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Theory and Measurements of Thermal Properties in Nanowires and Carbon Nanotubes

Bifano, Michael F. P.
http://rave.ohiolink.edu/etdc/view?acc_num=case1339998535

Abstract Details

2012, Doctor of Philosophy, Case Western Reserve University, EMC - Mechanical Engineering.
The extraordinary potential of nanoscale materials and nano-constituent composite materials coupled with rapid progress in the capability to synthesize these materials has created a demand for their property characterization. To develop a better understanding of thermal transport in nanoscale wires and nanotubes, this thesis utilizes both theoretical and experimental techniques to determine how nanostructure geometry, surface properties, and heat treatment affect their ability to conduct thermal energy. As an example, the effect of heat treatment on the thermal conductivity of commercially available, chemical vapor deposition-grown, multiwalled carbon nanotubes (MWCNTs) is presented. The measurement device is implemented inside a scanning electron microscope equipped with nanomanipulators and a gas injected electron beam deposition system for repeatable in-situ sample characterization. MWCNT samples treated to 3000 C show a 5-fold increase in average thermal conductivity as compared to the as-synthesized samples. By including an estimation of thermal contact resistance due to phonon impedance mismatch, the average thermal conductivity of the heat-treated MWCNT specimens is estimated to be 228 W/m-K. The results suggest that heat treatment is a viable method to improve thermal conductivity and highlight the importance of MWCNTs quality in thermal management applications. Classical MD simulations are used to investigate the sensitivity of thermal conductivity to side-wall defects in SWCNTs. Vacancy repair is evident with heat treatment. 3000 C heat treatment of SWCNTs having varying degrees of defect concentrations is found to generally increase thermal conductivity by at least 10 %. The results suggest that phonon mean free path in (6,6) SWCNTs is nearly equally impeded by side-wall functionalization as compared to atomic vacancies. Classical MD simulations estimate that 2 atom % of hydrogenation and 1.5 - 2 % vacancy concentrations reduce thermal conductivity to the same degree. As compared to non-functionalized SWCNTs, the results suggest that the use of chemically functionalized SWCNTs as a second phase material in multifunctional CNT-polymer composites may reduce thermal transport. Nanoscale effects on thermal properties are further evaluated by an investigation into the dependency of specific heat and ballistic thermal conductance on cross-sectional geometry in free-standing isotropic non-metallic crystalline nanostructures. Analysis of phonon confinement is performed using dispersion relations found by numerically solving the Pochhammer-Chree frequency equation for a tube. These dispersion relations are used to evaluate the specific heat and ballistic thermal conductance in the nanostructures as a function of the nanostructure geometry, size, and surface stiffness. 1D, 2D, and 3D geometric phonon confinement regimes are recognized and found to depend on both the nanostructure’s wall thickness and outer radius. Compared to nanowires, the frequency reduction of acoustic phonon modes in thinner walled nanotubes is shown to elevate the ballistic thermal conductance of the thin-walled nanotube between 0.2 K and 150 K. At 20 K, the ballistic thermal conductance of the thin-walled nanotube is found to be 300% greater than that of a solid nanowire. Surface modification of nanotubes and nanowires is performed using a multilayer elastic model to increase the average Young’s modulus of the first three atomic surface layers. The acoustic stiffening of the interior and exterior lateral walls of nanotubes is found to have a contrasting effect on specific heat and thermal conductivity as compared to the outer surface modification of the nanowire. A 10% reduction in specific heat and a 2% reduction in lattice thermal conductivity at 50 K occur in a crystalline nanotube having a 10 nm outer and 5 nm inner diameter. In contrast, at the same temperature, an approximate 30% increase in thermal conductivity and specific heat occurs when the acoustically stiffened surface is applied to the outer diameter of a nanowire with a solid cross-sectional area. The simplified model has the potential to investigate the acoustic engineering of nanowires and nanotubes by inducing surface stiffening or softening via appropriate surface chemical functionalization protocols or coatings.
Vikas Prakash (Committee Chair)
Iwan Alexander (Committee Member)
Kamotani Yasuhiru (Committee Member)
Mathur Harsh (Committee Member)
Dai Liming (Committee Member)
308 p.
Aerospace Engineering; Aerospace Materials; Engineering; Mechanical Engineering; Physics
Carbon Nanotube; Heat Treatment; Phonon Dispersion; Phonon Confinement

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Citations

  • Bifano, M. F. P. (2012). Theory and Measurements of Thermal Properties in Nanowires and Carbon Nanotubes [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1339998535

    APA Style (7th edition)

  • Bifano, Michael. Theory and Measurements of Thermal Properties in Nanowires and Carbon Nanotubes. 2012. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1339998535.

    MLA Style (8th edition)

  • Bifano, Michael. "Theory and Measurements of Thermal Properties in Nanowires and Carbon Nanotubes." Doctoral dissertation, Case Western Reserve University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=case1339998535

    Chicago Manual of Style (17th edition)

case1339998535
582
© 2012, all rights reserved.
This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.