Physica C 415 (2004) 158–162

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Influence of heat-treatment schedules on magneticcritical current density and phase formation

in bulk superconducting MgB2

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

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

2041 College Road, Columbus, OH 43210, USAb Hyper Tech Research Inc., Troy, OH 45373, USA

Received 20 October 2003; received in revised form 18 June 2004; accepted 12 July 2004

Abstract

Various heat-treatment schedules have been applied to the optimization of Tc and Jc,m in bulk MgB2. Samples were

prepared by first mixing in a �V� shaped mixer and then planetary milling a stoichiometric mixture of Mg and B pow-

ders. These powders were then compacted and heat-treated at different schedules under 200 Torr of Ar. The heat-treat-

ing schedules investigated involved different time-temperatures, heating rates, and cooling rates. Magnetic critical

current densities (at 20 K) of more than 2 · 105 A/cm2 in 10 kOe and 4 · 105 A/cm2 in 5 kOe were obtained. The influ-

ence of sodium silicate additions on the bulk properties has been studied.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

In general, two basic approaches are being fol-

lowed for the preparation of bulk MgB2: the

ex-situ approach, which involves compaction and

sintering of the preformed MgB2 powder, andthe in-situ approach where a mixture of elemental

Mg and B powders are reacted in the final wire.

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

doi:10.1016/j.physc.2004.07.009

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

6883677.

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

Different research groups, following these two ap-

proaches and their variants, have reported numer-

ous different optimal heat-treatment (HT)

schedules based on their specific preparation pro-

cedures. Giunchi et al. [1] infiltrated liquid Mg into

porous preforms of B and heat-treated them for950 �C/3 h. Hinks et al. [2] studied stoichiometric

variations for in-situ materials. Larbalestier et al.

[3] reported a multi-step heat-treatment, which in-

cluded HT at 600, 800 and 900 �C sequentially for

1 h at each temperature, followed by crushing and

compacting, and subsequent heat-treating under

ed.

M. Bhatia et al. / Physica C 415 (2004) 158–162 159

pressure at temperatures ranging from 650 to 800

�C. Dou et al. [4] heat-treated their SiC doped

MgB2 for 950 �C/3 h followed by LN2 quenching.

This paper describes the preparation and proper-

ties of in-situ processed bulk pellets. In particularwe show the effect of various heat-treatment sched-

ules on superconducting transition temperature, Tc

and magnetic current density, Jc,m. We further

show the effect of Na2O ÆSiO2 addition on various

properties like transition temperature, Tc, mag-

netic current density, Jc,m and the irreversibility

field, Hr.

ty, χ

dc (

emu/

Oe

cm3 )

-0.15

-0.10

-0.05

0.00 675oC/15 min 675oC/30 min 675oC/60 min

2. Sample preparation

The samples were prepared by an in-situ reac-

tion of a stoichiometric mixture of 325 mesh 99.9

% pure Mg and amorphous B powders with a typ-

ical size of 1–2 lm. Powders were mixed in a �V�shaped jar and then planetary milled for 48 minafter which an uncompacted powder density of

0.5039 g/ml was obtained. The milled powder

was then compacted in the form of a cylindrical

pellet in a steel die. The pellets along with the die

were then encapsulated in a quartz tube under

200 Torr of Ar. A small amount of Ta powder

was added to the capsule as an oxygen getter.

These capsules were then heat-treated at 650–900�C for 10–30 min. The HT was performed using

a step ramp protocol. In a few cases longer times

were also applied. After sintering, the capsules

were opened and the pellets removed as cylinders,

4 mm in diameter and 10 mm in length. They

where then reshaped into 5 · 2 · 2 mm3 cuboids.

Samples with sodium silicate [Alfa Aesar (Na2O ÆSiO2 99.9%)] additions were prepared in the sameway except that during milling 10 wt% of the com-

pound was added.

Temperature , T (K)0 5 10 15 20 25 30 35 40

Susc

eptib

ili

-0.35

-0.30

-0.25

-0.20

Fig. 1. vdc vs. T for HT at 675 �C.

