An electric field triples heat flow through ceramic -- by letting phonons live longer

Five to ten percent. That was the modest improvement in thermal conductivity that previous experiments had achieved by applying electric fields to ferroelectric materials. The new number from Oak Ridge National Laboratory is close to 300%. The difference is large enough to suggest that the earlier results were not just small -- they were measuring a fundamentally limited version of the effect.

The study, published in PRX Energy, was a collaboration between Oak Ridge, The Ohio State University, and Amphenol Corporation. It focused on relaxor-based ferroelectric ceramics, a class of materials whose internal electrical charges realign when exposed to an electric field. That realignment, the researchers found, does not just change the material's electrical properties. It changes how heat moves through it.

What phonons do, and why their lifespan matters

Heat travels through solid materials via phonons -- quantized vibrations of the atomic lattice. A phonon's ability to carry heat depends on how far it can travel before scattering off defects, boundaries, or other phonons. The longer a phonon survives, the more efficiently it transports thermal energy.

When the researchers applied an electric field to their ferroelectric crystals (a process called "poling"), they found that phonons vibrating along the field direction lived significantly longer than phonons vibrating perpendicular to it. The result was a nearly threefold difference in thermal conductivity between the two directions. The material became, in effect, a thermal one-way street.

"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.

Seeing atoms move at the Spallation Neutron Source

The experiments were carried out at the Spallation Neutron Source, a Department of Energy Office of Science user facility at Oak Ridge. Using inelastic neutron scattering, the team captured both the static arrangement of atoms and their dynamic behavior -- where the atoms sit and how they vibrate. This dual capability is what made it possible to connect changes in phonon lifetime directly to changes in thermal conductivity.

ORNL senior researcher Michael Manley designed and led the neutron scattering experiments. "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," he said.

The crystals were grown and poled by Raffi Sahul at Amphenol Corporation. The thermal conductivity measurements were designed by the late Professor Joseph Heremans of Ohio State, who guided doctoral candidate Delaram Rashadfar through the data interpretation before his passing.

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

Why controlling heat matters

Thermal management is a bottleneck in several high-performance systems. Solid-state electronic coolers, thermoelectric energy converters, chip-based circuits, and cogeneration systems that repurpose industrial waste heat all depend on precise control over where and how fast heat flows. The Carnot cycle, the idealized upper bound on heat engine efficiency, is defined by how well heat transfer between hot and cold reservoirs can be regulated.

What the Oak Ridge results suggest is that electric fields could provide a tunable, solid-state mechanism for directing heat flow without moving parts. Unlike traditional approaches that rely on material composition or geometry, this method allows the same piece of ceramic to conduct heat efficiently in one direction while resisting it in another -- and the effect can be switched on or off by changing the field.

Early-stage caveats

The work was performed on single crystals carefully prepared in a laboratory setting. Translating the effect into practical devices would require demonstrating it in polycrystalline materials, at relevant operating temperatures, and over sustained time periods. The study also does not address how quickly the thermal conductivity responds to changes in the applied field -- a critical parameter for any switching application.

Still, the gap between the previous 5-10% improvements and the 300% seen here is wide enough to warrant serious follow-up. If the mechanism holds in more practical configurations, it could open a path toward solid-state thermal switches and directional heat conductors that have no equivalent in current technology.

The work was funded by the DOE Basic Energy Sciences program.