David B. Brown will give a research presentation on his work as Chancellor’s Postdoctoral Fellow in mechanical engineering at UCLA. His current research focuses on high-temperature thermal properties and cooling of hypersonic leading edges. 

ABSTRACT

Increased power density caused by the miniaturization of modern microelectronic devices has led to thermal management challenges which degrade performance and reliability. At small length scales, the thermal properties of electronic materials can differ significantly from their bulk counterparts. The associated rise of surface area to volume ratio heightens the importance of interfaces, a bottleneck to heat removal quantified by the thermal boundary conductance (TBC). New materials such as two-dimensional (2D) graphene and transition metal dichalcogenides (TMDs) are under investigation for applications in next-generation devices.

The interfaces will play a critical role in the overall performance of these materials. A fundamental understanding and precise characterization of the thermal transport properties at the interfaces of these materials is essential to ensure energy-efficient operation and long lifetime in future electronic devices. Time-domain thermoreflectance, a pump-probe optical technique, was used to explore the TBC at the interface of metals and graphene, 2D hexagonal boron nitride and graphene, and its spatial variation at the interface of the 2D semiconducting TMD molybdenum diselenide and metals. The results highlight important considerations for the design and performance of next-generation devices featuring 2D materials.

Hypersonic vehicles, which travel at Mach 5 or higher, are an active area of research around the globe. In the hypersonic flight regime, mechanical, chemical, and thermal phenomena, which are less significant at low speeds, become more influential. These vehicles are subjected to extreme temperatures, one of the main factors limiting their further development. As a result, the aerodynamic design requirements must be balanced with thermal management concerns for sustained flight at hypersonic speeds, with specific attention given to the leading edge because of the significant heat fluxes in this region.

Thermionic cooling for hypersonics has been proposed and modeled but not yet characterized experimentally in detail. When combined with evaporative transpiration cooling of the molten thermionic material, the vehicles are predicted to survive higher sustained heat fluxes while experiencing reduced temperatures. An analytical, multi-mode cooling model is developed to estimate the surface temperature around the stagnation point, the location of maximum heat flux near the leading edge of hypersonic vehicles.

Experimentally, the presence of heat spreading by reflected thermionic electrons is readily evaluated by indirect measurements of apparent thermal diffusivity, e.g., the ability of a planar solid to spread heat radially, via a modified Ångström’s method featuring a concentrated light source to measure high-temperature thermal diffusivity that has been validated using various materials. Through combined multiphysics modeling and experiments, new design criteria can be established for hypersonic leading edges.