1904 IEBE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, NO. 2, JUNE? 1999

A New Symmetrical Arrangement of Tape-Shaped Multifilaments for Bi-2212/Ag Round-Shaped Wire

Michiya Okada, Kazuhide Tanaka, Tsuyoshi Wakuda, Hitachi Research Laboratory, Hitachi, Ltd., Hitachi, Ibaraki 319-1292, Japan

Katsumi Ohata, Junichi Sato, Advanced Research Center, Hitachi Cable, Ltd., Tsuchiura, Ibaraki 300-0026, Japan

Hiroaki Kumakura, Tsukasa Kiyoshi, Hitoshi Kitaguchi, Kazumasa Togano and Hitoshi Wada National Research Institute for Metals, Tsukuba, Ibaraki 305-0047, Japan

Abstract - A new Bi-2212fAg round-shaped wire with tape- shaped multifilaments has been successfully developed. The wire includes 126-960 tape-shaped filaments with triple rotation symmetry, having a good crystal alignment in each filament. We refer the the new wire at ROSATwire, (mtation-Symmetric Arranged Tape-in-tube e). Since the ROSATwire structure yields complete symmetrical arrangement of the tape-shaped filaments, it is eliminates the need of a rolling machine, but al- lows us to use a drawing or extrusion machine. We found that the present wire fabrication process markedly improves not only productivity and lowers cost, but also enhances the transport J, of the Bi-21212IAg wire. The I, and J, reached >340A and 1000A/mmz at 28T and 4K.

I. I ~ O D U C T I O N

Since silver (Ag) sheathed Bi-system superconducting tape is one of the most promising materials for practical applica- tions, a lot of effort has been made in improving the supercon- ducting properties of the tape prepared by powder-in-tube (PIT) methods. Since it is well known that the transport J, increases with improvements in the crystal alignment of the tape, mechanical deformation processes, such as rolling or pressing, are considered essential in obtaining good perfor- mance from the Bi-2212/Ag PIT tapes. The deformation processes, however, also confgure the conductor shape into a tape shape. From the viewpoint of practical application, round- or rectangular-shaped wire is preferable rather to tape- shaped wire, since round wires are very easy to handle and also easy to fabricate in accurate dimensions. Although there already exists reports describing Bi-system/Ag wires with round-shaped multi-filaments [ 1-31, almost all the previous works showed rather low transport Jc's compared to tape- shaped conductors due to their poor crystal alignment. In this paper, we show a new Bi-2212/Ag round-shaped wire, RO- SATwire (mtation Symmetric Arranged Tape-in-tube d) , with tape-shaped multi-filaments. The wire includes 126-960 tape-shaped filaments with a triple rotation symmetry, having good crystal alignment in each filament. Since the present wire structure yields complete symmetrical arrangement of the tape-shaped filaments, it is no longer necessary to use a rolling machine, but allows us to use a drawing machine. In the fol- lowing, we show how the present wire fabrication process markedly improves the productivity, lowers cost, and also improves the anisotropy of the Bi-22 12lAg conductor.

Manuscript received September 14, 1998.

The fabrication process of the ROSATwzre is schematically summarized in Fig. 1 The process is divided into the following three major steps,

1 preparing multifilamentary tape with accurate cross- sectional dimensions 2 stacking the tapes to form a segment with a dumond-rad shape 3 restacking the neighbouring three segments with triple rotation symmetry

Following the ROSATwzre process, it is easy to design various kinds of filament arrangements, i e , 3 , 12 and 27 segment structures as shown in Fig 1, without changing the cross- sectional dimensions of the starting tapes It is also easy io optimize the thickness of the filament and Adoxide interface area, both of which have been reported to dominate the J, of the Bi-21212/Ag tapes[4] Since this work is a preliminary study, we tried to fabricate simple structure, i e the wire with 3 segments, described as follows

3 segments 12 segments 27 segments

Fabrication process of ROSATwire (rotation-symmetric arranged Fig. 1 tape-in-tube wire) with various segments.

