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OBJECTIVE
Our research is on
atomic-level material design (metrics) for innovative heat
transfer functions. The fundamentals are transport and
transformation kinetics of thermal energy involving principal
carriers: phonon, electron, fluid particle, and photon.
Descriptions of our current projects are given below.
Thermoelectric materials used
for cooling and power generation. While small electronic
bandgaps and relatively high carrier concentrations help with
desirable, enhancing electrical properties (Seebeck coefficient
and electrical conductivity), enhancing phonon scattering helps
with desirable lowering of phonon conductivity. Using quantum
and molecular dynamics computations, we search for molecular
structures (including nanostructures) with high thermoelectric
figure of merit.
In MEMS cryocooler project, we
use a novel multistage, planar micro thermoelectric cooler
designed to cool functional microstructures. The thermoelectric
materials used are co-evaporation deposited telluride compound
films which have small phonon conductivity. Using
microfabrication we optimize the film-support structure to
minimize heat and electrical losses to achieve the lowest
coldstage temperatures.
In laser cooling of iondoped
crystals, the absorbed photon has a deficit in overcoming the
electronic transition gap and this is made up by absorbing
phonons (thus cooling the crystal). These are cooperative
processes where multiple principal carriers are designed to
assist for a efficient net thermal function. We look at
increasing the efficiency (and extending the cooling range to
cryogenic temperatures) of this laser cooling by optimizing the
photon and phonon absorptions using atomic-level design of the
host and ion atoms and also nanostructures (e.g., nanopowders).
In our microheatspreader, we
use distributed capillary-artery/evaporator design to remove
large heat flux from concentrated sources, such as
microprocessors, with smallest overall thermal resistance. We
use micromachined porous structures with surface treatments
aimed at maximizing capillary flows, while delivering the liquid
directly over the heat source.
In our polymer electrolyte fuel
cell project, we examine optimal nanopore size in the polymer
electrolyte leading to porewater state transitions which
enhances proton and water transport. The poresurface proton
conductivity undergoes a critical increase when the porewater is
increased passed a first threshold which allows for the adsorbed
water molecules to be continuous. Also, for liquid connectivity
or interpore bridging (capillary), adsorbed water layers join
across the pore (due to overlapping surface forces) causing a
second water content threshold (or transition). Due to small
pore size, this second threshold corresponds to high liquid
saturation in the electrolyte. In fuel cell operation, since the
liquid water saturation in electrolyte influences the water
contents of other layers in the cell, it is desired to have the
least amount of water in the pores, while avoiding polymer
electrolyte water content below the first threshold. | 
