Researchers at the NOMAD Laboratory at the Fritz Haber Institute of the Max Planck Society have shed light of the microscopic mechanisms that determine thermal conduction in heat insulators. Powered by the advances made in the NOMAD CoE, their computational research has shown that even short-lived and microscopically localized defect structures have a substantial impact on macroscopic transport processes. This discovery could contribute to more energy-efficient technologies by allowing for the tailoring of nanoscale thermal insulators through defect engineering.

Temporary formation of a defect pair in copper iodide. Although these defects only survive for a couple of picoseconds, i.e., for a trillionth of a second, they substantially influence macroscopic heat transport processes.

The NOMAD Laboratory researchers have recently elucidated on fundamental microscopic mechanisms that offer to tailor materials for heat insulation. This development advances the ongoing efforts to enhance 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 conservation and the development of sustainable technologies, materials with high thermal insulation capabilities are of utmost importance. These materials make it possible to retain and use heat that would otherwise go to waste. Therefore, improving the design of highly insulating materials is a key research objective in enabling more energy-efficient applications.

However, designing strong heat insulators is far from trivial, despite the fact that the underlying fundamental physical laws have been known for nearly a century. At a microscopic level, heat transport in semiconductors and insulators was understood in terms of the collective oscillation of the atoms around their equilibrium positions in the crystal lattice. These oscillations, called “phonons” in the field, involve zillions of atoms in solid materials and hence cover large, almost macroscopic length- and time-scales. In turn, an accurate description of the relevant dynamics across such a wide range of scales requires sophisticated methodological and computational advancements.

In a recent joint publication in Physical Review B (Editors’ Suggestion) and Physical Review Letters, researchers from the NOMAD Laboratory at the Fritz Haber Institute have exploited the algorithmic and computational improvements made by the NOMAD CoE to compute thermal conductivities without experimental input at unprecedented accuracy. They demonstrated that for strong heat insulators the above-mentioned phonon picture is not appropriate. Using large-scale calculations on supercomputers at of the Max Planck Society, the North-German Supercomputing Alliance, and the Jülich Supercomputing Centre, they scanned over 465 crystalline materials, for which the thermal conductivity had not yet been measured. Besides finding 28 strong thermal insulators, six of which featured an ultra-low thermal conductivity comparable to wood, this study shed light on a hitherto typically overlooked mechanism that can be used to systematically lower thermal conductivity. “We observed the temporary formation of defect structures that massively influences the atomic motion for an extremely short period of time”, says Dr. Florian Knoop (now Linköping University), first author of both publications. “Such effects are typically neglected in thermal-conductivity simulations, since these defects are so short-lived and so microscopically localised compared to typical heat-transport scales, that they are assumed to be irrelevant. However, the performed calculations showed that they trigger lower thermal conductivities”, adds Dr. Christian Carbogno, a senior author of the studies.

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

Read the full publications here:

Florian Knoop, Thomas A. R. Purcell, Matthias Scheffler, and Christian Carbogno,
Anharmonicity in Thermal Insulators: An Analysis from First Principles
Phys. Rev. Lett. 130, 236301 (2023).
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.236301

Florian Knoop, Matthias Scheffler, Christian Carbogno
Ab initio Green-Kubo simulations of heat transport in solids: Method and implementation
Phys. Rev. B 107, 224304 (2023).
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.107.224304