“It’s been realized for several years that a layered material might show very low thermal conductivity if the interfaces are close enough together and the differences in the vibrational properties of the two materials are sufficiently large,” David Cahill told nanotechweb.org. “We finally identified a system which shows that expected strong reduction in conductivity.”

Cahill and colleagues made their structures by atomic-layer deposition or magnetron sputter deposition, producing nanolaminates with a total thickness of between 40 and 70 nm. The structures had a thermal conductivity of around 0.6 W/sq.m/K, a figure that’s about three times smaller than for a conventional insulator. What’s more, the thermal conductivity of the structures decreased as the interface density increased.

The team measured the thermal conductivity of the material using a technique based on time-domain-thermoreflectance, an ultrafast optical metrology tool. They modified the technique slightly to obtain more information from one of the laser signals.

“The reflectivity of a metal is a very subtle function of its temperature,” said Cahill. “By measuring how fast the reflectivity, and therefore the temperature, changes over time we can determine the thermal conductivity. I am most proud of our ability to measure the thermal conductivity of thin layers (total thickness around 50 nm) of essentially any type of material; we were previously limited to electrically insulating films and the old method was much more time consuming.”

In the long term, nanolaminates could have applications as high-performance thermal barriers in, for example, turbine-engine blade coatings. However, the relatively high interface energy of the materials could limit their stability at high temperatures. “The results will also guide thinking in the research and development of energy-conversion materials called ‘solid-state thermionics’,” added Cahill.

Now the researchers, who reported their work in Science, are trying other combinations of materials to produce nanolaminates with an even lower thermal conductivity, and are carrying out fundamental studies of heat transport at solid-solid and solid-liquid interfaces.