Smart Foam

A spongelike shape-memory alloy could find use in communications, robotics, and aerospace.

Memory foam: A foam of nickel-manganese-gallium has the alloy’s shape-memory properties but is lighter and cheaper to make than other forms of the material. Credit: P. Mullner, M. Chmielus, and S. Donovan, Boise State University, and D. C. Dunand and Y. Boonyongmaneerat, Northwestern University

Researchers have made a lighter and potentially cheaper kind of shape-memory alloy: materials that change shape in response to a magnetic field but remember their original shape. The new material, a porous foam made from a nickel-manganese-gallium alloy, stretches slightly when exposed to a magnetic field. It retains its new form when the field is turned off, but it goes back to its original shape when the field is rotated 90 degrees.

Most shape-memory alloys are driven by temperature changes. Magnetically driven alloys, however, respond faster than those that respond to temperature. Another important advantage of materials that change shape under a magnetic field is that they can be activated from a distance, says Robert O'Handley, a materials-science and engineering researcher at MIT. Because magnetic shape-memory materials can be remotely changed, he says that they are particularly promising for biomedical applications. "You could make a stent, where you apply a magnetic field to it from outside the body and gradually open up an artery," he says.

But magnetic shape-memory alloys have been difficult and expensive to make. The new alloy could be cheaper and easier to synthesize.

And it could be useful in devices that need very precise, repeatable, and rapid positioning, says David Dunand, a materials-science and engineering professor at Northwestern University, who led the work on the new alloy. These devices include microscopes, tiny mirrors used in optical communication, and robots used in medicine. Because the foam is light, it could lead to aerospace applications, such as airplane wings that morph to become more aerodynamic.

The alloy that Dunand and his colleagues used is not new. Single crystals of nickel-manganese-gallium are known to stretch by 10 percent when exposed to a magnetic field. But single crystals, in which all the atoms are packed in a regular, repeating pattern, are expensive and time consuming to make.

Normally, the problem is that in polycrystalline metals, the individual crystals have random orientations. In the presence of a magnetic field, they stretch along different directions, pushing against each other and canceling out each other's motion, Dunand says. "The dream is to make a polycrystal but somehow give space to [the individual crystals] so they can move and not cancel each other's motions." This is precisely what happens in the foam because of the pores. The tiny crystals in the alloy get room to stretch, and the foam changes shape. The change is tiny right now--only 0.12 percent--but it's a start, Dunand says.

Making the foam is cheap and easy. The researchers pour molten alloy into a porous piece of sodium aluminate salt. After the alloy cools, the researchers dissolve the salt using acid, leaving behind a spongelike structure of the alloy. "The foam is a quite promising preparation route--significantly more efficient compared to the growth of single crystals," says Sebastian Fahler, who studies shape-memory alloys at the Leibniz Institute for Solid State and Materials Research, in Dresden, Germany. But the shape change will have to be much higher than 0.12 percent to have practical applications, he says.

Dunand and his colleagues have a plan for increasing the foam's shape change. Just like a sponge, the foam has struts connected at nodes, he explains. Each strut right now contains multiple tiny crystals. These crystals are still canceling out each other's motion to some extent, which is why the overall change in the foam is only 0.12 percent.

To get a larger shape change, Dunand says, the trick will be to make each strut behave like a single crystal, so that the foam on the whole will be more like a single crystal. That means the researchers would have to make individual crystals span each of the struts in the foam.

The material will still face competition. Nickel-titanium shape-memory alloys, which are suitable for use inside the body and are driven by temperature, are already employed to make stents.

For micropositioning applications, says O'Handley, the material will have to compete with piezoelectric materials such as quartz and lead titanate, which deform in response to electric current. But because the process to make the foam is easy and cheap, he says that it brings nickel-manganese-gallium closer to being cost competitive with piezoelectric materials.

http://www.technologyreview.com/Nanotech/20016/