Old material makes a new debut
For the most part, it was a wallflower for 50 years. Plain and simple, no one paid it much attention. No one suspected there might be more to the nondescript tan powder than met the eye.
Magnesium diboride is a relatively inexpensive metal compound that can be purchased in powder form from most standard chemical supply companies. Until this year, there was nothing remarkable about it — at least nothing that was known. But the material had never been investigated for superconductivity — whether it had the ability to conduct electricity perfectly, without resistance, when cooled to temperatures near absolute zero (minus 459 degrees Fahrenheit).
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 (minus 389 F), nearly twice the temperature of current intermetallic, low-temperature superconductors. The news 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, was the first to describe the mechanism of superconductivity in the material. "One of the things that allowed us to make progress quickly was that we figured out how to make high-purity powders of magnesium diboride simply, in a two-hour, turnaround process," says Canfield. "We ran several cycles a day, and then made isotopic substitutions." (Isotopes of an element have the same number of protons, but differ by how many neutrons are within each atomic nucleus.)
Figuring out the superconductivity
The physicists were very quick in getting the highest purity samples of MgB2 with the highest transition temperature — the temperature at which a material becomes superconducting. Their experiments showed that MgB2 sample pellets containing boron isotopes with an atomic mass of 11 became superconducting at 39.2 K (minus 389 F), while pellets containing boron isotopes with an atomic mass of 10 became superconducting at 40.2 K (minus 387 F). By changing the mass of the boron, the physicists saw a 1.0 K upward shift in transition temperature. "We wanted to understand the mechanism of superconductivity in the material," says Canfield. "And we found that the shift in transition temperature caused by the change in boron mass is consistent with standard models of intermetallic superconductivity."
Conventional intermetallic superconductors conform to the Bardeen, Cooper, Schrieffer theory of superconductivity, commonly called the BCS theory, which explains that the electrons in superconductors are grouped in pairs and that the movements of all the pairs in a single superconductor are interrelated — they make up a system that acts as a single entity.
When an electrical voltage is applied to a superconductor, all the electron pairs move together coherently as a unit. The movement creates a current that will flow indefinitely, even when the voltage is removed, because the electron pairs meet no opposition. This pairing takes place because the electrons are attracted to each other by creating and detecting vibrations in the lattice. These vibrations determine a material’s superconducting transition temperature. The smaller the mass of the superconducting material, the higher the frequency of the vibrations and the higher the transition temperature.
If a superconductor is allowed to warm up past its transition temperature, the electron pairs separate and the superconductor becomes a normal conductor. In all previously studied intermetallic compounds, the electron pairs usually break apart when the temperature climbs above 20 K (minus 424 F). But, amazingly, the electron pairs remain together in MgB2up to 40 K (minus 387 F).
The 1.0 K shift in superconducting transition temperature produced in the boron isotope experiments of Canfield and his colleagues is strong evidence that the electrons in MgB2 are interacting as described in the BCS theory of superconductivity. This would classify MgB2 as a conventional superconductor, but with an extremely high and unconventional transition temperature. And this is, as Canfield says, "a big, hairy deal."
How good is MgB2?
After addressing the mechanism of superconductivity in MgB2, the experimentalists then mapped out the basic properties of the material. "We do this for every material that comes down the pike," says Finnemore. "We wanted to see to what fields MgB2 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."
Data the researchers collected on the properties of the MgB2 pellets showed the intermetallic material would carry enormous electrical currents, even though the currents had to jump across a dense array of grain boundaries. This property is far different from that found in ceramic high-temperature superconductors. Ceramic superconductors operate at higher temperatures than the intermetallics, some over 100 K (minus 280 F), and they require less costly cooling arrangements. But intermetallic compounds are better at carrying current across grain boundaries, making it easier to synthesize superconducting wires that can carry large currents.
Soon after mapping the properties of MgB2, the experimentalists took on and met the challenge of creating wire from the material, starting with boron fibers. In essence, the researchers 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," says Canfield. "We’ve been making lengths of five centimeters (2 inches). We call the growth ‘angel hair’ because it looks like angel-hair pasta."
The researchers found that MgB2 wire has as sharp and as high a superconducting transition temperature as the powders they developed and that the wire is at least 80 percent dense. Development of the wire also allowed the researchers to measure the material’s resistivity — its ability to carry electricity in the nonsuperconducting state. The measurements revealed that MgB2 has a resistivity approximately 20 times lower (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 in kilo-meter lengths. This opens the possibility of developing a continuous process in which boron monofilament is made and then transformed into this wire," says Canfield. But he cautions, "Magnesium diboride is brittle."
Finnemore adds, "You can’t spool it and give someone a mile of the stuff. It will present challenges to coat and protect the wire, similar to those for previous intermetallic superconductors containing niobium and tin."
The work of the Ames Lab physicists to understand the physics of MgB2 progressed at an amazing pace. Following the January 10 announcement of superconductivity in the material, the Ames team 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 is quick to point out that there is much more work to be done, but he notes that MgB2 holds promise for being the next low-temperature superconductor of choice. Because of its higher superconducting transition temperature, wire made from the material would not have to be cooled to temperatures as low as for niobium-based superconductors, reducing cooling costs associated with the quantities of liquid helium required to operate such systems. Also, there is a greater abundance of magnesium and boron than 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 deter-mine the material’s usefulness."
For more information:
Paul Canfield, (515) 294-6270
Research funded by:
DOE Office of Basic Energy Sciences