3696 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 11, NO. 1, MARCH 2001

Pinning Characteristics of (Nb,Ta)$n Superconductors Produced by Nb/Ta-Sn

Composite Process Nobuya Banno, Takao Takeuchi, Kikuo Itoh, Hitoshi Wada, Hidehiro Matsumoto and Kyoji Tachikawa

Abstract-Pinning properties in (Nb,Ta)sSn superconductors produced by a Nb/Ta-Sn composite process are studied through the measurement of the temperature and magnetic field dependences of J, and its anisotropy with respect to the field direction, referring to the grain morphology and the compositional analysis of the (Nb,Ta)sSn phase. The experimental result of the anisotropy in J, and the observation of the grain morphology suggest that the contribution of the grain boundary to the pinning force density is not large. Two regions having different grain morphologies can be seen in the (Nb,Ta)pSn layer, where the boundary of the two regions roughly corresponds to the boundary between the Nb-Ta sheath and the Ta-Sn core in the initial Nb-Ta/Ta-Sn mono-core composite before reaction. Such a boundary between the two regions inside the (Nb,Ta)sSn phase and/or a S-N interface of the boundary between the Nb-sheath and the (Nb,Ta)3Sn superconducting phase would influence the pinning property. Both are possible reasons for the anisotropy in J,. On the other hand, the so-called "peak effect" is recognized in the flux pinning, suggesting that another pinning mechanism acts in high field, differing from that in low field.

Index Terms-anisotropy in J,, grain morphology, high field, (Nb,Ta)sSn, peak effect, pinning.

I. INTRODUCTION IDELY available high field superconductors at present w include the bronze-processed Nb3 Sn superconductors

with small amount of Ti addition [1]-[3]. However, much higher J, is required in high magnetic field for development of superconducting magnets capable of generating field beyond 20 T, e.g. 1 GHz NMR magnet. As one of the promising superconductors, the (Nb,Ta)& superconductors fabricated by the Nb/Ta-Sn composite process have been developed [4], [ 5 ] . The (Nb,Ta)sSn superconductors are fabricated through a reaction around 9OO0C, 2OOOC higher than the conventional bronze-processed Nb3Sn superconductors. The formed (Nb,Ta)3Sn layer is thick and shows the stoichiometric composition. Furthermore, the J, property in high magnetic field has a small shoulder, improving J, in high field. It seems to be the so-called "peak-effect". Such behavior differs considerably from the

Manuscript received September 18,2000. N. Banno, T. Takeuchi, K. Itoh and H. Wada are with the Tsukuba Magnet

Laboratory, the National Research Institute for Metals, 3-1 3 Sakura, Tsukuba, Japan (telephone: +8 1-298-59-5036, smail: banno @nrim.go.jp).

H. Matsumoto and K. Tachikawa are with the Tokai University, Hiratsuka, Japan.

conventional bronze-processed Nb3Sn superconductors. In this paper, first the temperature and field dependences of J, and its anisotropy with respect to the field direction are measured. Then, the pinning property is discussed, referring to the grain morphology and the result of a compositional analysis of the (Nb,Ta)sSn phase.

11. EXPERIMENTAL PROCEDURE Details of the preparation of (Nb,Ta)3Sn superconductors

have been described in [ 5 ] . In brief, the (Nb,Ta)& superconductors were made by reacting mono-core composites consisting of a Nb-Ta sheath and Ta-Sn powder core. The Ta-Sn compound powders were prepared by mixing powders of Ta and Sn with an atomic ratio of 6:5 and reacting at 95OOC for 10 h under a vacuum of 1 ~ 1 0 ' ~ Torr. The Ta-Sn powders were then encased in Nb-Ta tubes, 8 mm in outer diameter and 5 mm in inner diameter, and after grooved rolling and drawing, the composite wires were fabricated into tapes by flat rolling.

The Nb-Ta/Ta-Sn composites were reacted at 885°C to 950°C for 80 h to form a (Nb,Ta)3Sn phase by the diffusion reaction between the Nb-Ta sheath and the Ta-Sn powder core. The specifications and the reaction conditions of the samples are listed in Table I.

The critical current was measured by a four probe resistive method and determined with a I ,uV/cm criterion. The microstructure of the samples was studied through observation of the fractured cross-sections with a scanning electron microscope (SEM). Further, an energy dispersive x-ray (EDX) microanalysis was used for the compositional analysis.

