Physica C 407 (2004) 153–159

www.elsevier.com/locate/physc

Influence of heat-treatment schedules on the transportcurrent densities of long and short segments

of superconducting MgB2 wire

Mohit Bhatia a,*, M.D. Sumption a, Mike Tomsic b, E.W. Collings a

a Department of Materials Science and Engineering, LASM, Ohio State University, 477 Watts Hall, 2041 College Road,

Columbus, OH 43210, USAb Hyper Tech Research Inc., Troy, OH 45373, USA

Received 7 March 2004; received in revised form 15 April 2004; accepted 19 April 2004

Abstract

Various MgB2/sheath composite strands have been tested using transport and magnetic measurement techniques.

The strands had bimetallic sheaths of (1) Fe/monel, (2) Fe/Cu and (3) Cu/Cu. In strands (1) and (2) the Fe played the

role of diffusion barrier. Transport current densities of more than 5· 105 A/cm2 at 4.2 K in self-field and 2.7· 104 A/cm2

at 5 T were obtained on short lengths of Fe/monel strand. In coils, 6· 104 A/cm2 in 1 T at 4.2 K for Cu/Cu sheath

strands and 2.2· 105 for Fe/monel sheath strands have been measured. The strands had diameters of 0.8–1.1 mm and

superconducting fractions ranging from 27% to 40% of the total wire cross-section. They were heat-treated (HT) at

temperatures of from 675 to 900 �C under various heat-treatment schedules. The best results were obtained at 675 �Cfor Cu/Cu and 700 �C for Fe/monel strands.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

Practical applications of MgB2 superconductors

require the fabrication of dense wires (both mono

and multi-filament) with high current densities at

the desired operating temperatures. For MgB2

wires, the sheath material (barrier and outer

sheath) plays a very important role in determiningthe transport properties. This work is a compara-

tive study between the use of double Fe/monel, Cu/

Cu and Fe/Cu sheaths in the fabrication of MgB2

* Corresponding author. Tel.: +1-614-6885344; fax: +1-614-

6883677.

E-mail address: bhatia.27@osu.edu (M. Bhatia).

0921-4534/$ - see front matter � 2004 Elsevier B.V. All rights reserv

doi:10.1016/j.physc.2004.05.013

wires. We report magnetic and transport current

density measurements performed after heat-treat-

ment using various schedules.

2. Fabrication of the wires

2.1. Powder and strand fabrication

The strands studied were of the monocore

powder-in-tube (PIT) type, manufactured using a

continuous tube forming and filling (CTFF)

method by Hyper Tech Research Inc. (HTR). A

stoichiometric mixture of Mg (99.86% pure, fine)

and B (‘‘amorphous’’) powders were mixed in a ‘V’

shaped jarand then planetary milled in a Fritcsh

ed.

154 M. Bhatia et al. / Physica C 407 (2004) 153–159

planetary mill, model LC106A. A ball to powder

ratio of 1.7:1 by mass was used.

After milling the powder was dispersed onto thin

metal strip (either Fe or Cu) formed into a tube

during filling. This CTFF powder filled tube was

then inserted into either one or two outer sheathtubes and drawn to final size. Fig. 2(a) and (b) show

the cross-sections of the double Fe barrier monel

sheath (Fe/Fe/M) wire and the Cu/Cu sheath wire.

The wires tested had diameters of 0.8–1.1 mm.

2.2. Double iron/monel sheathMgB2 wire (Fe/Fe/M)

These strands consisted of CTFF (Mg+B)/Fe inserted into single wall Fe-tubes, 12 mm

