Proc. Schl. Eng. Tokai Univ., Ser. E 33（2008）41-46

Vol. XXXⅢ, 2008 ― 41 ―

*1 Graduate Student, Course of Metallurgical Engineering*2 Professor, Department of Materials Science *3 National Institute for Materials Science

Superconducting Properties and Structure of MgB2 Tapes

with SiC Addition Prepared by the Hot Pressing by

Akio Nitta*1, Shigehito Ohki*1, Yutaka Yamada*2, Kyoji Tachikawa*2 and Hiroaki Kumakura*3

(Received on March 31, 2008 and accepted on July 9, 2008)

Abstract

In-situ powder-in-tube processed MgB2 tapes have been prepared by a hot pressing. Magnesium hydride MgH2 and amorphous B powders were mixed to synthesize MgB2 superconductor. The mixed powder was encased in a steel tube and rolled into a monocore tape of about 4 mm in width and 0.5 mm in thickness. Nano-sized SiC powders were added to the MgB2 core. The tapes were hot-pressed under 100 MPa during the heat treatment 630-800℃ for 1-10 h in Ar gas atmosphere. The Ic for the hot-pressed tapes with 5 mass% SiC addition exceeds 300 A at 4.2 K and 5 T, which corresponds to the core Jc of nearly 1,000 A/mm2. The core Jc is enhanced more than a factor of two compared to that of tapes without pressing during the heat treatment. The higher Jc in hot pressed tapes results from the improvement of the denser MgB2 structures with less voids and better linkage among MgB2 grains.

Keywords: MgB2 Superconductor, Hot pressing, SiC addition, critical current

1. Introduction Since the discovery of superconductivity in MgB2 with critical temperature Tc of 39 K [1], development of conductors and thin films as well as fundamental research on the new superconductor have been studied. In particular, improvement of critical current Ic and critical current density Jc in MgB2 superconducting wires and tapes have gained world-wide interest for practical applications. There are two different methods for fabricating MgB2 wires and tapes by the powder in tube (PIT) process. First is the so-called ex-situ process, in which a commercially available MgB2 powder is utilized as raw material. Second is the in-situ process, in which a mixed powder of Mg and B is used to synthesize MgB2 superconductor. Many papers have been already published on the ex-situ PIT process [2]-[5] and in-situ PIT process [6]-[10]. The former process is simpler since no heat treatment is necessary to synthesize MgB2, while the superconducting properties depend sensitively on

the qualities of the starting MgB2 raw powder. The latter process is more attractive due to higher performance under magnetic field.

In this paper the effect of SiC nano-sized powder addition to the MgB2 core and hot pressing on the enhancement of the Ic and Jc in steel-sheathed in-situ PIT processed MgB2 tapes has been studied relating with the improvement in their microstructures.

2. Experimental

Fig. 1 shows the preparation procedure of the in-situ PIT

processed MgB2 tapes. Magnesium hydride MgH2 and amorphous B powder mixed with SiC powder addition was encased in a carbon steel tube outside diameter of 8 mm and inside diameter of 4.5 mm to form a core/sheath composite. The composite was fabricated into 1.8 mm square rod through grooved-rolling, then into a monocore tape 4 mm in width and 0.5 mm in thickness through flat rolling. The hot pressing was performed under 100 MPa in the temperature range of 630-800℃ for 1-10 h in Ar gas atmosphere. The heat treatment without pressing was similarly performed at 630-800℃ for 1-10 h in Ar-5%H2 gas atmosphere.

The grain size and the purity for the starting MgH2 and amorphous B powder, and additional SiC ceramic powder are listed in Table 1. MgH2 was used to avoid oxidized Mg

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A．NITTA S．OHKI, Y．YAMADA K．TACHIKAWA and H．KUMAKURA

Proceedings of the School of Engineering Tokai University, Series E ― 42 ―

Fig. 1. Preparation procedure of the in-situ PIT processed MgB2 tapes. Table 1 Grain size and purity of starting of MgH2 and amorphous B powders, and SiC additional powder. Fig. 2. Optical micrographs of the cross-section of MgB2 tapes heat-treated at 630℃ for 10 h. (a) Heat treated without pressing, (b) Hot pressing at 100 MPa. powder such as MgO. The MgH2 is thermally decomposed at near 450℃ into Mg metal to react with B powder. SiC added to the MgB2 core is nano-sized powder.

