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Physics Procedia 58 (2014) 106 – 109

26th International Symposium on Superconductivity, ISS 2013

Effects of sintering condition on the trapped magnetic fieldproperties for MgB2 bulks fabricated by in-situ capsule method
T. Yoshida*, T. Naito, H. Fujishiro
Department of Materials Science and Engineering, Faculty of Engineering, Iwate University, Morioka 020-8551, JapanAbstract
We have studied the trapped field properties of MgB2 bulks which were fabricated by the in-situ capsule method under various
sintering temperatures of 700-900 °C for 1-24 h. The trapped field, BT (T), was 1.8-1.9 T for all the bulks approximately at 16 K.
The BT (T) values on the bulks sintered at 900 °C for 6 h and at 800°C for 24 h were slightly lower than those of other bulks
because of the decrease of critical current density, Jc, which originates from both the lower connectivity, K, and the promotion of
the grain growth. The high temperature and/or the long periods sintering decrease the BT (T) value. From the obtained results, thesintering at 700-800 °C for 1-6 h is the optimum condition to fabricate MgB2 bulks by the capsule method.
© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license© 2014 The Authors. Published by Elsevier B.V.
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Selection and peer-review under responsibility of the ISS 2013 Program Committee.
Peer-review under responsibility of the ISS 2013 Program Committee

Keywords: Superconducting bulk magnet; MgB2 superconductors; Trapped field properties

1. Introduction
MgB2 has the highest superconducting transition temperature Tc=39 K among intermetallic compounds. Since its
coherence length is long [1] in comparison with RE-Ba-Cu-O (RE: rare earth elements) system, which allows us todiscard the weak-links at grain boundaries, we can produce a superconducting bulk magnet using a polycrystalline
MgB2.However, MgB2 must be fabricated in vacuum or Ar-atmosphere because Mg and B are oxidized in air.
Therefore the complicated fabrication process is needed.

* T. Yoshida. Tel.:+81-19-621-6362; fax: +81-19-621-6362
E-mail address: t2213027@iwate-u.ac.jp

1875-3892 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the ISS 2013 Program Committee
doi:10.1016/j.phpro.2014.09.038

107T. Yoshida et al. / Physics Procedia 58 (2014) 106 – 109

Several groups reported that the MgB2 bulks trapped the tesla-order magnetic field [2-5]. We reported the trappedmagnetic field BT of 1.77 T at 15.5 K on the MgB2 bulk with 38 mm in diameter and 9 mm in thickness, which wasfabricated by the in-situ capsule method [2]. Viznichenko et al. reported the BT value of about 2.3 T at 6 K for theMgB2 bulk with 28 mm in diameter and 11 mm in thickness which was sintered under high pressure of 2 GPa [3].
Tomita et al. reported the BT of 2.1 T at 20 K for the in-situ MgB2 bulk with 60 mm in diameter and 10 mm inthickness and 3.5 T at 13.5 K for the disc-pair bulks with 40 mm in diameter and 10 mm in thickness [4]. And also,Durrel et al. reported the BT value of over 3 T at 17.5 K was obtained for stack of the disc-pair MgB2 bulks ofdiameter 25 mm and thickness 5.4 mm [5].
The sintering condition of the reports is different from each other. The culminant pinning centers are the grainboundaries in MgB2, therefore the trapped field properties should be optimized by controlling the grain size andboundaries. In this paper, to optimize the grain condition, we have studied the trapped field and superconducting
properties of MgB2 bulks sintered under various conditions.
2. Experimental
2.1. Sample preparation
MgB2 bulks were fabricated by the capsule method [2]. Figure 1
shows the schematic image of the capsule. The precursor pellet wasinprepared from mixed powders of Mg (99.5% in purity, ̰180grain size) and B (99% in purity, 300 mesh in grain size) with the molar
ratio of 1:2. It was set in the stainless steel (SUS) container, and wassealed with both soft iron gasket and SUS cap in Ar atmosphere. The
sealed capsule was sintered at the temperature Tsin = 700-900 °C forFig. 1. Schematic illustration of the closed capsule.
the duration tsin = 1-24 h in a box furnace, and cooled down to room
temperature by furnace cooling. The typical size of the bulk is 30 mm in diameter and about 10 mm in thickness.
Each bulk was named as listed in Table 1. The packing factor of the bulks was about 50%, which was also shown in
Table 1.2.2. Measurements
The MgB2 bulk was magnetized by field cooling (FC) in a magnetic field of 5 T, and then the applied field wasdecreased to 0 T at a rate of -0.222 T/min. Trapped field BT value at the center of the bulk surface was measured bya cryogenic Hall sensor (BHT-921, F.W. BELL). After the FC experiment, we measured the magnetization curve ofseveral small pieces cut from the bulk using a commercial SQUID magnetometer (MPMS-XL, Quantum DesignInc.), and evaluated the critical current density Jc using the extended Bean model. Electrical resistivity U wasmeasured by a standard four probe technique. The micro structure of the MgB2 bulks was observed by a scanningelectron microscope (SEM).
3. Results and discussion
Figure 2(a) presents the temperature dependence of the trapped field, BT (T), of the MgB2 bulks with the different
sintering temperature Tsin and for constant tsin (= 6 h). The maximum BT (T) values of the CAP1, CAP2 and CAP3bulks, respectively, were 1.92 T at 16.7 K, 1.95 T at 16.3 K and 1.82 T at 16.3 K. The BT (T) value of the CAP3 bulk
Table 1. Specification of the MgB2 bulk.
Sample name
CAP1
CAP2
CAP3
CAP4
CAP5