thickness, and again into a monel outer sheaths

Table 1

Fe/Fe/monel, Fe/Cu, Cu/Cu short sample specifications

Sample name Reference name Dia., m

Fe/Fe/monel sheath samples

FeFeM750-60a MWJ17 1.14

FeFeM700-30 MWJ16 0.984

FeFeM700-60 MWJ15 0.984

FeFeM750-120 MWJ14 0.984

FeFeM750-60b MWJ13 0.984

FeFeM750-30 MWJ12 0.984

FeFeM750-15 MWJ11 0.984

FeFeM850-15 MWJ10 0.984

FeFeM850-05 MWJ09 0.984

FeFeM725-240 MWJ08 0.984

FeFeM650-60 MWJ07 1.14

FeFeM650-60 MWJ06 0.984

FeFeM900-00a MWJ05 0.984

FeFeM900-05a MWJ04 0.984

FeFeM900-00b MWJ03 1.14

FeFeM900-05b MWJ02 1.1.4

FeFeM900-15 MWJ01 1.14

Fe/Cu sheath samples

FeCu675-15a MWJ36 1.038

FeCu675-15b MWJ37 1.038

FeCu700-05a MWJ39 1.038

FeCu700-05b MWJ40 1.038

FeCu700-15a MWJ42 1.038

FeCu700-15b MWJ43 1.038

Cu/Cu sheath samples

CuCu675-15 MWJ21 1.038

CuCu700-05 MWJ27 1.038

CuCu700-15 MWJ32 1.038

CuCu725-05 MWJ24 1.038

and drawn down to a final diameter of about

1 mm.

2.3. Iron/copper sheath MgB2 wires (Fe/Cu)

The Fe/Cu standard PIT-process wires weremade by loading milled powders into a 0.12 mm

wall thickness Fe tube inserted into a fully hard

Cu-101 tube and drawn down to the final diame-

ters of about 1 mm.

2.4. Copper/copper sheath MgB2 wire (Cu/Cu)

The Cu/Cu monocore wires consisted of CTFF(Mg+B)/Cu (annealed Cu-101, 0.25 mm thick)

inserted into a full hard Cu 101 tube (9.5 mm

m Heat-treatment,

�C/minSC area, mm2

750/60 0.306

700/30 0.228

700/60 0.228

750/120 0.228

750/60 0.228

750/30 0.228

750/15 0.228

850/15 0.228

850/05 0.228

725/240 0.228

650/60 0.228

650/60 0.228

900/IN 0.228

900/05 0.228

900/IN 0.306

900/05 0.306

900/15 0.306

675/15 0.286

675/15 0.286

700/05 0.286

700/05 0.286

700/15 0.286

700/15 0.286

675/15 0.286

700/05 0.286

700/15 0.286

725/5 0.286

M. Bhatia et al. / Physica C 407 (2004) 153–159 155

OD · 7.75 mm ID) and drawn down to a final

diameter of approximately 1 mm. All the wires

so produced were then heat-treated in flowing Ar at

Time, t (mins)

0 20 40 60 80 100

Tem

pera

ture

, T (

o C)

0

100

200

300

400

500

600

700

800

750oC / 30 mins 675 oC /15 min

Fig. 1. T vs t for the HT of wires.

Table 2

Fe/Fe/monel and cu/cu long sample specifications

Sample name Dia., mm Heat-treatment,

�C/minSC area,

mm2

Cu/Cu sheath samples

CuCu675-15L 1.038 675/15 0.286

Fe/Fe/monel sheath samples

FeFeM700-30L 0.984 700/30 0.228

Fig. 2. Cross-section of monofilamentary

various time–temperature schedules (Tables 1 and

2). Heat-treatment profiles are shown in Fig. 1.

3. Measurements

3.1. Transport current density measurement

The transport current density, Jc, has been

measured on both short samples and longer seg-

ments (about 1m). Short sample were 3 cm in length

with a 0.5 cm gauge length. A 1 lV/cm criterion was

used for Ic. Measurements were performed using

the four-probe method. The longer segments wereabout 1 m, and were wound on a barrel-like holder

following the holder design used for strand testing

under the International Thermonuclear Energy

Reactor, ITER, program. The test specifications

can be found in [1]. Some of the measurements were

performed in self-field and others in field up to 6 T.

In our variant of the ITER test about 1 m of wire

was rolled around a 3.0 cm diameter stainless steelbarrel furnished with copper end rings (Fig. 3) for

the proper dissipation of the heat produced by the

large amounts of currents passed on to the sample.

The ends of the wires were soldered to the copper

rings, which in turn were soldered to the current

probe. The gauge length in this case was 20 cm.

