

Grain Growth, Morphology, and Composition Profiles in Industrial

The A15 formation process in Nb3Sn wires has been revisited. The progress of the reaction was studied by a careful FE-SEM study after a series of heat treatments at various temperatures and times. It was found that in the very early stage of the reaction, the Sn flow is high and a very thin layer of equiaxed A15 grains is formed at the interface between Nb and the Cu-Sn bronze. Beyond the very first layer with equiaxed grains, the grains form with a columnar shape. With further progress of the A15 layer and the continuous flow of Sn through the already formed layers, a decomposition of the columnar grains to equiaxed grains was observed. By TEM/EDX, we have confirmed our earlier observation of a radial Sn gradient in the filaments. The equiaxed regions have a sufficiently high critical field to carry supercurrents at high magnetic fields, in contrast to the columnar regions, where the B values are lower, thus their contribution to the current transport at the higher fields is limited. We have also studied internal Sn wires and found equiaxed grains only, with a Sn content through the A15 layer reaching from 22 at.% to values close to 25 at.%.

{\hbox{Nb}}_{3}{\hbox{Sn}}

Superconducting Fe-sheathed MgB2 monofilamentary tapes have been fabricated by the powder-in-tube technique, varying the particle size of the starting MgB2 powder and applying either cold or hot rolling during the last deformation process. Measurements of the critical current density Jc with the magnetic field applied parallel or perpendicular to the tape surface revealed a pronounced anisotropy for the cold rolled tapes, which was found to increase with increasing particle size and magnetic field strength. The microstructural origin of the Jc anisotropy was confirmed by means of x-ray diffraction performed on the filaments after mechanical removal of the sheath. The local texture was studied in a series of diffraction patterns collected at different distances from the filament centre, removing each time some 25 µm of the filament thickness. In the cold rolled tapes, the average orientation of the MgB2 grains was found to approach a preferred orientation with the crystallographic c-axis perpendicular to the tape surface, near the interface with the sheath; however, the misalignment angle increased towards the centre of the tape. In the hot rolled tapes, for which no Jc anisotropy was observed, the grains were found to be approximately randomly oriented. Roughness measurements performed on the side of the Fe sheath in contact with the MgB2 filament are in agreement with the difference in texture observed for cold and hot rolled tapes.