Physics of Phonon Transport
Phonons, which are the quantized lattice vibrations in the crystal structure, are responsible for transferring thermal energy. The velocity and scatterings of phonons generally affect the thermal conductivity of materials, which is a fundamental property for heat transfer. We are interested in understanding phonon transport and engineering the thermal conductivity of various materials.
Thermal Transport at the Nanoscale
Thermal transport in nanoscale structures has not been fully understood yet. There remain unknown parts even in the thermal conductivity of silicon, which is a primary material in the semiconductor industry. We have been trying to uncover the contributions of phonons with different mean-free paths (MFP) to the thermal conductivity of silicon, enabling us to estimate the reduction of thermal conductivity in silicon-based nanoelectronics.
Thermal Transport in Nanomaterials
The limit of miniaturization in modern electronics necessitates the substitute of silicon. Two-dimensional materials are promising candidates because of their geometrical characteristics. Notably, recent studies revealed the excellent thermal and mechanical properties of graphene. Although numerous theoretical calculations predicted the unique thermal properties of two-dimensional materials, the thermal transport in two-dimensional materials has not been elucidated yet, e.g., hydrodynamic phonon transport. We have been conducting experimental and theoretical research to explore thermal transport in two-dimensional materials.
※ Related project:
- Study on Heat Vortex and Second Sound in Phonon Hydrodynamic Transport of Graphene
- Development of ultra-broadband sub-THz absorbing/shielding/heat-dissipating all-in-one MXeneX materials and process technology
Measurement & Analysis Technique
We have experimental and theoretical capabilities to quantify the thermal properties of micro and nanoscale structures, which are crucial for understanding their thermal engineering potential and applications in electronics and energy devices.
MEMS-based Electrothermal Measurement Techniques
The 3w method is a widely used technique for measuring the thermal conductivity of thin films and bulk materials. It relies on the principle of Joule heating and the temperature dependence of the electrical resistance. Also, we are employing ultrahigh-precision measurement techniques for determining the thermal conductivities of nanostructured materials. For example, suspended micro-bridge method is used to measure the thermal conductivity of 2D thin films and nanowire/fiber. It involves fabricating micro-bridge (typically with a serpentine shape) and measuring the temperature rise across the bridge.
Optical Pump-Probe Measurement Techniques
Time/Frequency domain thermoreflectance are highly time-resolved pump-probe techniques that utilize ultrafast lasers (~200 fs) to generate and detect heat on the sample surface. Modulated pump laser (0.2–20 MHz) is used to heat a material, and another probe laser measures the resulting temperature rise.
Theoretical Calculations
We employ both analytical and numerical calculation methods to characterize the behavior of phonon transport. When examining phonons as the dominant carriers of heat conduction, the utilization of the Boltzmann transport equation enables us to predict phonon heat transport in the sub-continuum space. Furthermore, density functional theory is applied to derive accurate phonon properties, including phonon dispersion, group velocity, and phonon-phonon scattering rates.
Thermal Management
Maintaining a proper temperature is necessary for numerous applications such as turbine blades, automobile engines, computer chips, and batteries. Failures in thermal management result in material degradation, reduced lifetime, and energy efficiency. Our research interest in thermal management ranges from small electronics to large aerodynamic vehicles.
Thermal Management in Nanoscale Electronic Devices
As electronic devices continue to shrink in size, their power densities increase, leading to higher heat dissipation challenges. Understanding micro and nanoscale heat transport (especially for heat conduction beyond the classical Fourier’s law) is essential for efficiently dissipating the generated heat. Through studying heat transfer mechanisms at micro and nanoscales, we have been developing advanced thermal management strategies to prevent overheating, improve performance and reliability, and optimize management conditions.
Reduction of Hypersonic Aerodynamic Heating
Hypersonic vehicles, traveling at velocities greater than Mach 5, experience extreme heating effects due to intense aerodynamic forces and compression of air in front of the vehicle. This leads to high temperatures that can potentially compromise the structural integrity of the vehicle and limit its operational capabilities. The microporous structure is one of the strategies to delay the turbulent transition and mitigate subsequent aerodynamic heating. We have been focusing on manufacturing the microporous structure based on MEMS technology.
