|
Surprising scientific research results suggest new shape memory applications
By Virginia Heffernan
Nickel Magazine, March 2008 -- Researchers in the U.S. claim they have
come up with a less expensive, faster and more energy-efficient way to make shape memory alloys by carving
out extra space between individual crystals.
The breakthrough, shared by foam experts at Northwestern University and shape memory experts at Boise
State University, converts a nickel-manganese-gallium alloy into a foam that changes shape when exposed to a
magnetic field, then pops back into its original form when the field is reversed.
The foam could be used to replace a complicated machine with a much simpler design using fewer parts,
improve the efficacy of tiny motion control devices, or better control the emissions from combustion engines
by speeding up the motion of the valves.
“European car manufacturers are looking into developing valves with a mechanism based on magnetic shape
memory alloys,” says Professor Peter Müllner of Boise State University. “In this application, the speed of
action is critical.”
One of the key advantages of magnetic shape memory alloys over those driven by temperature is faster
response time. Another is their ability to be activated from a distance, making them potentially useful for
biomedical applications: opening up an artery with a stent, for instance.
But so far, these materials have been functional only as single crystals, which are expensive and tricky
to grow.
So the Northwestern-Boise team set out to create a material that approximated the excellent deformation
properties of a nickel-manganese-gallium crystal, without the expense, time and energy consumption required
to produce crystals individually.
To achieve this goal, Professor David Dunand and Dr. Yuttanant Boonyongmaneerat at Northwestern’s
Materials Science and Engineering Department poured molten nickel-manganese-gallium into a porous compact of
sodium aluminate powders. Nickel constituted more than half the molten material. After the metal solidified,
they leached out the oxide in acid, leaving behind large voids in the alloy.
The resulting metal foam looks like a piece of sponge toffee, allowing space for the individual crystals
to move. In a typical polycrystalline metal, the crystals would stretch along different directions in the
presence of a magnetic field, cancelling out each other’s motion.
When Müllner and graduate student Markus Chmielus exposed the foam to a magnetic field, they found it
deformed 0.12% -- not nearly as much as a single crystal would but still cause for celebration since this
range of deformation is sixty times greater than what had been observed in a polycrystal before.
"The results will trigger new research directions with industrial relevance,” says Müllner.
The main competitor for the new metal foam is Terfenol D, another ferromagnetic alloy that was developed
for military sonar devices. It converts magnetic field to mechanical power but has a maximum deformation of
about 0.12%. If Dunand and Müllner could better this by tinkering with their new foam, they might provide a
lighter, less expansive and more effective alternative in applications such as actuators and
magneto-mechanical sensors.
Virginia Heffernan is a Toronto-based freelance science writer.
Photos: Boise State University
Dr. Peter Müllner
Director, Boise State Center for Materials Characterization
Associate Professor of Materials Science and Engineering
Boise State University
1910 University Drive, MS 2075
Boise, ID 83725
U.S.A.
Phone: 1-208-426-5136
Fax: 1- 208-426-2470
E-mail: petermullner@boisestate.edu
|
|
