Scientists have discovered that applying an electric field to certain ceramics can dramatically redirect how heat moves through them.

New research from the Department of Energy’s Oak Ridge National Laboratory, carried out with collaborators at The Ohio State University and Amphenol Corporation, is challenging long-held ideas about how heat can be directed through solid materials.

The findings, reported in PRX Energy, show that an electric field can significantly change how phonons (tiny vibrations that carry heat) move inside a ceramic. When atoms vibrate in the same direction as the applied electric field (poling direction), the phonons remain active longer than vibrations that move across the field.

Because of this difference, heat travels through the material nearly three times more efficiently along the direction of the electric field than it does in other directions. The researchers say this strategy could open the door to new solid-state technologies that guide heat in practical devices.

“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

Managing the movement of heat is essential for many advanced technologies. Examples include electronic cooling systems that operate without moving parts, devices that convert heat into electricity, chip-based circuits used in modern electronics, and cogeneration systems that capture industrial heat and reuse it.

Maintaining the correct flow of heat allows these systems to operate at their highest efficiency and performance.

The relationship between heat flow and efficiency is illustrated by the Carnot cycle, a theoretical model of a heat engine that defines the maximum efficiency possible when heat moves between hot and cold reservoirs in a controlled way. In this research, the electric field reduces obstacles that normally disrupt phonon motion.

With fewer interruptions, the vibrations can travel farther through the material, similar to how traffic moves more freely when congestion is reduced. This improved movement of phonons enhances heat conduction in the direction of the electric field and increases efficiency.

Neutron Experiments Reveal Atomic Motion

The experiments were carried out at the Spallation Neutron Source, a DOE Office of Science user facility located at ORNL.

Scientists used advanced inelastic neutron scattering techniques to observe both the arrangement of atoms in the material (structure) and their motion (dynamics). Neutrons allow researchers to determine where atoms are positioned and how they move within a crystal. This method builds on the Nobel Prize-winning work of Clifford Shull and Bertram Brockhouse.

The data collected at the facility provided detailed insight into how the electric field affects phonons. The results show that the field not only increases the speed of these vibrations but also lengthens their lifetimes. Both effects are important for improving the transport of heat.

The team focused on a specialized ceramic known as relaxor-based ferroelectrics. When exposed to an electric field, small electric charges inside these materials become aligned. This alignment reduces scattering that normally disrupts heat carrying vibrations, allowing energy to move through the crystal more efficiently.

The crystals examined in the study were carefully grown and later exposed to an electric field, a process called “poling,” by Raffi Sahul at Amphenol Corporation. The resulting materials made it possible to precisely control the movement of energy through the solid.

ORNL senior researcher Michael Manley designed and led the inelastic neutron scattering experiments 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.

Connecting Heat Flow to Atomic Vibrations

By combining thermal conductivity measurements with neutron scattering results, the researchers were able to directly link changes in heat transport to the behavior of atomic vibrations inside the crystal.

The late Professor Joseph Heremans of Ohio State developed the thermal conductivity experiments and mentored doctoral candidate Delaram Rashadfar during the analysis of the results.

“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.”