Electric fields can change how heat moves through materials used in advanced electronics, according to two Cornell University studies that could help engineers address overheating as devices become smaller and more powerful.
One study predicts that an electric field can increase or decrease heat conduction through aluminum nitride, depending on its direction. A separate experiment measured changes in the thermal resistance of devices containing hexagonal boron nitride, Cornell reported Nov. 11.
Both projects were led by Zhiting Tian, the John Brancaccio Professor in Cornell’s Sibley School of Mechanical and Aerospace Engineering. The findings suggest that engineers designing high-power electronics may need to account for how operating electric fields affect heat transport, rather than treating a material’s ability to conduct heat as constant.
Field direction changes the prediction
The computational study focused on aluminum nitride, an ultrawide-bandgap semiconductor that can withstand substantially higher electric fields than conventional semiconductor materials. Such materials could support smaller, more powerful high-voltage systems and high-power radio-frequency devices. Greater power density, however, also creates heat that can undermine efficiency, reliability and device lifespan.
Tian and her students, including first author and doctoral student Joyce Christiansen-Salameh, examined how electric fields affect phonons — atomic vibrations that carry heat through a crystal. Interactions and scattering among those vibrations influence how efficiently thermal energy travels.
The team combined density functional theory, which describes materials’ electronic structure, with a technique called nonequilibrium Green’s function. According to Cornell, the combined approach had previously been used to study electron transport but had not been applied in this way to calculate phonon-based thermal conductivity under an electric field.
The researchers simulated aluminum nitride crystals between electrodes and applied fields in opposite directions. Their calculations traced how the fields redistributed electrons, changed forces between atoms and affected phonon scattering.
Without an applied field, the calculated thermal conductivity perpendicular to the film was approximately 334 watts per meter-kelvin. A field in one direction increased that conductivity; reversing it decreased conductivity. The direction relative to the crystal’s built-in polarity determined whether certain atomic bonds strengthened or weakened.
The findings, published in npj Computational Materials under the title “Thermal Conductivity of Wurtzite Aluminum Nitride Under External Electric Field,” point to a possible design strategy: orient semiconductor films so that the electric fields present during operation help heat escape.
“We could intentionally grow the film in the direction where the electric field is favorable to help dissipate heat,” Tian said in Cornell’s account.
A separate experiment measures thermal changes
The experimental study examined hexagonal boron nitride, an ultrawide-bandgap material used as a dielectric in emerging electronic devices. Its results concern thermal resistance — how strongly a material and its interfaces impede heat flow — rather than the aluminum nitride conductivity predicted in the computational study.
Researchers placed thin, single-crystal boron nitride flakes between metal electrodes and applied strong electric fields. They used frequency-domain thermoreflectance, a laser-based measurement technique, to track changes in the thermal resistance of the flakes and their interfaces with the electrodes.
In one device, thermal resistance fell by as much as 25% during repeated tests. The changes developed over tens of minutes to hours, with recovery observed during or between measurement cycles. Conventional electrical measurements did not reveal the changes detected by the thermal measurements, Cornell reported.
The experiment provides evidence that heat transport in an ultrawide-bandgap material can change substantially under electrical stress. It also establishes an experimental approach for investigating those effects; it does not demonstrate a finished cooling technology for commercial electronics.
The study, “Reversible Electric-Field-Induced Changes in the Thermal Resistance of Hexagonal Boron Nitride,” was published Nov. 9 in Nano Letters. Doctoral student Amelia Schaeffer was its first author. Co-authors included Lena Fine, Class of 2025, and Aloyse Maille, Class of 2027, both undergraduate research grant recipients through Cornell’s Duffield Engineering Office of Inclusive Excellence.
The experimental work was conducted in collaboration with Kansas State University professor James H. Edgar. Both studies were sponsored by the Office of Naval Research.
FingerLakes1.com is the region’s leading all-digital news publication. The company was founded in 1998 and has been keeping residents informed for more than two decades. Have a lead? Send it to news@fingerlakes1.com.