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.
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.”
Magnet coil made with Bi-2212 wire using the new process.
“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 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.”
Magnets are the principal market for superconductors, but making attractive conductors out of the high-temperature cuprate superconductors (HTSs) has proved difficult because of the presence of high-angle grain boundaries that are generally believed to lower the critical current density, Jc. To minimize such grain boundary obstacles, HTS conductors such as REBa2Cu3O7−x and (Bi, Pb)2Sr2Ca2Cu3O10−x are both made as tapes with a high aspect ratio and a large superconducting anisotropy. Here we report that Bi2Sr2CaCu2O8−x (Bi-2212) can be made in the much more desirable isotropic, round-wire, multifilament form that can be wound or cabled into arbitrary geometries and will be especially valuable for high-field NMR magnets beyond the present 1 GHz proton resonance limit of Nb3Sn technology. An appealing attribute of this Bi-2212 conductor is that, being without macroscopic texture, it contains many high-angle grain boundaries but nevertheless attains a very high Jc of 2,500 A mm−2 at 20 T and 4.2 K. The large potential of the conductor has been demonstrated by building a small coil that generated almost 2.6 Tesla in a 31 Tesla background field. This demonstration that grain boundary limits to high Jc can be practically overcome underlines the value of a renewed focus on grain boundary properties in non-ideal geometries.
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