For release: March 5, 2001

Contacts: For release: March 5, 2001
Paul Canfield, Condensed Matter Physics, (515) 294-6270
Doug Finnemore, Condensed Matter Physics, (515) 294-3455
Saren Johnston, Public Affairs, (515) 294-3474

AMES, Iowa – Physicists at the U.S. Department of Energy’s Ames Laboratory have succeeded in making superconducting wire segments from magnesium diboride, a simple compound that can be purchased in powder form from most companies supplying standard laboratory chemicals. Their research, although in its earliest stages, may lead to magnesium diboride becoming the preferred low-temperature superconductor for applications such as high-field magnets used in magnetic resonance imaging machines.

Magnesium diboride has been known to scientists for approximately 50 years, however, no one had ever investigated superconductivity in the material – whether it could conduct electrical current perfectly, without resistance, when cooled to temperatures near absolute zero.

That all changed in January when Jun Akimitsu of Aoyama Gakuin University in Tokyo announced he and his research team had discovered that magnesium diboride becomes superconducting at 39 Kelvin (-389 F), nearly twice the temperature of current intermetallic superconductors. The announcement had experimentalists around the world rushing to duplicate and confirm the Japanese findings.

But a team of Ames Laboratory physicists, including Paul Canfield, Doug Finnemore and Sergey Bud’ko, figured out how to make high-purity powders of magnesium diboride simply, in a two-hour, turn-around process. "We ran several cycles a day," said Canfield. "Having done that, we made isotopic substitutions." By changing the mass of the boron, the researchers saw a 1.0 Kelvin upward shift in transition temperature – the temperature at which a material becomes superconducting. "We wanted to understand the mechanism of superconductivity in the material, and we were very quick in getting the highest purity, sharpest transition samples," said Canfield. (Paper appears in the Feb. 26 issue of Physical Review Letters.)

Next, the experimentalists mapped out the basic properties of the magnesium diboride. "We do this for every material that comes down the pike," said Finnemore. "We wanted to see to what fields it remains superconducting, what types of currents it can carry in the superconducting state, and how much current it can carry through the grain boundaries in the material. So we did conventional measurements on what we thought was an exotic material, only it turned out to be kind of ordinary – the exotic thing is its superconducting temperature."

The data the Ames Lab researchers collected on the properties of the magnesium diboride pellets showed that the material would carry enormous electrical currents, even though the currents had to jump across a dense array of grain boundaries. (Paper to appear in the March 12 issue of Physical Review Letters.) This property is far different from that found in ceramic high-temperature superconductors and one that allows the development of better wires.

That being the case, the Ames Lab team took on and met the challenge of creating magnesium diboride wire, starting with boron fibers. In essence, you might say, they came up with a way of turning straw into gold. They placed the boron fibers in a tantalum tube with excess magnesium and heated it up. "As the magnesium vapor diffused, the boron sucked it up – what we were able to do was turn boron fibers into magnesium diboride wire," said Canfield. (Paper to appear in the March 12 issue of Physical Review Letters.) "We’ve been making lengths of five centimeters (two inches). We call the growth ‘angel hair’ because it looks like angel-hair pasta."

The researchers discovered that the magnesium diboride wire segments have as sharp and as high a superconducting transition temperature as the powders they developed and that the wires are at least 80 percent dense. Development of the wires also allowed the researchers to measure the material’s resistivity – its ability to carry electricity. The measurements revealed that magnesium diboride has a resistivity approximately 20 times better than that of the reigning niobium-based superconductor at its transition temperature.

"We made the wire in five-centimeter segments, but boron monofilaments are made continuously in kilometer lengths. This opens the possibility of developing a continuous process in which boron monofilament is made and then transformed into this wire," said Canfield. But he cautioned, "The magnesium diboride is brittle."

Finnemore added, "You can’t spool it and give someone a mile of the stuff. It will present challenges to coat and protect the wires, similar to those seen for previous intermetallic superconductors containing niobium and tin."

The work of the Ames Lab physicists to understand the physics of magnesium diboride has progressed at an amazing pace. Since the January 10 announcement of the discovery of superconductivity in the material, the Ames team has addressed the mechanism of superconductivity in the material, mapped its properties and developed wire segments along with what appears to be a means of making long wires – all in a month’s time.

Canfield was quick to point out that there is much more work to be done, but he noted that magnesium diboride holds promise for being the next low-temperature superconductor of choice. Because of its higher superconducting transition temperature, wires made from the material would not have to be cooled to as low of temperatures as niobium-based superconductors, reducing cooling costs associated with the quantities of liquid helium required to operate such systems. Also, there is a much larger abundance of magnesium and boron than there is of niobium.

The bottom line, according to Canfield: "With magnesium diboride, we have something that maps onto the existing technology of intermetallic wires, but with a factor of two higher in transition temperature. However, only time will determine the material’s usefulness."

Ames Laboratory is operated for the DOE by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.

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Note to editors:

Find images of the magnesium diboride wires at:  ftp://ftp.external.ameslab.gov/marti/Magdib1.jpg and

Find more information on this research and related studies at:  http://www.iitap.iastate.edu/htcu/39K.html