The U.S. National Science Foundation (NSF) has recently reported that two research teams have developed a new porous foam of an alloy that changes shape when exposed to a magnetic field. The NSF states that this new material is able to remember its original shape after it's been deformed by a physical or magnetic force. This polycrystalline nickel-manganese-gallium alloy is potentially cheaper and lighter than other materials currently used in devices ranging from sonar to precision valves. It also could be used to design biomedical pumps without moving parts and even for space applications and automobiles.
You can see above an example of such a magnetic shape-memory foam. The "new material is full of voids (dark) between thin, curvy 'struts.' A magnetic field causes small single crystals along the struts to change length, which alters the macroscopic dimensions of the whole sample." (Credit: David Dunand and Peter Müllner, via Physical Review Focus)
This 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. Dunand was helped by Yuttanant Boonyongmaneerat (a.k.a. Vee), a member of his research group working on Nickel-Based Foams (scroll down to the section "Magnetic Shape-Memory Ni-Mn-Ga Foams").
Here are some details from the NSF news release. "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."
For more information, this research work has been published in Physical Review Letters under the title "Increasing magnetoplasticity in polycrystalline Ni-Mn-Ga by reducing internal constraints through porosity" (Volume 99, Number 24, Article 247201, December 14, 2007 issue). Here is a link to the the abstract. "Foams with 55% and 76% open porosity were produced from a Ni-Mn-Ga magnetic shape-memory alloy by replication casting. These polycrystalline martensitic foams display a fully reversible magnetic-field-induced strain of up to 0.115% without bias stress, which is about 50 times larger than nonporous, fine-grained Ni-Mn-Ga. This very large improvement is attributed to the bamboolike structure of grains in the foam struts which, due to reduced internal constraints, deform by magnetic-field-induced twinning more easily than equiaxed grains in nonporous Ni-Mn-Ga."
You'll find additional details by reading a Physical Review Focus article, "Stretching More with Pores" (Michael Schirber, December 5, 2007), which features several animations. Here is an excerpt about how these new materials work. "Imagine a two-dimensional crystal lattice made of parallelograms all 'leaning' to the left, but above a certain horizontal line -- called a twin boundary -- the parallelograms all lean to the right. This alternate leaning may repeat, with several layers of 'twins' above the first pair, forming a zigzag pattern. Each parallelogram has a magnetic moment, like a tiny bar magnet, pointing parallel to the tilted sides. A magnetic field aligned with the right-leaning parallelograms switches the left-leaning ones into right-leaning and straightens out the zigzags, which lengthens the crystal along the new direction of alignment.
And here is the conclusion of the article. "Vladimir Chernenko, of the Institute for Energetics and Interfaces in Lecco, Italy, believes this technological achievement--the first foam to exhibit magnetic shape memory -- has great potential. "With further improvement of architecture, the performance can be increased," Chernenko says. The authors will need to verify other mechanical properties of their material, but he thinks its large strain and small weight might make it useful for space-born applications."
Sources: National Science Foundation (NSF) news release, December 18, 2007; and various websites
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