E. Mossang

Effect of grain refinement on enhancing critical current density and upper critical field in undoped MgB2 ex situ tapes

Ex situ powder-in-tube MgB2 tapes prepared with ball-milled, undoped powders showed a strong enhancement of the irreversibility field H∗, the upper critical field Hc2, and the critical current density Jc(H) together with the suppression of the anisotropy of all of these quantities. Jc reached 104 A/cm2 at 4.2 K and 10 T, with an irreversibility field of about 14 T at 4.2 K, and Hc2 of 9 T at 25 K, high values for not-doped MgB2. The enhanced Jc and H∗ values are associated with significant grain refinement produced by milling of the MgB2 powder, which enhances grain boundary pinning, even if at the same time also reducing the connectivity from about 12% to 8%. Although enhanced pinning and diminished connectivity are in opposition, the overall influence of ball milling on Jc is positive because the increased density of grains with a size comparable with the mean free path produces strong electron scattering that substantially increases Hc2, especially Hc2 perpendicular to the Mg and B planes.

Enhancement of the critical current density and the irreversibility field in maleic anhydride doped MgB2 based tapes

A significant enhancement of Jc, Hirr, and Hc2 in MgB2 tapes has been achieved by using maleic anhydride (C4H2O3) as dopants. MgB2 tape samples with 0 up to 30wt% C4H2O3 added were prepared by an in situ processed powder-in-tube method. Compared to the pure tapes, Jc for the 10wt% C4H2O3 doped samples was improved by more than an order of magnitude in high fields up to 18T. At 4.2K, the transport Jc for the 10wt% doped samples reached 1.08×104, 5.42×103, and 2.18×103A∕cm2 at 12, 14, and 16T, respectively, and were better than those of the best MgB2∕Fe tapes reported by far. Furthermore, the Hirr for the 10wt% doped sample reached 9.6T at 20K, which was comparable to that of NbTi at 4.2K. The improvement of the superconducting properties in doped tapes can be attributed to the increase of Hc2 and the grain size refinement by C4H2O3 doping.

J/sub C/(B,T) characterization of NbTi strands used in ITER PF-relevant insert and full-scale sample

In the framework of the R&D for the ITER Poloidal Field Coils (PFC), a testing campaign was achieved on NbTi strands used in PFC-relevant modules. Three candidate NbTi strands have been used for manufacture of two Full-Size Joint Samples (PFIS and PF-FSJS) and the PF Conductor Insert (PFCI): - one strand manufactured by Europa Metalli (Italy), Ni plated and 0.81 mm diameter; - one strand manufactured by Alstom (France), with a CuNi barrier and 0.81 mm diameter; - one strand manufactured by Bochvar VNIINM (Russia), Ni plated and 0.73 mm diameter. An extensive test campaign has been performed at GHMFL (CNRS, Grenoble) in 2003 using the CEA VTC facility at temperatures ranging from 4.2 K to 7.5 K and at magnetic fields ranging from 2.5 to 10.5 T. The results showed that all three strands have quite close performances in the ITER PFC operating range despite their structural differences and that due to the critical current density curvature with temperature, they fail to meet the 1.5 K PFC temperature margin criterion by 0.2 to 0.3 K. Additionally, applications of corresponding fits to PF-FSJS tests showed consistent results and finally a brief discussion on fitting formula is presented.

Doping with a special carbohydrate, C9H11NO, to improve the Jc-B properties of MgB2 tapes

A significant improvement in Jc at both 4.2 and 20 K has been achieved for MgB2 tapes using C9H11NO as a dopant. It is found that C9H11NO doping will cause a small depression in Tc while the Jc and Hc2 values are strongly enhanced. The highest Jc was obtained for a C9H11NO doped sample sintered at 800 °C, namely 3.7 × 104 A cm−2, at 4.2 K, 10 T. This is the highest Jc value obtained for a MgB2 tape using a carbohydrate as a doping material. Moreover, at 20 K, 5 T, a transport Jc value as high as 1.9 × 104 A cm−2 is observed, suggesting that MgB2 holds promise for practical applications. Besides the improvement of grain linkage, the clearly enhanced Hc2 is thought to be a major reason for the prominent Jc performance, in a field, for C9H11NO doped MgB2 samples.