X-ray diffraction (XRD) scans of the MgB2 core were performed, after crushing into powders, using Co-Kα line. The microstructures of the MgB2 core were observed using an optical microscope and scanning electron microscope (SEM), respectively. The Ic at 4.2 K of the specimens was measured by a four-probe resistive method, the criterion of the Ic measurement being 1 µV/cm. The magnetic field was applied

perpendicular to the specimen current and parallel to the tape surface at the time of the Ic measurement. The core Jc was calculated by dividing Ic by the cross-sectional area of the MgB2 core. The transport currents at 4.2 K up to 12 T were measured by the facilities of National Institute for Materials Science.

3. Results and discussion Figs. 2 (a) and (b) illustrate optical micrographs of the cross-section of MgB2 tapes heat treated at 630℃ for 10 h. In the core of the tape heat-treated without pressing shown in Fig. 2(a), large voids (black parts) are observed. On the other hand, almost no void is observed in the core of the tape hot pressed under 100 MPa seen in Fig. 2(b). The cross-sectional area of the core in hot-pressed tapes, being 0.35 mm2 in area, decreases by about 40% compared to that of the one heat treated without pressing. The reduction in core area almost corresponds to the volume reduction ratio of reacted MgB2 to raw Mg plus 2B powders in MgB2 synthesis.

Fig. 3. SEM micrographs of the cross-section of the fractured MgB2 core. (a) Heat treated without pressing, (b) Hot pressing at 100 MPa.

Steel tube

MgB2 Tapes

Hot Pressing

（4 mmw×0.5 mmt）

Heat Treatment

Grooved rolling and

Flat rolling

Powder Purity Grain Size

MgH2 98% < 45μm

B (amorphous) 97% < 1μm

SiC 95% 30nm

(a) Heat treated without pressing

(b) Hot pressing at 100 MPa

(a) Heat treated without pressing

(b) Hot pressing at 100 MPa

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Superconducting Properties and Structure of MgB2 Tapes with SiC Addition Prepared by the Hot Pressing

Vol. XXXⅢ, 2008 ― 43 ―

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SEM structures of the fractured cross-sectional core in MgB2 tapes heat treated at 630℃ for 10 h with no SiC addition and no pressing, and with 5 mass% SiC addition and hot pressing at 100 MPa are shown in Figs. 3(a) and (b), respectively. Fig. 3 (a) indicates that various-sized voids were formed in the core after the MgB2 synthesis. The structures seen in Fig. 3 (b) show that MgB2 core consists of fine grains including submicron-sized grains, and much less voids are formed. Thus, hot pressing during heat treatment prevents voids due to volume reduction in MgB2 synthesis and form the denser structure to strengthen the linkage among MgB2 grains. Furthermore, SiC nanopowder addition in the core may enhance the refining of MgB2 grains since the added SiC nanopowder depresses the MgB2 grain growth during synthesis of MgB2 in hot pressing. Fig. 4. XRD patterns of the MgB2 core powder for the SiC addition tapes hot- pressed at 630℃ for 10 h. SiC nanosize powder was added up to 10 mass% to the MgB2 core. Fig. 5. XRD patterns of the MgB2 core powder for the 5 mass% SiC addition tapes hot- pressed at 630~800℃ for 10 h.

Fig. 4 illustrates XRD patterns of the MgB2 core in SiC addition tapes hot-pressed at 630℃ for 10 h. MgB2 phases are synthesized and dominant in the whole range of 0-10 mass% SiC addition. However, the magnesium silicide such as Mg2Si increased with increasing SiC addition, and a small part of added SiC powder remains in the MgB2 core after the hot pressing in 7-10 mass% addition tapes.

Fig. 5 illustrates XRD patterns of the MgB2 core in 5 mass% SiC addition tapes hot-pressed at 630-800℃ for 10 h. The iron boride such as FeB reacted with steel sheath increases with increasing temperature of hot pressing. The formation of FeB deviates from the composition of B to be poorer from the stoichiometric MgB2 composition.

