LT 21 Proceedings of the 21st International Conference on Low Temperature Physics Prague, August 8-14, 1996

Part $5 -Teehniques and applications: Instrumentation

Overall Critical Current Density of Chevrel Wires in Magnetic Fields up to 24 tesla

M. Decroux a, N. Cheggour a, A. Gupta a, V. Bouquet b, R. Chevrel b, J.A.A.J. Perenboom c and 0. Fischer a

a University of Geneva, 24 quai Ernest Ansermet, 1211 Gen~ve 4, Switzerland b CSIM/URA 1495, University of Rennes, France c High Field Magnet Lab, University of Nijmegen, Netherlands

Wires of Pbo.6Sno.4Mo6S 8 with a protective sheet of Niobium have been carefully drawn and the critical current density Jc has been measured on small coils in magnetic fields up to 24 tesla. At 1.9K, the overall Jco, exceeds 100 A/mm 2 at 20 tesla and 60 A/mm 2 at 24 tesla. These large values highlight the good sintering between all the grains. However, the field dependence of Jc still indicates that the effective upper critical field Bc2 is too low especially at 4.2K (31.5 tesla instead of 50 tesla).

1. INTRODUCTION There is a strong need for a new technological superconducting wire able to produce magnetic fields well above 20 tesla. Among the known materials, the Chevrel phases have a great potential, since the upper critical field Be2 of several of the superconducting compounds of this family have values exceeding 35 tesla (35-40 tesla for SnMo6S 8 and 55-60 tesla for PbMo6S 8) [1]. However, it has been reported that the performance of this material may be limited at high fields by an irreversibility line whose origin should be similar to those observed with high temperature superconductors [2]. A detailed study of the critical current density on bulk samples gave evidence that this limitation has its origin in the degradation of superconducting properties at the grain boundaries[3]. Transmission electron microscope studies of the grain boundaries have revealed the presence of MoS 2 at the surface of the grains as a consequence of the decomposition of Mo6S 8 when Pb escaped from the surface [4]. Therefore the preparation of superconducting wires with optimal superconducting properties requires: a) production of non contaminated superconducting powder especially by oxygen which would consume the molecular Mo coming from the Mo6S 8 decomposition; b) good densification of the powder prior to the heat treatment to avoid losses of lead.

2. WIRES PROCESSING The wires are manufactured with the powder-in-tube process. The billet consists of an external sheet of Cu or CuNi30 and an internal one of Niobium which act as an anti diffusion barrier to protect the superconducting

Figure 1.- Micrography of wire#1 and wires#2.

Figure 2.- Micrography of wire#1 and wires#2

PSMS powder from Cu and Ni contamination. The powder used for these wires is obtained from the decomposition/reaction of Mo6S 8 with the appropriate amount of Pb and Sn [5]. The powder is outgassed at 200~ under secondary vacuum, inserted into the billet and mechanically compacted to insure a density of the

Czechoslovak Journal of Physics, Vol. 46 (1996), Suppl. S5 2757

order of 60-70%. The billet is swaged down to a diameter of 8 mm and then drawn with conventional dies. At a diameter of 2ram, the wire is reinforced with stainless steel and drawn to a diameter of 0.6ram(wire#1 SS/Cu/Nb/PSMS and wire#2 SS/CuNi/Nb/PSMS). On wire#3, CuNi was removed prior to the reinforcement with stainless steel (SS/Nb/PSMS). Two meters of wire are wound on a stainless steel support for the final heat treatment under an isostatic pressure of 1.9 kbar of argon for 0.5 to 2 hours at 900~

3. E X P E R I M E N T A L R E S U L T S The Figure 3 presents the critical current density Jc at 4.2K for the three types of wires investigated. At low magnetic field, wire#2 has a lower Jc as compared to the two other wires. This is the manifestation of the poor thermal stability of the wire with CuNi which shows thermal excursions [6]. At higher magnetic field, all the wires have approximately the same Jc demonstrating the good reproducibility of these wires. In Figure 4, the magnetic field dependence of the overall

~: i.2 10 9

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z s l0 ~ t3 h- 6 tO ~ Z

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,.. OlO o

. . . . i . . . . l . . . . i . . . .

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O WIRE#2 : SS/CuNi/Nb/PSMS

Z~ WIRE#3 : SS/Nb/PSMS

. . t t l t I , , | | , , , I , , , l

5 10 15 20 MAGNETIC FIELD [tesla]

Figure 3.- Magnetic field dependence of Jc.

25

t 0 ~

E

o

\ B =38 tesla 1.9 K \

. . . . -,., , , , , , , , . , , , , . , . \ . . . . . , , , , 1o' 10 15 20 25 30 35 40

MAGNETIC FIELD [tesla] Figure 4.- Magnetic field dependence of Jco vs. Br

critical current density Jco of wire#3 is reported. At 1.9K, Jco exceeds 100 A/mm 2 at 20 tesla and 60 A / m m 2 at 24 tesla. However, as the temperature is raised to 4.2K the strong decrease of Jco indicates that we are getting closer to the effective upper critical field.

4. D I S C U S S I O N The solid lines in Figure 4 represent the best fit of the data assuming a pinning mechanism at the grain boundaries with Be2 as a free parameter (Bc2 = 31.5 tesla and 38 tesla at 4.2 and 1.9K respectively). Assuming that the intrinsic Bc2 of 50 tesla at 4.2 K can be achieved at the grain boundaries, an appreciable increase of Jco is expected. In this case the barrier of 100A/mm 2 will be crossed above 30 tesla. These very encouraging results for Chevrel wires show simultaneously that the critical current can still be improved especially if superconducting properties at the grains boundaries as T c and Bc2 can be restored at their intrinsic values. Since lead is escaping easily from the phase at the grain boundaries, we expect that a similar mobility may also homogenise its content between adjacent grains and in consequence the superconducting properties across them.

5. C O N C L U S I O N This work has clearly demonstrate that Chevrel phase wires fulfil the technological criteria of an overall critical current of 100A/mm 2 at field higher than 20 tesla. The very large values of Jc obtained at fields smaller than 15 tesla (Jc>500A/mm 2) demonstrate that the superconducting grains are now very well connected. However at higher magnetic field, the sudden drop o f J c shows that the superconducting properties at the grain boundaries are still degraded. By an appropriate heat treatment the superconducting properties of the grain boundaries could be improved.

6. R E F E R E N C E S [1] 0 . Fischer and B. Maple, Superconductivity in

Ternary Compounds I and II, Topics in Current Physics, Vols. 3 .32 and 34, (Springer, 1982).

[2] C. Rossel et al, Physica C 165 (1990) 233. [3] A. Gupta et al, Physica C 234 (1994) 219. [4] S. Even-Boudjada, PhD thesis University of

Rennes, 1994. [5] M. Decroux et al, lEE Trans. Applied

Superconductivity 3 (1993) 1502. [6] N. Cheggour et al, Physica C 258 (1996) 21

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