A USC engineering research team has developed a material that contracts when heated, completing its first step toward developing a material that is unresponsive to heat.

Typical materials, like those used to make buildings or bridges, expand when they get hot. For this reason, expansion joints are needed to allow the materials to swell without buckling. In cases with mismatched materials, like some cooktops or dental fillings, one material will expand faster than another, causing it to crack.

“We wanted to solve all these thermal mismatch problems,” said Qiming Wang, assistant professor in the Sonny Astani Department of Civil and Environmental Engineering, who developed the contracting material with his research team. “Imagine if you can design some material that has zero expansion, no expansion at all.”

The team’s work, in collaboration with Christopher Spadaccini of the Lawrence Livermore National Laboratory and Associate Professor Nicholas Fang from the Massachusetts Institute of Technology, was sponsored by the National Science Foundation Manufacturing Machines and Equipment program and the DARPA Materials with Controlled Microstructural Architectures program. Physical Review Letters published an article detailing the team’s findings Oct. 21.

3-D printing

To create a composite material that contracts when heated, Wang designed a manufacturing technique that enables the user to 3-D print a structure consisting of more than one special material. In the process, thin layers of liquid are solidified by UV light one layer at a time, switching between the different materials. This creates a 3-D structure of any design with as many materials as needed.

For Wang’s contracting material, he designed a 3-D lattice structure, consisting of beams oriented at certain angles, to take advantage of the materials’ typical expansion behavior. As the two materials expand at different rates, the beams are pulled inward, making the structure as a whole contract.

“Overall, the structure will contract in volume, rather than expand in volume,” Wang said. “That’s the basic mechanism.”

Fine-tuning

The degree of contraction can be fine-tuned by altering the composition of the structure or altering the angles of the beams. In this way, the material can be manipulated to achieve the desired performance, and even zero thermal expansion.

Wang believes that this is just one way of obtaining their final goal. The other is by combining their novel material with another.

“We can design a zero expansion material by creating a composite of a positive expansion material with a negative inside it. Then you can achieve zero,” Wang said. “This was the first step. You first design a negative, and then you try to create a composite of these two to achieve zero. That will be the next step of the research.”

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