Phonon thermal transport at the nanoscale

Wilson, Richard B

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https://hdl.handle.net/2142/78354 Copy

Description

Title

Phonon thermal transport at the nanoscale

Author(s)

Wilson, Richard B

Issue Date

2015-04-08

Director of Research (if dissertation) or Advisor (if thesis)

Cahill, David G.

Doctoral Committee Chair(s)

Cahill, David G.

Committee Member(s)

• Trinkle, Dallas R.
• Sinha, Sanjiv
• Krogstad, Jessica

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Phonons
• Thermal Transport

Abstract

A comprehensive description of how heat and temperature evolve on nanometer to submicron length-scales does not yet exist because of gaps in our fundamental understanding of interfacial thermal transport and nondiffusive thermal transport. In this dissertation, I address these gaps in fundamental understanding. Interfaces often dominate the thermal response in nanoscale systems. However, a microscopic description of how heat is transported across crystal boundaries remains elusive. I present time-domain thermoreflectance (TDTR) experiments that improve our fundamental understanding of interfacial thermal transport. I show that, for clean interfaces between the two crystals, G derived from TDTR data usually lies in the range 0.25 Gmax < G < 0.7Gmax , where Gmax is the maximum possible conductance predicted by simple theory. Notable exceptions are Al/Si0.99Ge0.01, and Al/Si0.2Ge0.8, where G < 0.25Gmax . Analyzing TDTR data of Al/SiGe alloys with either a two-channel diffusive model or a two-channel ballistic/diffusive model explains the unusually low thermal conductances. Both models predict a significant reduction in the effective thermal conductivity of semiconductor alloys near an interface as a result of disparate heat flux boundary conditions for different groups of phonons in combination with weak coupling between different groups of phonons in the near interface region of the crystal. While it is well established that Fourier theory can break down in nanoscale thermal transport problems, various theories for how and why Fourier theory breaks down do not adequately describe existing experiments. I characterize the relationship between the failure of Fourier theory, phonon mean-free-paths, important length-scales of the temperature-profile, and interfacial-phonon scattering by TDTR experiments on nonmetallic crystals. When crystals are heated by a laser with a radius of less than two microns, Fourier theory overpredicts the materials ability to carry heat away from the heated region. The presence of the interface and the anisotropy of the temperature-profile results in an effective thermal conductivity tensor that is anisotropic.

Graduation Semester

2015-5

Type of Resource

text

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Copyright and License Information

Copyright 2015 Richard Brian Wilson

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