Physica C 397 (2003) 95–98

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Stress/strain characteristics of PIT MgB2 tapeswith nickel sheath––effect of indium addition to the core

Kazumune Katagiri a,*, Akifumi Iwamoto b, Yoshitaka Shoji a,Kyoji Tachikawa c, Yutaka Yamada c, Kazuo Watanabe d, Toshiyuki Mito b

a Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japanb National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan

c Faculty of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka 259-1292, Japand Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan

Received 5 June 2003; accepted 6 August 2003

Abstract

The electro-mechanical characteristics of the nickel sheathed powder-in-tube MgB2 mono-core tapes with 10 vol%

indium powder were evaluated. No marked change in the stress–strain curve was found by indium addition. On the

other hand, the irreversible strain for the tape where a drastic degradation of critical current Ic begins increased from

0.25% to 0.50% by the addition of indium. The indium addition also suppressed the slight increase in Ic with strain.

These characteristics of Ic with strain were compared with those reported by other researchers. Scanning electron

microscope observations of the longitudinal section of degraded samples revealed that the number of micro-cracks was

appreciably smaller in the indium added tapes.

� 2003 Elsevier B.V. All rights reserved.

PACS: 74.70.Ad; 74.25.Fy; 74.25.Ld

Keywords: MgB2 tape; Indium addition; Mechanical property; Critical current

1. Introduction

MgB2 superconductors fabricated by the pow-

der-in-tube (PIT) process are expected to be less

expensive superconducting conductors because

simple and low cost fabrication procedures areapplicable. Many studies on the processing have

been conducted [1–4]. The transport critical cur-

* Corresponding author. Tel./fax: +81-19-621-6412.

E-mail address: katagiri@iwate-u.ac.jp (K. Katagiri).

0921-4534/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/j.physc.2003.08.006

rent density, Jc, of the core in the PIT MgB2

conductors, which depends on the raw powder

quality, sheath material, fabrication procedure,

heat treatment condition and so on, is in the range

of 104–105 A/cm2 at 4.2 K and 0 T. In order to

improve the Jc, addition of metal powders, such assilver, nickel, copper, indium and tin, to MgB2

core has been tried by some of the present authors

[5,6]. Among them, it has been shown that the 10

vol% indium addition and annealing at 473 K for

10 h enhances Jc by a factor of 6–7. It has also

been shown that the indium is well elongated in

ed.

96 K. Katagiri et al. / Physica C 397 (2003) 95–98

the MgB2 core by scanning electron microscope

(SEM) observations. Therefore, the addition of

indium was assumed to contribute for the im-

provement of weak MgB2 grain linkage.

For practical applications, it is important to

study the stress/strain characteristics of the con-ductor. Strain dependencies of critical current, Ic,in PIT MgB2 tapes or round wires with the stain-

less steel or the iron/stainless steel sheaths have

been investigated at 4.2 K and 5 T [7,8]. The Icvalues are reported to increase with the applied

tensile strain up to 0.4–0.5% in the stainless steel

sheathed tapes and 0.47% in the Fe/stainless steel

sheathed wires. The tape with the indium powderin the core is expected to have better stress/strain

characteristics than that without indium. In this

study, the effect of the indium addition to the

MgB2 core on the stress/strain characteristics is

investigated.

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

1.2

(b)

4.2K, 2T

I c/Ic0

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600

(a)

Tensile strain (%)

Tens

ile s

tres

s (M

Pa)

Ni/MgB2 10% In Ic0=87ANi/MgB2 0% In Ic0=31A

Fig. 1. Stress–strain curves (a) and strain dependence of Ic (b)for nickel sheath/MgB2 tapes with and without 10 vol% indium

addition.

2. Experimental

Two nickel sheathed mono-core PIT tapes with

10 vol% indium addition in the core and without

it, for reference, were prepared. The commercial

Alfa-Aesar MgB2 powder with the purity of 98%

and grain size less than 106 lm was used. The

purity and grain size of the indium powder are

99.9% and less than 40 lm, respectively. The MgB2

powder with or without 10 vol% indium powder

was encased in a nickel tube with an outer/inner

diameter of 8/6 mm to form a sheath/core com-

posite. The final tape width and thickness after

rolling were about 4 and 0.3 mm, respectively. The

nickel/MgB2 ratio was estimated to be about 1.3

by the SEM photographs. Then, only the tape with

10 vol% indium addition was annealed at 473 Kfor 10 h in flowing argon/5 vol% hydrogen gas.

The details of the specimen preparation are found

in Ref. [5].

The stress/strain characteristics evaluation of

the tapes at 4.2 K and 2 T was conducted by using

an apparatus inserting in the bore of the 15 T su-

perconducting magnet at the Institute for Materi-

als Research, Tohoku University. The detaileddescription of the apparatus is available in Ref. [9].

The gauge length and the voltage tap distance of

the specimen were 17.5 and 8 mm, respectively.

Both ends of the specimen were soldered to each

copper terminal. The specimen can be contracted

freely during cool down, so that we do not have to

consider the pre-strain caused by the difference of

thermal contraction between the specimen and theapparatus. The strain was measured by a clip on-

gage, which was set between the copper current

terminals. It was calibrated at liquid helium tem-

perature. The Ic was evaluated with the criterion of

1 lV/cm. Two specimens were measured for each

PIT tapes.

