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Superconducting properties of metal/MgB 2 composite wires

E. Martínez, L.A. Angurel, A. Millán, R. Navarro

The development of materials with the new MgB 2 superconductor [1], which has critical temperatures of T c =39 K, has opened great expectations. This is because it is a suitable candidate for large-scale electrical applications such us magnets for Magnetic Resonance Imaging, transformers, motors, generators… operating in the range of 20-30 K, easily reachable with cryocoolers.

A key support of this interest arose from the conformation of long metal/MgB 2 composite wires and tapes with high critical currents densities ( J c =10 9 -10 10 A/m 2 ) using well-known powder-in-tube (PIT) methods. Because its potential scalability and production flexibility, these technologies are attractive and inexpensive for long-wire fabrication.

J c , which is the maximum current per unit area that the superconductor wire can carry without energy losses, is one of the most important parameters for the characterisation of such conductors. J c decreases under applied magnetic fields, B , reducing to zero at a value called irreversibility field, B irr .

Currently, MgB 2 materials have relatively low B irr values (4-6 T at 20 K), hence special efforts are being made to improve the superconducting behaviour under high fields. Some groups have reported improvements of the J c ( B ) dependence by neutron irradiation of the samples [2], by using ball-milled powders [3] and by doping with nano-particles (10-30 nm) such as SiC [4] and diamond [5].

In the PIT technique, mixtures of unreacted Mg and B powders, the so called in situ reaction approach; or pre-reacted MgB 2 powders, ex situ reaction technique, are packed inside metal tubes (in our case, typically of 4.0 mm outer diameter and 2.5 or 3.0 mm inner ones). Subsequently the tubes are cold drawn in round dyes down to 1.2 mm of diameter in 0.1 mm reduction steps. The final wires have core diameters of 0.6 or 0.8 mm, depending on the initial inner diameter of the tubes. Finally the wires are annealed in argon, to prevent oxidation, in order to react ( in situ ) or sinter ( ex situ ) the core precursors.

The selection of adequate sheaths for these composites, giving thermal, electrical and mechanical stability, constitutes nowadays an open issue to be addressed prior to reach technologically useful metal/MgB 2 conductors. Different metal sheaths have already been used: Fe, Cu, Ni, Ag, Cu-Ni alloys and stainless steel (SS), as well as different metal combinations such as: (from outside to inside) Cu/Ta and SS/Cu/Fe (for a review see for instance [6]). Up to now, the highest J c ( B , T ) values are obtained with hard metal sheaths that do not react with Mg or MgB 2 , such as steel and iron, but these wires, as consequence of a poor thermal stability, easily transit to normal state (quench) at J c > 10 9 A/m 2 . Moreover, silver sheathed wires present very poor results compared with copper or nickel sheathed wires and therefore have been disregarded.

Here, we report on the superconducting properties of Cu- and Ni-sheathed MgB 2 mono-filament wires fabricated in the ICMA by the PIT technique using the in situ and ex situ procedures, respectively. Both are interesting sheath candidates due to their properties of ductility, soldability and good thermal conductivity.


Created by Digital Micrograph, Gatan Inc.

Created by Digital Micrograph, Gatan Inc.

Figure 1: Longitudinal SEM images of (a) ex situ Ni/MgB2 wire annealed at 850 °C during 0.5 h; and (b) in situ Cu/MgB 2 annealed at 700 °C during 0.5 h. The circle in b) indicates the main phase containing MgB 2 .

A typical SEM longitudinal cross-section of Ni/MgB 2 and Cu/MgB 2 composite wires after annealing is shown in Figure 1. In both cases the superconducting cores have irregular shape. This is partially produced during mechanical conformation because the lower hardness of the sheath with respect to the precursors (particularly, boron and MgB 2 ), but it would be also due to the reactivity of the precursors from the core with the inner sheath wall during the heat treatment.

A reaction layer adjacent to the superconducting core, with darker contrast and 20 to 30 m m thickness, is observed for both wires. EDX and X-Ray analyses have indicated that this layer corresponds to MgNi 2 and MgCu 2 while the rest of the sheath remains pure nickel and copper, respectively. Nevertheless, the ex situ Ni/MgB 2 wires presents more homogeneous microstructures than the Cu-sheathed ones, which also show MgCu 2 grains homogeneously dispersed inside the core, with sizes typically ranging from 20 to 80 m m.

