Boiling Heat Transfer on Surfaces with Multi-Length Scale Structures
Figure 1: Copper nano-rods for enhanced boiling heat transfer.
In current industrial and commercial applications, boiling heat transfer plays a critical role in thermal devices such as boilers. An important parameter for boiling heat transfer, is the critical heat flux (CHF). Critical heat flux determines the max power density that can be handled by a boiling heat transfer device. In physics, CHF is limited by: (1) far-field hydrodynamic instability, and/or (2) near-field surface properties. These limits can be largely suppressed by optimizing a boiling surface from the nano-scale (Fig. 1) to macro-scale. The advancement in boiling research can further benefit research in other relevant fields, such as surfaces with enhanced condensation.
Heat Transfer inside Li-ion Batteries
Figure 2: Typical charging process of a Li-ion battery, with Li ions moving from the cathode to the anode. Image is taken from howstuffworks.com.
Nowadays lithium-ion (Li-ion) batteries are widely adopted in portable devices, hybrid and electrical vehicles. Although random explosions are not a widespread problem, millions of Li-ion batteries have been recently recalled by various manufacturers (e.g. Sony, GM, A123) due to the thermal safety concern. Presently, the highest priority for manufacturers has been switched from the energy density and cost of Li-ion batteries to their thermal safety, reliability, and durability. To serve the urgent industry needs, heat transfer studies on Li-ion batteries receive more attentions now. The goal of this project is to better understand the thermal transport processes inside an operating Li-ion battery and help fundamentally solve the thermal safety problem of Li-ion batteries. Better thermal designs of the whole battery pack will also be addressed to improve heat dissipation to the environment.
Funding resources: Yardney Technical Products, Inc. (Navy STTR project) , 09/16/14 to 04/9/15
Figure 3: Principles of thermoelectric cooling and power generation [Vining, Nature 413, 577-578 (2001)].
Thermoelectric materials have the ability to directly convert heat into electricity (power generation) or instead to use electricity to drive a heat flow (refrigeration). These materials have raised renewed interest not only due to their potential as sustainable energy sources but also due to the lack of moving parts which makes thermoelectric devices highly reliable for sensitive applications such as spacecraft generators. The performance of a thermoelectric device depends on the dimensionless figure of merit (ZT) of the material, defined as ZT=S2σT/κ, where S, κ, σ, T are the Seebeck coefficient, thermal conductivity, electrical conductivity, and absolute temperature, respectively. As a general approach, a high ZT can be obtained by reducing κ with nanostructures (e.g., nanopores in a thin film), while keeping S2σ.
Studies of Structurally Modified Graphene
Figure 4: Atomic structure of single-layer graphene.
As a single sheet of carbon atoms, graphene possesses superior thermal, electrical, and mechanical properties. However, most investigations in the past were focused on pure graphene, while in many applications graphene can be chemically or structurally modified. This project is to better understand the phonon (heat carriers in dielectric materials) and electron transport inside modified graphene. Our effort combines the state-of-art measurement techniques with fundamental theoretical studies. The growth of this project will significantly enhance our knowledge on how to better engineering phonon transport inside graphene for various applications, such as thermoelectrics, Li-ion batteries, and nanoelectronics. The research will be further extended to other two-dimensional materials such as transition metal dichalcogenides.
Funding resources: AFOSR YIP Award, 11/01/2015 to 10/31/2018
Thermal Transport Studies of a Single Grain Boundary
Figure 5: Transmission electron microscopy image of nanograined bulk BiSbTe alloys [Poudel*, Hao* (*equal contribution) et al., Science 320, 634 (2008)].
At the nanoscale, interfaces can largely suppress the thermal transport by scattering heat carriers that are mainly phonons in nonmetal materials. Such interfacial phonon scattering leads to a thermal resistance, known as Kapitza resistance. A good example of such interfaces can be grain boundaries within a polycrystalline thin film or bulk material. Despite its importance to many research fields, detailed phonon transport across a single grain boundary is still not well understood for now, particularly when the exact atomic structure and defects of a real grain boundary are considered. Combining experiments with atomistic simulations, this project aims to reveal more details of phonon transport across a single grain boundary. The knowledge gained from this project will largely advance the research on interface engineering to tune phonon transport, by varying the interfacial atomic and nanoscale structures.
Funding resources: NSF CAREER Award, 03/01/2017 to 02/28/2022
Electrothermal Simulations of Electronic Devices
Figure 6: Simulated temperature distribution within an operating GaN device.
With continuous miniaturization of electronic devices, overheating now becomes critical and can largely deteriorate the device performance. In this aspect, electrothermal simulations can provide important guidance for the improvement of existing device designs. This project aims to provide the most accurate simulation results by incorporating the nanoscale electron and phonon transport near a transistor, while considering heat spreading across the whole millimeter chip. The developed simulation technique can be widely used for various electronic devices, such as GaN devices for high-power and high-frequency applications. In the future, our simulations will be combined with experiments to advance the thermal management of various electronic devices.
Funding resources: DARPA/Air Force, 04/08/2015 to 07/10/2017
High-Temperature Heat Transfer Fluids
Figure 7: Concentrated solar power plant.
This project focuses on developing high temperature heat transfer fluids (HTF) to meet several critical criteria that will advance the efficiency of concentrated solar power (CSP): 1) Low melting temperatures no higher than 250oC. 2) Thermal stability at temperatures up to 800oC, and low vapor pressure. 3) Low chemical corrosion to devices accommodating HTF in CSP plant. 4) Thermal and transport properties comparable to the current solar salts for CSP systems. 5) Reasonably low-cost and abundant natural reserves.
Funding resources: DOE MURI, 10/01/2012 to 09/30/2016
Some figures above are from the Internet and the original sources are linked to the figures.