The multi-filamentary Bi-2212/Ag tapes 0.5 mm thick and 2.8 mm wide have been prepared using a PIT method. The tapes with 7-55 filaments are then stacked to form a segment with a diamond-rod shape. These segments were then stack.ed into the Ag tubing again, keeping triple rotation symmetry between the neighbouring three segments. The stacked rods were then extruded or drawn into wire with diameters that ranged between 0.8-2mm and with 128-960 filaments. Figs. 2 and 3 show the cross section of the ROSATwire with 126 and

1051-8223/99$10.00 0 1999 IEEE

1905

960 filaments, respectively. Some of the wires were rolled into a rectangular-shape with an aspect ratio of around 2. After mechanical deformation, the wires were cut to short samples 30-50mm in length, and then partially melted at around 880 “c in a flowing oxygen atmosphere. The J, of the wires was measured in various magnetic fields of up to 28T in liquid helium at the Tsukuba Magnet Laboratory, NRIM.

Fig.2 A cross-sectional view of Bi-2212lAg ROSATwire with 126 fila- ments. (dia. 1.6mm)

m e n t v tapes. Further merits of the ROSATwire processing are:

a) marked increase of the flat A@i-22 12 interface area b) ease of optimizing filament thickness with tape-shaped

c) less sausaging in the longitudinal, and radial directions d) less anisotropy e) low cost with high performance 0 accurate wire dimensions g) ready for solenoidal winding.

filaments

Microstructural observation showed good crystal alignment in the tape-shaped filaments. The uniform cross sectional shape of the filaments, shown in Fig. 2 and 3 after final step, is also thought to be another merit. Since ROSATwire has a symmet- rical cross section, sausaging during the drawing process is expected to decrease not only in the radial direction but also in the longitudinal direction of the wire, leading to increase of J, in long length of wire.

The typical IC‘s of ROSATwire with 960 filaments of 1.6mm diameter reached 900A at 4.2K and self magnetic field condi- tions, corresponding to Jc,oxi& =2500A/mm2. We found the Jc’s of rectangular wire showed much better transport prop- erties than those of the round wire as shown in Fig.4. The maximum IC and J,,osde at 4.2K and 28T are 2 9 8 4 800A/mm2 for round-wire and 3 6 0 4 1000A/mm2 for rectangular Wire, respectively, showing enough current carrying capacity for practical applications. Although the J i s are still lower than the best short sample[4], they are better than that for the conventional long tapes used in magnets[S].

1M:

Fig.4 Magnetic field dependence ofJ, for Bi-2212/Ag ROSATwire. Fig3 A cross-sectional view of Bi-2212/Ag ROSATWire with 960 fila-

ments. (&a. 1.6mm)

B. Anisotropy of ROSATwire

Iu. RESULTS AND DISCUSSION

A. Merits of ROSA Twire Processing

From the viewpoint of engineering, the first advantage in using the ROSATwire processing is the ease of fabricating round wire with a good performance ofJc and low cost, since we can arrange the filaments using the same size of multifila-

Since the present ROSATwires are expected to be less an- isotropic compared to the conventional tapes, we have made the rough estimation for the angular dependence of J, . The .JC,,,lcide of Bi-2212lAg tapes can be described using the fol- lowing equation [6],

Jc,oxlde = A[1- (-)2]l”BY-’(1 - -)& B

(1) T r, 4 2

1906

4,000

- "E 3,000 z. 4 2,000

t.000

where A, m, y dare the pinning parameters, and the Jc,oxide

represents the Jc of each Bi-2212 oxide having various orien- tations. Assuming T << T, and B << Bc2, JC,oxik is given by

Jc,ofide ( B ) = ABY-' . (2) Assuming the J,c,oXide for a Bi-2212 grain of a polycrystalline filament, in a segment, is dominated only by the magnetic field component Bp parallel to the c-axis of the grain, the Jc,s of the polycrystalline Bi-22 12 segment with various crystal align- ments can be given by the following equation,

-

-

-

-

(3)

= n,(2rro,)-f'2 exp(-es2 / 2 0 , ~ ) ( 5 ) where n, is the number of grains in a segment, 8 is the angle between external magnetic field B and a direction parallel to the tape surface in a segment, is the number of grains

with misalignment angle e, in a segment. We assume the c- axis misaligiunent of grains in a segment scattered with a Gaussian distribution F( 0,) , defined by a standard devia-

tion of o ( 3 o < x I2 ). Then the Jc of the ROSATwire con- sisting of multiple segments with various e in a magnetic field is given by,

nos

k

J c = c J C , , l k (6) s= 1

where k is the number of segment. For example, the angular dependence of J, for a mono-core tape is given by taking parameters k-1, A = 2536 and y = 0.73, using o =

5,10,15,17.5,20 as shown in Fig.5. The solid square in the figure shows the experimental data for a typical Bi-2212/Ag 54 filamentary tape. A ci s = 17.5 degree seems to give the best fit, almost corresponding to the reported orientation factor of 12-20 degree for Bi-2212/Ag multifilamentary' tapes measured by the transmission X-ray diffraction study [7].

