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Deformation behavior and mechanical analysis of vertically aligned carbon nanotube (VACNT) bundles

Citation

Hutchens, Shelby B. (2011) Deformation behavior and mechanical analysis of vertically aligned carbon nanotube (VACNT) bundles. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:05262011-141718914

Abstract

Vertically aligned carbon nanotubes (VACNTs) serve as integral components in a variety of applications including MEMS devices, energy absorbing materials, dry adhesives, light absorbing coatings, and electron emitters, all of which require structural robustness. It is only through an understanding of VACNT’s structural mechanical response and local constitutive stress-strain relationship that future advancements through rational design may take place. Even for applications in which the structural response is not central to device performance, VACNTs must be sufficiently robust and therefore knowledge of their microstructure-property relationship is essential. This thesis first describes the results of in situ uniaxial compression experiments of 50 micron diameter cylindrical bundles of these complex, hierarchical materials as they undergo unusual deformation behavior. Most notably they deform via a series of localized folding events, originating near the bundle base, which propagate laterally and collapse sequentially from bottom to top. This deformation mechanism accompanies an overall foam-like stress-strain response having elastic, plateau, and densification regimes with the addition of undulations in the stress throughout the plateau regime that correspond to the sequential folding events. Microstructural observations indicate the presence of a strength gradient, due to a gradient in both tube density and alignment along the bundle height, which is found to play a key role in both the sequential deformation process and the overall stress-strain response. Using the complicated structural response as both motivation and confirmation, a finite element model based on a viscoplastic solid is proposed. This model is characterized by a flow stress relation that contains an initial peak followed by strong softening and successive hardening. Analysis of this constitutive relation results in capture of the sequential buckling phenomenon and a strength gradient effect. This combination of experimental and modeling approaches motivates discussion of the particular microstructural mechanisms and local material behavior that govern the non-trivial energy absorption via sequential, localized buckle formation in the VACNT bundles.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:carbon nanotubes, finite element analysis, mechanical behavior, uniaxial compression, constitutive relation
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Awards:2011 Demetriades-Tsafka-Kokkalis Prize in Nanotechnology for best thesis in 'Nanotechnology and Related Fields' at Caltech. 2010 NSF fellowship recipient, "Mechanics of Soft Materials" short course. 2004-2008 Gordan and Betty Moore Teaching and Research Fellowship.
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Greer, Julia R. (advisor)
  • Wang, Zhen-Gang (advisor)
Thesis Committee:
  • Greer, Julia R. (chair)
  • Wang, Zhen-Gang
  • Daraio, Chiara
  • Flagan, Richard C.
Defense Date:16 May 2011
Funders:
Funding AgencyGrant Number
Gordon and Betty Moore FoundationUNSPECIFIED
Institute for Collaborative BiotechnologiesUNSPECIFIED
Record Number:CaltechTHESIS:05262011-141718914
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:05262011-141718914
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6454
Collection:CaltechTHESIS
Deposited By: Shelby Hutchens
Deposited On:31 May 2011 21:38
Last Modified:12 Dec 2014 18:04

Thesis Files

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[img] Video (MPEG) (In situ uniaxial compression of vertically aligned carbon nanotube micropillar.) - Supplemental Material
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[img] Video (MPEG) (Montage of initial buckling event during in situ uniaxial compression of vertically aligned carbon nanotube micropillars) - Supplemental Material
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