3. Measurements

All magnetization measurements were per-

formed with vibrating sample magnetometer(VSM) using field sweep amplitude of 1.7 T and

a temperature range of 4.2–40 K. A sweep rate

of 700 Oe/s was used for theM–H loops, while sus-

ceptibility, vdc vs. temperature, T measurements

were performed using a 5 Oe field sweep amplitude

after zero field cooling.

4. Results and discussion

Fig. 1 shows the volume susceptibility, vdc vs. Tat the optimal heat-treatment temperature, i.e. 675

�C. Note that here we are displaying volume sus-

ceptibility. This has dimensionless units, and in

the cgs-practical system we are using here the value

for complete exclusion is 1/(4pD) [5]. The demag-netization factor, D, based on the shape of our

samples and the tables of Osborn [6] for different

ellipsoids ranges from 0.276 to 0.40. Taking this

into consideration, we observe flux exclusion

which is complete and uniform. Together with

the sharp transitions this suggests good phase for-

mation. Fig. 2 is an expanded view. Recognizing

that the melting point, MP of Mg, is approxi-mately 650 �C we would expect that near 650 �C,the MgB2 formation reaction is struggling to com-

plete. This leads to a rise in Tc with the increasing

soaking times (from 30 to 120 min. at 650 �C). At

800 �C the Tc starts out maximized, and drops

with longer HT times. This can be attributed to

the complete formation of MgB2 and then

Temperature, T(K)28 30 32 34 36 38 40

Susc

eptib

ility

, χdc

(em

u/O

e cm

3 )

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

650˚C/120 min

650˚C/30 min

800C/30 min

850˚C/10 min

900˚C/10 min

Fig. 2. vdc vs. T for various HT.

160 M. Bhatia et al. / Physica C 415 (2004) 158–162

subsequent decomposition into secondary phases

depicted in the Mg–B phase diagram [7].

The Tc results can also be compared to the var-

iation of magnetic current densities Jc,m. Fig. 3

shows the M–H loop for MgB2 bulk sample HTfor 675 �C/30 min. The magnetic critical current

density, Jc,m, is extracted from the M–H loops

Magnetic field, B (kOe)-20 -15 -10 -5 0 5 10 15 20

Mag

netiz

atio

n, M

, 103 e

mu/

cm3

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

4.2K15K20K25K30K35K

Fig. 3. M–H loops for HT at 675 �C/30 min.

using the Bean critical-state model, according to

which [8–10]:

J c;m ¼ 20DM�

d 1� d3L

� �� �ð1Þ

where DM is the difference in the magnetization

between the upper and the lower branches of the

hysteresis loop, d is the width of the rectangular

slab in the direction perpendicular to the applied

field and L is the length of the sample. Fig. 4 shows

the variation of Jc,m with the applied magneticfield at various temperatures.

Fig. 5 is a comparison of the field variation of

Jc,m, extracted from theM–H loops for the various

samples at 20 K. It can be seen here that Jc,m in-

creases along with Tc at 650 and 675 �C. At 800

�C Tc increases while Jc,m drops. We attribute this

drop in Jc,m to grain growth. This can be very

clearly seen from Fig. 6, a plot of Jc,m at 20 K,10 kOe for various HT schedules. The highest

Jc,m value achieved is 2.2 · 105 A/cm2 at 20 K

and 10 kOe. This value is higher than the bulk val-

ues reported by Dou et al. [4] and is almost equal

to the Jc,m of their 10%SiC doped sample at the

same temperature and field [7].

At 900 �C, it can be noted that Jc,m and Tc de-

crease with longer heating times. This can be ex-

Magnetic Field ,B (kOe)0 2 4 6 8 10 12 14

J c,m

A/c

m2

1e+1

1e+2

1e+3

1e+4

1e+5

1e+6

20 K

25 K

30 K

35 K

Fig. 4. Jc,m vs. B at various measuring temperatures for HT at

675 �C/30 min.

