A. Tumino

Improvement of Magnetic Field Behavior of Ex-Situ Processed Magnesium Diboride Tapes

MgB2 tapes have been synthesized through the ex-situ powder-in-tube (PIT) method. This technique involves the cold working of tubes of various metals-in this case Ni-previously filled by suitably reacted MgB powders, followed by proper heat treatments. In particular, it allows the use of different starting powders and the control over their properties. We studied the influence of the starting powders on the superconducting properties of the final conductors, and we tried to improve their behavior in magnetic field by mastering their grain size and by inserting appropriate doping. In order to improve the pinning properties of the tapes, the granulometry has been varied through ball-milling of MgB, and different dopants-such as carbon or SiC nanoparticles-have been introduced either on the precursors or on MgB. Critical current measurements on the tapes are presented, both magnetic from the SQUID and transport in high magnetic field. In particular, IV characteristics have been measured up to 13 Tesla at GHMFL (Grenoble) in order to extract the critical current in two directions, i.e. with the field perpendicular and parallel to the tape surface.

Behavior of MgB2 react & wind coils above 10 K

React & wind magnesium diboride pancake coils and solenoids were constructed and tested. The solenoids (6 layers) were manufactured by winding a single length of MgB/sub 2/ tape (80 m) onto a 15 cm diameter, 15 cm high mandrel. The pancake coils were manufactured starting from a 40 meter long tape: the conductor was wound onto a 12 cm diameter coil former. The magnets were tested both in liquid helium and conduction cooled in a cryogen free apparatus. The results compared with the tape characteristics are reported.

MgB/sub 2/ tapes with non-magnetic sheath: effect of the sintering temperature on the superconducting properties

A nonmagnetic sheath, namely a Ni/Cr alloy (Ni80/Cr20), was used to fabricate MgB/sub 2/ tapes through the powder-in-tube (PIT) ex-situ technique. A series of samples sintered at different temperatures between 200/spl deg/C and 950/spl deg/C were prepared in order to correlate all the superconducting properties between them and with the sintering temperature. The possibility of using a nonmagnetic but at the same time hard sheath chemically compatible with the MgB/sub 2/ phase made possible a simpler interpretation of transport and magnetic measurements in magnetic field. With the chosen cold working parameters, we found that the amount of MgB/sub 2/ lattice strain was not sufficient to produce any enhancement of the upper critical field. The effect of the heat treatment temperature was on one hand to increase the grain connectivity but also to reduce the pinning properties of the superconductor.

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MgB2 tapes and wires have been synthesized through the ex-situ Powder-In-Tube (PIT) method. We studied the effects of using different metallic materials for the sheaths in new round or square shaped multifilamentary wires; enhanced critical currents in 19 filaments wires are reported. The influence of the starting powders on the superconducting properties of the final conductors was thoroughly investigated in order to improve their behavior in magnetic field. In particular we analyzed the effect of controlling the grain size in the powders by high energy ball milling and by lowering the synthesis temperature. TEM analysis results are shown that highlight the role played by the amorphous layer at the grain boundaries.

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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.

Powders and Study of the Properties and Architecture of Ex-Situ PIT Wires

Magnesium diboride (MgB/sub 2/) pancake coils and solenoids have been constructed and tested. The magnets have been wound using the react and winding technique. The magnets have been tested in liquid helium bath up to the quench currency. In this paper the coils and their behavior is discussed.

Magnesium Diboride Wires With Nonmagnetic Matrices—AC Loss Measurements and Numerical Calculations

Among perovskite oxides strontium titanate (STO) SrTiO3 undergoes a metal–insulator transition at very low carrier concentration and exhibits high mobility values at low temperature. We exploited such electrical properties and the structural compatibility of perovskite oxide materials in realizing ferroelectric field effect epitaxial heterostructures. By pulsed laser deposition, we grew patterned field effect devices, consisting of lanthanum doped STO and Pb(Zr,Ti)O3. Such devices showed a resistance modulation up to 20%, consistent with geometrical parameters and carrier concentration of the semiconducting channel.

Construction and tests of MgB/sub 2/ react & wound coils

Magnesium diboride (MgB2) is replacing some of the conventional superconductors due to its low cost and availability in kilometer lengths. MgB2 has also been considered for AC applications. In order to model the AC losses and the critical currents of the applications, intrinsic Jc(B)-dependence is an important factor also at low fields. In this work Jc(B)-dependence of an MgB2 sample is extracted from the standard in field voltage-current measurements. The proposed method is applied to a non magnetic titanium sheathed sample at 16 and 20 K and a simple formula for Jc(B) aligns with the measurements. In the fitting process, the critical current distribution inside the wire is numerically simulated in order to take the self field of the sample into account. Moreover, the same formula aligns with measurements of a different sample. These critical current measurements, performed at 4.2 K, were based on magnetization. In the self field computations, the superconducting cross section must be determined accurately. Therefore, we tailored an image processing tool for MgB2 wires to obtain the geometry from a photograph.

Strontium titanate resistance modulation by ferroelectric field effect

Among the characteristics required by a spaceborne superconducting magnet, lightness and reliability are the most important. The use of titanium clad magnesium diboride conductors fulfills both the requirements: it allows avoiding helium cryogenics, it guarantees good stability, and it has density below 4500 kg/m 3. A conductor based on a Ti-MgB 2 tape was developed in the framework of a study on superconducting shields to protect astronauts against cosmic rays, co-funded by the European Union (EU FP7 SR2S Project). The magnet is designed to operate at 10 K and below 4 T at the conductor. The prototype tape, manufactured via ex situ method, is 3 mm wide and 0.5 mm thick and has 19 MgB 2 filaments. According to the original design, a pure aluminum strip should be coupled to the tape in order to protect the conductor in case of quench. However, due to technical problems, the process was completed by soldering a copper strip on titanium. The conductor and its manufacture process are described in this paper.

Low field critical current density of titanium sheathed magnesium diboride wires

In spite of the relatively short time dedicated to the development of MgB2 conductors since its discovery in 2001, a remarkable progress has been already achieved in terms of critical current density, homogeneity of the conductors and reproducibility of the results over long lengths. Unlike other HTS and LTS materials, MgB2 conductor processing is more open to a number of improvements and modifications that help in making it more attractive for several DC and AC applications. Up to now our work has been focused on the improvement of the production capacity in order to allow for the assembly of large prototype magnets to test the capability of MgB2 to maintain the present level of performance over very long lengths. Long MgB2 tapes of about 1600 m with superconducting and mechanical properties suitable for making low field magnets are already available. Presently we are focusing on the enhancement of the irreversibility field. This would open the possibility of using MgB2 also for high field applications.