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Effects of sintering conditions on critical current properties and microstructures of MgB2 bulks
Physica C 426–431 (2005) 1220–1224 www.elsevier.com/locate/physc
Effects of sintering conditions on critical current properties and microstructures of MgB2 bulks Akiyasu Yamamoto *, Jun-ichi Shimoyama, Shinya Ueda, Yukari Katsura, Isao Iwayama, Shigeru Horii, Kohji Kishio Department of Superconductivity, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received 23 November 2004; accepted 6 March 2005 Available online 19 July 2005
Abstract The effects of heating conditions on critical current properties and microstructures of undoped MgB2 bulks were systematically studied. Strong correlation was observed between Jc and microstructures. The network structure with an excellent inter-grain connectivity of MgB2 grains contributed to high-Jc under low magnetic fields, and small grain size of MgB2 enhanced the grain boundary flux pinning. Long time heating at low temperatures below the melting point of magnesium was discovered to be most effective for synthesis of MgB2 bulks having strongly connected MgB2 network structure with small grains. The sample heated at 550 C for 1200 h recorded a high-Jc of 4.02 · 105 A cm2 at 20 K in self-field, while high-temperature and long time heating brought a significant grain growth which resulted in low Jc. 2005 Elsevier B.V. All rights reserved. PACS: 74.70.Ad Keywords: Magnesium diboride; Bulk; Critical current properties; Microstructures; PICT method
1. Introduction Superconductivity at 39 K of magnesium diboride (MgB2) enabled practical use at high operat* Corresponding author. Tel.: +81 3 5841 7713; fax: +81 3 5689 0574. E-mail address: [email protected] (A. Yamamoto).
ing temperatures around 20 K [1]. The MgB2 has several attractive characteristics for its application, such as high Jc properties(>105 A cm2 at 20 K under self-field) and essentially strong grain coupling due to its long coherence length. However, its Jc is suppressed under magnetic fields due to low upper critical fields, Hc2 and poor flux pinning. Therefore, improvement of Jc in magnetic field is needed for applications in extensive application fields.
0921-4534/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.03.075
A. Yamamoto et al. / Physica C 426–431 (2005) 1220–1224
Doping of various elements or compounds has been attempted for MgB2 and is proved to be effective for enhancement of Hc2 and/or flux pinning strength [2–7]. However, the resulting pinning effects by doping were not quantitatively consistent among many research groups [8]. Moreover, the flux pinning properties of the reference sample, undoped MgB2, varied largely in each paper, because superconducting properties of MgB2 are very sensitive to phase purity, size of starting boron powder and synthesis conditions [9]. Therefore, the intrinsic effects of impurity doping and/or microstructural control should be discussed under taking the effects of synthesis conditions for undoped MgB2 into account. In our previous study, strong correlation among crystallinity, Hc2 and Jc under high fields was found in undoped MgB2 bulks [10]. Samples heated below 650 C, which is the melting point of magnesium, were found to be composed of small grains with poor crystallinity. However, other microstructural factors such as grain size or connectivity of MgB2 grains which determine the flux pinning properties of undoped MgB2 have not been well clarified as functions of heating conditions, temperature and time. Based on these backgrounds, we have systematically examined the influences of heating conditions on Jc and microstructures of undoped MgB2.
2. Experimental The MgB2 bulks were synthesized by the powder-in-closed-tube (PICT) method starting from magnesium (99.9% purity, 10 lm in size) and boron (99%, 300 nm). Details of the PICT method will be found elsewhere [11]. The stainless steel (SUS316) tubes filled by powder mixture of magnesium and boron with a molar ratio of 1:2 were uniaxially pressed into tape shape. Their both ends were sealed by uniaxial pressing in order to prevent vaporization of magnesium during the heat treatment. The tubes were heated at 500–1100 C for 1–1200 h in evacuated quartz amopules, and then followed by quenching to room temperature. Constituent phases of the samples were analyzed by the powder X-ray diffraction (XRD)
1221
method using CuKa radiation. Microstructural observation was performed using a scanning electron microscope. Magnetization of the bulk samples were measured by a SQUID magnetometer. Jc was determined from magnetization loop using the extended Bean model.