Ic and core Jc at 4.2 K versus magnetic field in the SiC addition tapes hot-pressed at 630℃ for 10 h are shown in Fig. 6. Arrows in the figure indicate that the transport current exceeds the capacity limit of 300 A for the specimen probe and power source. Nanosize SiC powder addition enhances the Ic at 4.2 K, especially the Ic in higher magnetic field than 5 T. The Ic at 4.2 K and 5 T in the 3 and 5 mass% SiC addition tapes exceeds 300 A which corresponds to be higher than 850 A/mm2 in core Jc. 5 mass% SiC addition in the MgB2 core seems to be optimum, and the Ic of 5 % SiC addition tape is the highest under magnetic fields up to 12 T in 0-10 mass% SiC addition tapes. 7 and 10 % SiC addition improve the Ic at magnetic fields higher than 10 T. In the previous study for SiC addition tapes [10], the resistivity just above the Tc, ρn increases by a factor of two, while Tc slightly decreases by SiC addition due to the carbon substitution for Boron site in MgB2 crystal. Since the upper critical field Hc2 is proportional to the product of ρn and Tc, considerable increase of ρn enhances the Hc2 of the MgB2 core with SiC addition. Therefore, the improvement of core Jc at high magnetic field may result from the enhancement of Hc2 due to the incorporation of SiC nano-sized powder. Fig. 6. Ic and core Jc at 4.2 K versus magnetic field in the SiC addition tapes hot-pressed at 630℃ for 10 h.

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A．NITTA S．OHKI, Y．YAMADA K．TACHIKAWA and H．KUMAKURA

Proceedings of the School of Engineering Tokai University, Series E ― 44 ―

Fig. 7. Magnetic field dependence of Ic and core Jc at 4.2 K in 5 % SiC addition tapes hot-pressed at 630-800℃ for 1-10 h. Fig. 8. Magnetic field dependence of core Jc at 4.2 K for 5%SiC and no addition tapes hot-pressed and heat- treated at 630℃ for 10 h. Dashed line shows NbTi metallic superconducting wire.

Fig. 7 shows magnetic field dependence of Ic and core Jc at 4.2 K in 5 % SiC addition tapes hot-pressed at 630-800℃ for 1-10 h. The highest Ic at 4.2 K and magnetic field up to 12 T was obtained in the tape hot-pressed at 630℃ for 10 h. The Ic decreases with elevated temperature for hot-pressing time of 10 h, and slightly decreases with treated time for hot-pressing temperature of 700-800℃. According to [10], since SiC is strong FeB former, the degradation in Ic for the hot-pressed tapes at relatively high temperatures of 700-800℃ results from the FeB formation in core reacted with steel sheath. The hot-pressing at 630℃ for 1 h is too short to synthesize MgB2 in the tape.

The magnetic field dependence of core Jc at 4.2 K for

5% SiC and no addition tapes hot-pressed (H. P.) under 100 MPa and heat-treated (H. T.) without pressing at 630℃ for 10 h are summarized in Fig. 8 compared to NbTi metallic superconductor shown by dashed line. The addition of SiC nano-sized powder enhances the core Jc, especially the Jc at high magnetic field due to the improvement of Hc2 and refinement of the MgB2 grains. Furthermore, hot pressing during heat treatment appreciably enhances the core Jc for the tapes. The core Jc for 5 mass% SiC addition tape is 125 A/mm2 at 10 T, 500 A/mm2 at 7 T and close to 1,000 A/mm2

at 5 T, respectively. The difference in core Jc between hot-pressed and heat-treated without pressing tapes becomes larger under lower magnetic field than under high magnetic field. The enhancement of the core Jc in low magnetic field results from the improvement of MgB2 core structure, that is to say, denser structure without voids and strong linkage among MgB2 grains. Thus, the hot pressing during heat treatment and addition of SiC nano-sized powder performed higher core Jc at 4.2 K by a factor of 5 at 5 T and by a factor of 40 at 10 T in comparison with no pressing during heat treatment and no addition of effective nano-sized powders. The core Jc of the hot-pressed and SiC added MgB2 tapes is higher than that of NbTi wire above 10 T, while the Jc of NbTi still higher by a factor of 2-3 below 10 T.

