Superconducting MgB₂

MgB₂ is a BCS superconductor discovered in 2001. Its promising properties, such as the highT_c(~39 K), simple structure (see left), low costs of raw materials (Mg and B) and fabrication, and the absence of weak-link across grain boundaries, make MgB₂ a promising material for applications at above 20 K. However, to meet application requirements, the properties of MgB₂, especially the irreversibility field H* and high-field J_c, have to be improved through microstructure modifications: chemical doping, introduction of precipitates, and atomic-scale control of defects. The small length scale of these modifications means that structure and chemistry characterizations using the TEM are required to better understand the correlation between MgB₂’s superconducting properties and its microstructure.

Bulk MgB₂

Applications of MgB₂ are mostly likely to be in the form of tapes or wires created by bulk reaction of powders or sintering of pre-reacted powders. Improved H* and J_c from introduction of impurities such as carbon is also likely to be required.

Significant progress in creating MgB₂ wires has already been made, with Hypertech Inc and Columbus Inc both reporting long lengths and results from test magnet coils. However, impurity phases and porosity limit the connectivity and thus current carrying cross section and J_c of most bulk MgB₂.

We have extensively examined the microstructure and microchemistry of bulk samples prepared by Akiyoshi Matsumoto of NIMS in which nanoscale SiC particles were introduced into the precursor B and MgH₂ powders before reaction. These samples have the highest H_c2 of any bulk MgB₂.

Thin Film MgB₂

Publications

• “High-field properties of carbon-doped MgB₂ thin films by hybrid physical–chemical vapor deposition using different carbon sources” W. Dai, V. Ferrando, A. V. Pogrebnyakov, R. H. T. Wilke, K. Chen, X. Weng, J. Redwing, C. W. Bark, C.-B. Eom, Y. Zhu, P. M. Voyles, D. Rickel, J. B. Betts, C. H. Mielke, A. Gurevich, D. C. Larbalestier, Q. Li, and X. X. Xi, Superconductor Science and Technology 24, 125014 (2011).
• “MgB₂ / MgO / MgB₂ Josephson Junctions for High-Speed Circuits”, K. Chen, C. G. Zhuang, Q. Li, X. Weng, J. M. Redwing, Y. Zhu, P. M. Voyles, X. X. Xi, IEEE Transaction on Applied Superconductivity 21, 115-118 (2011).
• “Nanoscale disorder in pure and doped MgB₂ thin films,” Y. Zhu, A. V. Pogrebnyakov, R. H. Wilke, K. Chen, X. X. Xi, J. M. Redwing, C. G. Zhuang, Q. R. Feng, Z. Z. Gan, R. K. Singh, Y. Shen, N. Newman, J. M. Rowell, F. Hunte, J. Jaroszynski, D. C. Larbalestier, S. A. Baily, F. F. Balakirev, P. M. Voyles, Supercon. Sci. Tech. 23, 095008 (2010).
• “High-J_c MgB₂ Josephson junctions with operating temperature up to 40 K”, Ke Chen, C. G. Zhuang, Qi Li, Y. Zhu, P. M. Voyles, X. Weng, J. M. Redwing, R. K. Singh, A. W. Kleinsasser, and X. X. Xi, Appl. Phys. Lett. 96, 042506 (2010).
• “MgO platelets and high critical field in MgB₂ thin films doped with carbon from methane” Y. Zhu, F. Hunte, S. A. Baily, F. F. Balakirev, C. G. Zhuang, Q. R. Feng, Z. Z. Gan, X. X. Xi, D. C. Larbalestier, P. M. Voyles, Supercon. Sci. Tech. 22, 125001 (2009).
• TEM Characterization of Microstructure and Chemistry of Magnesium Diboride Superconductor, Y. Zhu, dissertation, Materials Science, University of Wisconsin – Madison, 2008.
• “Increased in-field critical current density in neutron irradiated MgB₂ films” I. Pallecchi, V. Ferrando, C. Tarantini, M. Putti, C. Ferdeghini, Y. Zhu, P.M. Voyles, X.X. Xi, Supercon. Sci. Tech. 22, 015023 (2009).
• “Nanoscale Disorder in MgB₂ Thin Films Grown by Hybrid Physical-Chemical Vapor Deposition”, Y. Zhu, D. C. Larbalestier, A. Gurevich, X. X. Xi, and P. M. Voyles, Microsc. Microanal. 14 (Suppl. 2), 212 (2008).
• “Nanoscale grains, high irreversibility field, and large critical current density as a function of high energy ball milling time in C-doped magnesium diboride”, B. J. Senkowicz, R. J. Mungall, Y. Zhu, J. Jiang, P. M. Voyles, E. E. Hellstrom, and D. C. Larbalestier, Supercond. Sci. Technol. 21, 035009 (2008).
• “Nanoscale disorder in high critical field, carbon-doped MgB₂ hybrid physical-chemical vapor deposition thin films” Y. Zhu, D. C. Larbalestier, P. M. Voyles, A. V. Pogrebnyakov, X. X. Xi, J. M. Redwing, Appl. Phys. Lett. 91, 082513 (2007).
• “Microstructures of SiC nanoparticle-doped MgB₂/Fe tapes”, Y. Zhu, A. Matsumoto, B. J. Senkowicz, H. Kumakura, H. Kitaguchi, M. C. Jewell, E. E. Hellstrom, D. C. Larbalestier, P. M. Voyles, J. Appl. Phys 102, 013913 (2007).
• “Understanding the route to high critical current density in mechanically alloyed Mg(B1-xCx)2”, B. J. Senkowicz, A. Polyanskii, R. J. Mungall, Y. Zhu, J. E. Giencke, P. M. Voyles, C. B. Eom, E. E. Hellstrom, and D. C. Larbalestier, Supercon. Sci. Tech 20, 650 (2007).
• “Evaluation of connectivity, flux pinning and upper critical field contributions to the critical current density of bulk MgB₂”, A. Matsumoto, H. Kumakura, H. Kitaguchi, B. J. Senkowicz, M. C. Jewell, E. E. Hellstrom, Y. Zhu, P. M. Voyles, D. C. Larbalestier, Appl. Phys. Lett 86, 132508 (2006).