These New Shape-Shifting Materials Get Super Cool, Super Fast
After losing his sight to smallpox in 1759 at the age of 2, John Gough developed a heightened sense of touch. The budding naturalist soon learned to identify plants by feel, touching their hairs with his lower lip and their stamens and pistils with his tongue. So when as an adult he quickly stretched a piece of natural rubber and felt its sudden warmth on his lip—and its subsequent coolness as it relaxed—he gained what he considered the most direct and convincing proof of a curious phenomenon.
He described his observations in 1802, providing the first record, in English at least, of what’s now known as the elastocaloric effect. It’s part of a broader category of caloric effects, in which some external trigger—a force, pressure, a magnetic or electric field—induces a change in a material’s temperature.
But caloric effects have become more than a curiosity.
Over the past couple of decades, researchers have identified increasingly mighty caloric materials. The ultimate goal is to build environmentally friendly refrigerators and air conditioners—caloric cooling devices won’t leak harmful refrigerants, which can be thousands of times more potent than carbon dioxide as a greenhouse gas. But better cooling devices require better materials.
The more a material can change its temperature, the more efficient it can be. And in the last year, researchers have identified two unique types of materials that can change by an unprecedented amount. One responds to an applied force, the other to pressure. They are both capable of temperature changes—“delta T” for short—of a dramatic 30 degrees Celsius or more.
“Who would’ve thought you would get a material to give you a delta T of 30 by itself?” said Ichiro Takeuchi, a materials scientist at the University of Maryland, College Park, who wasn’t part of the new research. “That’s enormous.”
Hot Flash
Gough didn’t know it, but when he stretched his piece of rubber more than two centuries ago, he lined up the long molecules inside. The alignment reduced the disorder in the system—disorder measured by a quantity called entropy.
According to the second law of thermodynamics, the total entropy of a closed system must increase, or at least remain constant. If the entropy of the rubber’s molecular configuration decreases, then the entropy must increase elsewhere.
In a piece of rubber like Gough’s, the increase in entropy happens in the vibrational motion of the molecules. The molecules shake, and this boost in molecular movement manifests itself as heat—a seemingly hidden heat called latent heat. If the rubber is stretched quickly enough, the latent heat stays in the material and its temperature goes up.
Many materials have at least a slight elastocaloric effect, warming up a bit when squeezed or stretched. But to reach temperature changes large enough to be useful in a cooling system, the material would need a much larger corresponding change in entropy.
The best elastocaloric materials so far are shape memory alloys. They work because of a phase change, akin to liquid water freezing into ice. In one phase, the material can warp and stay warped. But if you crank up the heat, the alloy’s crystal structure transitions into a more rigid phase and reverts to whichever shape it had before (hence the name shape memory alloy).
