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Handling Heat with Shrinking Metamaterials


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this sudden heating may affect their performance,” Fang says. “So you real- ly have to take great care in account- ing for this thermal stress or shock.” The researchers have published


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their results in the journal Physical Review Letters. Fang’s co-authors in- clude former MIT postdoc Qi Ge, along with lead author Qiming Wang of the University of Southern Califor- nia, Jonathan Hopkins of the Univer- sity of California at Los Angeles, and Julie Jackson and Christopher Spadaccini of Lawrence Livermore National Laboratory (LLNL).


Printing Ingredients In the mid-1990s, scientists pro-


posed theoretical structures whose arrangement should exhibit a prop- erty called “negative thermal expan- sion,” or NTE. The key to the arrangement was to build three-di- mensional, lattice-like structures from two types of materials, each with a different NTE coefficient, or rate of expansion upon heating. When the whole structure is


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heated, one material should expand faster and pull the other material in- ward, shrinking the entire structure as a result. “These theoretical papers were


talking about how these types of structures could really break the con- ventional limit of thermal expan- sion,” Fang says. “But at the time, they were limited by how things were made. That’s where we saw this as a good opportunity for micro-fabrica- tion to demonstrate this concept.” Fang’s lab has pioneered a 3D


printing technique called micro- stereolithography, in which the re- searchers use light from a projector to print very small structures in liq- uid resin, layer by layer. “We can take the same idea as


an inkjet printer, and print and so- lidify different ingredients, all on the same template,” Fang says. Taking inspiration from the


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general framework proposed previ- ously by theorists, Fang and his col- leagues printed small, three-dimen- sional, star-shaped structures made from interconnecting beams. They fabricated each beam from one of two ingredients: a stiff, slow-to-expand copper-containing material, and a more elastic, fast-expanding polymer substance. The internal beams were made from the elastic material, while the outer trusses were composed of stiff copper. “If we have proper placement of


these beams and lattices, then even if every individual component ex- pands, because of the way they pull each other, the overall lattice could actually shrink,” Fang says. “The problem we’re treating is a


thermal mismatch problem,” says Wang. “These materials have differ- ent thermal expansion coefficients, so once we increase the temperature, they interact with each other and pull inward, so the overall struc- ture’s volume decreases.”


Room to Experiment The researchers put their com-


posite structures to the test by plac- ing them within a small glass cham- ber and slowly increasing the cham- ber’s temperature, from room tem-


This 3D printed structure is


designed to shrink when exposed to heat, based on the stretching and pulling of its internal beams and trusses. Image: Qiming Wang.


“It shrinks by about one part in


a thousand, or about 0.6 percent,” Fang says. While that may not seem significant, Fang adds that “the very fact that it shrinks is impressive.” For most applications, Fang says de- signers may simply prefer structures that do not expand when heated. In addition to their experi-


ments, the researchers developed a computational model to characterize the relationships between the inter- connecting beams, the spaces be- tween the beams, and the direction and degree to which they expand with heat. The researchers can con- trol how much a structure will shrink by tuning two main “knobs” in the model: the dimensions of the individ- ual beams, and their relative stiff- ness, which is directly related to a material’s rate of heat expansion. “We now have a tuning method


for digitally placing individual com- ponents of different stiffness and thermal expansion within a struc- ture, and we can force a particular beam or section to deflect or extend in a desired fashion,” Fang says. “There is room to experiment with other materials, such as carbon nan- otubes, which are stronger and lighter. Now we can have more fun in the lab exploring these different structures.” This research was sup- ported, in part, by the Defense Ad- vanced Research Projects Agency. Web: www.news.mit.edu r


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perature to about 540°F (282°C). They observed that as structure was heated, it first maintained its initial shape, then gradually bent inward, shrinking in size.


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