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Temporary formation of a pair of defects in copper iodide. Although these defects survive for only a couple of picoseconds, i.e. for a trillionth of a second, they substantially affect the macroscopic processes of heat transport. Credit: Florian Knoop, NOMAD Laboratory

Researchers in the NOMAD laboratory recently shed light on the fundamental microscopic mechanisms that can help tailor thermal insulation materials. This development advances ongoing efforts to improve energy efficiency and sustainability.

The role of heat transport is crucial in various scientific and industrial applications, such as catalysis, turbine technologies and thermoelectric heat converters that convert waste heat into electricity.

Particularly in the context of energy saving and the development of sustainable technologies, materials with high thermal insulation capabilities are of the utmost importance. These materials allow heat to be retained and used that would otherwise be wasted. Therefore, improving the design of highly insulating materials is a key research goal to enable more energy-efficient applications.

However, designing highly thermal insulators is far from trivial, despite the fact that the underlying fundamental physical laws have been known for almost a century. At the microscopic level, heat transport in semiconductors and insulators has been understood in terms of the collective oscillation of atoms about their equilibrium positions in the crystal lattice. These oscillations, called “phonons” in the field, involve enormous numbers of atoms in solid materials and thus cover large, almost macroscopic length and time scales.

In a recent publication joined in Physical review b AND Physical Review Letters, researchers at the NOMAD Laboratory of the Fritz Haber Institute have advanced the computational possibilities to calculate thermal conductivities without experimental input with unprecedented accuracy. They showed that for strong thermal insulators the above-mentioned phonon picture is not appropriate.

Using large-scale calculations on supercomputers from the Max Planck Society, the North-German Supercomputing Alliance and the Jlich Supercomputing Center, they scanned over 465 crystalline materials, for which thermal conductivity had not yet been measured. In addition to finding 28 strong thermal insulators, six of which have very low thermal conductivity comparable to wood, this study sheds light on a hitherto typically supervised mechanism that systematically lowers thermal conductivity.

“We have observed the temporary formation of faulty structures that massively affect atomic motion for an extremely short period of time,” says Dr. Florian Knoop (now Linkping University), first author of both publications.

“Such effects are typically neglected in thermal conductivity simulations, because these defects are so short-lived and so microscopically localized relative to typical heat transport scales, that they are assumed to be irrelevant. However, calculations performed have shown that they trigger a minor thermal conductivities,” adds Dr. Christian Carbogno, senior author of the studies.

These insights may offer new opportunities to tune and design nanoscale thermal insulators through defect engineering, potentially contributing to advances in energy-efficient technology.

More information:
Florian Knoop et al, Anharmonicity in Thermal Insulators: An Analysis from First Principles, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.236301

Florian Knoop et al, Ab initio Green-Kubo simulations of heat transport in solids: method and implementation, Physical review b (2023). DOI: 10.1103/PhysRevB.107.224304

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Physical review b

Physical Review Letters

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