|
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 band
gaps 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 nano-structures) with high thermoelectric
figure of merit.
In laser cooling of ion-doped
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 an 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 nano-structures (e.g., nano
powders).
In MEMS cryo-cooler project, we
use a novel multi-stage, 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
micro-fabrication we optimize the film-support structure to
minimize heat and electrical losses to achieve the lowest
cold-stage temperatures.
In our micro heat spreader, 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 micro-machined 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 pore size in the polymer
electrolyte leading to pore-water state transition which
enhances proton transport. The pore-surface proton conductivity
undergoes a critical increase when the pore-water is increased
passed a threshold which allows for the adsorbed water molecules
to form bridged (capillary) condensate across the pore (due to
overlapping surface forces). In fuel cell operation it is
desired to have the least amount of water in the pores, while
preventing polymer electrolyte water content below this
threshold.
| 
