Superconductors can carry electricity without any resistance and with many times the current density of conventional materials such as copper wire. Since their discovery at the beginning of the 20th century, these properties of superconductors have led scientists and engineers to dream of exploiting them in transmission lines to carry electrical power over vast distances with zero resistance, and therefore perfect efficiency, with far fewer cables than are needed in the conventional electric grid. However, the high cost of making and maintaining superconducting cables has limited their use to a few critical applications, such as delivering power to high-density urban areas or creating better magnets for MRI machines.
Irradiating superconductors. One way to improve the efficiency of superconducting wires is to raise their critical temperature thus reducing the cost to cool them. Scientists at the Center for Emergent Superconductivity (CES) in collaboration with researchers in Switzerland have successfully demonstrated the feasibility of a second approach. Rather than raising the critical temperature of a superconductor, Lei Fang and co-workers report a method for irradiating superconductors that increases the amount of electric current they can carry by a factor of 10. A superconducting cable made of such a treated material would require 10 times less wire to carry the same amount of current, dramatically reducing the total cost.
The CES team working to increase superconducting currents focused on a new iron-based superconductor discovered in 2008. The material is made of samarium, iron, arsenic, oxygen, and fluorine (abbreviated SFAOF). They focused on this material because SFAOF becomes a superconductor at the highest temperature of all the materials in the iron-containing family of superconductors, although this is still a chilly -360 degrees Fahrenheit. However, the material is susceptible to the formation of nanometer-scale current loops, called vortices, when used. (A nanometer is one billionth of a meter, about the length of five to ten atoms lined up in a row.) While vortices are not in themselves bad for superconductivity, when electrical current is passed through the superconductor, the vortices get dragged along like pieces of driftwood in a stream. This dragging in turn causes energy losses and ultimately ruins the perfect efficiency of a superconducting cable.
One method of resolving the vortex drag problem is to engineer ways to pin the vortices in place so they can’t move when current flows. When this is done, the superconducting current can flow around the stationary vortices without any loss.
Pinning vortices. In their study, the CES team describes how nanoscale tracks--punched vertically through the layered structure of the SFAOF crystals--can be tailored to provide vortex pinning centers. Tiny nanometer-sized columns were made by irradiating single-crystal samples of SFAOF with high-energy beams of ionized lead atoms produced at the Argonne Tandem Linac Accelerator System (ATLAS). By adjusting the beam energy and the sample’s exposure time, and by choosing the right type of atom with which to bombard crystals, a large number of damage tracks are created that "pin" the vortices in place. Maximum currents as high as 20,000,000 amps per square centimeter are achieved with this method, representing an increase of 10 times over the maximum current the pristine material can carry.
Surprisingly, even though irradiation by heavy particles such as lead is a very violent process that damages materials on the atomic level, it was found that the temperature at which SFAOF became a superconductor was not reduced, meaning this other aspect of how well a superconductor performs was not harmed by improving its current-carrying capability.
The future? The CES, which also has researchers located at Brookhaven National Laboratory and the University of Illinois at Urbana-Champaign, is looking to apply the same heavy ion irradiation techniques to improve useful properties of other high-temperature superconductors, such as the copper-based ones traditionally used to make transmission lines, as well as improving the performance of existing commercially made superconducting wires.
"For these conductors, we think we can get this process down to a few seconds of irradiation, something that could be installed into existing reel-to-reel manufacturing processes," said Karen Kihlstrom, a graduate student who has since joined the Argonne-based group. "That would make this a really commercially viable process for improving superconducting wires."
