In the world of commercial materials, lighter and cheaper is
usually better, especially when those attributes are coupled with
superior strength and special properties, such as a material's ability
to remember its original shape after it's been deformed by a physical
or magnetic force.
The porous nature of nickel-manganese-gallium alloy gives it "shape-memory" properties. The material lengthens, or strains, up to 10 percent when subjected to a magnetic field. The NSF-funded researchers believe the porous alloy has great potential for uses that require light weight and a large strain, such as space and automotive applications and tiny motion control devices or biomedical pumps with no moving parts.
A new class of materials known as "magnetic shape-memory
foams" has been developed by two research teams headed by Peter
Müllner at Boise State University and David Dunand at
Northwestern University, both funded by the National
Science Foundation (NSF).
The foam consists of a nickel-manganese-gallium alloy whose
structure resembles a piece of Swiss cheese with small voids of space
between thin, curvy "struts" of material. The struts have a bamboo-like
grain structure that can lengthen, or strain, up to 10 percent when a
magnetic field is applied. Strain is the degree to which a material
deforms under load. In this instance, the force came from a magnetic
field rather a physical load. Force from magnetic fields can be exerted
over long range, making them advantageous for many applications. The
alloy material retains its new shape when the field is turned off, but
the magnetically sensitive atomic structure returns to its original
structure if the field is rotated 90 degrees--a phenomenon called
"magnetic shape-memory."
Making large single crystals of the alloy material is too slow
and expensive to be commercially viable -- one of the reasons why gems
are so costly -- so the researchers make polycrystalline alloys, which
contain many small crystals or grains. Traditional polycrystalline
materials are not porous and exhibit near zero strains due to
mechanical constraints at the boundaries between each grain. In
contrast, a single crystal exhibits a large strain as there are no
internal boundaries. By introducing voids into the polycrystalline
alloy, the researchers have made a porous material that has less
internal mechanical constraint and exhibits a reasonably large degree
of strain.
The researchers created the new material by pouring molten
alloy into a piece of porous sodium aluminate salt. Once the material
cooled, they leached out the salt with acid, leaving behind large
voids. The researchers then exposed the porous alloy to a rotating
magnetic field. The level of strain achieved after each of the over 10
million rotations is consistent with the best currently used magnetic
actuators, and Müllner and Dunand expect to significantly
improve the strain when they have further optimized the foam's
architecture.
"The base alloy material was previously known, but it wasn't
very effective for shape-memory applications," Dunand said. "The porous
nature of the material amplifies the shape-change effect, making it a
good candidate for tiny motion control devices or biomedical pumps
without moving parts." NSF Program Director Harsh Deep Chopra agrees.
"It's the first foam to exhibit magnetic shape memory - it has great
potential for uses that require a large strain and light weight such as
space applications and automobiles. These materials are able to do more
with less material given their foamy structure and provide a
sustainable approach to materials development."
The work was funded by NSF through grant DMR-0502551 to expand
basic knowledge about the microstructural properties of shape memory
alloys influenced by magnetic fields and through grant DMR-0505772 to
develop new shape-memory foams.