T_c and J_c distribution in in situ processed MgB₂ bulk superconductors with/without C doping

pISSN 1229-3008 eISSN 2287-6251

Progress in Superconductivity and Cryogenics

Vol.16, No.2, (2014), pp.36~41 http://dx.doi.org/10.9714/psac.2014.16.2.036

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1. INTRODUCTION

Since MgB

has a high superconducting critical temperature (T

) of 39 K [1], MgB

is considered as a promising material for the applications of superconductor magnets operating without a use of liquid helium [2]. In addition to the high T

, MgB

has many other merits [3-6].

They are the low isotropic current flow in the crystal structure of MgB

, the strongly coupled grain boundary, the low material cost, and the simple fabrication method into a wire [4-6]. Because of the advantages, MgB

is expected to replace NbTi which is the conventional superconducting wire using liquid helium as a coolant in near future [2, 3].

Two different well-known fabrication methods, an in situ process [7-9] and an ex-situ process [10, 11], are widely used to fabricate MgB

bulk materials and superconducting wires. In the in situ process magnesium (Mg) and boron (B) powders are used as raw materials [7], whereas in the ex situ process the readily synthesized MgB

powder is used [10]. In the in situ processed MgB

, the MgB

formed via the solid or liquid reaction between Mg and B and the resulting microstructure was very porous [12-15]. In comparison with the in situ processed MgB

, a few pores was present in the microstructure of the ex situ processed MgB

. The critical current density (J

) of the in situ processed MgB

was higher than that of the ex situ processed MgB

[9]. The high J

of the in situ processed MgB

seems to be attributed to the lattice distortion, strongly-coupled grain boundaries and defects present inside MgB

grains, which act as flux pinning sites for the applied magnetic fields [6, 16-18]. The impurity additions such as silicon carbide [6], carbon (C) [16] and organic

materials including carbon to MgB

[17-19] were effective in enhancing the current properties at the high magnetic fields. For the commercial use of MgB

superconductors to the magnet applications, the long length MgB

wires with a high T

and J

should be fabricated. The superconducting properties of MgB

are fairly dependent on the fabrication process [20-23]. To improve the J

of the in situ processed MgB

, processing parameters such as initial size and composition of raw materials, a type of dopants, and reaction temperature should be optimized. Additionally, reliable current properties should be achieved.

In this study, MgB

bulk superconductors with/without C doping were prepared through the in situ reaction process using Mg and B powder. To understand the property reliability of MgB

bulk superconductors, ten test samples were taken from the MgB

bulk superconductors and their superconducting properties were examined.

2. EXPERIMENTS

Mg (purity of 99.7 %, particle size of 4-6 µm, Tangshan Weihao Co. Ltd., China) and B (purity of 95-97 %, particle size less than 1 µm, Tangshan Weihao Co. Ltd., China) were used as raw powders to prepare MgB

bulk pellets.

To enhance of the J

of MgB

through grain refinement, B powder was ball-milled at 200 rpm for 2 h using ZrO

balls (2 mm in diameter). Toluene (C

H

, 99.5%) was used as a solvent for milling. The ball-milled B powder was dried at 100 ℃ in a vacuum oven. Two different batches of undoped MgB

and C-doped MgB

were prepared. The C doping to MgB

was known to be effective to increase the J

[16-19]. C was added to MgB

samples through the glycerin (C

H

O

) treatment for B powder, which was

T <sub>c</sub> and J c distribution in in situ processed MgB 2 bulk superconductors with/without C doping

C.-J. Kim

, Y. J. Kim, C.-Y. Lim, B.-H. Jun, S.-D. Park, and K. N. Choo .

Neutron Utilization Technology Division, Korea Atomic Energy Research Institute, Daejeon, Korea (Received 9 June 2014; revised or reviewed 17 June 2014; accepted 18 June 2014)

Abstract

Temperature dependence of magnetic moment (m−T) and the magnetization (M−𝐻) at 5 K and 20 K of the in situ processed MgB

bulk pellets with/without carbon (C) doping were examined. The superconducting critical temperature (T

), the superconducting transition width (∆T) and the critical current density (J

) were estimated for ten test samples taken from the MgB

bulk pellets. The reliable m−T characteristics associated with the uniform MgB

formation were obtained for both MgB

pellets.

