Researchers at the Department of Energy’s Oak Ridge National Laboratory (ORNL), in collaboration with The Ohio State University and Amphenol Corporation, have discovered a surprising new way to control how heat moves through solid materials. Their findings challenge long-held assumptions about heat transport and could lead to more efficient cooling systems, energy devices, and electronic technologies.
Published in PRX EnergyThe study found that applying an electric field to a special ceramic changes the behavior of phonons, the tiny atomic vibrations responsible for transporting heat. When atoms vibrate in the same direction as the electric field (the direction of polarization), those phonons persist much longer than vibrations traveling across the atom. As a result, heat is transferred nearly three times more efficiently along the direction of the electric field than in other directions.
“Being able to control both the rate and manner in which heat flows could lead to devices that manage thermal energy much more efficiently,” said ORNL postdoctoral researcher Puspa Upreti.
Why heat control is important
The ability to efficiently conduct heat is essential to many advanced technologies. These include solid-state electronic cooling systems with no moving parts, devices that convert heat to electricity, chip-based electronics, and cogeneration systems that capture and reuse waste heat from industrial processes.
Better control of heat transfer can improve both performance and energy efficiency. This concept is illustrated by the Carnot cycle, an idealized model that defines the theoretical maximum efficiency of a heat engine by carefully controlling the transfer of heat between hot and cold regions.
In the new study, an electric field reduced the obstacles that normally block the movement of phonons. This allows heat-carrying vibrations to travel farther through the material, making heat transfer in the direction of the electric field more efficient, much like easing congestion on a busy highway.
Neutron experiments reveal atomic motion
To understand exactly what was happening inside the material, the research team conducted experiments at the Spallation Neutron Source, a DOE Office of Science User Facility operated by ORNL.
The researchers used advanced inelastic neutron scattering techniques to observe the positions of atoms within the crystal and how those atoms move. Neutrons are uniquely suited for this type of analysis because they can reveal both the structure and atomic dynamics of materials, based on techniques recognized by the work of Nobel Prize winners Clifford Shull and Bertram Brockhouse.
The measurements showed that applying an electric field not only increased the phonon’s velocity, but also significantly extended the phonon’s survival time before scattering. These longer lifetimes are the main reason why this material conducts heat so well.
Ceramics with excellent heat conduction
The researchers focused on a type of ceramic known as relaxer-based ferroelectrics. When exposed to an electric field, the tiny charges within these materials align. This arrangement reduces scattering of heat-carrying phonons, allowing thermal energy to move through the crystal more efficiently.
The crystals used in the experiment were carefully grown by Raffi Sahul of Amphenol Corporation and then exposed, or “polarized,” to an electric field. The resulting material demonstrated highly controllable heat transport.
ORNL Senior Research Scientist Michael Manley led the inelastic neutron scattering experiment along with ORNL Senior Research and Development Staff member Raphael Hellman.
“Previous studies on bulk ferroelectric materials achieved modest improvements in thermal conductivity of 5 to 10 percent, but the new measurements reveal improvements of nearly 300 percent, primarily because phonons can travel much longer before stopping,” Manley said.
Researchers were surprised by the three-fold increase
By combining thermal conductivity measurements with neutron scattering data, the researchers were able to directly link the dramatic increase in heat flow to changes in atomic vibrations inside the crystal.
The late Ohio State University professor Joseph Herremans designed the thermal conductivity experiment and guided the analysis to doctoral candidate Delaram Rashadfar.
“Previous studies had expected only modest effects, but the observed three-fold difference proves to be an important result,” Rashadfar said. “Professor Hemans always emphasized the importance of trusting the data first and following the theory.”
This research was supported by the DOE Basic Energy Sciences Program and additional contributing partners.