3. Results and discussion The formation reaction of MgB2 from powder mixture of magnesium and boron is sensitive to heating conditions. First, we varied the heating temperature from 600 C to 1100 C and fixed the heating time for 1 h, in order to clarify the influence of heating temperature on the phase formation of MgB2. Powder XRD analysis revealed that nearly single-phase MgB2 bulks containing a small amount of MgO were obtained for the samples heated at above 700 C for 1 h, while the samples heated below 650 C for 1 h showed strong diffraction peaks due to unreacted magnesium. Since the melting point of magnesium is 650 C, the MgB2 phase is formed by the solid(Mg)– solid(B) reaction when heating temperature is below 650 C. Therefore, the phase formation of MgB2 below 650 C required longer time comparing with the synthesis at higher temperatures above 700 C, where the liquid(Mg)–solid(B) reaction occurs. For example, very long heating periods of 1200 and 60 h were needed to obtain single phased MgB2 samples when they were heated at 550 and 600 C, respectively. Since the particle size of magnesium and boron used in the present study were relatively large, heating time for complete reaction between magnesium and boron can be reduced dramatically by using fine starting materials. Samples heated above 700 C exhibited high Tc(onset) over 38 K, while the Tc s of the samples heated for 1 h at 600 and 650 C were 20 and 35.2 K, respectively, possibly due to extremely poor crystallinity of MgB2. Relationship between heating temperature and Jc at 20 K under self-field were summarized in Fig. 1. The heating temperatures which showed the maximum Jc for the same heating time were shifted systematically towards lower temperatures with an increase of heating time. For the samples
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A. Yamamoto et al. / Physica C 426–431 (2005) 1220–1224
Jc / kA cm-2 (self-field)
600 500
at 20 K
400
1h 3h 6h 12 h 24 h
60 h 120 h 240 h 600 h 1200 h
300 200 100 0 500
600
700 800 900 1000 Heating temperature / C
1100
Fig. 1. Relationships between heating conditions and Jc at 20 K under self-field for undoped MgB2 bulks.
heated for 1 h, the sample heated at 850 C showed the highest Jc of 3.05 · 105 A cm2. The optimal heating temperatures for the heating times of 3, 6, 24 and 240 h were 800, 700, 650 and 600 C, respectively. For the samples heated below 650 C, where the solid–solid reaction occurs, Jc increased systematically with an increase of heating time, and furthermore, the maximum Jc systematically enhanced by lowering heating temperature. In fact, the highest Jc of 4.02 · 105 A cm2 was obtained for the sam-
ple heated at 550 C for very long time of 1200 h. On the other hand, long time heating at high temperatures above 900 C dramatically degrades Jc. For example, the sample heated at 900 C for 60 h showed quite low Jc of 4.7 · 104 A cm2. Strong correlation was also found between Jc and microstructures of MgB2. Secondary electron images of the fractured surface of the samples heated at 600–1100 C for 1 h and at 550 C for 1200 h are shown in Fig. 2. In Fig. 2(a), unreacted particles of magnesium and boron with the same sizes as the starting powders were observed in the sample heated at 600 C for 1 h. Many large voids with a typical size of several ten micrometers were seen in the perfectly reacted samples heated at 700–900 C for 1 h, as shown in insets of Fig. 2(b)–(d). However, a network structure with strongly and densely connected MgB2 grains, which is believed to be excellent current pass, can be seen around the voids. In fact, the samples with a strongly linked network structure (Fig. 2(b)–(f)) showed high Jc except the sample heated at 1100 C for 1 h. As shown in Fig. 2(e), significant grain growth which resulted in large MgB2 grains with the size of several lm was observed in the sample heated at 1100 C. Since the grain boundaries act as predominant pinning centers in MgB2 [12], large grain
Fig. 2. Secondary electron images of the fractured surface of MgB2 bulks: (a) heated at 600 C for 1 h; (b) 700 C, 1 h; (c)800 C, 1 h; (d) 900 C, 1 h; (e) 1100 C, 1 h; and (f) 550 C, 1200 h.
