A group of scientists has discovered how to make better wires using a promising material known as Bi-2212. With this discovery comes the possibility of creating magnetic fields in excess of 30 Tesla, three to four times higher than those generated by present accelerator magnet technology.
Bi-2212 stands out as the only HTS (high temperaure superconductor) that can be fabricated as a round wire. This makes Bi-2212 a perfect candidate for winding cables and coils without significantly changing present magnet technology.
Researchers have developed a technique that prevents bubble formation almost entirely by performing the melting and re-solidification of materials under high external gas pressure. The group observed five times higher current in a long wire sample made by the new method compared to an identical sample made by the standard recipe.
Importantly, the electrical currents in the improved Bi-2212 wires were two times larger than the usual benchmarks used for the development of superconducting magnet coils.
This breakthrough signifies the birth of a new high-performance, very-high-field magnet conductor. Shen and his collaborators are optimistic that they now have the recipe for a new class of superconducting magnets that could help unlock information about chemical and biological processes, create new materials and reveal the fundamental processes of the universe.
High engineering current density JE of over 500 A/mm2 at 20 T and 4.2 K can be regularly achieved in the Ag sheathed multifilamentary Bi2Sr2CaCu2Ox (Bi-2212) round wire when the sample length is several
centimeters. However, JE(20 T) in Bi-2212 wires of several meters length, as well as longer pieces wound
in coils, rarely exceeds 200 A/mm2. Moreover, long-length Bi-2212 wires often exhibit signs of leakage
after melt processing that are rarely found in short, open-ended samples. Expanding on work by
Malagoli et al., combined with detecting gas releases during heat treating the state-of-the-art
powder-in-tube (PIT) Bi-2212 wire using mass spectroscopy and modeling gas transport in Bi-2212 wire,
we explicitly show that JE degradation with length is a direct consequence of Bi-2212 de-densification
due to wire swelling produced by high internal gas pressures at elevated temperatures. We examined
the wire expansion at critical stages of the melt processing of as-drawn PIT wires and the wires that
received a degassing treatment or a cold-densification treatment before melt processing, further
showing that wire swells because under the influence of internal gas pressure, silver creeps and
ruptures, and that the creep rupture of silver sheath naturally leads to the leakage of Bi-2212 liquid.
Identifying the mechanisms of JE degradation and leakage led us to conclude that it is feasible to develop a long-length, leak-free Bi-2212 conductor that carries a JE of over 500 A/mm2 for very high field superconducting magnet technology through a combination of reducing or removing the source of
internal gases with techniques such as applying an external gas pressure during heat treatment to inhibit de-densification of Bi-2212 or to even induce compressive Ag creep to further densify Bi-2212.