4. Conclusions

The combination of hot pressing during the heat treatment and the addition of SiC nano-sized powder appreciably enhanced the core Jc for the in-situ PIT processed MgB2 tapes with steel sheath. The core Jc at 4.2 K for the 5 mass% SiC addition tape hot pressed under 100 MPa at 630℃ for 10 h is 125 A/mm2 at 10 T, 500 A/mm2 at 7 T and close to 1,000 A/mm2 at 5 T, respectively. Hot pressing during heat treatment prevents voids due to the volume shrinkage in MgB2 synthesis reaction, and strengthen the linkage among MgB2 grains. The addition of SiC nano-sized powder enhances the core Jc, especially the Jc at high magnetic field due to the improvement of Hc2 and refinement of the MgB2 grains.

Acknowledgement The authors wish to thank Mr. Kyuna and Mr.

Miyamoto of Future Science and Technology Joint Research Center of Tokai University for their SEM image observations.

References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani,

and J. Akimitsu, “Superconductivity at 39K in magnesium diboride,” Nature 10, (2001), pp. 63-64.

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H.P. 5% SiCH.T. 5% SiCH.T. no add.

… NbTi

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Superconducting Properties and Structure of MgB2 Tapes with SiC Addition Prepared by the Hot Pressing

Vol. XXXⅢ, 2008 ― 45 ―

[2] G. Grasso, A. Malagoli, C. Ferdeghini, S. Roncallo, V. Braccini, M. R. Cimberle, and A. S. Siri, “Large transport critical currents in unsintered MgB2 superconducting tapes,” Appl. Phys. Lett., vol. 79, (2001), pp. 230-232.

[3] H. Kumakura, A. Matsumoto, H. Fujii, and K. Togano, “High transport critical current density obtained for powder-in-tube processed MgB2 tapes and wires using stainless steel and Cu-Ni tubes,” Appl. Phys. Lett., vol. 79, (2001), pp. 2435-2437.

[4] K. Tachikawa, Y. Yamada, M. Enomoto, M. Aodai, and H. Kumakura, “Effects of Metal Powder addition on the Structure and Critical Current of Ni-Sheathed PIT MgB2 Tapes,” IEEE Trans. Appl. Supercond., vol. 13, (2003), pp. 3269-3272.

[5] K. Katagiri, A. Iwamoto, Y. Shoji, K. Tachikawa, Y. Yamada, K. Watanabe, and T. Mito, “Stress/strain characteristics of PIT MgB2 tapes with nickel sheath,” Physica C, vol. 397, (2003), pp. 95-98.

[6] S. X. Dou, S. Soltanian J. Horvat, X. L. Wang, S. H. Zhou, M. Ionescu, H. K. Liu, P. Munroe, and M. Tomsic, “Enhancement of the critical current density and flux pinning of MgB2 superconductor by nanoparticle SiC doping,” Appl. Phys. Lett., Vol. 81, (2002), pp. 3419-3421.

[7] H. Kumakura, H. Kitaguchi, A. Matsumoto, and H. Hatakeyama, “Improved Superconducting Properties of In-Situ PIT-Processed MgB2 Tapes,” IEEE Trans. Appl. Supercond., vol. 15, (2005), pp. 3184-3187.

[8] S. X. Dou, S. Soltanian, W. K. Yeoh, and Y. Zhang, “Effect of Nano-Particle Doping on the Upper Critical Field and Flux Pinning in MgB2,” IEEE Trans. Appl. Supercond., vol. 15, (2005), pp. 3219-3222.

[9] K. Tanaka, H. Kitaguchi, H. Kumakura, M. Hirakawa, H. Yamada, and M. Okada, ”Fabrication and Transport Properties of MgB2 Mono-Core Wire and Solenoid Coil,” IEEE Trans. Appl. Supercond., vol. 15, (2005), pp. 3180-3183.

[10] Y. Yamada, M. Nakatsuka, Y. Kato, K. Tachikawa, and H. Kumakura, ”Superconducting Properties of In-situ PIT MgB2 Tapes with Different Ceramic Powder,” Adv. in Cryo. Engr., vol. 52, (2006), pp. 631-638.

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