IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, NO. 2, JUNE 1999 1437

Properties of fiber-reinforced niobium- tin superconductor fabricated by bronze process

H. Tateishi, K. Agatsuma, K. Arai and M. Umeda Electrotechnical Laboratory, Tsukuba, IBARAKI, JAPAN

K. Gotoh, N. Sadakata and T. Saitoh Fujikura, Co. Ltd., Kotoku, TOKYO, JAPAN

Abstruct- We are developing a fiber-reinforced type of superconductor for large scale, high-field magnets. Formerly we used the sputtering processa to develop a niobium-tin conductor reinforced with a tungsten fiber and showed that this type of conductor has excellent stress tolerance. For practical applications however, a conventional fabrication process like bronze process is desirable. Hence we have tried to fabricate a fiber-reinforced type niobium-tin conductor utilizing tantalum fiber as the reinforcing member. In this conductor, each niobium-tin filament has a tantalum core of about 20- ,u m diameter. We adopted tantalum as the core material since it has both good ductility and mechanical strength comparable to stainless steel. This conductor showed a reasonable critical field of about 22 T and good mechanical strength compared to a niobium-tin conductor fabricated by the conventional bronze process.

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

In order to generate high magnetic fields exceeding 10 T, compound superconducting materials must be used because of their high upper critical fields. Large-scale magnets used for generation of such high fields are subjected to high tensile stress due to the huge hoop forces generated during operation. The conductors must therefore be reinforced in order to avoid degradation of critical currents due to strain exerted by high stress. As one possible solution to achieve reinforcement, we have been developing a Fiber-Reinforced Nb3Sn Superconductor(FRS). The concept of FRS is to reinforce each superconducting filament directly with a high-elastic- modulus fiber such as a carbon fiber, an alumina fiber or a metallic fiber. We have shown excellent stress-tolerance of Nb3Sn FRS fabricated by the sputtering process and using a tungsten fiber as reinforcement[ 1],[2]. Tungsten fibers which have the highest elastic modulus among pure metals, approximately 400 GPa, were adopted because our principle is to minimize the strain exerted on the conductor due to hoop stress with high elastic modulus of reinforcing fibers and share a part of elctromagnetic force by the conductor itself. Conventional drawing methods however, can not be applied to fabricate FRS with tungsten because the hardness of tungsten is very much higher than that of niobium and bronze which react to form Nb3Sn. Hence we have tried to fabricate Nb3Sn FRS with bronze process utilizing tantalum as material for reinforcement. Tantalum has been chosen since i t has good ductility suitable for the drawing process as has already been shown with commercial bronze-processed

Manuscript received September 14, 1998.

conductors and also has high Young’s modulus comparable to stainless steel.

11. PROPERTIES OF a PROTOTYPE CONDUCTOR

As the first step in conductor development, we tried to fabricate a prototype conductor with composition shown in Fig.1. The purpose of this prototype fabrication was to investigate whether this type of conductor can be fabricated industrially or not and whether it shows reasonable superconducting properties compared with conductors fabricated by the conventional bronze process. We selected this composition since it is supposed to be easy to fabricate and the reaction process between niobium and bronze will be the same as that used in conventional bronze process.

Each filament consists of a tantalum core 16 p m in diameter and a niobium layer surrounding this core with the thickness of 1.311 m. 85 units of this filament are embedded in a matrix of Cu-8wt.%Sn, which is to be reacted with niobium. Bronze with low tin concentration of 8% was adopted because of good ductility. There is no copper- stabilizer. Ductility of the composite was good and we suffered no problem in drawing the conductor. Wire specifications are summarized in Table I.

T A B L E I SPEC[FICATIONS OF A PROTOTYPE CONDUCTOR

wire diameter 0.3 mm number of cores 85 diameter of Ta-core 16 p m thickness of Nb 1.3 ,u m matrix Cu-8wt %Sn Ta/Nb/bronze(%) 25.9/8.7/65.4 twist pitch length 6 mm

Fig. I Schematic illustration of cross section of a fiber-reinforced type NbiSn conductor fabricated by the bronze process, using tantalum as the reinforcing material

1051-8223/99$10.00 0 1999 IEEE

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Fig.2 applied field. Jc is normalized by total cross-sectional area of a composite.

Critical current density of prototype conductors as a function of

We heat-treated this conductor under several conditions and measured critical currents in background magnetic fields up to 14.5 T. Fig.2 shows typical results of I, measurement. From the Kramer plot of these data, critical fields are estimated to be about 22 T. However, overall critical current density at 14.5 T is about 42 Nmm2 and this is still rather low compared to that of commercial wires. J, should be improved by using bronze with a higher tin concentration and adopting a suitable conductor design.

Thus i t has been shown that a fiber-reinforced type of Nb3Sn conductor that shows a reasonable upper critical field, can be fabricated using the bronze process, which means that it can be used for a conductor for a high field magnet.

