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贡献 MIT纳米微薄膜声子传输博士论文一篇
Phonon Heat Conduction in
Nano and Micro-Porous Thin Films
这是一篇博士论文.我认为很不错.
ABSTRACT OF THE DISSERTATION
Phonon Heat Conduction in
Nano and Micro-porous Thin Films
by
David Won-Jun Song
Doctor of Philosophy in Mechanical Engineering
University of California, Los Angeles, 2003
Professor Gang Chen, Chair
In this dissertation, the phonon size effect in the experimental and theoretical studies of random and
periodic porous media are reported. First, a literature review on the past modeling studies on porous
media are presented that covers both the earlier works that use the traditional effective medium
approach and the few existing recent works that consider the low-dimensional effects. Next, the
experimental characterization of the cross-plane thermal conductivity of randomly nano-porous
bismuth thin films is presented. Fabricated in search for more efficient thermoelectric materials, the
nano-porous bismuth films use nano-scale pores to impede phonon transport more than electron
transport. Their cross-plane thermal conductivity characterization using the differential 3ω
technique revealed an order-of-magnitude reduction in the thermal conductivity values of the porous
bismuth over those of non-porous bismuth films and a potential for the independent tuning of their
electrical conductivity and thermal conductivity, but the defect-laden structure was difficult to
model. Therefore, a new study was undertaken that focused on simpler periodic micro-porous
single-crystal silicon membranes. A batch of such membranes were fabricated from both a plain
silicon wafer and a silicon-on-insulator wafer using MEMS techniques, including bulk chemical
etching and deep-reactive ion etching. The resulting samples contained periodically arranged pores
of controlled dimension and orientation, but the pore dimension and orientation was varied from
sample to sample to experimentally isolate the phonon size effect due to pore boundary scattering.
The in-plane thermal conductivity of the micro-porous silicon membranes is characterized by a
modified version of Völklein’s DC method. The resulting thermal conductivity reduction in porous
films compared to the solid silicon film strongly suggest phonon size effect. The three-dimensional
phonon transport in porous silicon membranes were modeled using the Monte Carlo method to yield
an equivalent solution to the three-dimensional Boltzmann transport equation.. Since the focus of
the modeling is the effect of pore boundary scattering on phonon transport, a simplified view of grayxxx
body medium is studied. The thermal conductivity result predicts a thermal conductivity reduction
in porous silicon films compared to solid silicon films by an amount beyond that predicted from
porosity and comparable to that observed in experiments. |
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