By KATHLEEN LAUFENBERG
TALLAHASSEE, Fla. — MagLab researchers have invented a groundbreaking new way to process Bi-2212 — one that makes it far more useful for building high-powered magnets including very high-field NMR magnets, a Muon Accelerator at Fermilab or a new upgrade for the powerful Large Hadron Collider at CERN.
A breakthrough discovery made by MagLab scientists and a researcher at CERN, the particle accelerator laboratory in Switzerland, will be published in the April issue of Nature Materials. An image of the superconductor that’s causing all the excitement — a newly processed form of Bi-2212, or “bisco” — will also be featured on the journal’s cover.
Bi-2212 is a complex high-temperature superconducting material made of bismuth, strontium, calcium, copper, and oxygen that is well known to superconduct (or transmit electricity without loss) at super-cold temperatures up to 90 degrees Kelvin (or negative 183 degrees Celsius).
Since most superconductors are used to make magnets, what matters even more than the temperature at which they become superconducting, is the density of supercurrent (supercurrent flows without resistance and thus generates no heat or electrical loss) that can flow though wires made of the material.
Magnet engineers were previously using a form of bisco constructed in a superconducting ribbon processed in a very complex way to minimize the grain boundary density and raise the supercurrent density.
Now, by employing the MagLab’s new, pioneering process, they can make Bi-2212 into round wires. Put another way, engineers were previously limited to wide “fettuccini” ribbons to build magnets, but now can choose skinny “spaghetti” wires. Magnet builders much prefer “spaghetti” to “fettuccine” because high-current cables and complex winding shapes are much more feasible with round than with flat wires.
"This is the first time that any high-temperature superconductor has been made in the form that is the most useful for creating high-field magnets — a form that is round, multifilament, twisted and capable of being made in multiple architectures and sizes — without giving up the high-current density that is needed for making powerful magnets,” said David Larbalestier, the director of the Applied Superconductivity Center and the lead investigator on the journal article. “For the very long lengths that are needed for magnet coils — hundreds of meters to kilometers in length — we have figured out a way to increase the critical current density by almost a factor of 10.”
That means the newly processed MagLab bisco “spaghetti” can also carry far more electricity than its “fettuccini” predecessor.
“We’re talking current density of well over 500 amps per square millimeter,” said Larbalestier of the increase. By contrast, copper wiring operates at about 1 amp per square millimeter
What makes the breakthrough even more valuable is that this technology already has industry-wide appeal. Oxford Superconducting Technology, for example, has a number of interested customers and the researchers involved are providing processing details or process support so that the results can be replicated.
The breakthrough in processing came through very careful study of the complex microstructure of the wires and correlation to the supercurrent density. The multiple investigators on the article — “Isotropic round-wire multifilament cuprate superconductor for generation of magnetic fields above 30 T” — included processing experts, microstructural experts and superconducting property measurement experts ( J. Jiang; U. P. Trociewitz; F. Kametani; M. Dalban-Canassy; M. Matras; P. Chen; N.C. Craig; P. J. Lee; E. E. Hellstrom at the MagLab and CERN scientist C. Scheuerlein).
The key breakthrough was to discover that current was not primarily being blocked by grain boundaries, but rather by internally generated gas, which was blowing the filaments apart. The way to remove residual porosity and to make the filaments fully dense is to react the wire into the superconducting state under 50-100 atmosphere pressure, greatly increasing the connectivity and supercurrent density. A small insert coil of Bi-2212 wire treated in this way generated 3 T in the 31 T Bitter coil at the MagLab.
“We want to see this process used,” Larbalestier said. “We want to build lots of magnets out of Bi-2212, get the wire cost down, useable lengths way up and make Bi-2212 the precursor of new generations of round, twisted, multifilament, high-temperature superconductor wires that will be revolutionize superconducting applications.”
The conductor research underpinning the breakthrough was supported by a grant from the U.S. Department of Energy’s Office of High Energy Physics in the framework of a multilab collaboration (Very High Field Superconducting Magnet Collaboration) in which groups at Fermilab, Brookhaven and Lawrence Berkeley labs played major roles. The high-field magnet work at the MagLab was financed by the National Science Foundation and the State of Florida.
The National High Magnetic Field Laboratory is the world’s largest and highest-powered magnet facility. Located at Florida State University, the University of Florida and Los Alamos National Laboratory, the interdisciplinary National MagLab hosts scientists from around the world to perform basic research in high magnetic fields, advancing our understanding of materials, energy and life. The lab is funded by the National Science Foundation (DMR-1157490) and the state of Florida. For more information, visit us online at nationalmaglab.org or follow us on Facebook, Twitter, Instagram and Pinterest at NationalMagLab.