|| 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 , 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
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 , by using ball-milled powders  and by doping with nano-particles
(10-30 nm) such as SiC  and diamond .
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
). 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.
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
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 , 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.
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.
The financial support of the Spanish CICYT projects MAT-1999-1028
and MAT-2002-04121-C03-02 is acknowledged.
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