Characterization of transport properties variations with magnetic field and temperature of ITER-candidate NbTi strands

Abstract While the International Thermonuclear Experimental Reactor (ITER) conceptual design retained the Nb 3 Sn for toroidal field (TF) and central solenoid (CS) coils, the low working field (around 6 T) promoted the choice of NbTi for the poloidal field (PF) coils. EU has carried out the experimental study of industrial NbTi strands and cables with different internal structures in order to choose the one which generate the lowest losses when used in the PF operating conditions (i.e. pulsed field). CEA has contributed to this project through the experimental study of the transport properties variations with respect to magnetic field and temperature of two candidate strands. One of them contains an internal CuNi barrier and the other is Ni-plated. A homemade cryostat is used to control the temperature of the sample which is wound on a VAMAS-like mandrel. J c measurements are presented here and subsequently the parameters deduced from scaling laws and their variation with temperature between 4.2 and 7 K and with field up to 11 T. A comparison between the two strands characteristics and ITER PF coils criteria is also discussed. The results are in good agreement with literature and lie inside an acceptable range in spite of some discrepancy with the ITER PF criteria: a recent thermo-hydraulic simulation confirmed it. In the future, this study, completed by AC losses measurements on cabled strands, should help to optimise the strands performances below the ITER PF security margins.

Growth and properties of MOCVD YBa2Cu3O7−x thin films

Abstract Epitaxial thin layers of YBa 2 Cu 3 O 7−x are synthesised by thermal decomposition (750 – 830 °C) of tetramethylheptanedionates of yttrium, barium and copper in the presence of oxygen. Argon is used as a carrier gas and the partial pressures of the different precursors are monitored via a careful control of the sources temperatures. The superconducting films with thicknesses ranging between 40 nm and 200 nm are grown on (100) SrTiO 3 , (012) LaA1O 3 or (100) MgO. The growth rate varies between 2.7 nm/min and 4 nm/min. The layers are analysed by scanning and transmission electron microscopy, x-ray diffraction and Rutherford backscattering spectrometry. The normal — superconductor transition is investigated via DC and AC resistance, magnetization and AC susceptibility measurements as a function of temperature. Magnetisation hysteresis loops recordings, I–V measurements on microbridges and non linear susceptibility analysis are used to explore the irreversible properties of the layers. Typical parameters for MOCVD films grown on LaA1O3 are as follows: T c = 90 K, Δ T c = 0.4 K and J c (77 K) = 2 10 6 A cm −2 .

Structure and morphology of YBa2Cu3O7−x LPCVD layers

Abstract Thin films of YBa 2 Cu 3 O 7−x have been grown by chemical vapor deposition at 825 °C (5 Torr). The starting materials are tetramethylheptanedionates of Y, Ba and Cu. The superconducting properties of the films obtained on monocrystalline MgO have been studied by AC resistivity, AC susceptibility and magnetization measurements. Superconducting layers with zero resistance at 85 K and a transition width lower than 2 K are obtained. This communication will address more specifically the structural properties as well as the morphology of the layers. The films are highly orientated with their c-axis perpendicular to the substrate plane, and the pole figures show a very good in-plane orientation. The morphology of the layers has been investigated by scanning electron microscopy: we do observe a strong correlation between the roughness of the surface and the composition of the gas phase.

Effects of NbC addition on the critical current density of MgB2 tapes

NbC-added MgB2 tapes with different heating temperatures were prepared through the in situ powder-in-tube method. The effect of NbC additions on the MgB2/Fe tapes was investigated. The results show that the sintered temperature has a remarkable effect on the critical current density (JC) of NbC-added samples. Compared to pure tapes, the field JC at either 4.2 or 20 K is much improved in the NbC-added samples sintered at 800 °C. The mechanism of JC enhancement by NbC addition is also discussed.

Measurements of critical currents as a function of temperature in YBa2Cu3O7-x thin films: a comparative study

The critical current density (Jc) for zero magnetic field has been measured as a function of temperature in c-oriented thin layers of YBa2CU3O7-x prepared by different synthesis methods. One direct and two indirect Jc(T) methods were employed for each film: transport measurements on patterned film constrictions, DC remanent magnetization studies using the Bean model and non-linear AC screening analysis. Results from the indirect methods were calibrated against transport data. Possible origins of observed dependences are suggested and calibration procedures are discussed. The investigation demonstrated the feasibility of using AC screening analysis in rapid determinations of Jc(T) close to Tc.

Geometry Optimization for SMES Solenoids Using HTS Ribbons

SMES devices are an attractive solution for pulsed current sources. The specific stored energy is moderate therefore it has to be improved. HTS materials offer opportunities in terms of current carrying capabilities as well as mechanical strength. The specific magnetic storage capability is presented as a function of the aspect ratio of solenoid. Shape optimization is discussed using the strongest superconducting tapes available.