October 20, 2010

FRONTIERS IN MATERIALS SCIENCE
VINSE COLLOQUIUM SERIES

Dr. Li Shi
Associate Professor and Myron L. Begeman Fellowship in Engineering
Department of Mechanical Engineering and Texas Materials Institute
University of Texas at Austin

"Thermal Transport and Thermoelectric Energy Conversion in Nanostructured and Complex
Materials"

Abstract:  High and low thermal conductivities, respectively, are desirable for increasing the energy
efficiency of electronic and thermoelectric devices. Recently, ultrahigh thermal conductivity
has been reported in mechanically exfoliated monolayer graphene. Our recent measurements
show that even large-area graphene grown by chemical vapor deposition possesses higher
thermal conductivity than graphite, but contact with a dielectric support suppresses the thermal
conductivity of graphene because of phonon leakage across the interface. Despite the still high
thermal conductivity of supported graphene, spatial mapping of the low-frequency phonon
temperature distribution in graphene electronic devices reveals that the major heat dissipation
path is across the dielectric support instead of lateral heat spreading along the graphene to the
metal electrodes. In the other end of the thermal conductivity spectrum, our measurements
reveal highly anisotropic thermal transport in disordered-layered thin films that were found to
possess ultralow cross-plane thermal conductivity. We have also found that the lattice thermal
conductivity of nanowire structures is suppressed considerably for III-V semiconductors, but
only slightly for bismuth telluride with already useful thermoelectric figure of merit (ZT).
In contrast, the thermal conductivity of higher manganese silicide (HMS) nanowires can be
suppressed from the already low bulk values to the amorphous limit. This finding of a glassy
thermal conductivity in a crystal is attributed to the combined effect of a complex crystal
structure and phonon-interface scattering in the HMS nanowires, and points to a potential
approach to enhancing the ZT.

Bio:  Prof. Li Shi received the B.E. degree in Thermal Engineering from Tsinghua University, Beijing
in 1991, M.S. degree from Arizona State University in 1997, and Ph.D. degree in Mechanical
Engineering from University of California at Berkeley in 2001. Dr. Shi was a Research Staff
Member at IBM Research Division from 2001 to 2002. He has held positions of assistant
professor between 2002 and 2006 and associate professor since 2006 at the Department of
Mechanical Engineering and Texas Materials Institute, University of Texas at Austin. Dr.
Shi specializes in thermal transport and thermoelectric energy conversion in nanostructured
and complex materials. His other current research efforts include nanotechnologies for drug
delivery and biomedical imaging. He received the CAREER award from the National Science
Foundation in 2003, the Young Investigator Award from Office of Naval Research in 2004, the
ASME Transaction Journal of Heat Transfer Outstanding Reviewer Award in 2005. He has been
appointed as a Myron L. Begeman Fellow in Engineering at UT Austin since 2007.