Four-phonon breakthrough overcomes thermal-conductivity conundrum

Researchers believe they can predict four-phonon scattering, an advance in quantum mechanical science that could have an impact in applications that include electronics cooling.

(Purdue University image/Tianli Feng, Xiulin Ruan)

Phonons are quantum-mechanical phenomena that describe how vibrations travel through a material’s crystal structure. The phonons interact, sometimes combining and splitting into new phonons, changing direction and behaviour.

This “scattering” is fundamental to how a material conducts heat. Until now, researchers have been able to realistically model only the interactions of three phonons. In new findings, researchers from Purdue University and Oak Ridge National Laboratory have shown how to accurately model the interactions of four phonons and their effect on heat flow.

The discovery could help efforts to improve a range of technologies including thermoelectric devices; thermal-barrier coatings; heat sinks for electronics cooling; nuclear fuels; and research into solid-state heat transfer in general.

“Being able to predict four-phonon scattering has been a decades-long challenge,” said Xiulin Ruan, a Purdue professor of mechanical engineering.

Four-phonon interactions have long been ignored, in part because they were considered to be negligible and researchers didn’t know how to model them.

“Now we have clearly shown the importance of four-phonon scattering,” he said.

Findings were detailed in Physical Review B. The paper was co-authored by former Purdue doctoral student Tianli Feng, and ORNL researcher Lucas Lindsay; and Ruan.

According to Purdue University, simulating four-phonon scattering has so far required 10,000 times the computational resources as three-phonon scattering, making it unfeasible to perform quality theoretical predictions. However, the Purdue team said it has developed a new method for carrying out the theoretical computations and optimised the simulation of four-phonon scattering, reducing the computational resources needed.

“It’s a new physical picture,” Feng said. “The mechanism of four-phonon scattering was already known, but nobody knew how to make the theoretical predictions or how to assess its importance, which are what we have achieved.”

Being able to incorporate four-phonon data into calculations will help researchers develop new materials. Materials that have ultrahigh thermal conductivity are ideal for heat sinks, while those with low thermal conductivity are suited for thermoelectric applications and thermal barrier coatings.

The new findings demonstrate that only using three-phonon scattering in calculations produces results that overestimate the performance of some materials while underestimating the performance of others.

“Their findings shed important light on previous theoretical predictions and experimental measurements, and will help to guide the development of new materials for a broad range of applications,” said Alan McGaughey, a professor of mechanical engineering at Carnegie Mellon University. “Of particular note is the potential to set limits on how high or low thermal conductivity can be across a range of temperatures.”