McCormick Magazine

Metallic Foam Shifts Shape


David DunandResearchers have turned a stubborn alloy into a shape-shifting foam by just giving it a little breathing room. David Dunand, the James N. and Margie M. Krebs Professor of Materials Science and Engineering at the McCormick School, has teamed up with Boise State University professor Peter Müllner and McCormick postdoctoral research fellow Vee Boonyongmaneerat to create a foam from a nickel-manganese-gallium (Ni-Mn-Ga) alloy that changes shape when exposed to a magnetic field. The new foam could translate to smaller, lighter pumps and more aerodynamic airplane wings.

When exposed to a magnetic field, a single crystal of the Ni-Mn-Ga alloy will deform approximately 10 percent. The alloy then retains its new shape when the field is turned off but returns to its original shape if the magnetic field is rotated 90 degrees. That's what scientists call "magnetic shape memory."

But single crystals of materials like this are extremely expensive and time consuming to make (just like gems), and most materials are created as "polycrystals" — a collection of randomly oriented crystals. In polycrystals each individual grain moves in a different direction when exposed to the magnetic field, and the overall deformation is cancelled so that the alloy ends up not moving at all.

When Dunand and Müllner met at a conference in late 2006, they decided to combine Müllner's knowledge of magnetic shape memory materials with Dunand's expertise in metallic foam. They hoped a polycrystalline foam of the alloy, which looks like a sponge, would allow more space for individual crystals, letting them move more like a loosely connected collection of single crystals.

Boonyongmaneerat was working on a project using nickel foams for fuel cells, so the researchers decided to use his process to create the Ni-Mn-Ga foam. The researchers took powders of oxide and pushed the liquid metal alloy between the grains of the powders, creating a composite. They then removed the oxide powders from the composite with acid — leaving behind a metallic foam.

That just left the question: Would the foam change shape? To find out, Dunand shipped the foam to Boise State, where Müllner and his student Markus Chmielus tested the foam in a rotating magnetic field. Though it didn't move the 10 percent that a single crystal would, it did move 0.12 percent.

"It was very exciting because we went from zero to an actual value, albeit small, but comparable to the best competitor," Dunand said.

That competitor is Terfenol D, another material that also changes shape magnetically under a different mechanism. It has been around for a long time, is expensive to make, and is already maxed out at 0.12 percent deformation.

Dunand thinks that once the Ni-Mn-Ga foam is optimized — through casting it differently or perhaps heat-treating it differently — it will provide better results. He believes that such a foam could be a lighter, cheaper replacement for Terfenol D applications such as sonar, actuators, and magnetomechanical sensors. Dunand and Müllner also believe the foam could replace applications that require small, rapid movement, like very small actuators consisting of a foamed rod and a coil, replacing bulkier classical linear motors with rotors, stators, gears, and shafts, possibly for biomedical applications or for opening and closing valves in energy-efficient car engines.

The material could have even bigger applications, however. Dunand said it's possible that the foam could eventually be used to control slight changes in the shape of airplane wings to make them more aerodynamic based on the speed of the aircraft. "The goal is to be able to change, ever so slightly, the aerodynamics of the flow of air to make the flight more efficient at all speeds," he said.

Dunand and Müllner coauthored a paper on the research that was published in the December 14, 2007, edition of Physical Review Letters. For now, researchers will continue to optimize the strains of the foams by examining both processing and foam architecture. In the meantime, Boise State and Northwestern have jointly filed an application for a patent.

The research of both Dunand and Müllner was sponsored by the National Science Foundation.

—Emily Ayshford