Current Research Topics Of Interest

Switchable and Nonlinear Thermal Devices

Thermal switches, diodes, and transistors can overcome the limitations of traditional thermal resistors. Each of these thermal components has a signature functionality: Thermal switches have a tunable thermal resistance, thermal diodes rectify heat currents, and thermal transistors can switch and amplify heat flows. We are researching mechanisms to achieve tunable and nonlinear thermal performance, and demonstrating applications of these devices in spacecraft thermal management and building climate control. One mechanism of interest focuses on leveraging the temperature dependent magnetic response of ferromagnets near the Curie temperature to make and break thermal contact between surfaces. leading to thermal rectification, thermal regulation, and negative differential thermal resistance. We also are constructing liquid-vapor phase change thermal diodes leveraging droplet interactions with superhydrophobic and superhydrophillic interfaces.

Publications: Castelli et al., Cell Rep. Phys. Sci. (2024); Shimokusu et al., Adv. Matls. Int. (2024); Castelli et al., Cell Rep. Phys. Sci. (2023); Shimokusu et al., Appl. Thermal. Eng. (2023); Castelli et al., Nat. Comm. (2023); Zhu et al., Adv. Funct. Matls. (2022); Shimokusu et al., IJHMT (2022);  Wehmeyer et al., Appl. Phys. Rev. (2017)

Thermal properties of carbon nanotube materials

Rice University has a strong history of carbon nanomaterial research dating back to the pioneering work of Rick Smalley and the discovery of buckeyballs. We collaborate with other Rice faculty members to study the thermal, electrical, and thermoelectric properties of carbon nanotube fibers (CNTFs), which consist of many aligned nanotubes oriented along the axial direction of the fiber. We utilize 3ω and steady-state electrothermal methods to experimentally quantify the thermal conductivity and diffusivity of suspended CNT fibers, and perform measurements on CNT ribbons to explore the anisotropic thermal properties. These CNT materials also offer promise for active cooling and heat switching applications due to their high thermal conductivity and high thermoelectric power factor.

Publications: Liao et al., ACS AEM (2024); Marquez Peraca et al., APL (2023); Taylor et al., Carbon (2021) , Komatsu et al., Nat. Comm. (2021), Lee et al., Science Advances (2022)

Nanoscale temperature mapping in TEM

Common macroscopic thermometers such as IR cameras  do not have sufficiently high spatial resolution to map microscopic temperature gradients. Similarly, nanoscale temperature gradients remain inaccessible to microscopic thermometry methods using far-field optics or electrothermal methods. We are developing new thermometry techniques with ultrahigh spatial resolution to probe heat transfer at the nanoscale.  We  use the scanning transmission electron microscopes and sample heating holders at Rice’s state-of-the-art Electron Microscopy Center to explore temperature mapping in the STEM, with the ultimate goal of achieving <10 nm spatial resolution thermal maps. These capabilities will offer new insight into heat transport across interfaces, and could enable improved hotspot measurements in electronic devices.

Publications: Wehmeyer et al., Appl. Phys. Lett. (2018) , Wehmeyer, J. Appl. Phys. (2019)

Sub-continuum phonon transport simulations

Since classic continuum models such as Fourier’s law break down at the nanoscale, we develop new numerical simulations to model thermal transport in complicated nanostructures. For example, we use a ray tracing technique to track phonons as they scatter off the surfaces of a nanostructure, and use this information to calculate the thermal conductivity. Quantifying the size effects on the thermal conductivity allows us to test different models for phonon transport against experiments, and to design nanostructures to have a desired thermal performance.

Publications: Song and Wehmeyer, Matls. Today Phys. (2023), Song and Wehmeyer, J. Appl. Phys. (2021), Lee et al., Nat. Comm. (2017) , Wei et al. , Nanoscale (2016), Wehmeyer et al., Phys. Rev. B (2018)