The T

s and ∆Ts of all test samples of the undoped MgB

were the same each other as 37.5 K and 1.5 K, respectively. The T

s and

∆Ts of the C-doped MgB

were 36.5 K and 2.5 K, respectively. Unlike the m−T characteristics, there existed the difference among the M−H curves of the test samples, which might be caused by the microstructure variation. In spite of the slight T

decrease, the C doping was effective in enhancing the J

at 5 K.

Keywords: Superconducting critical temperature, critical current density, in situ process MgB

bulk, property reliability

* Corresponding author: [email protected]

C.-J. Kim, Y. J. Kim, C.-Y. Lim, B.-H. Jun, S.-D. Park, and K. N. Choo

already reported in our previous work [24, 25]. B powder was mixed with a liquid glycerin, heated to 100 ℃, maintained at this temperature for appropriate time periods and dried at 200℃ for 20 h in a vacuum oven. The B powders with/without the glycerin treatment were mixed with Mg powder to a composition ratio of Mg:B=1:2. The powder mixtures were uniaxially pressed in a steel mold into pellets (15 mm in diameter and 12 mm in thickness).

The prepared MgB

pellets were encapsulated using titanium (Ti) tube to suppress the possible oxidation of Mg during heat treatment. The Ti-encapsulated pellets were heated at 850℃ at a heating rate of 5℃/min in flowing Ar gas in a tubular furnace, maintained at this temperature for 0.5 h and then furnace-cooled. Since a melting point (m. p.) of Mg is 649 ℃, Mg powders in the (Mg+B) pellet melts during the heat treatment and the Mg melt reacts with B to form MgB

through the liquid-solid reaction of eq. (1).

When the glycerin-treated B powder is used as a raw material, a small amount of C is included in glycerin substitutes for boron sites and then the C is incorporated with MgB

according to eq. (2).

Mg(l) + 2B(s) → MgB

(s) (1) Mg(l) + 2B

C

(s) → Mg(B

C

)

(s) (2) where l and s denote a liquid and a solid, respectively.

After the heat treatment, the phase formation in MgB

pellets was analyzed using a powder X-ray diffraction (XRD) method. Microstructure for fracture surfaces of MgB

pellets was examined using a scanning electron microscope (SEM). Not illustrated here, the XRD results for the undoped MgB

and C-doped MgB

showed that the main formed phase was MgB

and the minor impurity phase was MgO in both MgB

pellets.

Ten rectangular samples with a dimension of 3×3×3 mm

(approximate values) were taken from the heat-treated undoped and C-doped MgB

pellets for the superconducting property measurement (see Fig. 1). The MgB

pellets were cut into several pieces using a diamond saw. To understand the T

and J

distribution of MgB

pellets, the temperature dependence of magnetic moment ( m−T ) and magnetization (M−H) for applied magnetic field up to 7 T at 5 K and 20 K were measured for the ten test samples using a Magnetic Property Measurement System (MPMS, Quantum Design). The magnetic J

was calculated from the width of ∆M of the M−H loops using an extended Bean’s critical model [26] for a rectangular sample given by eq. (3).

] 3 [ 1

] / )[

( ] 20 / )[

(

b cm a a

cm emu H cm M

A H J

 

 

  −

= ∆ (3)

Where a and b (b≥a) are the lengths of sides of a rectangular sample used for the M−H measurement. The average J

and the standard deviation were also calculated from the data estimated for the ten test samples.

(a) (b)

Fig. 1. (a) photo of the in situ processed MgB

pellet, heat-treated at 850℃ for 0.5 h and (b) schematics of ten samples (1-10) taken from sample (a) for magnetization measurement.

