Portable superconductors: Boost for NMR and MRI

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  • Published: Feb 15, 2017
  • Author: David Bradley
  • Channels: NMR Knowledge Base
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Material confluence

The confluence of novel materials and a pioneering US discovery with innovation in cryogenics could soon lead to portable and lower-cost NMR and MRI machines, thanks to work at Cambridge University (Credit: AIP/ Appl Phys Lett/Durrell et al)

The confluence of novel materials and a pioneering US discovery with innovation in cryogenics could soon lead to portable and lower-cost NMR and MRI machines, thanks to work at Cambridge University.

Superconductivity is an intriguing phenomenon wherein an electrical current can flow unhindered by resistance through an appropriate material. It was discovered on the 8th April 1911 by Heike Kamerlingh Onnes who was investigating the electrical properties of solidified mercury at the cryogenic temperature achievable with the recently developed liquid helium. During the same laboratory session Onnes inadvertently stumbled upon the superfluid phase of helium although did not recognise its significance. Since that period, scientists have searched for and found many other materials that are superconductors at cryogenic temperatures. Lead, for instance, is a superconductor at 7 Kelvin and niobium nitride at 16 K.

Pioneering approach

It was many years before the discovery of so-called high-temperature superconductors. Indeed, until 1986 it was assumed that superconductivity could not exist at temperatures above about 30 K. So, it was with great excitement that Bednorz and Müller observed superconductivity in a lanthanum-based cuprate perovskite material at a transition temperature of 35 K in 1986, which earned them a rapid Nobel Prize in Physics in 1987. Swapping out the lanthanum for yttrium in this structure shoved the transition temperature up to 92 K, which is the realm of the much more accessible liquid nitrogen. Experiments with a wide range of materials at different temperatures and pressures have nudged the transition temperatures of these high-temperature superconductors up and down the Kelvin scale over subsequent years. Always in mind is the possibility that somewhere in materials chemical space there exists an exotic compound, a complex alloy or perhaps even some weird previously untested combination of elements that will have a transition temperature closer to the freezing point of water, if not above that.

Meanwhile, governments, industry, health care centres and scientific researchers continue to make use of superconductivity in applications extending from magnetic resonance imaging (MRI) in hospitals and high-resolution nuclear magnetic resonance (NMR) spectroscopy to the cavities of particle accelerators in which scientists explore the fundamentals of matter. And yet, the incredibly chilly climes of the current superconducting world aside, there are challenges to the wider application of these materials.

Fundamentally, NMR spectroscopists would like to integrate the potential of superconducting instruments into much smaller, perhaps even portable systems. Now, researchers at Cambridge University, UK, in research led by John Durrell and his team have demonstrated how a portable superconducting magnetic system, essentially, a high performance substitute for a conventional permanent magnet, can sustain a 3 Tesla magnetic field. The work evolved largely from the pioneering discoveries of University of Houston physicist Roy Weinstein, who has shown how conventional electromagnets and pulsed field magnetization can be used to activate superconducting magnetic fields which are "captured" and sustained as part of a superconductive arrangement.

Portability

In the Cambridge approach, they avoid the need for large and costly superconducting magnets to "activate" what could be a portable system. Durrell points out that the work also exploits the more readily available and cheaper cooling systems now available to get those materials down below their transitions.

"For example, the leap with advances in cryogenics, allows you to do interesting things in other areas, too," Durell explains. "There is a lot coming together to make this possible." While large industrial-size superconducting systems do generate 20 T magnetic fields, Durrell’s 3 T magnetic field is a breakthrough for a portable system.

Weinstein and his colleagues had used yttrium barium cuprate doped with uranium and subjected to an irradiation treatment. Durrell's team opted for the less expensive gadolinium barium cuprate and avoided the uranium doping. "It was a surprise to us that we managed to see in a not quite so cutting-edge material the same giant flux leap effect as Weinstein demonstrated," Durrell adds. "The key thing that made this possible is that we have looked at what Roy has done to get it to work but for this kind of portable system. Before we were using conventional superconducting magnets to charge our bulks. This will make access to these high fields cheaper and more practical." Low cost NMR and MRI systems for hospitals are also a strong possibility for use, Durrell suggests. There might also be the possibility of magnetically directed drug delivery that exploits these superconductors.

Related Links

Appl Phys Lett 2017, online: "A portable magnetic field of >3 T generated by the flux jump assisted, pulse field magnetization of bulk superconductors"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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