A. Yamamoto et al. / Physica C 426–431 (2005) 1220–1224
50 550 600 700 800 900 1000 1100
40 Fk / kA0.5 T 0.25 m-1
growth corresponds to reduction of effective pinning sites. Low Jc despite the excellent grain connectivity observed in this sample is believed to be due to the large grains. Note that development of grain connectivity always accompanies grain growth of MgB2 in the high temperature synthesis above 900 C. Similarly in the case of the high temperature synthesized samples, network structure with excellent grain connectivity of MgB2 and voids was also observed for the sample heated at 550 C for 1200 h as shown in Fig. 2(f). We found that the initial grain size of MgB2 remained almost unchanged by heating for such a long time when heating temperature was below 650 C. In this sample, MgB2 grains with a typical size of approximately 300 nm, which was almost identical to the size of the starting boron particle, were clearly seen. Therefore, the reasons why the sample heated at 550 C for 1200 h showed the highest Jc was explained by the improved intergrain connectivity and highly dense grain boundaries which act as effective pinning sites. All these results suggest that the low-temperature solid–solid reaction is favorable to synthesize high Jc MgB2 bulks, while apparent grain growth occurs in the case of the solid–liquid reaction. Kramer plot F K ¼ J c0.5 H 0.25 at 20 K for the samples heated at 600–1100 C for 1 h, at 550 C for 1200 h and at 600 C for 24 h is shown in Fig. 3. As was mentioned in the literatures [10,13], the linear dependence of FK on magnetic fields under 0.5–2.5 T were observed except for the samples heated at 1000 C and 1100 C for 1 h. The magnetic field dependence of FK for the samples heated at 700 C, 800 C and 900 C for 1 h were very similar to each other, which corresponds to their similar microstructure shown in Fig. 2(b)–(d). Compared to these samples, enhancement of FK below 2.5 T was not found in the sample heated at 600 C for 24 h which contained unreacted magnesium and boron and, therefore, a densely packed MgB2 structure was not formed. These suggest that the grain connectivity, i.e. a strongly linked MgB2 network structure observed in Fig. 2(b)–(f), is responsible for high Jc below 2.5 T with the predominant pinning by dense grain boundaries. On the other hand, the FK for the sample heated at 600 C for
1223
30
C, C, C, C, C, C, C,
1200 h 24 h 1h 1h 1h 1h 1h
20
10 at 20 K 0
0
1
2
µ 0H / T
3
4
5
Fig. 3. Kramer plots F K ¼ J c0.5 H 0.25 at 20 K for MgB2 bulks heated at 600 C, 700 C, 800 C, 900 C, 1000 C and 1100 C for 1 h, at 550 C for 1200 h and at 600 C for 24 h.
24 h becomes larger than those of the samples heated at 700–900 C for 1 h under high fields above 3 T. This means poor crystallinity introduced by the solid–solid reaction contributes to enhanced pinning strength under high fields, as pointed out in our previous paper [10]. The highest FK performance under whole fields was observed for the sample heated at 550 C for 1200 h. It should be emphasized that small grain size, poor crystallinity and a strongly connected MgB2 network structure formed through heating at low temperature for long time is desirable to produce MgB2 materials with excellent critical current performance. The samples heated at 1000 C and 1100 C for 1 h showed anomalous behavior of FK and low irreversibility fields 3 T. Since the grain sizes of these samples were very large, typically several lm, the grain boundary pinning was less effective. Therefore, the flux pinning properties of the samples heated at over 1000 C are rather similar to those of single crystals [14].
4. Conclusions The influences of heating conditions on critical current properties and microstructures of undoped
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A. Yamamoto et al. / Physica C 426–431 (2005) 1220–1224
MgB2 bulks were systematically studied. Grain growth of MgB2 was found to be suppressed by the low-temperature heating below 650 C where solid–solid reaction of magnesium and boron occurs, and small grains are contributed to strengthen the grain boundary flux pinning. Moreover, a strongly connected MgB2 network structure observed in high-temperature heated or low-temperature and long time heated samples is believed to be effective current pass. Since the high-temperature long time heating resulted in crystal growth of MgB2 and suppressed grain boundary flux pinning due to the large grain size, the long time heating below 650 C is more desirable to synthesize MgB2 bulks having dense grain boundaries, excellent inter-grain connectivity and poor crystallinity, which are essentially important factors for high Jc up to high fields. Further improvement of critical current performance will be achieved by lowering the heating temperature which provides more dense grain boundaries and grains with poorer crystallinity, i.e. highly dense pinning site in high Hc2 matrix. Using fine starting powders [15–17] or boron and/or magnesium contained compounds [18,19] are believed to promising approaches to decrease formation temperature of MgB2.
