Scientists are taking the heat

Moving heat around where you want it to go — adding it to houses and hairdryers, removing it from car engines and refrigerators — is one of the great challenges of engineering.

All activity generates heat because energy escapes from everything we do. But too much heat can wear out batteries and electronic components, such as parts in an aging laptop that runs too hot to actually sit on your lap. If you can’t get rid of heat, you’ve got a problem.

Scientists at University of Chicago have invented a way to funnel heat around at the microscopic level: a thermal insulator made using an innovative technique. They stack ultrathin layers of crystalline sheets on top of each other but slightly rotate each layer, creating a material with atoms that are aligned in one direction but not in the other.

“Think of a partly finished Rubik’s Cube with layers all rotated in random directions,” said Shi En Kim, graduate student at the university’s Pritzker School of Molecular Engineering and first author of the study. “What that means is that within each layer of the crystal, we still have an ordered lattice of atoms. But if you move to the neighboring layer, you have no idea where the next atoms will be relative to the previous layer. The atoms are completely messy along this direction.”

The result is a material that is extremely good at both containing heat and moving it, albeit in different directions — an unusual ability at the microscale and one that could have very useful applications in electronics and other technology.

“The combination of excellent heat conductivity in one direction and excellent insulation in the other direction does not exist at all in nature,” said study lead author Jiwoong Park, professor of chemistry and molecular engineering at University of Chicago. “We hope this could open up an entirely new direction for making novel materials.”

Scientists constantly search for materials with unusual properties because they can unlock capabilities for devices.

Park’s group had been investigating ways to make extremely thin layers of materials that are a few atoms thick. Materials used for devices normally are made up of highly regular, repeating lattices of atoms, which makes it easy for electricity and heat to move through the materials. But the scientists wondered what would happen if they instead slightly rotated each successive layer as they stacked them.

The team measured the results and found that a microscopic wall made of this material was terrific at preventing heat from moving between compartments.

“The thermal conductivity is just amazingly low — as low as air, which is still one of the best insulators we know,” Park said. “That in itself is surprising because it’s very unusual to find that property in a material that is a dense solid. Those tend to be good heat conductors.”

But what was really exciting for the scientists was when they measured the material’s ability to transport heat along the wall and found that the material easily could do so.

Those two properties combined could be useful. For example, making computer chips smaller and smaller results in more and more power running through a small space, creating an environment with a high power density — a dangerous hot spot, Kim said.

“You’re basically baking your electronic devices at power levels as if you are putting them in a microwave oven,” she said. “One of the biggest challenges in electronics is to take care of heat at that scale because some components of electronics are very unstable at high temperatures. But if we can use a material that can both conduct heat and insulate heat at the same time in different directions, we can siphon heat away from the heat source — such as the battery — while avoiding the more fragile parts of the device.”

That capability could open doors to experiment with materials that have been too heat-sensitive for engineers to use in electronics. In addition, creating an extreme thermal gradient — in which something is hot on one side and cool on the other — is difficult, particularly at such small scales, but could have many applications in technology.

The scientists tested the layering technique only in one material, molybdenum disulfide, but think that this mechanism should be general across many others.   

— Louise Lerner