TABLE 11 SPECIFICATIONS OF A FIBER-REINFORCED TYPE OF NBiSN C:ONDUCTOR

wire diameter 0.39 mm number of cores 85 diameter of Ta-core 21 11 m thickness of Nb 2 p m matrix Cu-l Iwt.%Sn TalNb/bronze/Cu(%) 34/9/26/3 I twist pitch length 7.8 mm 3 120 m

Each tantalum core is 21 ,U m in diameter and the niobium layer is 2 ,U m thick. 85 units of this filament are em- bedded in a matrix of Cu-I lwt.%Sn. The bronze has a tin concentration of 11% in order to obtain higher Jc. Copper stabilizer is located at the surface and a 36 ,U m thick tantalum barrier separates the filament bundle and copper. Copper ratio is 0.44. Tantalum accounts for 34% of the total cross sectional area. The specifications are summarized i n

111. C R I T I C A L C U R R E N T DENSITIES 0 0 1 : f : ! : I : ~ : I : ! : I 875 900 925 950 975 1000 1025 1050 1075

Heat treatment temperature(K)

With the success of prototype conductor fabrication, we proceeded to fabricate a more practical conductor with the specifications shown in Table 11. This time copper stabilizer and tantalum barrier against diffusion of tin from bronze matrix are added to the composition shown in Fig. 1 .

Fig.4 Critical current density of a fiber-reinforced type of Nb3Sn conductor with copper stabilizer, as a function of heat treatment temperature. Heat treatment time is fixed at 48 hours. J, is normalized by cross-sectional area excluding copper and tantalum barrier, which is so-called non C'u-Jc and is equivalent to the overall Jc shown in Fig.2.

Fig. 3 A photograph of a cross section of a fiber-reinforced type of Nb3Sn conductor with copper stabilizer and tantalum barrier.

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Fig 5 Krainei plot of Jc-data foi ditterent heat tieatment temperatures Upper ciitical field obtained by extrapolation I S about 20 T tor temperature below 971 K, but i t becomes higher with incieasing tempeiatiire above 971 K and seems to exceed 25 T

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Fig.4 shows non-Cu J, at 14.5 T for different heat-treatment temperatures with the same heat-treatment time of 48 hours. As is easily seen from the figure, J, becomes higher when the temperature exceeds 973 K. This phenomena is supposed to be due to high upper critical fields of these samples, which fact is shown in Fig.5. Critical fields estimated from the Kramer plot are about 20 T for samples heat-treated below 973 K. Yet for higher heat treatment temperature, critical field seems to exceed 25 T. This may be caused by tantalum diffusion from the core region into the niobium. These results indicate the possibility of using this conductor for very high magnetic field generation. Yet this needs more detailed research.

With longer heat-treatment time(up to 200 hours), the critical current density seems to increase further(data are not shown in this paper) but this increment is 25% at most and is not worth the necessary time and accompanying degradation of mechanical strength, which is to be discussed in the next section,

I V . MECHANICAL PROPERTIES

We measured two kinds of mechanical properties in order to check the mechanical strength of the conductor, which is a key issue of our conductor development.. One is critical current measurement under tensile load and the other is measurement of stress-strain characteristics.

A. Critical Current Under Tensile Load

Cri t ica l c u r r e n t was m e a s u r e d u n d e r uniax ia l t ens i le load with the equipment originally designed by Ekin.

Fig.6 shows the measured critical currents at 14 T as a function of applied strain. The heat-treatment temperature is 973 K and heat-treatment times are between 12 and 192 hours. The usual peak effect is not so clear, but a slight peak is observed at the strain of about 0.2%. An interesting result is that longer heat-treatment brings higher J, but degradation with applied strain becomes sharper. The strain level at which critical current becomes equal to that under no strain is about 0.6% for heat treatment times below 48 hours but this decreases to about 0.4% for longer heat treatments.

TABLE III[3] PARAMETERS NECESSARY TO CALCULATE PRESTRAIN OF A COMPOSITE

component Young’s modulus(GPa) (%;from IOOOK to 4.2K) Bronze I24 I .68 Ta I78 0.6 I

thermal contraction

Nh& 167 n 74

From the composition shown in Table 11, we can calculate thermal prestrain exerted on a Nb3Sn filament assuming a simple combination rule for composite material[3]. Using parameters shown in Table 111, prestrain of NbiSn at 4.2K is calculated to be 0.25%(compressive). As is well known, this value corresponds to the strain at which I, reaches the maximum on a I, vs. applied strain curve. The measured

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Applied Strain(%)

Fig.6 Critical currents at 14T as a function of applied tensile strain. Heat- treatment temperature is 973K and heat-treatment time are between 12 and 96 hours

value is 0.2% and agreement is reasonable. It is not clear yet why the peak value of I, is not much enhanced from that at no load. Murase et al. reported similar results with a Nb3Sn composite which is reinforced by an external tungsten wire[4] and this phenomenon may be specific to a composite with additional reinforcement using high elastic modulus materials.

B. Stress-Strain Characteristics

Stress-strain curves are measured using a Instron 4505 test machine. Fig.7 shows stress-strain curves of samples heat- treated at 973 K and measured at 4.2K. In the figure, the properties of non-heat-treated conductor is also shown for comparison. We soldered a sample of about 6cm-length into a 0.5mm groove shaved on a copper strip Imm thick. Then a whole sample was set into a chuck in the Instron test head.

