Microscopic pathways for efficient heat transfer in energy materials

(Nanowerk News) Researchers at the NOMAD Laboratory have recently made significant strides in elucidating the underlying microscopic phenomena that can guide the adjustment of materials intended for heat insulation. This breakthrough drives sustainable initiatives aimed at increasing energy efficiency and promoting sustainability.

Heat transport plays an important role in a variety of scientific and industrial applications, such as catalysis, turbine technology and thermoelectric heat converters that convert waste heat into electrical energy. Especially in the field of energy conservation and continuous technological evolution, materials having superior thermal insulation capabilities are considered paramount. These materials facilitate the capture and utilization of heat that may be lost to the environment. As a result, optimizing the design of high insulation materials stands as a key research objective, playing an important role in driving more energy efficient applications. Formation of transient pair defects in copper iodide. Although these defects persist for only a few picoseconds, i.e. as long as a trillionth of a second, they substantially affect macroscopic heat transfer processes. (Image: Florian Knoop, NOMAD Laboratory)

Designing a strong heat insulator, however, is by no means easy, even though the basic physical principles behind it have been understood for nearly a century. The microscopic heat transport in semiconductors and insulators is usually understood in terms of the collective vibration of atoms around their equilibrium positions in the crystal lattice. These oscillations, referred to as “phonons” in this art, span large numbers of atoms in solid materials, thus spanning large, nearly macroscopic, temporal, and spatial scales.

A recent collaborative publication on physical review B (Editor’s suggestion) and Physical Review Letter (“Anharmonics in Thermal Insulators: Analysis from First Principles”), from researchers at the Fritz Haber Institute-based NOMAD Laboratory, have greatly extended the computational power to calculate thermal conductivity with unprecedented precision, and without relying on experimental inputs.

The researchers demonstrated that for a strong heat insulator, the phonon paradigm is inadequate. They used expansive computations on supercomputers belonging to the Max Planck Society, the North German Supercomputing Alliance, and the Jülich Supercomputing Center, examining more than 465 crystalline materials, whose thermal conductivities have not yet been measured.

In addition to identifying 28 strong thermal insulators, six of which exhibit very low thermal conductivities similar to wood, this study sheds light on a normally overlooked mechanism capable of systematically reducing thermal conductivity.

“We detected the transient formation of structural defects that significantly alter the motion of the atoms for a very short duration,” said Dr. Florian Knoop (now at Linköping University), lead author of both papers.

“This effect is generally ignored in thermal conductivity simulations, given that these defects are so fleeting and microscopically localized relative to the typical heat transfer scale that they are considered unimportant. However, our calculations reveal that they lead to lower thermal conductivity,” added Dr Christian Carbogno, a senior author of the study.

Such disclosures could open new avenues for careful adjustment and design of thermal insulators at the nanoscale level through defect engineering, potentially contributing to advances in energy-efficient technologies.