Fig. 6 compares the calculation (solid line) and the typical magnetic field dependence of J, for a typical 54 filamentary tape (solid squares) . The calculation made on a grain orienta- tion factor ci = 17.5 degree. These results suggest that the present calculations basically explain the transport properties in magnetic fields. Then, we tried to estimate the angular dependence of Jc for the ROSATwire using the above men- tioned parameters ( B s =17.5 degree), but with the segments of triple rotation symmetry of the ROSATwire structure. We

, assume a ROSATujire structure with a 3 segment structure has perfect triple rotation symmetry (k3), and current flows uniformly in the cross sectional area of the wire.

5 3

06 d

-150 -100 -50 0 50 1oc1 150

Angle 0 (degree)

Fig.5 Comparison of angular dependence of calculated and measured .Ic for Bi-2212/Ag multifilamentary tapes. The solid line indicates the calcula- tions made on various crystal alignment factors of U (standard deviation). The solid square in the figure shows the expenmental data for a Bi-2212/A.g 54 filamentary tape

/. Gaussion

= 17.5

B l t a p e surface

"0 5 10 15 20 Magnetic Field B (T)

Fig.6 Calculated magnetic field dependence of J, compared with experi- mental data. The solid h e indicates the calculations made on U = 17.5..

0 8 1 0.4 ~

02 - 4.2K. 1OT

o . ~ , r , > , , . , ,

-150 -100 -50 0 50 1W 150

Angle 8 (degree)

Fig.7 Comparison of angular dependence of,J, for ROSATwire (solid line) and a conventional tape (dotted line). The solid squares indicate experimental results for Bi-2212/Ag 54 filament tape.

1907

5,000

4,000

C P E 3,000 E 3 4 2,000 a 50

1,000

O

Best short sample

0 5 10 15 20 25 30 Magnetic field B(T)

Fig. 8 Estimated magnetic field dependence of J, for the ROUTwire. J, distributed between the solid and broken lines, dependmg on the magnetic field hection.

Fig. 7 shows the calculated results for ROSATwire com- pared with a conventional multifilamentary tape. Solid squares indicate the experimental results for the Si-2212lAg 54 fila- mentary tape. We found the ratio of minimum to maximum J, to be 1.61 for tape but 1.05 for the ROSATwire. Thus, the anisotropy of the present ROSATwire was found to be almost negligible. The maximum and minimum J, of the ROSATwire should appear in 60 degrees of rotational-interval, and is expected to change within only 5%.

Fig. 8 also represents the estimation for the magnetic field dependence of J, for the ROSATwire, assuming the Jc,oxide of the segments is equal to that of the best short sample tape. Since ROSATwire has a round-shaped cross section, the measured J,‘s are expected to locate somewhere between the solid and broken lines in Fig. 8, depending on the magnetic field direction. Although the estimated J, of ROSATwire is still lower than the best short sample tape[8], we believe there is still a lot of room for improving the transport Jc’s. It should be pointed out that when we construct a magnet using a HTS conductor, the performance of the magnet will be dominated

, by the minimum J, , i.e. the maximum magnetic field compo- nent of B I tape. Therefore, applying the ROSATwire to high magnetic field applications should effectively improve the performance of the magnet, due to the less anisotropy. Thus, the present ROSATwire is considered to be the most promis- ing conductor for constructing a high field magnet with ex-

ceflent field-homogeneity which is required for solenoidal winding , i.e. the next generation of HTS magnet.

IV. CONCLUSION

A new Bi-22 12lAg round-shaped wire, ROSA Twire, with tape-shaped multifilaments has successfully been developed. The wire includes 126-960 tape-shaped filaments with a triple rotation symmetry having good crystal alignment in the fila- ments. Since the ROSATwire structure yields complete sym- metrical arrangement of the tape-shaped filaments, less aniso- tropy and good transport properties are realised. We found present wire fabrication processes markedly improve pro- ductivity, reduce costs and also enhance the transport J, of the Si-21212lAg wire. The I, and J, of the ROSATwire reached >340A and 1000A/mmz at 28T and 4K. The anisotropy of the wire was also estimated to be around 1.05, showing marked improvement when compared with conventional tape-shaped wires. The ROSATwire is considered to be the most promising conductor in constructing the next generation HTS magnet with excellent field homogeneity.

REFERENCES

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