Microscopic ways of efficient heat transfer in energetic materials

June 10, 2023

(News from Nanowerk) Recently, NOMAD researchers have made significant progress in illuminating microscopic phenomena that can guide the customization of materials for thermal insulation. This breakthrough will advance ongoing initiatives to improve energy efficiency and promote sustainability.

Heat transfer plays an instrumental role in many scientific and industrial applications, such as catalysis, turbine technology, and thermoelectric heat converters that convert waste heat into electricity. In particular, in the context of the evolution of energy saving and sustainable technologies, materials with high thermal insulation capabilities are considered in the first place. Such materials facilitate the capture and utilization of heat that would otherwise be dissipated into the environment. Therefore, optimizing the design of highly insulating materials is a key research goal to help develop energy-efficient applications. Transient formation of a defect pair in copper iodide Transient formation of a defect pair in copper iodide. Although these defects only live for a few picoseconds, or trillionths of a second, they have a significant effect on macroscopic heat transfer processes. (Photo: Florian Knoop, NOMAD Lab)

However, designing robust thermal insulators is a simple matter, despite the fact that the fundamental physical principles behind them have been understood for nearly a century. Microscopic heat transfer in semiconductors and insulators is usually understood in terms of collective oscillations of atoms around their equilibrium positions in the crystal lattice. These oscillations, called “phonons” in the field, involve large numbers of atoms in solid materials and thus span large, almost macroscopic, temporal and spatial scales.

A recent joint publication Physical examination B (Editor’s recommendations) and Physical Review Letters Researchers at the Fritz Haber Institute’s NOMAD Laboratory (“Anharmonicity in Heat Insulators: An Analysis from First Principles”) have significantly expanded the ability to calculate thermal conductivities with unprecedented accuracy and without relying on experimental data.

Researchers have shown that the aforementioned phonon paradigm is insufficient for robust thermal insulators. Using extensive calculations on supercomputers belonging to the Max Planck Society, the North German Supercomputing Alliance and the Jülich Supercomputing Center, they studied more than 465 crystalline materials whose thermal conductivity had not yet been measured.

In addition to identifying 28 robust thermal insulators, six of which exhibited very low thermal conductivity similar to wood, this study highlighted commonly overlooked mechanisms capable of systematically reducing thermal conductivity.

Dr. Florian Knoop (now at Linköping University), lead author of both papers, says: “We have detected the transient formation of defect structures that significantly change the atomic motion for an incredibly short time.”

“These effects are usually not taken into account in heat conduction simulations because these defects are localized very fast and microscopic relative to conventional heat transfer scales, so they are assumed to be insignificant. However, our calculations revealed that they reduce thermal conductivity,” adds Dr. Christian Carbogno, senior author of the study.

Such discoveries could open up new ways to fine-tune and design thermal insulators at the nanoscale through defect engineering, which could contribute to advances in energy-efficient technology.

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