ReviewDevelopment of ex situ processed MgB2 wires and their applications to magnets
Introduction
Just a few months after the discovery of superconductivity in magnesium diboride MgB2, it became clear that this material would have been promising in the future to compete with the existing superconductors for industrial applications, in particular in the field of superconducting MRI magnets. To date, only five years after, the impressive advancement of industrially manufactured wires and prototype magnets based on MgB2 is a practical demonstration that those initial ideas were mostly truthful.
A number of techniques have been developed to improve the wire processing and to achieve higher critical current densities Jc. Several groups [1], [2], [3], [4] have followed the so-called ex situ technique, while the majority [5], [6], [7], [8], [9], [10] have preferred to use the in situ technique. Both ways are based on the powder in tube (PIT) method but while the first uses fully reacted MgB2 powders, the latter uses a mixture of unreacted Mg and B. On the other hand, first promising results have been also achieved on MgB2 coated conductors manufactured by means of various thin film growth processes.
The work presented in this paper refers entirely to conductors manufactured by the ex situ PIT process. Indeed, while the in situ route presents several advantages as low cost, high fill factor, high speed process, low temperature reaction processes, and relatively easy MgB2 nanoparticle doping, the ex situ technique currently appears to be more suitable for the development of long conductors and complex multifilamentary wire geometry, it allows for a better control of the powder granulometry and purity degree as well, and finally it leads to more robust conductors that can be readily employed to realize magnets by the Wind and React process.
Recently, Nakane et al. [11] have also proposed a novel method to transfer the enhanced superconducting properties of the in situ MgB2 powders to the ex situ conductors in order to capitalize on the advantages afforded by both methods.
Due to the aforementioned reasons, so far the Jc(B) behaviour for the ex situ conductors has not been as good as in the in situ case. Therefore, a deeper development of the starting MgB2 powders in the ex situ process is needed in order to further enhance Jc, at least at magnetic fields of the order of 2–4 T at 20 K and 5–10 T at 4.2 K. In fact these are most likely the typical operating conditions of the conductors when we consider their most effective use in a magnet for MRI.
Nevertheless, especially from the point of view of the industrial applications, improving the starting MgB2 powders for the ex situ fabrication process seems to be a feasible way to make these tapes definitely competitive. The doping and the granulometry control are two straightforward ways to run along at the beginning of such optimisation process.
In this paper, an overview of the results reached in the recent past on MgB2 conductors is briefly reported. First, we will present the technique developed for the fabrication of mono- and multifilamentary tapes in long lengths and an overview of their main properties. Then we will show how the tape performances can be improved by modifying the properties of the starting MgB2 powders: in this case, the transport Jc vs. B behaviour of monofilamentary tapes fabricated through the ex situ technique will be reported up to very high fields. We will focus on the addition of SiC nanoparticles to the B before the reaction with Mg, on the high-energy ball milling of MgB2 powders alone and with the addition of SiC or C. Finally, we will review the large-scale application devices developed with our multifilamentary tapes up to this moment.
Section snippets
Preparation of long multifilamentary MgB2 tapes
Through the ex situ technique, suitable MgB2 powders are packed inside metallic tubes in order to prepare both mono- and multifilamentary conductors: the powders are currently prepared from commercial amorphous B (95–97% purity) and Mg (99% purity): these precursors are mixed and then they undergo a heat treatment at 900 °C in Ar, reacting almost entirely into MgB2 with some residual traces of MgO in a quantity well below 10 wt%.
Ni tubes are filled with such reacted powders with a packing density
Superconducting properties of the ‘standard’ multifilamentary tapes
The transport critical current Ic was measured using the standard four-probe method in varying magnetic field applied both perpendicular and parallel to the tape surface direction and at different temperatures. In Fig. 3, Fig. 4 the Ic behaviour as a function of the temperature and the magnetic field is reported as measured by Kitaguchi at NIMS in Tsukuba, Japan, on short multifilamentary conductors cut from a unit length exceeding 1.6 km.
The Ic value drops quickly as the magnetic field
Improvement of the performances of the conductor
It is well known that in the production process of MgB2 conductors, several parameters have a direct influence on Jc in the superconducting filaments, and that the achievement of relevant transport properties can be achieved only after optimising the superconducting and/or microstructural properties of the initial powders and of the constituents composing the conductor itself.
The first example of long length (1.53 km) of multifilamentary MgB2 conductor was demonstrated in April 2005. This was an
MRI magnets
ASG Superconductors started to design an open MRI magnet based on the use of MgB2 conductors well in advance compared with the expected industrialization of the conductor. Several times during the engineering study, the design was changed to follow the evolution of the conductor. On the other hand, this work gave the possibility to start winding the first double pancake composing the magnet just few weeks after the production of the conductor by Columbus Superconductors in its first real and
Superconducting joints
The fact that it was possible to use the powder in tube (PIT) technique to easily obtain superconducting wires with MgB2 filaments gave to everybody the impression that a superconducting joint between MgB2 conductors would be successfully developed sooner or later.
The solution to this problem was not easily individuated though, but after several attempts ASG Superconductors has been able to develop a technique to join individual strands that produce an electrical joint resistance not measurable
Conclusions
We have presented an overview of the results reached in the recent past on MgB2 conductors. In the last two years, the MgB2 compound has demonstrated the potential to approach a solid industrial development. Several long lengths of conductor were already produced and delivered to customers by different manufacturers. A large bore MRI magnet, operating in a cryogenic-free environment at 20 K, was built and successfully tested. The possibility to perform superconducting joints was demonstrated for
Acknowledgements
C. Bernini, A. Malagoli, A.S. Siri, A. Tumino, M. Vignolo, are acknowledged for the work performed at LAMIA; C. D’Urzo, A. Laurenti, R. Marabotto, M. Modica, M. Tassisto for the work performed at ASG. E. Mossang from GHMFL is acknowledged for technical support.
Financial support from “Transnational Access – Specific Support Action” Program – Contract no. RITA-CT-2003-505474 of the European Commission and from the EU-FP6 STRP ‘HIPERMAG’ – Contract no. NMP3-CT-2004-505724 – is acknowledged.
References (16)
- et al.
Physica C
(2001) - et al.
Physica C
(2001) - et al.
Physica C
(2002) Superconcond. Sci. Technol.
(2001)- et al.
Appl. Phys. Lett.
(2001) - et al.
Supercond. Sci. Technol.
(2002) - et al.
Supercond. Sci. Technol.
(2003) - et al.
Appl. Phys. Lett.
(2002)