Tsin (°C)
700
800
900
800
800tsin (h)666124

Packing factor (%)
46.6
50.8
48.7
49.5
47.7

Tc (K)
38.8
38.9
39.039.0
38.8

Connectivity K (%)
16.1
19.0
17.3
17.6
14.0

Max of BT
1.92 T at 16.7 K
1.95 T at 16.3 K
1.82 T at 16.3 K
1.42 T at 23.6 K
1.79 T at 15.2 K

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T. Yoshida et al. / Physics Procedia 58 (2014) 106 – 109

Fig. 2. Temperature dependence of trapped field BT (T) for the (a) variation of temperature 700-900 °C (tsin = 6 h) and (b) variationof sintering period 1-24 h (Tsin = 800 °C).

Fig. 3. Temperature dependence of resistivity.
Inset shows the expanded view near Tc.

Fig. 4. Magnetic field dependence of the critical current density Jc
at 20 K for the MgB2 bulks.

was slightly lower than those of the other bulks. Figure 2(b) shows the BT (T) of the MgB2 bulks with the different
sintering time tsin and at constant Tsin (= 800 °C). The maximum BT (T) values of the CAP4 and CAP5 bulks,
respectively, were 1.42 T at 23.6 K and 1.79 T at 15.2 K. The BT (T) value of the CAP5 bulk was lower than those ofother bulks. Therefore, these results suggest that the sintering at higher temperature and/or for the long-time,decrease the BT (T) value. The BT (T) of the CAP1, CAP2 and CAP4 bulks were comparable.
The observed XRD peaks for all the bulks were mainly indexed by the MgB2 phase, but a small amount ofimpurity such as Mg was observed. Both CAP3 and CAP5 bulks with lower BT (T) value did not necessarily containmany impurities compared with other bulks.
Figure 3 shows the temperature dependence of the resistivity U (T) for each bulk. Inset shows the expanded viewnear the critical temperature, Tc. The onset Tc of each bulk was 38.8-39.0 K. The electrical connectivity K, which isthe effective cross section of the bulk for the supercurrent path, was evaluated from the following expression [6], K= 'Ug / 'Uh 100, where 'Ug = Ug (300 K) - Ug (40 K) 6.32 P:cm [7], and 'U = U(300 K)- U (40 K), i.e., the ratioof the resistivity difference between resistivity of the ideal MgB2 grains and that of the present sample. The
connectivity value of each bulk is shown in Table 1. The K value of CAP5 was 14%, which is lower than those ofother bulks and is consistent with the lower BT (T) of the CAP5 bulk.
Figure 4 shows the magnetic field dependence of the critical current density Jc at 20 K for each bulk. The Jc value
in the zero field of each bulk was 1.2-1.6 h 105 A/cm2. Jc and irreversibility fields, Birr, of the CAP1 bulk aresomewhat higher, and those of the CAP3 bulk are lower among these bulks. Although both packing factor andconnectivity values of the CAP1 bulk were slightly lower than other bulks, the higher Jc and Birr give rise to the highBT (T) similarly to that of the CAP2 bulk. On the other hands, for the CAP3 and CAP5 bulks, the BT (T) values wereT. Yoshida et al. / Physics Procedia 58 (2014) 106 – 109

109

Fig. 5. SEM images of the MgB2 bulks

low relatively, which originate from the low P, K, Jc and Birr values for the CAP3 and from the low P and K values
for the CAP5, respectively.
Figure 5 shows the SEM images of the bulk surface. The sizes of the grains were smaller than several
for all the bulks. However, some of the grains in the CAP3 and CAP5 bulks became large by gain growth, which
was promoted for the bulks fabricated at higher temperature and for the long-time sintering. As a result, the grainboundaries, which act as pinning centers in MgB2 bulk, were decreased by the grain growth and the BT (T) value
decreased in both CAP3 and CAP5 bulks.
4. Conclusion
We have investigated the trapped field BT, the critical current density Jc, connectivity K and microstructure of theMgB2 bulks fabricated by the capsule method. The important obtained results are described in the followings.
(1) The MgB2 bulks fabricated at 700-900 °C for 1-24 h show nearly the single phase, and the BT (T) was 1.8-1.9 T
for all the bulks approximately at 16 K.
(2) The BT(T) values on the bulk sintered at 900 °C-6 h and at 800 °C-24 h bulk were slightly lower than those ofother bulks because of the low K, and Jc values and the enhancement of the grain growth. The sintering at thehigh temperature and/or for the long period, decrease the BT (T) value.
(3) The Jc value in the zero field of each bulk was 1.2-1.6 h 105 A/cm2. Jc and Birr of the 700 °C-6 h bulk aresomewhat higher, and those of the CAP3 bulk are lower among these bulks.
(4) The obtained results suggest that the optimum fabricating condition in the capsule method is the sintering
temperature of 700-800 °C and the sintering period of 1-6 h.
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