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John O | July 2018

Research supports Einstein theorem about how heat moves through solids

By Josh Perry, Editor


Scientists at the U.S. Department of Energy Oak Ridge National Laboratory (Oak Ridge, Tenn.) have found evidence that shows heat hopping randomly from atom to atom in thermal insulators, which supports a theory that Einstein first proposed in 1911.


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.
(Jill Hemman and Adam Malin/Oak Ridge National Laboratory, U.S. Dept. of Energy)


According to a report from the ORNL, the hopping is in addition to the normal heat flow through atom vibration and is not seen in materials with high thermal conductivity. The scientists believe that this discovery will lead to new materials to be used in thermoelectric devices to recover waste heat or barrier coatings.


To observe the hopping, scientists used vibration-sensing tools to detect the motion of atoms and supercomputers to simulate how heat moved between the atoms of a thallium-based crystal. The article explained, “Their analysis revealed that the atomic vibrations in the crystal lattice were too sluggish to transmit much heat.”


“Many useful materials, such as silicon, have a chemically bonded latticework of atoms,” the article noted. “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.”


This research points to a two-channel movement of heat, which could improve material choices in thermal management, improving efficiency and reducing energy costs.


The research was recently published in Science. The abstract stated:


“Solids with ultralow thermal conductivity are of great interest as thermal barrier coatings for insulation or thermoelectrics for energy conversion. However, the theoretical limits of lattice thermal conductivity (κ) are unclear.


“In typical crystals a phonon picture is valid, whereas lowest κ values occur in highly disordered materials where this picture fails and heat is supposedly carried by random walk among uncorrelated oscillators. Here we identify a simple crystal, Tl3VSe4, with a calculated phonon κ [0.16 Watts per meter-Kelvin (W/m-K)] one-half that of our measured κ (0.30 W/m-K) at 300 K, approaching disorder κ values, although Raman spectra, specific heat, and temperature dependence of κ reveal typical phonon characteristics.


“Adding a transport component based on uncorrelated oscillators explains the measured κ and suggests that a two-channel model is necessary for crystals with ultralow κ.”

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