References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (2001) 63. [2] R.J. Cava, H.W. Zandbergen, K. Inumaru, Physica C 385 (2003) 8. [3] E. Ohmichi, T. Masui, S. Lee, S. Tajima, T. Osada, J. Phys. Soc. Jpn. 73 (2004) 2065.
[4] S.X. Dou, S. Soltanian, J. Horvat, X.L. Wang, S.H. Zhou, M. Ionescu, H.K. Liu, Appl. Phys. Lett. 81 (2002) 3419. [5] A. Yamamoto, J. Shimoyama, S. Ueda, Y. Katsura, I. Iwayama, S. Horii, K. Kishio, Appl. Phys. Lett. 86 (2005) 212502. [6] N.E. Anderson, W.E. Straszheim, S.L. BudÕko, P.C. Canfield, D.K. Finnemore, R.J. Suplinskas, Physica C 390 (2003) 11. [7] A. Yamamoto, J.I. Shimoyama, S. Ueda, Y. Katsura, S. Horii, K. Kishio, IEEE Trans. Appl. Supercond. 15 (2005) 3292. [8] S.X. Dou, S. Soltanian, W.K. Yeoh, Y. Zhang, IEEE Trans. Appl. Supercond. 15 (2005) 3219. [9] R.A. Ribeiro, S.L. BudÕko, C. Petrovic, P.C. Canfield, Physica C 385 (2003) 16. [10] A. Yamamoto, J. Shimoyama, S. Ueda, Y. Katsura, S. Horii, K. Kishio, Supercond. Sci. Technol. 18 (2005) 116. [11] A. Yamamoto, J. Shimoyama, S. Ueda, Y. Katsura, S. Horii, K. Kishio, Supercond. Sci. Technol. 17 (2004) 921. [12] H. Kitaguchi, A. Matsumoto, H. Kumakura, T. Doi, H. Yamamoto, K. Saitoh, H. Sosiati, S. Hata, Appl. Phys. Lett. 85 (2004) 2842. [13] D.C. Larbalestier, L.D. Cooley, M.O. Rikel, A.A. Polyanskii, J. Jiang, S. Patnaik, X.Y. Cai, D.M. Feldmann, A. Gurevich, A.A. Squitieri, M.T. Naus, C.B. Eom, E.E. Hellstrom, R.J. Cava, K.A. Regan, N. Rogado, M.A. Hayward, T. He, J.S. Slusky, P. Khalifah, K. Inumaru, M. Haas, Nature 410 (2001) 186. [14] Z.X. Shi, A.K. Pradhan, M. Tokunaga, K. Yamazaki, T. Tamegai, Y. Takano, K. Togano, H. Kito, H. Ihara, Physica C 378–381 (2002) 550. [15] H. Yamada, M. Hirakawa, H. Kumakura, A. Matsumoto, H. Kitaguchi, Appl. Phys. Lett. 84 (2004) 1728. [16] A. Matsumoto, H. Kumakura, H. Kitaguchi, H. Hatakeyama, H. Yamada, M. Hirakawa, IEEE Trans. Appl. Supercond. 15 (2005) 3333. [17] O. Perner, J. Eckert, W. Ha¨ßler, C. Fischer, K.H. Mu¨ller, G. Fuchs, B. Holzapfel, L. Schultz, Supercond. Sci. Technol. 17 (2004) 1148. [18] A. Kikuchi, Y. Yoshida, Y. Iijima, N. Banno, T. Takeuchi, K. Inoue, Supercond. Sci. Technol. 17 (2004) 781. [19] L.D. Cooley, K. Kang, R.F. Klie, Q. Li, A.M. Moodenbaugh, R.L. Sabatini, Supercond. Sci. Technol. 17 (2004) 942.