3. Results and discussion

Fig. 1(a) shows the stress–strain curves of the

tapes. Square symbols represent the points on

unloading. They are almost linear excepting for

K. Katagiri et al. / Physica C 397 (2003) 95–98 97

high and low stress region. Although some dis-

crepancy is seen in the initial region of straining,

the stress–strain curves for two tapes are almost

comparable within the accuracy of the strain

measurement, 0.05%.

Fig. 1(b) shows the strain dependence of Ic forthe two tapes. The Ic is normalized by Ic0, Ic at 0%external strain. The tapes with and without indium

addition have Ic0 of 87 and 31 A, respectively. In

the case of indium added tape, Ic is constant,

within the accuracy of measurement, irrespective

of strain applied and then suddenly degraded be-

yond 0.5% strain. On the other hand, the Ic of thetape without indium increased very slightly up to0.2% with the external strain and then gradually

decreased until 0.25% where it begins to decrease

rather steeply. After the degradation, Ic could not

reversibly recovered on unloading. The compres-

sive strain levels of the cores of MgB2 subjected

from Ni sheath for indium added tape and that

without it appear almost the same. The increase in

the strain for the irreversible degradation onset byindium addition will be ascribed to the improved

connectivity of MgB2 grains as evidenced by the

increase in the Ic before straining mentioned above.

The mechanisms how the indium addition sup-

presses the slight increase in Ic with strain is not

known at present.

The strain dependence of Ic in Fig. 1(b) is re-

plotted in Fig. 2 together with those available

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.0

0.2

0.4

0.6

0.8

1.0

1.2

I c/Ic0

Tensile strain (%)

Ni/MgB2 10 In addition [4.2K, 2T]Ni/MgB2 [4.2K, 2T]SS25/Fe/MgB2(Goldacker[8])[4.2K, 5T]SUS316/MgB2(Kitaguchi[7]) [4.2K, 5T]

Fig. 2. Comparison of strain dependence of Ic with those re-

ported by Kitaguchi et al. [7] and Goldacker and Schlachter [8].

reported by Kitaguchi et al. [7] and Goldacker

and Schlachter [8]. According to the higher ther-

mal contraction coefficient of nickel (12.8 · 10�6

K�1 at 300 K) as compared with that of MgB2

(8.3 · 10�6 K�1) [10], the slight increase in Ic in the

tape without indium in the present study is as-cribed to the relaxation of the compressive strain

generated in the MgB2 core during cooling down

to 4.2 K [7,8]. However, the amount of the in-

crease, less than 1.5%, is quite small compared

with those, 7% and 13%, in Refs. [7] and [8], re-

spectively. One of the main reasons for the dif-

ference in the amount of increase might be that

our measurement was performed at relativelylower magnetic field of 2 T as compared with the

measurements at 5 T [7,8]. Another possible rea-

son is the difference in the shape of the cross

section. The aspect ratio in the tapes used in the

present study and Ref [7] are larger than those of

round wire [8]. Difference in the three-dimensional

strain state due to the aspect ratio has already

been reported [11]. Amount of the pre-strainsubjected to thermal history has possibility to

explain both the degree of enhancement of Icmentioned above and the irreversible strain where

the Ic steeply decreases. Because the materials of

the sheath as well as the sheath to superconductor

ratio are different, the differences in the irrevers-

ible strains for the four tapes cannot be discussed

quantitatively at present.Fig. 3 shows SEM photographs of the longi-

tudinal section of the tape specimen without in-

dium addition after the tensile test. The axis of

tensile loading is horizontal in the photographs.

Many cracks could be observed for the tape

without indium addition. On the other hand, ra-

ther small number of cracks is found in the tape

with indium addition. The cracks appear topropagate mainly along the grain boundary in the

magnified view (Fig. 3(b)). The cracks are sup-

posed to have caused the irreversible Ic degrada-

tion. According to the transmission electron

microscopic study, it has been observed that the

indium distributed so as to fill the gap of the MgB2

grains [5]. We believe that the indium powder

addition contributes to the improvement of theweak MgB2 grain linkage and, therefore, both the

Ic and its strain characteristics.

Fig. 3. SEM photographs in longitudinal section for tapes

without indium powder after tensile test. (a) Low magnification

view and (b) magnified view.

98 K. Katagiri et al. / Physica C 397 (2003) 95–98

4. Summary

The stress/strain characteristics at 4.2 K and 2 T

of the PIT nickel sheathed MgB2 tapes with andwithout 10 vol% indium addition to its core were

evaluated. In the latter tape, the Ic slightly in-

creased as the strain is increased. Beyond the peak,

it gradually and then steeply degraded. On the

other hand, no peak of Ic is found in the tape with

indium addition. After the degradation, Ic did not

recover on unloading. The irreversible steep Icdegradation begins at the strains of 0.5% and

0.25% for the tapes with and without indium

powder, respectively. The cracks in the longitudi-

nal section of both tapes with and without indium

powder after the tensile test appeared to grow

mainly along the grain boundary of MgB2. Thus,

we believe that the indium powder addition con-tributes to the improvement of the mechanical

linkage of the MgB2 grain.

Acknowledgements

We wish to thank T. Tsukinokizawa, K. Hiroi

T. Araya and R. Takaya, Graduate students ofIwate University for their help in the experiments

at HFML, Tohoku University and SEM ob-

servations. This work was performed with the

support and under the auspice of the NIFS Col-

laborative Research Program.

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