No Resistance Needed!

By Manuel Ramos, UTEP and Allan J. Jacobson, TCSUH

In 1911, H.K. Onnes, a Dutch physicist, discovered superconductivity by cooling mercury metal to extremely low temperature and observing that the metal exhibited zero resistance to electric current. He received the Nobel Prize in 1913 “for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium.”¹

His discoveries opened up a new field to many physicists and materials scientists around the world who have worked subsequently in order to understand the phenomenon of superconductivity and to discover new materials; the field has continued to fascinate scientists and engineers for almost 100 years.

Before 1973, many metals and metal alloys were found to become superconducting at temperatures below 23.2 K. These became known as low temperature superconductor (LTS) materials. The LTS materials such as NbTi have been commercialized in superconducting magnets used in magnetic resonance imaging, nuclear magnetic resonance, high-energy physics accelerators, and plasma fusion reactors. The underlying mechanism of low-temperature superconductivity was shown experimentally and theoretically to depend on the coupling of electrons to the lattice vibrations. In 1957, based on this coupling mechanism, Bardeen, Cooper, and Schrieffer developed a theory of superconductivity (BCS theory) that gave a complete theoretical explanation of the phenomenon. ²

In 1986, Bednorz and Müller discovered oxide-based ceramic materials that demonstrated superconducting properties as high as 35 K.³ This was quickly followed in early 1997 by the announcement by C.W. Chu and colleagues of a copper oxide superconductor YBa₂Cu₃O₇ that became superconducting at 92 K, above the boiling point of liquid nitrogen 77 K. Further discoveries of copper oxides containing bismuth,⁴ thallium,⁵ or mercury,⁶ with even higher transition temperatures up to 138 K at ambient pressure in the mercurocuprate HgBa₂Ca₂Cu₃O_8+x, quickly followed. Today, the record stands at ~164 K for this phase under pressure.⁷ A superconducting material with a critical temperature above 23.2 K is known as a high-temperature superconductor (HTS), despite the need for cryogenic refrigeration.

After 20 years of research and development, HTSs, particularly YBCO, are poised for large-scale applications because of the development of the technology to produce long lengths of superconducting conductors in the form of thick films (~1 m) on a metallic substrate with high current carrying capability (~100 A cm width). Today, the only substantive commercial products incorporating HTS materials are electronic filters used in wireless base stations, which use HTS. However, much larger applications have either been successfully demonstrated or demonstrations are in progress.⁸ Examples include HTS transmission cables, fault current limiters and fault current limiting transformers, to aid our aging electric power grid, large electric motors for ship propulsion, and HTS generators to produce energy from wind power. High-field magnets that use HTS are also under development for magnetic resonance imaging and for use in high-energy physics projects. Several challenges remain to substantial commercial inroads of HTS technology into the new applications. Cost of HTS wire, refrigeration, demonstration of systems cost benefits, and long-term reliability are all issues that need to be addressed.

New superconductors continue to be discovered, for example MgB₂ (T_c= 39 K)⁹ and the iron arsenide family (with T_c up to 56 K),¹⁰ and while these materials have transition temperatures below the cuprates, they are still high-temperature superconductors under the definition given previously. Further, given that no complete theory exists at present as to how HTS materials work, many feel that it should be possible to raise the superconducting temperature to over 200 K in the next 10 years.¹¹ Consequently, in addition to pushing forward with the applications of the existing materials, particularly YBCO, a renewed interest in the search for new materials with higher transition temperatures has developed in the last few years.¹²

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8. Coalition for the Commercial Application of Superconductors, http://www.ccas-web.org/
9. J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (6824), 63 (2001).
10. Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130 (11), 3296 (2008).
11. H. Wienstock, APS Division of Condensed Matter Physics Newsletter, Summer 2009, p. 5.
12. U.S. D.O.E. Office of Basic Energy Sciences, Basic Research Needs for Superconductivity: http://www.sc.doe.gov/bes/reports/list.html.