Effects of sintering conditions on critical current properties and microstructures of MgB2 bulks Akiyasu Yamamoto *, Jun-ichi Shimoyama, Shinya Ueda, Yukari Katsura, Isao Iwayama, Shigeru Horii, Kohji Kishio Department of Superconductivity, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received 23 November 2004; accepted 6 March 2005 Available online 19 July 2005
Abstract The effects of heating conditions on critical current properties and microstructures of undoped MgB2 bulks were systematically studied. Strong correlation was observed between Jc and microstructures. The network structure with an excellent inter-grain connectivity of MgB2 grains contributed to high-Jc under low magnetic fields, and small grain size of MgB2 enhanced the grain boundary flux pinning. Long time heating at low temperatures below the melting point of magnesium was discovered to be most effective for synthesis of MgB2 bulks having strongly connected MgB2 network structure with small grains. The sample heated at 550 C for 1200 h recorded a high-Jc of 4.02 · 105 A cm2 at 20 K in self-field, while high-temperature and long time heating brought a significant grain growth which resulted in low Jc. 2005 Elsevier B.V. All rights reserved. PACS: 74.70.Ad Keywords: Magnesium diboride; Bulk; Critical current properties; Microstructures; PICT method
1. Introduction Superconductivity at 39 K of magnesium diboride (MgB2) enabled practical use at high operat* Corresponding author. Tel.: +81 3 5841 7713; fax: +81 3 5689 0574. E-mail address: [email protected] (A. Yamamoto).
ing temperatures around 20 K [1]. The MgB2 has several attractive characteristics for its application, such as high Jc properties(>105 A cm2 at 20 K under self-field) and essentially strong grain coupling due to its long coherence length. However, its Jc is suppressed under magnetic fields due to low upper critical fields, Hc2 and poor flux pinning. Therefore, improvement of Jc in magnetic field is needed for applications in extensive application fields.
0921-4534/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.03.075
A. Yamamoto et al. / Physica C 426–431 (2005) 1220–1224
Doping of various elements or compounds has been attempted for MgB2 and is proved to be effective for enhancement of Hc2 and/or flux pinning strength [2–7]. However, the resulting pinning effects by doping were not quantitatively consistent among many research groups [8]. Moreover, the flux pinning properties of the reference sample, undoped MgB2, varied largely in each paper, because superconducting properties of MgB2 are very sensitive to phase purity, size of starting boron powder and synthesis conditions [9]. Therefore, the intrinsic effects of impurity doping and/or microstructural control should be discussed under taking the effects of synthesis conditions for undoped MgB2 into account. In our previous study, strong correlation among crystallinity, Hc2 and Jc under high fields was found in undoped MgB2 bulks [10]. Samples heated below 650 C, which is the melting point of magnesium, were found to be composed of small grains with poor crystallinity. However, other microstructural factors such as grain size or connectivity of MgB2 grains which determine the flux pinning properties of undoped MgB2 have not been well clarified as functions of heating conditions, temperature and time. Based on these backgrounds, we have systematically examined the influences of heating conditions on Jc and microstructures of undoped MgB2.
2. Experimental The MgB2 bulks were synthesized by the powder-in-closed-tube (PICT) method starting from magnesium (99.9% purity, 10 lm in size) and boron (99%, 300 nm). Details of the PICT method will be found elsewhere [11]. The stainless steel (SUS316) tubes filled by powder mixture of magnesium and boron with a molar ratio of 1:2 were uniaxially pressed into tape shape. Their both ends were sealed by uniaxial pressing in order to prevent vaporization of magnesium during the heat treatment. The tubes were heated at 500–1100 C for 1–1200 h in evacuated quartz amopules, and then followed by quenching to room temperature. Constituent phases of the samples were analyzed by the powder X-ray diffraction (XRD)
1221
method using CuKa radiation. Microstructural observation was performed using a scanning electron microscope. Magnetization of the bulk samples were measured by a SQUID magnetometer. Jc was determined from magnetization loop using the extended Bean model.