Figure 2. Magnetic field dependence of the critical current density, J c,M (B), estimated from the M-B hysteresis loops at different temperatures for the Cu- (a) and Ni-sheathed wires (b) annealed at 850 ºC during 0.5 h. ‘q' in (a) means quenched at room temperature.

The field dependences of the critical current density estimated form the magnetization hysteresis curves, J c,M ( B ), are shown in Fig. 2. Note that for these wires, J c values of 10 9 A/m 2 would correspond to currents of 500-600 A.

Ni-sheathed wires show a superconducting behaviour better than Cu ones, having higher J c values and less sharp J c ( B ) decays. This way, J c,M decreases down to 10 8 A/m 2 for magnetic fields of 3 and 2 T at 20 K, for the Ni- and Cu-sheathed wires, respectively.

For the same sheath material, the annealing conditions only affect the J c values but not their field dependence, indicating that these parameters, for the used range here, do not change the pinning mechanisms. For the Cu-sheathed wires, it has been observed that an excess of Mg over the stoichiometric proportions (x in fig. 2-a, given in atom.%) results in an improvement of J c by the increase of the amount of superconducting phases

From our results (Fig. 2-b), it is clear that doping with SiC nano-particles (20 nm average size) is also effective in improving the J c ( B ) decays when using the ex situ procedure instead of the in situ reaction originally used in [4], obtaining J c,M >108A/m2 for B <3.7 T at 20 K.

The superconducting behaviour of Ni-sheathed wires shows that nickel may be a valuable alternative to iron for practical applications. Nevertheless, non-magnetic Ni based alloy would be preferable. Moreover, it has to be noted that Ni would not be suitable with in situ procedures because of the important reactivity of Mg and Ni. Therefore, doping would require either the ex situ method or the use of diffusion barriers between the core and the Ni sheath.

The lower performance of Cu/MgB 2 wires would be due to the in situ reaction it-self and to the reactivity between the Cu sheath and the Mg precursors together with the insufficient hardness of copper. Nevertheless, main advantages of these MgB 2 /Cu wires are certainly related to the good thermal stability given by the high thermal conductivity of the sheath. This allows carrying out transport measurements up to high currents (500-700 A at 15-20 K) without quenching, even without surrounding cryogenic liquid or gas to thermalise the sample, which is very unlikely on wires with others sheaths such as iron, stainless steel, nickel, etc.

Our future efforts are focused in the search of conductors thermally stable and with high performance at fields between 5 to 10 T, both necessary for technical applications.

Principal publications

E. Martínez, L.A. Angurel, R. Navarro, Supercond. Sci. Technol 2002, 15, 1043.

E. Martínez, A. Millán, R. Navarro, European Conference of Applied Superconductivity, EUCAS , 2003.

Acknowledgements

The financial support of the Spanish CICYT projects MAT-1999-1028 and MAT-2002-04121-C03-02 is acknowledged.

References

  • [1] Nagamatsu J, Nakagawa N, Murakanaka T, Zenitani Y, Nature 2001, 410, 063.
  • [2] Eisterer M, Glowacki B A, Weber H W, Greenwood L R and Majoros M, Supercond. Sci. Technol. 15 , 1088 (2002).
  • [3] Flükiger R, Lezza P, Beneduce C, Musolino N, Suo H L Supercond. Sci. Technol. 16 , 264 (2003)
  • [4] Zhou S H, Pan A V, Qin M J, Liu H K, Dou S X, Physica C 387 321 (2003).
  • [5] Cheng C H, Zhang H, Zhao Y, Feng Y, Rui X F, Munroe P, Zeng H M, Koshizuka N, Murakami M, Supercond. Sci. Technol. 16 , 1182 (2003).
  • [6] Flükiger R, Suo HL, Musolino N, Beneduce C, Toulemonde P, Lezza P, Physica C 385 286 (2003)

 

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