3. RESUTLS AND DISCUSSION

Figure 2 shows the m−T curves of the ten test samples of (a) undoped MgB

and (b) C-doped MgB

, heat-treated at 850℃ for 0.5 h, respectively. As can be seen in Fig. 2(a), the m−T curves of the ten test samples are similar each other in shape. The ten test samples show the same superconducting transition temperature (T

) of 37.5 K as well as the same transition width (∆T) of 1.5 K (from 37.5 K to 36 K). The T

of 37.5 K of the undoped MgB

samples is slightly lower than 39 K that was reported in other study [1], which seems to be attributed to the impurity elements included in the low purity raw materials (Mg: 99.7% and B: 95−97%) of this study.

30 32 34 36 38 40

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

1 2 3 4 5 6 7 8 9 10

(a) Undoped MgB

Temperature (K)

Normalized magnetic moment (emu)

30 32 34 36 38 40

-1.0 -0.8 -0.6 -0.4 -0.2

0.0

(b) Carbon-doped MgB

Normalized magnetic moment (emu)

Temperature (K)

1 2 3 4 5 6 7 8 9 10

Fig. 2. Normalized m−T curves for ten test samples of (a) undoped MgB

and (b) C-doped MgB

.

37

T

and J

distribution in in situ processed MgB

bulk superconductors with/without C doping

The ten test samples of the C-doped MgB

also show the similar m−T characteristic behavior with the same T

and the same ∆T (see Fig. 2(b)). The T

is 36.5 K and the ∆T of 2.5 K (from 36.5 K to 34 K), which is lower or larger than that of the undoped MgB

. The lower T

and the larger ∆T are attributed to the C incorporation with MgB

[16, 24]

from the glycerin treated B power which was used as a raw material.

The m−T curves of each ten test samples of the undoped MgB

and the C-doped MgB

pellets also showed the reliable m−T characteristics with the same T

and the same ∆T. The results indicates that the in site reaction process using Mg and B powders leads to the homogeneous reaction to form the superconducting MgB

phase in the MgB

pellets.

Figure 3 shows the critical current density vs. magnetic field (J

−H) curves at (a) 5 K and (b) 20 K of the ten samples of the undoped MgB

, calculated using a Bean model. It was difficult to get the reliable J

data at the magnetic fields below 2 T at 5 K and below 1 T at 20 K owing to the data scattering by flux jump. Therefore, only the J

data at 2-7 T, 5 K are shown here. The J

−H curves at 5 K up to 7 T and 20 K (1−5 T) show the typical magnetic field (H) dependence of J

. As H increases, the J

decreases monotonically. The J

decrease at 20 K with increasing H is more rapid than at 5 K. There exist differences in J

−H curves among the test ten samples. This is comparable to the m−T curves of the same samples with the same T

and the same ∆T observed in Fig. 2. The J

differences among samples indicate that the current-carrying capacity of a part of the MgB

sample is different from that of other parts. The J

difference among the ten test samples seems to be associated with the microstructural variation regarding the current flow, which will be mentioned later.

1 2 3 4 5 6 7

10<sup>3</sup> 10<sup>4</sup> 10<sup>5</sup>

1 2 3 4 5 6 7 8 9 10

(a) Undoped MgB

, at 5 K

Critical current density (A cm-2)

Applied magnetic field (T)

1 2 3 4 5

10<sup>3</sup> 10<sup>4</sup>

10<sup>5 1 2</sup> <sub>3</sub>

4 5 6 7 8 9 10

(b) Undoped MgB

, at 20 K

Critical current density (A cm-2)

Applied magnetic field (T)

Fig. 3. J

−H curves at (a) 5 K and (b) 20 K estimated for ten test samples of undoped MgB

.

Figure 4 shows the J

−H curves at (a) 5 K and (b) 20 K of the ten test samples of the C-doped MgB

. The J

−B curves at 5 K, (2−7 T) and 20 K, (1−5 T) are also characterized by the J

decrease with increasing H. There also exist differences in J

among the ten test samples.

