Cheap to make, simple to cool, easy to shape into wires, magnesium diboride could throw the field of superconducting applications wide open. "It could replace niobium-titanium, a conventional low-temperature superconductor, in future MRI magnets," says visionary Paul Grant, science fellow at the Electric Power Research Institute (Palo Alto, Calif.). "It has to be given serious attention."
Besides magnetic resonance imaging (MRI), practical uses of the zero-resistance material in transformers and fault current limiters seem not too much to hope for. Much further off, perhaps, could be networks of superconducting power lines cooled by liquid hydrogen, as proposed by Grant.
Until the magnesium diboride revelation, engineers trying to apply superconductivity to the real world had to grapple with distinctly un-ideal materials. Low-temperature (metallic) superconductors, while not too costly and with excellent mechanical properties, require cooling down nearly to zero—4.2 K—a pricey proposition. High-temperature (ceramic) superconductors can be cooled at far less expense to a less chilly 77 K, but their manufacturing process requires a great deal of silver, a costly material.
Magnesium diboride falls between the two types on the temperature scale. With a superconducting transition temperature of 39 K, it can be conveniently cooled with commercial cryocoolers or liquid hydrogen (boiling point: 20.2 K).
A powder that can be found in any well-stocked chemistry laboratory, it had never been tested for superconductivity until very recently. The five Japanese researchers who did so announced their discovery in January 2001 at a small conference in the Japanese city of Sendai. The news spread among superconductor cognoscenti like wild fire. Lights burned late into the night in laboratories worldwide as scientists worked overtime studying the inexpensive metallic compound's properties with a view to harnessing its surprising potential.
Within two weeks, Paul C. Canfield of Iowa State University (Ames) and his physics/astronomy department collaborators had made 3-cm-long wires out of the compound by passing magnesium vapors over boron fibers. "This material is just full of good news," he said.
Two months after the initial announcement, the American Physical Society held its annual meeting in Seattle, Wash., and arranged a special evening session of magnesium diboride papers. The large hotel banquet hall chosen for the event was jammed with physicists; the session, which began after dinner, included dozens of oral presentations and went on until 1 a.m.
The attendees were fascinated by the wealth of data indicating the compound was a conventional (low-temperature metallic) superconductor. This augured well for the future. Such a material can be explained by the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, which says that the interactions between the electrons in a superconductor are governed by the thermal vibrations of atoms in the crystal structure of the material. So the behavior of magnesium diboride would be easy to understand. (The physical mechanism responsible for superconductors of the so-called high-temperature type is not yet clearly understood.)
"It's rare that we get so excited," said Robert J. Cava of Princeton University, an expert on superconductivity and an attendee at the conference.
But initial data also showed researchers that they had some work to do. In order for the material to be truly useful, its current-carrying capability and magnetic field tolerance would have to be improved and a commercially viable and economical method of manufacturing wires would have to be developed. Today, little more than a year later, the news is very encouraging.