Mechanism of Enhancement in Electromagnetic Properties of MgB2 by Nano SiC Doping

A comparative study of pure, SiC, and C doped MgB2 wires has revealed that the SiC doping allowed C substitution and MgB2 formation to take place simultaneously at low temperatures. C substitution enhances Hc2, while the defects, small grain size, and nanoinclusions induced by C incorporation and low-temperature processing are responsible for the improvement in Jc. The irreversibility field (Hirr) for the SiC doped sample reached the benchmarking value of 10 T at 20 K, exceeding that of NbTi at 4.2 K. This dual reaction model also enables us to predict desirable dopants for enhancing the performance properties of MgB2

Adiabatic normal zone development in MgB2 superconductors

A-priori knowledge of the normal zone development in MgB/sub 2/ conductors is essential for quench protection of applications. Therefore the normal zone propagation in a monofilament MgB/sub 2//Fe conductor under near-adiabatic conditions at 4.2 K has been measured and simulated. The results show normal zone propagation velocities up to several meters per second. In addition, by including the voltage-current relation into the computational model, the influence of the n-value on the normal zone propagation is determined. The simulations show that lower n-values suppress the normal zone propagation velocity due to lower heat generation in the MgB/sub 2/ filaments

Two-gap superconductivity in MgB2_{2}: clean or dirty?

A large number of experimental facts and theoretical arguments favor a two-gap model for superconductivity in MgB$_{2}$. However, this model predicts strong suppression of the critical temperature by interband impurity scattering and, presumably, a strong correlation between the critical temperature and the residual resistivity. No such correlation has been observed. We argue that this fact can be understood if the band disparity of the electronic structure is taken into account, not only in the superconducting state, but also in normal transport

SupernetNL program: 3.4 km 110 kV AC underground superconducting cable in the Dutch grid

TenneT, a leading European electricity transmission system operator (TSO) is planning to install a 3.4 km long underground superconducting 110 kV cable as part of the Dutch electricity grid, in the city of Enschede. HTS cables have already been demonstrated on a relatively small scale in other countries, but they are usually not part of the meshed high-voltage grid and the length of the relevant cable section generally does not exceed 1 km. In 2009, a 600-meter section of HTS cable was installed in New York, and in 2014 a 1-km long section was taken in operation in Essen, Germany to replace a 10 kV AC medium-voltage line. In the Supernet NL program, TenneT is working together with several leading knowledge institutes including University of Twente, Delft University of Technology, the Institute of Science and Sustain- able Development (IWO), HAN University of Applied Sciences and RH Marine. These institutes have been investigating control engineering aspects and the requirements the cable must meet. In the meantime, the tender process has been started which consists of two phases. In the fi rst phase (summer 2017) appropriate candidates are selected directly followed by a call for tender in August. Receipt of the best and fi nal offer is scheduled for the end of November. In the presentation, the project will be introduced and requirements will be discussed, specifi cally focusing on the cryogenic aspects

Unusual effects of anisotropy on the specific heat of ceramic and single crystal MgB2

The two-gap structure in the superconducting state of MgB_2 gives rise to unusual thermodynamic properties which depart markedly from the isotropic single-band BCS model, both in their temperature- and field dependence. We report and discuss measurements of the specific heat up to 16 T on ceramic, and up to 14 T on single crystal samples, which demonstrate these effects in the bulk. The behavior in zero field is described in terms of two characteristic temperatures, a crossover temperature Tc_pi ~ 13 K, and a critical temperature Tc = Tc_sigma ~ 38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field Hc2_pi ~ 0.35 T, and an anisotropic upper critical field with extreme values Hc2_sigma_c ~ 3.5 T and Hc2_sigma_ab ~ 19 T, where the indexes \pi and \sigma refer to the 3D and 2D sheets of the Fermi surface. Irradiation-induced interband scattering tends to move the gaps toward a common value, and increases the upper critical field up to ~ 28 T when Tc = 30 K.Comment: 31 pages, 9 figures. Accepted in the Physica C special issue on MgB

Distinct voltage–current characteristics of Nb3Sn strands with dispersed and collective crack distributions

Two ITER-type Nb3Sn superconductor strands, one prepared with the bronze route and the other with the internal-tin route, were used to investigate the impact of filament cracking on the strand's transport properties. Careful mechanical polishing allowed unambiguous identification of the microscopic fractures of filaments caused by axial straining of the strands. After application of high axial tensile strain, densely and uniformly spaced cracks were observed in the bronze strand, while fewer but more correlated cracks occurred in the internal-tin strand. Crack initiation was observed in the bronze strand after an applied tensile strain of more than 0.8%, while for the internal-tin strand cracks were found already in the unloaded specimen, with further crack growth beyond 0.3% applied strain. With the Pacman strain device, the voltage–current characteristics at zero applied strain were measured after several successive applications of incrementally increasing tensile strain. Distinct dissimilarities in the voltage–current characteristics were observed between the dispersed and the collective crack distributions. We also modelled the influence of cracks on the voltage–current characteristics of the two strands by considering two limiting cases of the crack behaviour

Pressure-induced critical current reduction in impregnated Nb3_{3}Sn Rutherford cables for use in future accelerator magnets

The measured critical current reduction in Nb3Sn Rutherford cables under magnet-relevant transverse pressure levels is analyzed in terms of the strain state of the filaments inside their strands. Several straightforward mechanical 2D FE models of the cables’ cross-section are used to translate the stress that is applied to the surface of the impregnated cables into a strain distribution on its strands. The resulting critical current reduction of the cable is then estimated from the average deviatoric strain in the strands’ filamentary zone, using the well-established strain scaling relations obtained for isolated strands. This allows to identify the main factors that influence the pressure response of impregnated Nb3Sn accelerator cables. The analysis is presented for state-of-the-art cable samples that were measured at the University of Twente and shows how especially stiff and incompressible resins reduces the deviatoric strain in the filamentary zone of the cable strands, but also how relatively small alignment errors can lead to stress concentrations that reduce the critical current density significantly

Role of c-axis field components in making the transport E(J) characteristics of (Bi,Pb)2Sr2Ca2Cu3Ox tapes insensitive to the direction of field within the tape plane

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