Title: Nanoscale Structure and Chemistry to Control Heat Transport Through Composites and Across Interfaces
Speaker: Professor Patrick Hopkins, Department of Mech. And Aero. Engineering, University of Virginia
(Research Website: patrickehopkins.com)
Date: Tuesday, May 23, 2017
Time: 1:00 – 2:00 p.m.,
Location: NASA / LaRC, Bldg. 1200, Rm. 227
Abstract: High heat fluxes and increased temperatures have led to major road blocks in the advancement of materials and technologies. For example, high frequency switches and optical links, energy storage and conversion devices, and high power laser systems have all demonstrated thermal failures that prevent functional material composites from reaching their full theoretical potential. These composites devices, which rely on a multitude of systems and hence a high density of material interfaces in a given volume, reach thermal failure not due to energy dissipation in the materials comprising the device, but due to the material interfaces. As I will discuss, the key to overcoming power and energy concerns and increasing device efficiency therefore relies heavily on mitigating and controlling the thermal boundary conductance across material interfaces.
While many of us are quite familiar with the thermal conductivity of a material, the thermal boundary conductance across material interfacial regions is much less frequently studied. The thermal boundary conductance (TBC), the inverse of which is the thermal boundary resistance, represents the thermal resistance associated with the interfacial region between two materials. This interfacial property, which was originally realized by Kapitza in 1941, is well known to be intimately related to the electronic and phononic densities of states in each material comprising the boundary. However, more recently, we have shown that the TBC across material interfaces is intimately related to the structures, masses and bonding environments of the atomic species near the interface. This variably of TBC with atomic interfacial properties provides a mechanism to increase or decrease the TBC based on engineering the atomic properties around an interface.
In this talk, I will discuss our recent work in which we are demonstrating the ability to manipulate the heat transfer in alloys, composite materials and across interfaces from understanding the local atomistic environment (atomic order and bonding). We use time-domain thermoreflectance (TDTR), a pump-probe technique centered around short pulsed laser systems to measure the thermal properties of these various systems. TDTR offers the capability to resolve the thermal properties (thermal conductivity, thermal boundary conductance, heat capacity) in thin films and nanoscale interfaces due to the nature of the short laser pulses and high frequency of modulation. I will begin by presenting an overview of TDTR, discussing its capabilities and recent application in measuring thermal properties of materials at temperatures up to ~1,800 K. I will then present a series of experimental works highlighting the role of chemical composition and defects on the thermal properties of a variety of material systems, lending insight to how to control the thermal properties through designing material defects and interfaces. Specifically, I will discuss:
-Interface effects on the thermal conductivity of silicon alloys and alloy nanocomposites, highlighting the role of alloy composition on thermal boundary conductance
-Phonon scattering processes and thermal conductivity of entropy stabilized oxides in which 5+ different atomic species in a single crystal structure lead to exceptionally low thermal conductivities in relatively high modulus materials
-The ability to create thermal phonon filters based on phonon wavelengths, interface densities, crystallinity and interfacial chemistry in superlattices
Bio: Patrick E. Hopkins is an Associate Professor in the Department of Mechanical and Aerospace Engineering at the University of Virginia. Patrick received his Ph.D. in Mechanical and Aerospace Engineering at the University of Virginia in 2008. His dissertation research was focused on developed pulsed laser-based diagnostics to measure temperature and energy transfer in solid materials. Prior to his PhD, Patrick graduated from the University of Virginia with majors in Mechanical Engineering and Physics. After his Ph.D., Patrick was one of two researchers in the nation to receive a Truman Fellowship from Sandia National Laboratories. Under this Fellowship, Patrick worked at Sandia in Albuquerque, NM from 2008 – 2011 developing novel laser-based diagnostics to measure temperature and energetic processes in solid nanosystems and across interfaces adjacent to solid and liquids. In 2011, Patrick returned to the University of Virginia as an Assistant Professor, and was promoted to Associate Professor with Tenure in 2015. Patrick’s current research interest are in energy transport, charge flow, laser-chemical processes and photonic interactions with condensed matter, soft materials, liquids, vapors and their interfaces. Patrick’s groups at the University of Virginia uses various optical thermometry-based experiments to measure the thermal conductivity, thermal boundary conductance, thermal accommodation, strain propagation and sound speed, and electron, phonon, and vibrational scattering mechanisms in a wide array of bulk materials and nanosystems. In the general fields of nanoscale heat transfer, laser interactions with matter, and energy transport, storage and capture, Patrick has authored or co-authored over 140 technical papers (peer reviewed) and been awarded 3 patents focused on materials, energy and laser metrology. Patrick has been recognized for his accomplishments in these fields via an Air Force Office of Scientific Research Young Investigator Award, an Office of Naval Research Young Investigator Award, the ASME Bergles-Rohsenow Young Investigator Award in Heat Transfer, and the Presidential Early Career Award for Scientists and Engineering