3. Results and discussion The formation reaction of MgB2 from powder mixture of magnesium and boron is sensitive to heating conditions. First, we varied the heating temperature from 600 C to 1100 C and fixed the heating time for 1 h, in order to clarify the influence of heating temperature on the phase formation of MgB2. Powder XRD analysis revealed that nearly single-phase MgB2 bulks containing a small amount of MgO were obtained for the samples heated at above 700 C for 1 h, while the samples heated below 650 C for 1 h showed strong diffraction peaks due to unreacted magnesium. Since the melting point of magnesium is 650 C, the MgB2 phase is formed by the solid(Mg)– solid(B) reaction when heating temperature is below 650 C. Therefore, the phase formation of MgB2 below 650 C required longer time comparing with the synthesis at higher temperatures above 700 C, where the liquid(Mg)–solid(B) reaction occurs. For example, very long heating periods of 1200 and 60 h were needed to obtain single phased MgB2 samples when they were heated at 550 and 600 C, respectively. Since the particle size of magnesium and boron used in the present study were relatively large, heating time for complete reaction between magnesium and boron can be reduced dramatically by using fine starting materials. Samples heated above 700 C exhibited high Tc(onset) over 38 K, while the Tc s of the samples heated for 1 h at 600 and 650 C were 20 and 35.2 K, respectively, possibly due to extremely poor crystallinity of MgB2. Relationship between heating temperature and Jc at 20 K under self-field were summarized in Fig. 1. The heating temperatures which showed the maximum Jc for the same heating time were shifted systematically towards lower temperatures with an increase of heating time. For the samples
1222
A. Yamamoto et al. / Physica C 426–431 (2005) 1220–1224
Jc / kA cm-2 (self-field)
600 500
at 20 K
400
1h 3h 6h 12 h 24 h
60 h 120 h 240 h 600 h 1200 h
300 200 100 0 500
600
700 800 900 1000 Heating temperature / C
1100
Fig. 1. Relationships between heating conditions and Jc at 20 K under self-field for undoped MgB2 bulks.
heated for 1 h, the sample heated at 850 C showed the highest Jc of 3.05 · 105 A cm2. The optimal heating temperatures for the heating times of 3, 6, 24 and 240 h were 800, 700, 650 and 600 C, respectively. For the samples heated below 650 C, where the solid–solid reaction occurs, Jc increased systematically with an increase of heating time, and furthermore, the maximum Jc systematically enhanced by lowering heating temperature. In fact, the highest Jc of 4.02 · 105 A cm2 was obtained for the sam-
ple heated at 550 C for very long time of 1200 h. On the other hand, long time heating at high temperatures above 900 C dramatically degrades Jc. For example, the sample heated at 900 C for 60 h showed quite low Jc of 4.7 · 104 A cm2. Strong correlation was also found between Jc and microstructures of MgB2. Secondary electron images of the fractured surface of the samples heated at 600–1100 C for 1 h and at 550 C for 1200 h are shown in Fig. 2. In Fig. 2(a), unreacted particles of magnesium and boron with the same sizes as the starting powders were observed in the sample heated at 600 C for 1 h. Many large voids with a typical size of several ten micrometers were seen in the perfectly reacted samples heated at 700–900 C for 1 h, as shown in insets of Fig. 2(b)–(d). However, a network structure with strongly and densely connected MgB2 grains, which is believed to be excellent current pass, can be seen around the voids. In fact, the samples with a strongly linked network structure (Fig. 2(b)–(f)) showed high Jc except the sample heated at 1100 C for 1 h. As shown in Fig. 2(e), significant grain growth which resulted in large MgB2 grains with the size of several lm was observed in the sample heated at 1100 C. Since the grain boundaries act as predominant pinning centers in MgB2 [12], large grain
Fig. 2. Secondary electron images of the fractured surface of MgB2 bulks: (a) heated at 600 C for 1 h; (b) 700 C, 1 h; (c)800 C, 1 h; (d) 900 C, 1 h; (e) 1100 C, 1 h; and (f) 550 C, 1200 h.
