“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 researchers needed to study the heat flow in a material that's really poor at conducting heat, and chose a compound based on the element thallium, a rather uncommon metal, wedged between mercury and lead in the periodic table. Researchers said the crystal they used, “has one of the lowest thermal conductivities of any crystal.” It sounds as if they weren't deliberately trying to prove or disprove this prediction, but that they were trying to make sure they could predict the heat flow with a computer model.
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.
Lindsay and his colleagues used sophisticated vibration-sensing tools to detect the motion of atoms and 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.The predicted heat flow from the computer simulations was about half of the heat flow observed in the experiments. This is what led them to conclude another, additional heat transfer mechanism was at work.
“Much of the vibrating energy is confined to single atoms, and the energy then hops randomly through the crystal.”This is the dawn of trying to use this phenomenon in the engineering toolbox, like all other known heat transfer mechanisms.
Many useful materials, such as silicon, have a chemically bonded latticework of atoms. Heat is usually carried through this lattice by atomic vibrations, or sound waves. These heat-bearing waves bump into each other, which slows the transfer of heat.
“Both the sound waves and the heat-hopping mechanism first theorized by Einstein characterize a two-channel model, and not only in this material, but in several other materials that also demonstrate ultralow conductivity,” said ORNL materials scientist David Parker.First comes detecting it, then comes trying to benefit from it, and finally attempts at controlling it. Without control, it's just a phenomenon.
For now, heat-hopping may only be detectable in excellent thermal insulators. “However, this heat-hopping channel may well be present in other crystalline solids, creating a new lever for managing heat,” he said.
(New research about the transfer of heat—fundamental to all materials—suggests that in thermal insulators, heat is conveyed by atomic vibrations and by random hopping of energy from atom to atom. Here the vibrational coupling is shown as the light blue springs and thermal hopping as the yellow arcs. - ORNL)