TABLE I

Samplenumber # t #2 #3 #4 #5

Concentration of Ta in NbTa 4.2at% 3.3at% 3.3at% 4.2at% 3.3at% tube Configuration Tape Tape Tape Tape Wire Dimension 1.75" 2.47" 1.92" 2.49" 1.2mm4

X X X X 0.45" 0.36" 0.57" 0.45"

Reaction 8 8 5 T 950°C 900°C 900°C 9 0 0 T condition X X X X X

80h 80h 80h 80h 8 0 h

1051-8223/01$10.00 0 2001 IEEE

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111. EXPERIMENTAL RESULTS

Fig. 1 shows J,-B characteristics of the samples, including in comparison the result for a conventional bronze-processed (Nb,Ti)3Sn superconductor (wire diameter: 1 .O mm, filament diameter: 4.5 pn, number of filaments: 5047) used as a standard sample of VAMAS [6 ] . The J,-B characteristics of the (Nb,Ta)3Sn samples have a shoulder at high fields. Further, in the tape samples, a small anisotropy in J, was found below 23 T. On the other hand, the anisotropy in J, was not found above 23 T. The J, in magnetic field parallel to the tape surface (10 was higher than in perpendicular (I) as shown in Fig. 1. Fig. 2 shows the temperature dependence of the J,-B characteristics for samples #1 and #2: #2 was reacted at much higher temperature in comparison to #l. For all temperatures, a similar tendency for the anisotropy in J, was found for both samples. Further, relatively larger differences in J, with respect to the field direction can be seen in the samples with large aspect ratio, i.e. #2 and #4 as shown in Fig. 1.

Fig. 3 shows SEM photographs of the (Nb,Ta)$3n phases formed in samples #1 and #2, where the superconducting (Nb,Ta)3Sn layer was identified fiom the EDX result. It is clearly seen that there are two regions in the (Nb,Ta)& layer. The first region is near the sheath, showing columnar grain morphology in the direction of the diffusion. On the other hand, the other region at the core side shows equiaxial grain morphology. The columnar grains near the sheath are fine, while the equiaxial grains at the core side are coarse. The boundary between the two regions roughly corresponds to the initial boundary between the Nb-Ta sheath and the Ta-Sn powder core before reaction, so that the difference of the grain morphologies depends on each reaction process whether at the Nb-Ta sheath or at the Ta-Sn core. Furthermore, the grain size in the sample #2 was slightly

200

150 N h

E E B ronzr -procsssr d - 5

Y

U100 7 a

0

50

I C 2 K . I , I , , \\I 0 12 14 16 18 20 22 24 26

B (T I Fig. 1 measured samples.

Comparison of non-Cu J, as a hnction of magnetic field among

100

80

- 5 60

U c

-12K (1)

-14K (I)

-1BK (I) w-

100 E E

-4BK ( I / ) - 5

E

U 7 - -

50 0

0 10 15 20 25 0 0 5

(T)

40

20

70

60

50

9 40

30

U -

20

I O

0

-14K (I) +IBK (//)

U

0 5 10 15 20 25

Fig. 2. Temperame dependence of I,-B characteristics for (a) sample # I annealed at 885OCx80h and (b) sample #2 annealed at 95O0Cx80h.

B (1)

coarser than in #1, depending on the reaction temperature. Fig. 4 is the result of the EDX composition analysis on the

polished cross-sectional plane in the (Nb,Ta)3Sn layer for samples #1 and #2. The Sn concentration in the (Nb,Ta)3Sn layer is almost 25 at%. The Ta concentration shows the tendency to be large towards the core in both samples. The Ta concentration in (Nb,Ta),Sn phase considerably differs fiom each region in the (Nb,Ta)d3n phase.

Fig. 5 shows the normalized pinning force Fp as a hnction of BIBc2 for sample #1 and #2, where Bc2 denotes an upper critical field. At least two peaks are seen there, i.e. the so-called peak effect in Fp can be recognized. This peak effect leads to enhancement of J, near the upper critical field. On the other hand, it is noticeable that the peak field at the lower reduced magnetic field varies slightly for both samples. This indicates that the dominant pinning center might slightly change as the temperature change.

