Carbon doping induced imperfections on MgB2 superconducting wire
© Kim and Choi; licensee Springer. 2015
Received: 12 January 2015
Accepted: 15 January 2015
Published: 4 March 2015
Carbon is known to be the most effective dopant for magnesium diboride, MgB2, especially for enhancing high-field electrical properties. Carbon increases the impurity scattering rate, and this, combined with the two-band nature of MgB2, raises the upper critical field and thereby the high-field critical current. In addition, microscopic analysis has shown that carbon doping induces crystalline imperfections as the origin of impurity scattering. Herein, further detailed transmission electron microscopy analysis has been applied to find more clues for understanding the role of carbon.
KeywordsCarbon doping Impurity scattering MgB2 superconducting wire Microscopic origin
Magnesium diboride, MgB2, has been regarded as a very promising candidate for alternatively commercial superconducting materials (Nagamatsu et al. 2001). Together with its simple crystalline structure and low material cost, the transition temperature around 40 K enabling a cryogen-free operation is definitely attractive for engineering applications (Patel et al. 2014). Even if MgB2 has many advantages, it still has drawbacks in terms of material synthesis. For example, two key precursors such as magnesium and boron are very sensitive to air, especially oxygen, resulting in MgO and B2O3 as impurity phases (Kim et al. 2007). In addition, a large difference in the melting temperature between the two precursors makes MgB2 a complicated reaction (Hata et al. 2013).
For making a wire/tape conductor, a normal ‘in situ’ process has been applied with a specific dopant, i.e., carbon. The introduction of carbon dopant for impurity scattering is to increase the upper critical field and thereby enhance the high-field critical current density. When carbon substitutes boron sites of the MgB2 structure, shrinkage of the a-axis parameter occurs (Kazakov et al. 2005; Kortus et al. 2005). There can be a decrease in the density of state (DOS) and hardening of the optical E 2g phonons which is known to be strongly related with the superconducting properties of MgB2 (Eisterer 2007). According to our previous report (Kim et al. 2012), however, the studies employing microscopic tools such as scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) did not show direct evidence on how boron sites are replaced by carbon. The main reason is the limitation (<1 at.% carbon) of the EELS resolution. A recent work on the STEM observation of a MgB2 specimen sintered at a low temperature (650°C) and short sintering time (<1 h) still remains unclear because the majority of carbon was found to be mainly located at the grain boundaries (Susner et al. 2014). In this work, as an extension of our previous work, detailed structural analysis and microstructure observations have been further carried out with an electrical current carrying property of a MgB2 superconductor.
The chemical solution process via a carbohydrate has become popular and advantageous in terms of both cost and performance (Kim et al. 2012). For this study, MgB2 was fabricated by the in situ powder-in-tube process (Jin et al. 2011). After dissolving 10 wt.% malic acid (C4H6O5, 99.0%) in toluene (C7H8, 99.5%), the solution was mixed with an appropriate amount of boron (B, 99.9%) powder in a Spex mill (Spex Industries Inc., Metuchen, NJ, USA) for 10 min (100 rpm). The slurry was dried at 150°C in a dry oven to form a coating around boron powder particles (that is, carbon encapsulated boron). This uniform composite was then mixed with magnesium (Mg, 99%) powder and mixed in a Fritsch planetary mill (Fritsch, Idar-Oberstein, Germany; 500 rpm) for 10 min. The mixed powder was packed into a 140-mm-long iron (Fe) tube. The outer and inner diameters of the Fe tube were 10 and 8 mm, respectively. The packed tube was drawn till the final outer diameter became 0.83 mm. The fabricated wires were sintered at 650°C for 30 min under high-purity argon gas with a ramp rate of 5°C min−1.
For microstructures, we used a JEOL Cs-corrected dedicated scanning transmission electron microscope (STEM; JEM-2500SES, JEOL Ltd. Tokyo Japan) equipped with a Gatan 766 EELS spectrometer (Enfina 1000, Gatan, Inc., Pleasanton, CA, USA). The specimens for transmission electron microscopy (TEM) observation were prepared by a wedge polishing method and then ion-milled with a low accelerating voltage (0.1 keV). High-angle annular dark-field (HAADF) images were obtained with a beam size of about 1 Å at the condition of the HAADF detector inner cutoff angle in 113 mrad. In addition, high-resolution TEM observation was conducted in a JEOL 300-keV field emission transmission electron microscope (FE-TEM, JEM-3000F).
