Magnesium diboride could replace niobium-titanium, a conventional low-temperature superconductor, in future MRI magnets and could also find use in transformers and fault current limiters. 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 are expensive because the manufacturing process requires a great deal of silver.
Magnesium diboride falls between the two types on the temperature scale; it is a conventional (low-temperature metallic) superconductor. It can be conveniently cooled with commercial cryocoolers or liquid hydrogen. 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 materials that go into making magnesium diboride, magnesium and boron, are both dirt-cheap. So cheap in fact that magnesium diboride cable may eventually be comparable in price with copper cable. But to be useful, scientists must be able to form the material, a powder in its original form, into wires. As early as May of 2001, scientists at Agere Systems (Allentown, Pa.), a spinoff of Lucent Technologies, had made magnesium diboride tape in lengths of almost a meter. Using a more manufacturing-friendly process, Hypertech Research has already made 100-meter-long wires. In the United Kingdom, Diboride Conductors is also working to commercialize the technology.
Magnesium diboride needs to be chilled to 20-30 K for it to be useful. While this is colder than liquid nitrogen, it is within the range of a standard commercial cryocooler, and the cost is not that high.
Magnesium diboride does have some shortcomings. The material's inability to carry much current was one that surfaced early. Reports indicated that the wire could carry only 35 000 A/cm2. (Real-life superconductor applications require a larger value, at least 80 000 A/cm2.) However, that figure has crept upward, and is currently at around 200 000 A/cm2 for a magnetic field of 1 tesla, typical of transformers and motors.
The material also does not stand up very well to strong magnetic fields. Early data showed that its superconductivity vanished in fields greater than 2 T, which produces magnetic vortices inside the alloy. These vortices move under the Lorenz force created by the current, and their motion dissipates energy, which shows up as electrical resistance. The solution turns out to lie in making the material less structurally perfect. Several approaches, which deliberately introduce structural defects and impurities, have improved magnesium diboride's magnetic properties.
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