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40

35

Fig 3 SEM photographs of fractured cross-sections for (a) sample #I annealed at 885OCx80h and (b) sample #2 annealed at 950Tx80h denote the posittans of the boundary between the each region with different grain morphologies

and

IV. DISCUSSIONS If the grain boundary is the dominant pinning center, J, in

a magnetic field perpendicular to the tape surface should be much higher than in parallel, because the pin density would be larger along the axis parallel to the tape surface than perpendicular: the grain boundary density is proportional to the inverse of the grain size, However, the experimental results exhibit the opposite trend. This suggests that there is the contribution of some pinning sites parallel to the tape surface. One such possible pinning site is the boundary between the (Nb,Ta)$n superconducting layer and the Nb sheath, the so-called S-N interface [7]. In addition, the boundary between the two regions having the columnar and the equiaxial grain morphologies inside the (Nb,Ta)$3n phase might also contribute to the volume pinning force density. Such pinning sites parallel to the tape surface are widened as the aspect ratio of the tape sample becomes higher, compatible with samples #2 and #4 showing a relatively large difference in J, with respect to the magnetic field direction.

--_ Boundary of Nb-Ta sheath 0 " " ~ . " ' " ' ~ ' " '

0 20 40 60 80

and (Nb,Ta)3Sn phase (pm) Distance from boundary between (Nb,Ta) sheath

Fig. 4. EDX results for samples # 1 and #2

1 -

0.8 -

X

2 d 0.6 - 5- LL

0.4 -

0.2 -

0 -

- - 4 ~ ( u 17 4 16K ( U ) -16K (I) -C- 17K (U) -17K (I) -4.2K (//J -4.2K (I)

0 0.2 0.4 0.6 0.8 I

BIB 52

I I . , l , , , . I , , . . . , . . .

(b) #2

d 0.6

U.

0.4

0.2

0

-12K ( U ) -12K (I)

14K ( / I ) --t 14K (I) - 16K (//) -16K I l l

-4.2K '(//) -t- 4.2K (I)

0 0.2 0.4 0.6 0.8 1

Fig. 5. Normalized pinning force as a function of BIB,, for (a) sample # 1 and (b) #2.

B'BD2

The peak effect at high reduced magnetic field shown in Fig. 5 has been observed also for other A15-type compound superconductors having a coarse grain morphology, e.g. bronze processed V3Ga superconductors [SI, Nb3(A1,Ge) superconductors fabricated by irradiation with laser beam [9] etc.. The peak of Fp in the high field region appears as the temperature lowers in V3Ga [SI, which looks like the present result on the (Nb,Ta)3Sn. It is indicated that sofienhg of the flux lattice near the upper critical field enables the flux to ‘match the pin structure and thereby causes the peak effect for V3Ga [SI and Nb-Ta [lo]. Thus, such flux-line-lattice (FLL) I pinning center synchronization is one of the reasons for the peak effect in the (Nb,Ta)3Sn samples. It is believed that as the grain becomes coarser, the peak effect occurs more significantly. Considering the coarse grain morphology on the core side, the peak effect of the (Nb,Ta)8n superconductors may come from the (Nb,Ta)3Sn grains in the region near the core.

V. CONCLUSION Pinning properties on (Nb,Ta)3Sn superconductors

fabricated through Nb/Ta-Sn composite process was investigated. From the experimental results on the anisotropy in J, and the observation of the grain morphology, it was found that the contribution of grain boundary pinning was not large in low magnetic field region. Other possible pinning sites are S-N interfaces, e.g. the boundary between the Nb-Ta sheath and the (Nb,Ta)$n superconducting phase, and the boundary between the columnar and equiaxial grain morphology regions inside the (Nb,Ta)3Sn layer. Such pinning sites parallel to the tape surface could result in the anisotropy in J, with respect to the magnetic field direction.

The pinning property shows that there were at least two peak fields. Such peak effect is ofien observed for other

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A15-type compound superconductors, where the peak of Fp in the high field region appears as the temperature lowers. A similar tendency can be seen in the results on the (Nb,Ta)3Sn superconductors. Thus, as reported previously, a FLL / pinning center synchronization might be one of reasons for the peak effect in the (Nb,Ta)3Sn superconductors.

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