The J

−H data at 5 K of the undoped MgB

was compared with that of the C-doped MgB

. Figure 5 shows the J

histogram of the ten test samples at 5 K, 4 T of (a) undoped MgB

and (b) C-doped MgB

. The J

s at 4 T of the undoped MgB

and the C-doped MgB

are similar each other as about 10

A/cm

. As H increases, the J

difference of between two MgB

pellets becomes large. The J

s at high fields of the C-doped MgB

are relatively higher than those of the undoped MgB

. For example, the average J

at 5 T of the undoped MgB

, calculated from the ten test samples, is 1.95×10

A/cm

and the standard deviation is 4.2×10

A/cm

(21% of the average J

). In comparison with the undoped MgB

, the average J

at 5 T of the C-doped MgB

is 2.32 × 10

A/cm

and the standard deviation is 3.6×10

A/cm

(15% of the average J

). The standard deviation data indicates that the C-doped MgB

is more reliable than that of the undoped MgB

in current-carrying capacity. The J

enhancement in the C-doped MgB

attributed to the microstructural defects induced by the C incorporation with the MgB

lattices [17, 18, 24].

1 2 3 4 5 6 7

10<sup>3</sup> 10<sup>4</sup>

10<sup>5 1 2</sup> <sub>3</sub>

4 5 6 7 8 9 10

Applied magnetic field (T) Critical current density (Acm-2)

(a) Carbon-doped MgB

, at 5 K

1 2 3 4 5

10<sup>3</sup> 10<sup>4</sup> 10<sup>5</sup>

1 2 3 4 5 6 7 8 9 10

(b) Carbon-doped MgB

, at 20K

Applied magnetic field (T) Critical current density (Acm-2)

Fig. 4. J

−H curves at (a) 5 K and (b) 20 K estimated for ten test samples of C-doped MgB

.

The J

−H data at 20 K of the undoped MgB

and the

C-doped MgB

were also analyzed and the result was shown

in Fig. 6. The histogram shows the J

distribution of the ten

test samples at 20 K, 2 T of (a) undoped MgB

and (b)

C-doped MgB

. Unlike the J

data at 5 K, 4 T, the J

at 20 K,

2 T of the undoped MgB

is higher than that of the C-doped

38

C.-J. Kim, Y. J. Kim, C.-Y. Lim, B.-H. Jun, S.-D. Park, and K. N. Choo

1 2 3 4 5 6 7 8 9 10

0 5000 10000 15000 20000 25000 30000

Average

(a) Undoped MgB

, at 4 T, 5 K

Sample number Critical current density (Acm-2)

1 2 3 4 5 6 7 8 9 10

0 5000 10000 15000 20000 25000

30000 (b) Carbon-doped MgB<sub>2</sub>, 4 T, 5 K

Sample number Critical current density (Acm-2)

Average

Fig. 5. Histogram of the J

of the ten test samples at 5 K, 4 T of (a) undoped MgB

and (b) C-doped MgB

.

1 2 3 4 5 6 7 8 9 10

0 5000 10000 15000 20000 25000 30000 35000 40000

Sample number Critical current density(Acm-2)

(a) Undoped MgB

, 2 T, 20 K

Average

1 2 3 4 5 6 7 8 9 10

0 5000 10000 15000 20000 25000 30000 35000 40000

Sample number Critical current density (Acm-2)

(b) Carbon-doped MgB

, 2 T, 20 K

Average

Fig. 6. Histogram of the J

of the ten test samples at 20 K, 2 T of (a) undoped MgB

and (b) C-doped MgB

.

MgB

. The average J

at 20 T of the undoped MgB

is 2.7×10

A/cm

and the standard deviation is 5.8×10

A/cm

(21% of the average J

). The average J

at 2 T of the C-doped MgB

is 1.88 × 10

A/cm

and the standard deviation is 2.8×10

A/cm

(15% of the average J

). The C doping effect on the J

enhancement is not observed at 20 K. These results imply that the flux pinning mechanism at

20 K and the low magnetic fields is different from that at 5 K and the high magnetic fields.

The T

is a function of the carrier density in MgB

, whereas the J

is a function of the microstructural defects.

Since the current anisotropic factor of MgB

is small, the coherency length is as large as 2 nm (ab plane) −6 nm (c axis) and the grain boundary is strongly coupled [3-5], the grain boundaries act as effective flux pinning centers with other defects. Therefore, the J

of MgB

can be represented as follows,

J

MgB

= f(s, d, g,….) (4) where s, d and g denote the lattice strain, the defect density inside MgB

grain and the grain size, respectively.