3.2. Magnetic current density measurements

Magnetic current density measurements were

performed using a vibrating sample magnetometer

MgB2 wire (a) Fe/Fe/M, (b) Cu/Cu.

Magnetic Field, B (T)0

J c, A

/cm

2 of

SC

104

105

FeFeM900-15LL

FeFeM900-05LFeFeM900-00L

FeFeM900-05FeFeM900-00FeFeM850-05

FeFeM850-15FeFeM750-15FeFeM750-30

FeFeM750-60FeFeM750-120

FeFeM750-60LFeFeM725-240MW J08 (723/4)

FeFeM700-60FeFeM700-30LHT#1 FeFeM700-30

Magnetic Field, B (T)0 2 4

J cA

/cm

2 S

C

103

104

105

106

CuCu700-15CuCu675-15

(a)

Lower Temperatures

1 2 3 4 5 6

1 3 5 6

(b)

Fig. 3. Transport Jc (4.2 K) vs B for (a) Fe/Fe/M and (b) Cu/Cu samples after various HTs.

156 M. Bhatia et al. / Physica C 407 (2004) 153–159

(VSM) on the Fe/Fe/M and Cu/Cu strands at

temperatures ranging from 4.2 to 35 K. With Fe/

Fe/M samples the magnetization of the Fe (mea-

sured above 40 K) was subtracted from the total in

order to obtain that of the superconducting core.

4. Results

4.1. Short samples

The HT schedules of the short samples are lis-

ted in Table 1.

The Jc of the Fe/Fe/M samples is plotted against

the magnetic field, B, in Fig. 3(a). The measure-

ments were performed between 4 and 6 T and the

data extrapolated to lower fields. It can be noted

from the figure that the samples with less aggres-

sive heat-treatments have the higher Jcs. This maystem from several sources. In the first case, mod-erate temperatures may lead to grain growth while

at higher ones, MgB2 phase decomposition is likely

depending upon the Mg vapor pressure [2]. How-

ever, it may also be that the sheath/powder inter-

action (at incipient sheath defects for Fe/Fe/M)

causes degradation at more aggressive HTs.

M. Bhatia et al. / Physica C 407 (2004) 153–159 157

The less aggressively HTd samples FeFeM700-

30 and FeFeM750-15 samples have 4.2 K Jcs of2.5 · 104 and 2.4 · 104 A/cm2 respectively at an

applied field of 5 T. These values are higher than

those reported by Zhou et al. [3] for their seven-

filament wires and equal to the high Jc values re-ported by Kumakura et al. [4] for their ZrSi2 and

ZrB2 doped MgB2 wires. Our extrapolated trans-

port data are in accordance with the low field

magnetic current density data deduced from the

VSM measurements and depicted in Fig. 4(a) for

the FeFeM700-30 sample. Comparable curves

for the Cu/Cu sample are plotted in Fig. 3(b) and

Fig. 4(b). Fig. 3(b) shows the variation of trans-port Jc with applied field. Here again it can be

seen that the low time–temperature HT, i.e. 675

�C/15 min, is optimal. These data are in accor-

dance with those for bulk MgB2, published else-

where [5].

Fig. 5. BSE of (a) an FeFeM sam

Magnetic Field, B (T)0.5 0.6 0.7 0.8 0.9 1.0 1.1

J c, A

/cm

2 of

SC

104

105

106

4.2K 10K 15K 20K 25K

(a)

Fig. 4. Magnetic Jc vs B for (a) FeFeM

With the Cu/Cu strands, it has been seen that at

higher temperatures the Cu sheath tends to react

with the powder inside, to form unwanted Mg–Cu

phases. The electron back-scattering image (BSE),

Fig. 5(b), shows this reaction layer between the Cu

sheath and the MgB2 powder. Hence, during HTthe Mg–Cu and Mg–B reactions compete. Thus

the optimal time–temperature schedule for Cu/Cu

wire is lower than that of Fe/Fe/M wire, i.e., 675

�C/15 min compared to 700 �C/30 min. For Fe/Fe/M a characteristic problem faced is the formation

of pinholes in the inner Fe layer but this can be

overcome by increasing the thickness of the Fe

sheath.Fig. 6 is a comparison of the FeFeM700-30 and

FeFeM750-15 transport-Jc data with some of the

best published magnetic data [6]. It can be noted

that the transport-Jcs of these samples are com-

parable to the 10% SiC doped bulk samples

ple and (b) a CuCu sample.

