MgB2 is a BCS superconductor discovered in 2001. Its promising properties, such as the highTc(~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 Jc, 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.
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 Jc 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 Jc 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 Hc2 of any bulk MgB2.
Thin Film MgB2
- “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-Jc 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).