Boron (mole %)0 20 40 60 80 100

Tem

pera

ture

(˚C

)

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

Solid + MgB2

Liquid + MgB2

Gas + MgB2

Gas + MgB4

Gas + MgB7

Gas + Liquid

MgB

2 +

MgB

4

MgB

4 + M

gB7

B(S

) +

MgB

7

Fig. 7. Theoretical phase diagram of Mg–B [10].

H/T Schedule

650/30 650/120675/15675/30675/60 800/30 850/10 900/10 900/30

J c,m

A/c

m2

104

105

Fig. 6. Jc,m vs. HT at 20 K and 10 kOe for different HT

schedules.

Magnetic Field,B (kOe)0 2 4 6 8 10 12 14

J c,m

A/c

m2

104

105

106

675/30675/15675/60

650/120650/30850/10800/30900/10

900/30

Fig. 5. Jc,m vs. B for different HT schedules.

M. Bhatia et al. / Physica C 415 (2004) 158–162 161

plained on the basis of the Mg–B phase diagram,

Fig. 7 [11]. Liu et al. [11] in their theoretical study

of this diagram have shown that MgB2 decom-

poses into MgB4 and Mg vapor at a wide range

of temperatures, with pressure having a significant

influence on the decomposition temperature. In

our samples, we expect the decomposition tem-

perature to be around 900 �C. Detailed thermo-

dynamic and microstructural studies will be

presented in subsequent papers. Based on our pre-

sent study we find, for our preparation procedure,

an optimal HT temperature of 675 �C, associatedwith a soaking time of 30 min.

5. Na2O ÆSiO2 doping

We have added a small amount of Na2O ÆSiO2

to our optimized sample and studied its effect.

Fig. 8 compares the vdc of the doped and undopedsample. We find that there is a drop in the Jc,m,

especially at the lower temperatures. However, it

can be shown that the actual fields at which Jc,mare non-zero are increased by the presence of

Na2O ÆSiO2, see Fig. 9. That is, Hr is increasing

at 30 and 35 K. We can get a value for Hr for these

two samples at 35 K using the criterion of 100 A/

cm2, in which case we get 5.5 Oe for the binary and6.5 Oe for the doped sample. However, the pres-

ence of the Na2O ÆSiO2 is also lowering Jc,m. It

could be that the Na2O ÆSiO2 is inhibiting grain-

to-grain connection. In the future we intend to

look into whether the Na2O ÆSiO2 is segregating

Temperature, T (K)

0 5 10 15 20 25 30 35 40

Nor

mal

ized

χdc

-0.25

-0.20

-0.15

-0.10

-0.05

0.00 Pure MgB2

MgB2 + Na2O.SiO2

Fig. 8. vdc vs. T for Na2O ÆSiO2 doped and pure MgB2 sample

HT at 675 �C/30 min.

Magnetic Field, B (kOe)0 2 4 6 8 10 12 14 16

J c (

A/c

m3 )

101

102

103

104

105

106

MgB2 + Na2O.SiO2

Pure MgB2

25 K

30 K

35 K

Fig. 9. Jc,m vs. B for Na2O ÆSiO2 doped and pure MgB2 sample

HT at 675 �C/30 min.

162 M. Bhatia et al. / Physica C 415 (2004) 158–162

to the grain boundaries, and if lower dopant levels

would be more optimal.

6. Summary and conclusion

The optimum heat-treatment schedule for our

powder compacts was 675 �C/30 min. We have

achieved Jc,m values of 2.3 · 105 A/cm2 at 20 K,10 kOe. We further showed the effect of Na2O ÆSiO2 doping which lowered Jc,m, but seemed to in-

crease Hr at higher temperatures. It may be that a

lower dopant level would be of interest.

Acknowledgments

A State of Ohio Technology Action Fund

Grant has supported this research work, per-

formed at the Ohio State University, USA. The

authors also wish to thank Prof. S.X. Dou, ISEM,

University of Wollongong, Australia.

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