Compressive response of vertically aligned carbon nanotube films gleaned from in situ flat-punch indentations

Pathak, Siddhartha; Mohan, Nisha; Abadi, Parisa Pour Shahid Saeed; Graham, Samuel; Cola, Baratunde A.; Greer, Julia R.

doi:10.1557/jmr.2012.366

Cited by 22 publications

(15 citation statements)

References 46 publications

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“…This causes the CNTs directly under the indenter to collectively separate from the rest of the CNT forest as a block, presumably because of their low resistance to shear as has been previously reported. 4,8,9 In contrast, the rigid constraint at the top of the coated CNT forests drives their deformation via out-of-plane bending of the buckled area, similar to Mode I crack opening. Larger crack-opening displacements correspond to lower radii of curvature, which in turn generate a greater bending moment and higher tensile stress acting on the interface between the CNT forest and the substrate.…”

Section: -3mentioning

confidence: 99%

Buckling-driven delamination of carbon nanotube forests

Abadi

Hutchens

Greer

et al. 2013

Applied Physics Letters

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We report buckling-driven delamination of carbon nanotube (CNT) forests from their growth substrates when subjected to compression. Macroscale compression experiments reveal local delamination at the CNT forest-substrate interface. Results of microscale flat punch indentations indicate that enhanced CNT interlocking at the top surface of the forest accomplished by application of a metal coating causes delamination of the forest from the growth substrate, a phenomenon not observed in indentation of as-grown CNT forests. We postulate that the post-buckling tensile stresses that develop at the base of the CNT forests serve as the driving force for delamination.

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Section: -3mentioning

confidence: 99%

Buckling-driven delamination of carbon nanotube forests

Abadi

Hutchens

Greer

et al. 2013

Applied Physics Letters

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“…Vertically aligned carbon nanotube (CNT) films may combine mechanical compliance with high thermal conductivity (7-18), but there have been few reports of the in-plane modulus of these films and little physical explanation for the wide range for the data (2-300 MPa) (19)(20)(21)(22)(23)(24)(25). Our previous data for the thickness-dependent in-plane modulus of multiwalled CNT films indicated a strong dependence on the nanotube density and alignment (21), which are linked to the detailed growth details (26)(27)(28).…”

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Zipping, entanglement, and the elastic modulus of aligned single-walled carbon nanotube films

Won

Gao

Panzer

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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Reliably routing heat to and from conversion materials is a daunting challenge for a variety of innovative energy technologies--from thermal solar to automotive waste heat recovery systems--whose efficiencies degrade due to massive thermomechanical stresses at interfaces. This problem may soon be addressed by adhesives based on vertically aligned carbon nanotubes, which promise the revolutionary combination of high through-plane thermal conductivity and vanishing in-plane mechanical stiffness. Here, we report the data for the in-plane modulus of aligned single-walled carbon nanotube films using a microfabricated resonator method. Molecular simulations and electron microscopy identify the nanoscale mechanisms responsible for this property. The zipping and unzipping of adjacent nanotubes and the degree of alignment and entanglement are shown to govern the spatially varying local modulus, thereby providing the route to engineered materials with outstanding combinations of mechanical and thermal properties.nanostructured materials | van der Waals | thermal interface materials | thermoelectrics | energy conversion N anostructured materials provide unique combinations of properties that promise performance breakthroughs for applications ranging from energy conversion to data storage and computation (1-4). In many cases it is the very unusual combination of two properties (2), neither of which is an extreme value when considered alone, that leads to adoption and major performance benefits. An example is the search for a mechanically compliant thermal conductor that can, for example, link semiconducting materials with the metals used for heat spreading and exchange. A particularly compelling case is thermoelectric waste heat recovery systems (3), for which the interfaces between the thermoelectric materials, electrodes, insulators, and heat exchangers must accommodate enormous, repetitive thermomechanical stress. However, nature does not provide a material combining the necessary high thermal conductivity with the required low elastic modulus (5, 6).Vertically aligned carbon nanotube (CNT) films may combine mechanical compliance with high thermal conductivity (7-18), but there have been few reports of the in-plane modulus of these films and little physical explanation for the wide range for the data (2-300 MPa) (19-25). Our previous data for the thickness-dependent in-plane modulus of multiwalled CNT films indicated a strong dependence on the nanotube density and alignment (21), which are linked to the detailed growth details (26-28). Other approaches, such as mesoscopic simulations or atomistic models, found that CNT networks exhibit unique self-organization, including bending and bundling (29-31). Therefore, relating the nanoscale morphological details to the mechanical properties is critical.Combined experimental, theoretical, and computational techniques applied to the more complex and fundamentally challenging single-walled CNT system are presented in this paper, along with the in-plane data for the modulus for sin...

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“…Uniaxial compression experiments on VACNT samples with micrometer dimensions ,,,, and indentations into VACNT forests ,− have demonstrated their mechanical deformation characteristics to be similar to open-cell foams. , In the spirit of a foam-like response, the compressive stress–strain relationship for VACNTs has three distinct regimes: (1) initial linear elastic loading, (2) an oscillatory plateau, where each undulation in the data corresponded to a localized buckle formation in the material, and (3) a densification, manifested by a rapid stress increase. In contrast to cellular foams, which typically deform by bending and collapsing of cell walls, , postelastic flow in VACNTs is accommodated by the formation of localized folds or buckles of the entangled arrays, usually oriented orthogonally to the compression direction (Figure S1).…”

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Local Relative Density Modulates Failure and Strength in Vertically Aligned Carbon Nanotubes

et al. 2013

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Micromechanical experiments, image analysis, and theoretical modeling revealed that local failure events and compressive stresses of vertically aligned carbon nanotubes (VACNTs) were uniquely linked to relative density gradients. Edge detection analysis of systematically obtained scanning electron micrographs was used to quantify a microstructural figure-of-merit related to relative local density along VACNT heights. Sequential bottom-to-top buckling and hardening in stress–strain response were observed in samples with smaller relative density at the bottom. When density gradient was insubstantial or reversed, bottom regions always buckled last, and a flat stress plateau was obtained. These findings were consistent with predictions of a 2D material model based on a viscoplastic solid with plastic non-normality and a hardening–softening–hardening plastic flow relation. The hardening slope in compression generated by the model was directly related to the stiffness gradient along the sample height, and hence to the local relative density. These results demonstrate that a microstructural figure-of-merit, the effective relative density, can be used to quantify and predict the mechanical response.

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Compressive response of vertically aligned carbon nanotube films gleaned from in situ flat-punch indentations

Cited by 22 publications

References 46 publications

Buckling-driven delamination of carbon nanotube forests

Buckling-driven delamination of carbon nanotube forests

Zipping, entanglement, and the elastic modulus of aligned single-walled carbon nanotube films

Local Relative Density Modulates Failure and Strength in Vertically Aligned Carbon Nanotubes

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