Our detailed microscopic analyses can be summarized into the following consistent picture on the effect of homogeneous carbon doping during the wire heat treatment process: Magnesium vapor permeates into carbon-shielded boron and forms nanocrystalline MgB2 seeds. As these seeds grow further, grains merge together, carbon is extruded outside MgB2 phases, and a small amount of remaining carbon causes lots of defects. Carbon also prevents boron agglomeration, resulting in a dense wire core. Already, the critical current density at low field is comparable to Nb-Ti, and we expect that further optimization can lead the un-reacted boron phase into a fully reacted MgB2 phase while maintaining its small grain size and a drastic increase in the critical current density far beyond Nb-Ti can be realized.
This work was supported by KBSI (Korea Basic Science Institute) Grant T35519 to S. Choi.
- Eisterer M (2007) Magnetic properties and critical currents of MgB2. Supercond Sci Technol 20:R47–R73View ArticleGoogle Scholar
- Gurevich A (2003) Enhancement of the upper critical field by nonmagnetic impurities in dirty two-gap superconductors. Phys Rev B 67:184515View ArticleGoogle Scholar
- Hata S, Sosiati H, Shimada Y, Matsumoto A, Ikeda K, Nakashima H, Kitaguchi H, Kumakura (2013) Imperfection of microstructural control in MgB2 superconducting tapes fabricated using an in-situ powder-in-tube process: toward practical applications. J Mater Sci 48:132View ArticleGoogle Scholar
- Jin S, Mavoori H, Bower C, Dover RBV (2011) High critical currents in iron-clad superconducting MgB2 wires. Nature 411:563–565View ArticleGoogle Scholar
- Kazakov SM, Puzniak R, Rogacki K, Mironov AV, Zhigadlo ND, Jun J, Soltmann C, Batlogg B, Kotus J, Dolgov OV, Kremer RK, Golubov AA (2005) Carbon substitution in MgB2 single crystals: Structural and superconducting properties. Phys Rev B 71:024533View ArticleGoogle Scholar
- Kim JH, Dou SX, Shi DQ, Rindfleisch M, Tomsic M (2007) Study of MgO formation and structural defects in in situ processed MgB2/Fe wires. Supercond Sci Technol 20:1026–1031View ArticleGoogle Scholar
- Kim JH, Oh S, Kumakura H, Matsumoto A, Heo YU, Song KS, Kang YM, Maeda M, Rindfleisch M, Tomsic M, Choi S, Dou SX (2011) Tailored materials for high-performance MgB2 wire. Adv Mater 23:4942–4946View ArticleGoogle Scholar
- Kim JH, Oh S, Heo YU, Hata S, Kumakura H, Matsumoto A, Mitsuhara M, Choi S, Shima Y, Maeda M, MacManus-Driscoll JL, Dou SX (2012) Microscopic role of carbon on MgB2 wire for critical current density comparable to NbTi. NPG Asia Mater 4:E3View ArticleGoogle Scholar
- Kortus J, Dolgov OV, Kremer RK, Golubov AA (2005) Carbon substitution in MgB2 single crystal: structural and superconducting properties. Phys Rev Lett 94:027002View ArticleGoogle Scholar
- Maeda M, Kim JH, Oh S, Li WX, Takase K, Kuroiwa Y, Dou SX, Takano Y (2013) Enhancing the superconducting properties of magnesium diboride without doping. J Am Ceram Soc 96:2893–2897View ArticleGoogle Scholar
- Nagamatsu J, Nakagawa N, Muranaka T, Zenitani Y, Akimitsu J (2001) Superconductivity at 39 K in magnesium diboride. Nature 410:63–64View ArticleGoogle Scholar
- Patel D, Hossain MSA, Motaman A, Barua S, Shahabuddin M, Kim JH (2014) Rational design of MgB2 conductors toward practical applications. Cryogenics 63:160–165View ArticleGoogle Scholar
- Shahabuddin M, Alzayed NS, Oh S, Choi S, Maeda M, Hata S, Shimada Y, Hossain MSA, Kim JH (2014a) Microstructural and crystallographic imperfections of MgB2 superconducting wire and their correlation with the critical current density. AIP Adv 4:017113View ArticleGoogle Scholar
- Shahabuddin M, Alzayed NS, Oh S, Choi S, Maeda M, Shah MS, Motaman A, Hossain MSA, Kim JH (2014b) Percolative nature of current transport in polycrystalline MgB2 wires. Solid State Commun 181:20–23View ArticleGoogle Scholar
- Susner MA, Bohnenstiehl SD, Dregia SA, Sumption MD, Yang Y, Donovan JJ, Collings EW (2014) Homogeneous carbon doping of magnesium diboride by high-temperature, high-pressure synthesis. Appl Phys Lett 104:162603View ArticleGoogle Scholar
- Ummarino GA, Daghero D, Gonnelli RS, Moudden AH (2005) Carbon substitutions in MgB2 within the two-band Eliashberg theory. Phys Rev B 71:134511View ArticleGoogle Scholar
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