Critical currents and pinning mechanism of superconducting Nb3Sn multifilamentary tape

PHYSICA ELSEVIER

Physica C 341-348 (2000) 1139-1140 www.elsevier.nl/Iocate/physc

Critical Currents and Pinning Mechanism of Superconducting Nb3Sn Multifilamentary Tape H.I~ Shaoa, S.A. Anma a, Y.M. Caib, X.L. Sun b, C.J. Wanga, J.R. Fangc aDepartment of Physics and National Laboratory of Solid State Microstructures, Center for Advanced Studies in Science and Teohnology of Mierostruetures, Nanjing University, Nanjing 210093, P.R. China ~)opartment of Basic Sciences, Nanjing University of Chemical Technology, Nanjing 210009, China ¢Changsha Research Institute of Mining and Metallurgy, P.O. Box 67, Changsha 410012, China Results of transmission electron microscopy examination as well as critical current J~ measurements at 4.2K under fields of up to nearly He2 for superconducting Nb3Sn multifilamentary tape optimized with a new method of multiple heat treatment and processing cycles are presented. The pinning mechanism of the optimized samples is discussed with respect to these results, demonstrating the theoretical basis for the optimization process.

Keywords:Critical current, Pinning mechanisms, Wire and tape, Vortex dynamics. Since multifilamentary Nb3Sn tape and V~Ga are mostly used under high magnetic fields, we desire new superconducting materials that can be used under such high magnetic fields. Metallurgical factors (heat trealrnent, alloy elements and mechanical main) can obviously influence the J¢, but the effective control of these factors still requires further investigation. Here, we described the current density and pinning properties of a pre-reaction supeeondueting Nb3Sn tape. The measured Jc were used to plot curves of Jcl/2Hl/4-H. The scaling law derived by Krarner [1,2] is applicable to Nb3Sn tape. Using its original form to plot the curve F/Fm~- hl/2(1-h) 1/2, (where F=J~-I is body pinning force, h=H/Hc2 is reduced magnetic field), the scaling law can be determined through these curves. But there are difficulties in plotting these curves. To overcome these difficulties, we used a convenient form for reaching high field properties ofJc ;

Jcl/2Hl/2= 0.371cl(l_aopl/2)q(Hc2_H),

O)

where ao is magnetic flux lattice distance, p is the density of pinning centers and K the GL parameter. For small grain size between 0.1-0.2pro, we can assume ao << p-1/2,and the Jci/2Hl/4-H curve is nearly

a straight line, which can be extrapolated to Hco. This curve suits Kramer's law [1], but he assumed that flux motion under high field is eonlrolled only by the shear motion, having no relation with the pinning strength. However, experiments have shown

lo

,

13

14

16

14

t7

1|

tl1

H (T)

Fig.1. Effect of strain on J¢1/2H1/4-H curve. Strain effect in curve (a) is greater than that in curve (b). Samples a and b are the same but under different strains. that grain size and other factors influence Jc under bigh field, hence Kramer's assumption is incorrect and we have therefore modified eq.(1) as follows;

0921-4534/00/$ - see front matter © 2000 ElsevierScience B.V. All rights reserved. PI! S0921~534(00)00825-X

1140

H.M. Shao et al./Physica C 341-348 (2000) 1139-1140

Jcl/2H1/4= f(p,K) (Hc2-H) / (l-a0pI/2)

(2)

where f(p,~)=Apl/4/K, and A denotes pinning strength. To fully understand eq.(2), we supplement some experimental examples here. From Fig.l, Hc2 in curve (b) is larger than that in curve (a) because of the increase in T¢. If the increase in He2 is caused by the direct increase in T¢, we can assume K to be constant, i.e., 0c=7.5 ×103~/1/21~, Hc2= 3.11×103~ p, Te, where On is the normal residual resistivity and 7 is the specific heat constant). Strain only has a little influence on the pinning strength and grain size, i.e., the coefficient K is constant, implying that the slopes of lines (a) and (b) are the same since strain has little effect on rc and p.

~ ~

'

e,O . . . .

t

.

~ 6

I

L• 7|@~2XI5Itt (3,1u)

. . ~ t0

H(T)

. . • * =N.'N.~ke. IS

Fig.2. Temperature and heat treatment effects on J~V2HVn-H curve. From Fig.2, the grain size becomes smaller along with lower temperature and shorter heating time. The increase in He2 is caused by the increase of T¢ in this figure. In Fig.3, even when excess Ta is added to the alloy, there is still no obvious change in the grain size according to results of the scanning electron microscope. Ta doping has little effect on grain size but an appreciable effect on the flux pinning strength.

e/co • N'b 7 Tu O Nb3Ta • I~'b 10 T i + /¢o 20 T .

,i.e

2 .g

-F~ O,S

It

I0

H

111

Fig.3. The effect of Alloying (Ta doping) on Jcl/2Hl/4-H curve. From fig.3, we can obtain smaller grain size through alloying, thus leading to an obvious reduction in the slope of the J¢I/2HV4-H eurve. To achieve this, .we employed a new technology of temperature treatment in two stages, specifically: 640°Cx15h+800°Cx0.5h. If the temperature is higher than 800°C, T¢ will drop obviously, and if the temperature is lower than 650°C, it will drop a little. We chose a temperature of 640°C during the first stage and maintain the temperature in the second stage because we want to raise the degree of order of Nb3Sn so that we can get high He2. We chose the heating time as 15hours and a temperature of 800°C at the second stage. Our results indicate that Hc~ increases and Tc does not reduce when we treat the sample using above procedures. The experimental results are shown in table 1. REFERENCES

1. E.J. Kramer, J. Appl. Phys.44 (1973) 1360 2. D.P. Hampshire, Physica C 296(1998) 153-166

Table 1: Effect of the new technology on the samples. Sample 8101-1 8101-2 8111-13

Heat treatment System 650°Cx20h 650°Cx20h 650°Cx20h

Diameter of core (!J em~ 4.1 4.1 2.4

2O "

(T)

Average grain ~iTe 826A 826A 750A

Tc 17.5K 17.5K 17.7K

Hc2 21.1T 20.8T 22.3T