Bernd Seeber

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 thermal precompression on the mechanical behaviour of Ag-sheathed (Bi,Pb)2223 tapes with different matrices

Abstract The behaviour of the critical current in longitudinally strained Bi,Pb(2223) tapes shows a strain-insensitive plateau up to an irreversible strain limit e irr . For higher strains, the formation of cracks induces an abrupt decrease of the critical current. We investigate the relationship between precompression and irreversible strain limit with a set of tapes made with different filling factors as well as an in situ Inconel 600-reinforced tape. I c ( e ) curves were measured in a longitudinal strain apparatus at 77 K. The precompression at the measurement temperature is numerically estimated for each sample as well as the evolution of precompression during the cool-down. These calculated values are compared to the fracture susceptibility of extracted filaments, which gives an empirical estimate of the precompression. The main hypothesis of the “Irreversible I c Reduction Model” is confirmed, i.e. the irreversible strain limit essentially depends on precompression. However, we also found that the regime where I c remains constant contains a tensile component: the plateau extends beyond the external tensile strain needed to relieve the thermal precompression and includes a regime where the ceramic is further elongated non-destructively. This non-destructive deformation can be understood as a “connected-grains” behaviour, and extends the strain-insensitive plateau ∼0.1% beyond the precompression strain. This value is confirmed with a three points bending experiment performed on single filaments which gives a similar value for the bending failure strain. A comparable regime was found to exist also under compressive strain. These non-destructive regimes are of great importance for practical applications since up to a certain level the precompression can develop without any I c degradation.

New ternary Mo(II) - compounds InxMo15Se19 containing Mo6Se8 and Mo9Se11 units

Abstract A new ternary molybdenum chalcogenide has been found with composition In x Mo 15 Se 19 . This compound crystallizes in the hexagonal space group P6 3 /m, and shows both types of blocks Mo 6 Se 8 and Mo 9 Se 11 . To our knowledge, this is the first Me 9 X 11 unit ever observed in a material. It presents a homogeneity domain: 2, 9 3.3 Mo 15 Se 19-y X y (X = S, Te) is possible in the limits 0

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.

Anisotropy of Hc2 in the Chevrel phases

Abstract The anisotropy of the upper critical field of PbMo6S8, PbMo6Se8 and SnMo6Se8 has been measured. All three compounds show an anisotropy of about 20 %, with the maximum Hc2 value when H is perpendicular to the ternary axis. The effective mass model with the mass ratio e2≈ 0.69 fits well our results.

Mechanical properties of Bi,Pb(2223) single filaments and Ic(ε) behaviour in longitudinally strained tapes

The Youngs modulus and fracture stress of isolated Bi,Pb(2223) filaments were deduced from three-point bending tests performed at different stages of the tapes preparation. These results were introduced in the model describing the evolution of critical current of tapes submitted to a longitudinal strain in view to predict their irreversible strain limit eirr. These calculated irreversible strain limits were compared to measured values, taken from a set of tapes made with different filling factors and composite matrices. This experiment shows that the predicted irreversible strain limits correspond to the measured ones. Presenting the Ic behaviour of highly stressed tapes in a magnetic field, we discuss the evolution of the ratio Istrongc0/Ic0 versus strain. This value, representative of the fraction of the critical current attributed to strongly connected grains, increases significantly during the crack formation regime at e > eirr. This indicates that mechanically weak links correspond to electromagnetically weak ones. This result is further confirmed by comparing the modulus of rupture obtained in single filaments extracted from tapes with different Ic values.

In∼3Mo15Se19: A new high field superconductor

Abstract We report the discovery of a new high field superconductor In∼3Mo15Se19 with a critical temperature Tc = 4.3 K. This compound crystallizes in a hexagonal crystal structure (space group P63/m) containing different building blocks of Mo6Se8 and Mo9Se11, respectively. The most important physical behaviour is the extremely high initial slope of the critical field at Tc, which varies between 68kG/K and 78kG/K. These are among the highest values ever observed in a bulk material.

Test Results of the First US ITER TF Conductor in SULTAN

The US Domestic Agency is one of six parties supplying TF cable-in-conduit conductors (CICCs) for ITER. Previous tests have shown that measured performance of the TF CICCs can be much lower than expected from the strand properties at the projected uniaxial strain and that the cabling pattern may also be an important factor. Worst of all, voltage signals well below the expected critical surface could not be reliably interpreted or canceled, making test results very suspect. The TFUS1 sample was prepared to achieve multiple goals: 1) to ensure uniform current distribution and to eliminate parasitic voltage signals by improving joints, 2) to explore the potential benefits of a different cabling pattern for better support of strain-sensitive strands, and 3) to explore the source of voltage development in the cable through the use of innovative penetrating diagnostics. Test results of the first US-made samples are presented and discussed.

Overview on the recent progress on Chevrel phases and the impact on the development of PbMo/sub 6/S/sub 8/ wires

The critical current density J/sub c/ of PbMo/sub 6/S/sub 8/ (PMS) wires has not reached a value required for industrial applications. However, large intragrain J/sub c/ has been observed, indicating that the grain connections play a dominant role in the J/sub c/ limitation. This behavior is similar to that of high-T/sub c/ oxides, where this limitation is thought to have an intrinsic origin related to their extremely short coherence lengths. But the coherence length of PMS lies between those of conventional superconductors and high-T/sub c/ materials. Therefore, it has been argued that an intrinsic granularity behavior is not expected for PMS and that the observed tendency towards granularity has another origin. Recent results on the phase stability, a possible structural transition, and the degradation of the grain boundaries of PMS point to different possible origins for the J/sub c/ limitation. A critical analysis of these investigations is presented, and their impact on the performance of wires is discussed.<<ETX>>

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.