#heat flow

A discovery by scientists at the Department of Energy’s Oak Ridge National Laboratory supports a century-old theory by Albert Einstein that explains how heat moves through everything from travel mugs to engine parts.

The transfer of heat is fundamental to all materials. This new research, published in the journal Science, explored thermal insulators, which are materials that block transmission of heat.

“We saw evidence for what Einstein first proposed in 1911—that heat energy hops randomly from atom to atom in thermal insulators,” said Lucas Lindsay, materials theorist at ORNL. “The hopping is in addition to the normal heat flow through the collective vibration of atoms.”

The random energy hopping is not noticeable in materials that conduct heat well, like copper on the bottom of saucepans during cooking, but may be detectable in solids that are less able to transmit heat.

Just as an electrical switch regulates the flow of electric current, thermal switches can control the flow of heat. These switches serve as thermal control devices and are useful for thermal management applications. For example, they can be used in industries to reduce waste heat, resulting in cost and energy savings. These switches require materials whose thermal conductivity (κ) can be modulated to a large extent. This would allow the switch to have an “on” and “off” state depending on the thermal conductivity. However, such materials are rare and challenging to develop, and those that have been developed show only small reversible variations in their κ.

Now, in a study published in Advanced Electronic Materials, researchers from Tokyo Institute of Technology (Tokyo Tech) and the National Institute for Materials Science, Japan, have taken things to the next level with a material that can achieve a large variation in its κ by changing its crystal structure dimensionality. The team achieved this remarkable feat by using a solid solution of lead selenide (PbSe) and tin selenide (SnSe), which can switch between a 3-dimensional (3D) cubic crystal structure and a 2-dimensional (2D) layered crystal structure with changes in temperature.