The sample without heat treatment showed plastic behavior up to a strain of 1.75% although Young’s modulus became gradually smaller and fractured after that. On the other hand, heat-treated samples seem elastic up to a strain of about 1.0% with almost constant Young’s moduli. After that, the Young’s moduli started to degrade and showed serration above a strain of about 1.25%. In general, longer heat treatment degrades the Young’s modulus of the elastic region slightly and reduces the strain at which serration starts but the effect is not very drastic(measurement of the sample heat-treated for 192 hours has not been carried out yet).

These tendencies are more clearly shown. in Fig.8, where effective Young’s moduli calculated by differentiating stress with strain are shown. Data point scatter is believed to be caused mainly by digital data acquisition process since original data are taken by computer control with digital processor. Table IV summarizes mechanical properties.

TABLE IV SUMMARY OF MECHANICAL PROPERTIES

property non-heat-treated sample heat-treated sample initial Young’s modulus - l00GPa - 70 GPa

Young’s modulus - 60 GPa - 50 GPa

yield strength I300MPa 700GPa

limit of strain for elastic region I .S % 1.0%

at the elastic limit

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Properties of heat-treated samples shown in Figures 7 and 8 are understood as follows. Before tensile load is applied, the copper stabilizer and bronze matrix are supposed to be already deformed plastically by thermal stress exerted by cooling from 1000 K to 4.2 K. Hence the components which can sustain applied load are Ta and Nb3Sn. If we assume that effective Young's modulus of. the composite is given by simple combination rule, it is calculated to be 75 GPa. As the tensile load is increased, Nb3Sn starts to behave like plastic because of its brittle nature and finally yields. At this point, Young's modulus of the composite is determined by Ta only and it should be 60 GPa. In practice however Ta also starts to be plastically deformed and the measured value is about 50 GPa as is shown in Table IV. The yield point of Nb3Sn is not clear from Figures.7 and 8 but it is supposed to be about 0.4% judging from the critical current degradation shown in Fig.7.

C. Critical current as a function of applied tensile load

We can obtain I, versus stress curves by combining results shown in Figures.6 and 7. For the case of 48 hours heat- treatment at 973 K, result is shown in Fig.8. The conductor can sustain the stress of about 300 MPa without significant degradation of critical currents. This stress limit is much improved from that of conventional bronze-processed wire, which fact shows the merit of the FRS Nb3Sn composite.

V. CONCLUSIONS

Fiber-reinforced Nb3Sn superconductor has been fabricated by bronze process using tantalum as reinforcing material. There was no problem in drawing process. The conductor showed a critical field of at least 25 T, which is enough for high field magnet application. Critical current measurement under tensile load and a tensile load test using a Instron test machine revealed that the conductor can sustain stress of about 300 MPa without significant degradation of critical current. This limitation is much higher than that of conventional bronze processed Nb3Sn conductor. These results .will bring a new application field of Nb3Sn, especially in high field magnet fabrication.

A c K N O W L E D G M E N T S

The authors are grateful to Dr. M. Kamimoto, Director, Energy Technology Division and Dr. S. Kosaka, Director, Frontier Technology Division, of the Electrotechnical Laboratory, for their support and encoura, oement.

REFERENCES

[ I ] K. Arai, H. Tateishi, M. Umeda, and K. Agatsunia, "Fiber-reinforced superconductors for a IST-class high-field pulsed magnet and their conceptual design," lEEE Trurrs. Applied Sic/'erco,rdricriv;l~i, vol. 3, pp.555-558, March I993 H. Tateishi, K. Arai, and K. Agatsunia, "Properties of niultifilamentary Nb3Sn fiber-reinforced superconductors for high-field pulsed magnets," lEEE Trurrs. Applied Sir/Jercond[rcti~if~. vol. 5 , pp. 1587- 1590, June 1995.

[2]

0 0 0 5 1 0 1 5 j 2 0

Strain(%)

Fig.7 conductor with copper stabilizer, measured at 4.2K with no magnetic field. Data of a sample with no heat treatment(as-drull.n)are also shown for comnari son.

Stress-strain characteristics of a fiber-reinforced type of Nb3Sn

0 0 0 5 1 0 1 5 2 0 Strain( Yo)

Fig.8 strain of the data shown in Fig.8. Scattering of data is supposcd to be mainly because of digital data acquisition.

Effective Young's modulus obtained by differentiating stress with

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Stress(MPa)

Fig9 Critical current as a function of applied stress calculated from the data shown i n Fig.6 and Fig.7.

[3] D. S. Easton, D. M. Kroeger, W. Specking, and C. C. Koch, " A prediction of the stress state in Nb3Sn superconducting composites," J . Appl. Phys., Vo1.51, pp.2748-2757 May 1980 S. Murase. H. Shiraki, 0. Horigami, M. Koizurni, S. Mine, H. Takeda and H. Baba, "STRESS EFFECTS ON W/Cu REINFORCED NbiSn COMPOSITE CONDUCTORS," Filanzentury A / 5 Superconducts, PI. Suenaga and A. F. Clark, Eds. New York:Plenum. pp.233-240, 1980

[4]