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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.
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