Scientists at the Oak Ridge National Laboratory (ORNL) made a discovery that supports a century-old theory by Albert Einstein that explains how heat moves through everything from travel mugs to engine parts. “We saw evidence for what Einstein first proposed in 1911—that heat energy hops randomly from atom to atom in thermal insulators,” says Lucas Lindsay, a 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 are good conductors of heat, such as copper, but may be detectable in solids less able to transmit heat.
This observation advances understanding of heat conduction in thermal insulators and could lead to the discovery of materials that recover waste heat or prevent transmission of heat.
Lindsay and his colleagues used sophisticated vibration-sensing tools to detect the motion of atoms and then turned to supercomputers to simulate the journey of heat through a simple thallium-based crystal. Their analysis revealed that the atomic vibrations in the crystal lattice were too sluggish to transmit much heat.
“Our predictions were two times lower than we observed from experiments. We were initially baffled,” Lindsay said. “This led to the observation that another heat transfer mechanism must be at play.”
Knowing that the second heat transfer channel of random energy hopping exists will help researchers choose materials for heat management applications. This finding could also drastically reduce energy costs, carbon emissions, and waste heat.
Many useful materials, such as silicon, have a chemically bonded latticework of atoms. Heat usually gets carried through this lattice by atomic vibrations, or sound waves. These heat-bearing waves bump into each other, which slows heat transfer.
“The thallium-based material we studied has one of the lowest thermal conductivities of any crystal,” Lindsay says. “Much of the vibrating energy is confined to single atoms, and energy then hops randomly through the crystal.”
“Both the sound waves and heat-hopping mechanism first theorized by Einstein characterize a two-channel model, and not only in this material, but in several other materials with ultralow conductivity,” explains ORNL materials scientist David Parker.
For now, heat-hopping may only be detectable in excellent thermal insulators. “However, this heat-hopping channel may be in other crystalline solids, creating a new way to manage heat,” Parker adds.