Keeping the costs of superconducting magnets down using ultrasound

Superconductivity already has a variety of practical applications, such as medical imaging and levitating transportation like the ever-popular maglev systems. However, to ensure that the benefits of applied superconductors keep spreading further into other technological fields, we need to find ways of not only improving their performance, but also making them more accessible and simpler to fabricate.

In this regard, magnesium diboride (MgB₂) has attracted the attention of researchers since its discovery as a superconductor with multiple advantages. It is a lightweight, easily processible material made from widely abundant precursors; these qualities combined, greatly lower the overall cost of working with MgB₂.

However, a key practical property of a superconductor is its critical current density (J_c)–the maximum current density at which it can operate without dissipating energy like conventional conductors do. Increasing the J_c of MgB₂ through affordable means has proven to be a notable challenge, which is usually tackled through materials engineering and by optimizing fabrication procedures and conditions.

In a recent study accepted for publication in Materials Science and Engineering: B, a team of scientists from Shibaura Institute of Technology, Japan, has developed a cost-effective technique to boost the J_c of bulk MgB₂: ultrasonication. Their approach involves dissolving cheap commercial boron in hexane and using ultrasound to disperse the solute thoroughly. Once hexane is evaporated and removed, one obtains a very fine boron powder, which is then sintered with magnesium to produce MgB₂. But why does using finer boron result in better superconducting properties?

The answer is magnetic flux pinning. Though superconductors generally repel external magnetic fields, some quantized amounts of magnetic flux sometimes penetrate the material under the right conditions, producing the strong force responsible for superconducting levitation. This penetration occurs only in pinning centers, which arise from various kinds of defects in the material; in the case of MgB₂, the pinning centers are located at the grain boundaries. Professor Muralidhar Miryala, who led the study, explains: “To put it briefly, the refined boron powder obtained via ultrasonication results in a higher density of grain boundaries by reducing overall grain size. In turn, the increment in grain boundaries equals an increase in flux pinning centers, which are responsible for the higher J_c we observed in our samples.”

The scientists’ synthesis procedure produced high-quality bulk MgB₂ mostly free of oxidation impurities. Compared with a non-ultrasonicated sample used as a reference, the J_c values increased by as much as 20%, depending on the ultrasonication time used. Moreover, the results of scanning electron microscopy and energy dispersive X-ray spectroscopy analyses revealed a secondary mechanism that could give rise to enhanced J_c. The team noted a layered structure of what seems to be Mg-B-O coating the walls of pores of boron deficiencies. This layered coating structure can not only act as a pinning center itself, but also have a restraining effect on the grain size.

Excited about the overall results, Miryala remarks: “Our study lays a foundation towards realizing affordable high-performance bulk MgB₂ for superconducting magnets. This will help reduce the cost of magnet-based technologies and make them more accessible to the general population, especially in the medical field.” Even though further studies will be needed to find the optimal solvent and ultrasonication parameters, the present findings are certainly promising and could promote the use of MgB₂ superconducting magnets in other areas, including space applications, water cleaning, and electric motors. Hopefully, and given enough time, we shall all benefit from accessible superconductors in one way or another!

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Title of original paper: Superconducting properties of sintered bulk MgB₂ prepared from hexane-mediated high-energy-ultra-sonicated boron

Journal: Materials Science and Engineering: B

DOI: 10.1016/j.mseb.2020.115030

Shibaura Institute of Technology (SIT) is a private university with campuses in Tokyo and Saitama. Since the establishment of its predecessor, Tokyo Higher School of Industry and Commerce, in 1927, it has maintained “learning through practice” as its philosophy in the education of engineers. SIT was the only private science and engineering university selected for the Top Global University Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology and will receive support from the ministry for 10 years starting from the 2014 academic year. Its motto, “Nurturing engineers who learn from society and contribute to society,” reflects its mission of fostering scientists and engineers who can contribute to the sustainable growth of the world by exposing their over 8,000 students to culturally diverse environments, where they learn to cope, collaborate, and relate with fellow students from around the world.

Website: https://www.shibaura-it.ac.jp/en/

Dr Muralidhar Miryala is currently Deputy President and Professor at the Graduate School of Engineering and Science of SIT. His work is mostly centered around superconducting materials, especially the development of powerful superconducting magnets, and understanding and exploiting pinning effects. He has published over 480 research items including patents, books, review articles, research articles etc., between 1990 and 2020.

This work was partly supported by Shibaura Institute of Technology (SIT) Research Center for Green Innovation and Grant-in-Aid FD research budget code: 112282. One of the authors (Sai Srikanth Arvapalli) acknowledges support from SIT for his doctoral program.