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Silicon Revisited: Understanding Pure Phonon Anharmonicity and the Effects on Thermophysical Properties

Citation

Kim, Dennis Sungtae (2018) Silicon Revisited: Understanding Pure Phonon Anharmonicity and the Effects on Thermophysical Properties. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9C827HR. http://resolver.caltech.edu/CaltechThesis:02202018-144811029

Abstract

Phonons, quantized lattice vibrations, govern most of the thermophysical properties of solid-state materials such that understanding the temperature dependent lattice dynamics is of great technological importance. I performed inelastic neutron scattering measurements at the Spallation Neutron Source on ARCS, a wide-angular chopper spectrometer, to measure phonon dispersions and density of states over a wide range of temperatures. Large phonon anharmonicities manifested by phonon energy shifts and broadenings were observed in both measured phonon dispersions and phonon density of states. The sources of deviations from the simple harmonic model with temperature were elucidated using experimentally assessed lattice dynamics coupled with ab initio methods. Pure anharmonicity dominates the changes in lattice dynamics with temperature and therefore drive the entropy and thermophysical properties of thermal expansion and thermal conductivity. Crystal structure, anharmonicity, and nuclear quantum effects all play important roles in the thermal expansion of silicon, and a simple mechanical explanation is inappropriate. The quantum effect of nuclear vibrations is also expected to be important for thermal expansion of many materials. My experimental techniques capture the linewidth broadenings from phonon anharmonicity needed to calculate thermal conductivity. The methods developed for data reduction on single crystal inelastic neutron scattering data and predicting macroscopic quantities should also be useful for understanding microscopic mechanisms behind thermophysical properties for materials.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:materials thermodynamics, inelastic neutron scattering, phonon anharmonicity
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Fultz, Brent T.
Thesis Committee:
  • Fultz, Brent T. (chair)
  • Schwab, Keith C.
  • Minnich, Austin J.
  • Nad Perge, Stavan
  • Hellman, Bjoern O.
Defense Date:21 August 2017
Record Number:CaltechThesis:02202018-144811029
Persistent URL:http://resolver.caltech.edu/CaltechThesis:02202018-144811029
DOI:10.7907/Z9C827HR
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1103/PhysRevB.92.054304DOIArticle adapted for ch. 2
http://dx.doi.org10.1073/pnas.1707745115DOIArticle adapted for ch. 3
ORCID:
AuthorORCID
Kim, Dennis Sungtae0000-0002-5707-2609
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:10726
Collection:CaltechTHESIS
Deposited By: Dennis Kim
Deposited On:09 Mar 2018 18:26
Last Modified:23 Jan 2019 17:14

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