Pedal to the Metal

by Mike Perricone

Developments in superconducting materials are accelerating, from the first superconducting plastic to metals that could some day bring down the costs of accelerator magnets for high-energy physics. The pages of the scientific journal Nature have been full of possibilities.

"Right now, there's lots of scientific excitement in superconductivity,"said David Larbalestier, director of the University of Wisconsin's Applied Superconductivity Center.

In the March 8 issue of Nature, New Jersey's Bell Laboratories announced the creation of the first superconducting plastic. Christian Kloc, Zhenen Bao and Ananth Dodabalapur of Bell Labs, along with three European colleagues, achieved superconductivity (at a temperature of about 2.6 kelvins) with a solution containing the plastic, polythiophene.

In the March 1 issue of Nature, a paper by Jun Akimitsu and colleagues described superconductivity at 39K in the simple compound magnesium boride (MgB₂). Again in the March 8 issue, Larbalestier and colleagues from Wisconsin and Princeton described their evaluation of the current-carrying capability of MgB₂. This material, Larbalestier and colleagues wrote, "offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications."

Magnesium diboride seems to act in ways similar to other low-temperature metallic superconductors. For example, its grain boundaries do not exhibit weakened superconductivity that blocks current flow from crystal to crystal, a quality that makes the high-temperature ceramic superconductors susceptible to weak magnetic fields. But magnesium diboride operates at a much higher temperature (39K vs. 18K) than another developmentally promising magnet material, niobium-tin (Nb₃Sn). The higher the temperature, the lower the need for liquid helium cooling, and the lower the cost of operating superconducting accelerator magnets.

Larbalestier concedes there are two immediate obstacles to using magnesium diboride for magnets. First, although the critical temperature (Tc, the temperature at which superconducting occurs) is much higher than that of niobium-titanium (Nb-Ti; 9K) or niobium-tin (Nb₃Sn; 18K), it is too low to replace ceramic high-temperature superconductors (HTS) which have Tc ranging from 90 to 110K. Second, the strengths of the magnetic field at which bulk supercurrents disappear (the irreversibility field H*), and which wipe out superconductivity (Hc2, or "upper critical fieldî), are too low to replace the niobium-based superconductors. But there is hope.

"It should be possible to raise the upper critical field by alloying,"Larbalestier said, "just as is done by adding titanium to niobium to make the niobium-titanium from which the Tevatron is made."

Larbalestier pointed out that without titanium, the upper critical field for niobium is about 0.3 tesla; with titanium added, the value increases to 11 tesla. In a second paper recently submitted to Nature, Larbalestier and other colleagues showed that thin films of magnesium boride demonstrate improved irreversibility fields. And he proposed that magnesium diboride could be highly useful for electronic applications where the magnetic field is not a factor.

If magnesium diboride proves useful for accelerator magnets, at a low cost, that development couldn't come at a better time.

"The state-of-the-art material for accelerator magnets is niobium titanium,"said Peter Limon, head of Fermilab's Technical Division. "But we're reaching the limits of that material, with the Large Hadron Collider quadrupoles we're making at Fermilab and the LHC dipoles being made in Europe. We need new materials to go higher in field strength."

The most promising prospect so far is niobium-tin alloy (Nb₃Sn), which Limon described as a better performer than niobium-titanium, but with the drawbacks of being brittle and expensive. Accelerators don't have the market leverage of biomedical technology, where the magnetic resonance imaging industry drives the market for niobium-titanium superconducting wire. Limon said the peak demands of LHC production would match MRI demands for a year or two, but not longer.

"The problem we have is that to drive a commercial market, you need a continuous demand,"Limon said. "Our demand is a pulse. It may be a huge pulse, lasting a few years, but it's still a pulse."

Larbalestier said it is too early to tell the role plastics might play in magnets and, by extension, the market.

"So far, only metallic superconductors are applicable for superconducting magnets or cavities,"he said. "But all of us in the applied community work off the basic science of the type that the Bell Labs group is doing. Magnesium diboride and polythiophene are both valuable."

Polymer plastics, chemical molecules with long strings of carbon atoms, came late to the electrical game. Conducting organic polymers were discovered in the mid-1970s, leading to the 2000 Nobel Prize in Chemistry for Alan J. Heeger of the University of California at Santa Barbara, Alan G. MacDiarmid of the University of Pennsylvania, and Hideki Shirakawa of Japan's University of Tsukuba.

Plastics are cheap to make, and they're every-where, creating immediate excitement about superconducting possibilities.

"With the method we used, many organic materials may potentially be made superconducting now,"Bao commented.

Stay tuned for more developments, and more papers.