In this study we have analyzed the m−T and M−H curves of the ten test samples taken from the in situ reaction processed MgB

with/without C doping. The undoped MgB

and the C-doped MgB

showed the reliable m−T characteristics with the same T

and ∆T. This result indicates the formation of the superconducting MgB

phase with property reliability. The T

of MgB

decreased and the ∆T became larger as C was doped to MgB

. In spite of the T

decrease, the C doping enhanced the J

at 5 K. The J

enhancement was more remarkable at the high magnetic fields. However, it was not effective at 20 K.

The T

decrease and the J

enhancement can be explained in terms of the C incorporation with MgB

It can be recognized from the J

histograms that the reliability of current-carry capacity was increased by C doping in spite of the small decrease of T

. The carbon can substitutes the boron site due to the similar atomic size between the two elements [16, 17]. The C substitution for B sites of MgB

decreases the T

of MgB

and the T

decrease was proportional to the C content. The C incorporation with MgB

can also affect the J

of MgB

as the carbon substitutes some of B atoms of MgB

. The C can affect the thermal event of MgB

such as the temperatures for the formation of MgB

, densification and grain growth. The C addition tends to reduce the grain size of MgB

. The micro-defects such as grain boundaries, the lattice strain and chemical substitution can act as flux pinning sites of MgB

, which increase the J

at magnetic fields. However, the J

enhancement by the C doping seems to be limited at the low temperature of 5 K and the high magnetic fields.

The C effect on the J

enhancement at the high temperature of 20 K and the low magnetic fields was not observed. This may be caused by the T or H dependence of the flux pinning of MgB

. To clarify it, further systematic study on T-H-J

is needed.

In comparison to the T

decrease and the J

enhancement by the C doping, the difference in J

among the ten test samples of the undoped MgB

and the C-dope MgB

is considered to be caused from the microstructure of the in situ process MgB

samples. From eqs. (1) and (2), we could recognize that MgB

formed through the solid state or liquid state reaction of Mg and B powders. Since the heat treatment temperature (850 ℃) is higher than m. p. of Mg, the used Mg powders melt. The melting of Mg

39

T

and J

distribution in in situ processed MgB

bulk superconductors with/without C doping

Fig. 7. SEM micrograph of the fracture surface of in situ reaction processed MgB

showing many spherical pores formed by the melting of spherical Mg powders .

produces many spherical pores with a shape similarity with Mg powders [13] (see Fig. 7). The pore is a non-superconducting phase and does not give any positive effect on the J

. If the special pore distribution in parts of the in situ processed MgB

pellets is different, it affects the current-carrying capacity of the local parts of the MgB

pellets. This is one possible explanation on the J

−H characteristic of the in situ processed MgB

pellets.

4. CONCLUSIONS

The T

and J

distribution of the undoped MgB

and the C-doped MgB

bulk pellets, prepared by an in situ process using Mg and B powders, were examined. For this, the m − T and M − H measurement were carried out for the ten test samples taken from both MgB

pellets. The T

and ∆T of all test samples of both MgB

pellets were the same each other. The T

s (36.5 K) of the C-doped samples were lower than that (37.5 K) of the undoped MgB

. In comparison of the m − T characteristics which showed the same T

and ∆T, there presented the difference in J

among the test samples of both MgB

bulk pellets. The M − H characteristic of the C-doped MgB

was more reliable than that of the undoped MgB

. The J

at 5 K and the high magnetic fields up to 7 T of the C-doped MgB

was higher than that of the undoped MgB

. The lower T

and the higher J

at 5 K of the C-doped MgB

appear to be due to the possible formation of the defects or the lattice stain induced by the C incorporation with MgB

. However, the C effect on the J

enhancement did not appear at 20 K. The presence of different amount of pores in samples is one possible cause for the J

difference among the test samples.

ACKNOWLEDGMENT

This work was financially supported by the National Research Foundation Grant funded by Ministry of Science, ICT and Future Planning (MSIP) of Republic of Korea (Project Number: NRF-2013M2A8A1035822).

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