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January 19, 2011

New Magnets Could Solve Our Rare-Earth Problems

MIT Technology Review - Researchers are working on composites that would make strong magnets that need less of the hard-to-get ingredients.

Stronger, lighter magnets could enter the market in the next few years, making more efficient car engines and wind turbines possible. Researchers need the new materials because today's best magnets use rare-earth metals, whose supply is becoming unreliable even as demand grows.

So researchers are now working on new types of nanostructured magnets that would use smaller amounts of rare-earth metals than standard magnets. Many hurdles remain, but GE Global Research hopes to demonstrate new magnet materials within the next two years.



The strongest magnets rely on an alloy of the rare-earth metal neodymium that also includes iron and boron. Magnet makers sometimes add other rare-earth metals, including dysprosium and terbium, to these magnets to improve their properties. Supplies of all three of these rare earths are at risk because of increasing demand and the possibility that China, which produces most of them, will restrict exports.

Nanocomposite magnet materials are made up of nanoparticles of the metals that are found in today's magnetic alloys. These composites have, for example, neodymium-based nanoparticles mixed with iron-based nanoparticles. These nanostructured regions in the magnet interact in a way that leads to greater magnetic properties than those found in conventional magnetic alloys.

The advantage of nanocomposites for magnets is twofold: nanocomposites promise to be stronger than other magnets of similar weight, and they should use less rare-earth metals. What enables better magnetic properties in these nanocomposites is a property called exchange coupling. The physics are complex, but coupling between different nanoparticles in the composite leads to overall magnetic properties that are greater than the sum of the parts.

Exchange coupling can't happen in pure magnet materials, but emerges in composites made of mixtures of nanoparticles of the same metals that are used to make conventional magnets. "The advantage of stronger magnets is that the machines you put them in can be smaller and lighter," says Johnson.

GE would not disclose which materials it's using to make the magnets, or what its manufacturing methods would be, but Johnson says the company will rely on techniques it has developed to work with other metals. The main problem the company faces, says Johnson, is scaling up production to make large magnets—so far it's only been possible to make thin films of the nanocomposites

The group is experimenting with a wide range of different types of nanoparticles, including combinations of neodymium-based nanoparticles with iron-cobalt nanoparticles.


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