A. Yamamoto et al. / Physica C 426–431 (2005) 1220–1224
50 550 600 700 800 900 1000 1100
40 Fk / kA0.5 T 0.25 m-1
growth corresponds to reduction of effective pinning sites. Low Jc despite the excellent grain connectivity observed in this sample is believed to be due to the large grains. Note that development of grain connectivity always accompanies grain growth of MgB2 in the high temperature synthesis above 900 C. Similarly in the case of the high temperature synthesized samples, network structure with excellent grain connectivity of MgB2 and voids was also observed for the sample heated at 550 C for 1200 h as shown in Fig. 2(f). We found that the initial grain size of MgB2 remained almost unchanged by heating for such a long time when heating temperature was below 650 C. In this sample, MgB2 grains with a typical size of approximately 300 nm, which was almost identical to the size of the starting boron particle, were clearly seen. Therefore, the reasons why the sample heated at 550 C for 1200 h showed the highest Jc was explained by the improved intergrain connectivity and highly dense grain boundaries which act as effective pinning sites. All these results suggest that the low-temperature solid–solid reaction is favorable to synthesize high Jc MgB2 bulks, while apparent grain growth occurs in the case of the solid–liquid reaction. Kramer plot F K ¼ J c0.5 H 0.25 at 20 K for the samples heated at 600–1100 C for 1 h, at 550 C for 1200 h and at 600 C for 24 h is shown in Fig. 3. As was mentioned in the literatures [10,13], the linear dependence of FK on magnetic fields under 0.5–2.5 T were observed except for the samples heated at 1000 C and 1100 C for 1 h. The magnetic field dependence of FK for the samples heated at 700 C, 800 C and 900 C for 1 h were very similar to each other, which corresponds to their similar microstructure shown in Fig. 2(b)–(d). Compared to these samples, enhancement of FK below 2.5 T was not found in the sample heated at 600 C for 24 h which contained unreacted magnesium and boron and, therefore, a densely packed MgB2 structure was not formed. These suggest that the grain connectivity, i.e. a strongly linked MgB2 network structure observed in Fig. 2(b)–(f), is responsible for high Jc below 2.5 T with the predominant pinning by dense grain boundaries. On the other hand, the FK for the sample heated at 600 C for
1223
30
C, C, C, C, C, C, C,
1200 h 24 h 1h 1h 1h 1h 1h
20
10 at 20 K 0
0
1
2
µ 0H / T
3
4
5
Fig. 3. Kramer plots F K ¼ J c0.5 H 0.25 at 20 K for MgB2 bulks heated at 600 C, 700 C, 800 C, 900 C, 1000 C and 1100 C for 1 h, at 550 C for 1200 h and at 600 C for 24 h.
24 h becomes larger than those of the samples heated at 700–900 C for 1 h under high fields above 3 T. This means poor crystallinity introduced by the solid–solid reaction contributes to enhanced pinning strength under high fields, as pointed out in our previous paper [10]. The highest FK performance under whole fields was observed for the sample heated at 550 C for 1200 h. It should be emphasized that small grain size, poor crystallinity and a strongly connected MgB2 network structure formed through heating at low temperature for long time is desirable to produce MgB2 materials with excellent critical current performance. The samples heated at 1000 C and 1100 C for 1 h showed anomalous behavior of FK and low irreversibility fields 3 T. Since the grain sizes of these samples were very large, typically several lm, the grain boundary pinning was less effective. Therefore, the flux pinning properties of the samples heated at over 1000 C are rather similar to those of single crystals [14].
4. Conclusions The influences of heating conditions on critical current properties and microstructures of undoped
1224
A. Yamamoto et al. / Physica C 426–431 (2005) 1220–1224
MgB2 bulks were systematically studied. Grain growth of MgB2 was found to be suppressed by the low-temperature heating below 650 C where solid–solid reaction of magnesium and boron occurs, and small grains are contributed to strengthen the grain boundary flux pinning. Moreover, a strongly connected MgB2 network structure observed in high-temperature heated or low-temperature and long time heated samples is believed to be effective current pass. Since the high-temperature long time heating resulted in crystal growth of MgB2 and suppressed grain boundary flux pinning due to the large grain size, the long time heating below 650 C is more desirable to synthesize MgB2 bulks having dense grain boundaries, excellent inter-grain connectivity and poor crystallinity, which are essentially important factors for high Jc up to high fields. Further improvement of critical current performance will be achieved by lowering the heating temperature which provides more dense grain boundaries and grains with poorer crystallinity, i.e. highly dense pinning site in high Hc2 matrix. Using fine starting powders [15–17] or boron and/or magnesium contained compounds [18,19] are believed to promising approaches to decrease formation temperature of MgB2.
References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (2001) 63. [2] R.J. Cava, H.W. Zandbergen, K. Inumaru, Physica C 385 (2003) 8. [3] E. Ohmichi, T. Masui, S. Lee, S. Tajima, T. Osada, J. Phys. Soc. Jpn. 73 (2004) 2065.
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