Magnetic field, B (T)0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

J c, A

/cm

2 of

SC

102

103

104

105

4.2K10K15K20K25K

(b)

700-30, (b) CuCu675-15 samples.

Magnetic Field, B (T) 0 2 4 6 8

J c A

/cm

2 SC

102

103

104

105

106

FeFeM850-5 FeFeM750-15 FeFeM750-30 FeFeM700-30

Dou (0%SiC) Dou (5%SiC) Dou (10%SiC)

FeFeM700-30 mag

Fig. 6. Jc;m vs B for Fe/Fe/M samples compared to published

data.

Temperature,T(oC)0 10 20 30 40 50

Nor

mal

ized

res

ista

nce

0.0

0.2

0.4

0.6

0.8

1.0

FeFeM700-30FeFeM700-15FeFeM700-05CuCu675-15

Fig. 7. Resistive Tc for Fe/Fe/M and Cu/Cu samples.

Magnetic Field, B (T)0 3 7

J c A

/cm

2 of

SC

103

104

105

106

CuCu675-15 L (Barrel)FeFeM700-30 L (Barrel)CuCu675-15 (Magnetic) FeFeM700-30 (Magnetic) FeFeM700-30 (Short) CuCu675-15 (Short)

Magnetic Field, B (T)0 1 5

J c1/

2 B1/

4

100

1000CuCu675-15LFeFeM700-30L

2 3 4 6 7

1 2 4 5 6

Fig. 8. Jc vs B for FeFeM700-30L and CuCu675-15L (inset:

J 1=2c B1=4 vs B [Kramer plot] for the same samples).

158 M. Bhatia et al. / Physica C 407 (2004) 153–159

reported by Dou et al. [6]. Fig. 7 shows the Tc ofthe optimized Fe/Fe/M and Cu/Cu wires.

Samples with the Fe/Cu barrier/sheath were

also investigated, but found to have low Jcsattributed to significant Fe-barrier breakage and

subsequent poisoning-by-Cu. The cause of the

breakage is thought to stem from the large flow

stress mismatch between the Fe and the Cu. It

should be possible to overcome this problem byusing a thicker Fe sheath. Property data for these

wires is to be reported.

4.2. Long samples

The transport Jcs of 1-m lengths of FeFeM700-

30L and CuCu675-15L were also measured. The Jcmeasurements were performed at 4.2 K in fields of1–6 T. Fig. 8 shows the variation of Jc with appliedfield for these wires. Those long sample Jcs are inaccordance with the transport Jcs measured on the

short (3 cm) samples. However, the reduction of

end-heating effects encountered with barrel

mounting enabled the long samples to be measured

at lower fields (higher transport currents) than was

possible under short-sample mounting. The inset inFig. 8 is the Kramer plot for these Jc values.

5. Conclusions

The optimized heat-treatment for the Fe/Fe/M

MgB2 strand, prepared as described above is 700

�C/30 min while that for Cu/Cu MgB2 strand is675 �C/15 min. Also a 4.2 K, 5 T Jc of 2.5 · 104 A/cm2 has been achieved for the double Fe/monel

M. Bhatia et al. / Physica C 407 (2004) 153–159 159

sheathed wire. Long lengths of wire have been

measured and the results are consistent with the

short length results thereby proving the effective-

ness of the fabrication process.

Acknowledgements

This research work was supported by a State of

Ohio Technology Action Fund Grant, U.S. Air

Force SBIR F33615-02-M-2267, and a Missile

Defense Agency SBIR administered by the U.S.

Air Force, contract no. F33615-02-M-2283. The

authors also wish to thank Dr. S.X. Dou, ISEM,University of Wollongong, Australia.

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