Engineers from the University of Southern California have developed a material that contracts when it is heated—a first step toward developing a material that is unresponsive to heat.Qiming Wang, assistant professor of civil and environmental engineering, hopes to develop a zero-expansion material. Image credit: USC.

Most materials, such as those used to make buildings or bridges, expand when they become hot. For this reason, expansion joints are needed to allow the materials to swell without buckling. In cases with mismatched materials, such as cooktops and dental fillings, one material can expand faster than another, causing it to crack.

“We wanted to solve all these thermal mismatch problems,” says Qiming Wang, assistant professor 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.”

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

He created a 3D 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.

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

While this is one way of reaching the researchers' ultimate objective, Wang says a second method 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,” he says. “This was the first step—you design a negative. Then you try to create a composite of these two to achieve zero—that will be the next step of the research.”