New metamaterial shrinks when the heat is on

A bi-material microlattice structure, 3D printed from polymer and a polymer/copper composite material, can flex inward, causing the structure to contract when exposed to heat over a range of tens to hundreds of degree(Credit: LLNL)

It's one of the basic facts of science: Heat something and it expands. But a team of US scientists has gone counterintuitive and invented a 3D-printed material that shrinks when heated. Developed as part of DARPA's program to study materials with controlled microstructure architecture, the lightweight metamaterial exhibits what the researchers call "negative thermal expansion."

Metamaterials are one of those things that come out of the lab with an air of enchantment about them. Basically, they're made up of composite materials, like metals, plastics, or ceramics, engineered into repeating, microscopic structures. Depending on how these structures are designed, they can give the metamaterial properties that aren't found in nature and may not even be derived from the source materials themselves.

The study by a team from the Lawrence Livermore National Laboratory's (LLNL) Additive Manufacturing Initiative in partnership with the University of Southern California, MIT, and the University of California, Los Angeles, used a 3D printing process called projection microstereolithograpy to form a polymer and a polymer/copper composite into a highly complex 3D bi-material microlattice structure. To put it more simply, they printed a material made of two substances to form a pattern by printing out the polymer in a layer, cleaning the surface to avoid contamination, then printing the polymer/copper composite, then repeating.

The end result was a sheet made of microscopic beams and voids that hook together to form cells. The geometry of each cell is such that when heat is applied, one material expands more than the other, causing the cell to flex inward and the entire material shrinks in all three dimensions. According to LLNL, the design can be tweaked to operate over a range of tens to hundreds of degrees, and the geometry and topology can also be engineered to not shrink at all or even expand.

The team sees the negative thermal expansion material has having a range of applications. These include passively securing parts in microchips and high precision optical mounts without the need for active heating or cooling to keep them from coming loose, making dental fillings more secure when the patient is eating hot foods, and as a way to replace thermal expansion spaces in bridges or buildings with a solid padding.

"The problem we're treating is a thermal mismatch problem," says USC professor Qiming Wang. "These materials have different thermal expansion coefficients, so once we increase the temperature, they interact with each other and pull inward, so the overall structure's volume decreases. The next step is to fabricate zero thermal expansion materials that could also solve these problems."

The research is published in Physical Review Letters.

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