Thermal conductivity measurement on individual SiGe nanowires
Date Issued
2012
Date
2012
Author(s)
Hsiao, Tzu-Kan
Abstract
In this thesis, I describe experimental thermal conductivity measurements on individual SiGe nanowires, which is a model alloy system that the role of alloy scatterings of phonons in nanostructures can be investigated for the first time.
Introducing efficient and yet independent methods of engineering phononic properties of a material are important topics for enhancing the energy conversion efficiency of thermoelectric materials. Recently, it has been pointed out theoretically that alloy scatterings of phonons can filter out most high frequency optical phonons and may lead to ballistic phonon phenomena at microscales. However, so far no experimental investigations have been conducted to verify the extraordinary effect. In my thesis, I will show direct evidence that the phonon mean free path of SiGe nanowires exceeds 8.3μm, which is the longest scale ever observed in all thermal conductors at room temperature.
In the first part of the thesis, I introduce the background of our research topic, and then I describe various techniques for experimental implementations, including setting up a temperature control system, building a gas injection system in the SEM chamber, designing experimental procedures and analyzing data.
In the second part of the thesis, I present the experimental findings of ballistic phonons in SiGe nanowires. Strong alloy scatterings in homo- or hetero-structures of SiGe nanowires effectively filter out most high frequency optical phonons but leave ~0.04% of the excited phonon modes responsible for heat conduction in the nanowire. It results in our observation of a linear length dependence of thermal conductivity with phonon mean free paths exceeding 8.3μm. In addition, thermal conductivities of SiGe nanowires exhibit weak diameter dependence. The absence of umklapp process features in the temperature dependence of thermal conductivity indicates that the phonon-phonon interactions are negligible and the heat transfer of SiGe nanowires is dominated by alloy scatterings. Remarkably, the low frequency ballistic phonons are immune to structural deformation, stacking faults, twin boundaries, local strains, and elemental variations.
Based on my result, the nearly monochromatic low frequency phonons in alloyed materials provide a unique opportunity to obtain much reduced thermal conductivities in SiGe via effects of phononic crystals. By introducing complete phononic band gap in a SiGe thin film, the thermal conductivity will likely reach 0.05 W/m-K at room temperature, and correspondingly, increase the figure of merit (ZT) of thermoelectric properties of SiGe. In the third part of my thesis, I design a porous thin film with 2D honeycomb structures. The ultralow thermal conductivity results from the phononic bandgap will likely realize ZT ~ 2.4 at room temperature and display a peak ZT = 8.8 at 1100 K.
Subjects
Silicon-Germanium nanowires
phonon
heat transfer
ballistic transport
thermoelectric
phononic crystal
Type
thesis
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