This electric field trick boosted heat flow by nearly 300%

Published in PRX Energy, the study found that applying an electric field to a specialized ceramic changes the behavior of phonons, the tiny atomic vibrations responsible for carrying heat. When the atoms vibrate in the same direction as the electric field (poling direction), those phonons persist much longer than vibrations moving across it. As a result, heat travels almost three times more efficiently along the direction of the electric field than it does in other directions.

"Being able to control both how fast and in what manner heat flows could lead to devices that manage thermal energy far more efficiently," said Puspa Upreti, an ORNL postdoctoral research associate.

Why Controlling Heat Matters

The ability to direct heat efficiently is essential for many advanced technologies. These include solid state electronic cooling systems with no moving parts, devices that convert heat into electricity, chip based electronics, and cogeneration systems that capture and reuse waste heat from industrial processes.

Better control over heat transfer can improve both performance and energy efficiency. The concept is illustrated by the Carnot cycle, an idealized model that defines the maximum theoretical efficiency of a heat engine by carefully regulating the movement of heat between hot and cold regions.

In the new study, the electric field reduced obstacles that normally interfere with phonon movement. That allowed the heat carrying vibrations to travel farther through the material, much like easing congestion on a busy highway, leading to much more efficient heat conduction in the direction of the electric field.

Neutron Experiments Reveal Atomic Motion

To understand exactly what was happening inside the material, the team carried out experiments at the Spallation Neutron Source, a DOE Office of Science user facility operated by ORNL.

Using advanced inelastic neutron scattering techniques, the researchers observed both the positions of atoms within the crystal and how those atoms moved. Neutrons are uniquely suited for this type of analysis because they can reveal both a material's structure and its atomic dynamics, building on methods recognized by the Nobel Prize winning work of Clifford Shull and Bertram Brockhouse.

The measurements showed that applying an electric field not only increased the speed of the phonons but also significantly extended how long they survived before scattering. Those longer lifetimes are a key reason the material became so much better at conducting heat.

A Ceramic With Remarkable Heat Transfer

The researchers focused on a class of ceramics known as relaxor based ferroelectrics. When exposed to an electric field, tiny electric charges within these materials become aligned. That alignment reduces scattering of the heat carrying phonons, allowing thermal energy to move through the crystal much more efficiently.

The crystals used in the experiments were carefully grown and then exposed to the electric field, or "poled," by Raffi Sahul at Amphenol Corporation. The resulting materials demonstrated highly controllable heat transport.

ORNL senior researcher Michael Manley led the inelastic neutron scattering experiments together with ORNL senior R&D staff member Raphaël Hermann.

"Earlier work on bulk ferroelectric materials achieved modest improvements in thermal conductivity of 5 percent to 10 percent, while the new measurements reveal an enhancement close to 300 percent -- mainly because the phonons are able to travel much longer before they stop," Manley said.

A Threefold Increase Surprised Researchers

By combining thermal conductivity measurements with neutron scattering data, the team was able to directly connect the dramatic increase in heat flow to changes in the atomic vibrations inside the crystal.

The late Professor Joseph Heremans of Ohio State designed the thermal conductivity experiments and guided doctoral candidate Delaram Rashadfar through the analysis.

"While earlier work led us to expect only a modest effect, observing a threefold difference turned out to be a significant result," said Rashadfar. "Professor Heremans always stressed the importance of trusting the data first and letting the theory follow."

The research was supported by the DOE Basic Energy Sciences program along with additional contributing partners.