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

Abstract

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...

The Evolution of Carbon Nanotube Network Structure in Unidirectional Nanocomposites Resolved by Quantitative Electron Tomography

“…17 As the CNTs are brought closer together, shortrange van der Waals attractions cause CNTs to bundle on different length scales. 44 Understanding the size and morphology of the bundles is, therefore, crucial to the interpretation of transport property measurements, and to the understanding of mechanical property enhancements, which depend strongly on These volumes are designed to be representative by selecting regions that are orders of magnitude larger than the critical microstructural dimensions (CNT diameter and average CNT spacing). Note that a larger volume is probed for the 0.44% V f sample since the CNT spacings are larger.…”

Effects of Nanostructure and Coating on the Mechanics of Carbon Nanotube Arrays

“…More promising are methods based on coarse grained molecular dynamics (CG MD), which can be employed to study the deformation mechanics of CNT arrays on a larger scale. [23][24][25] These idealized models do not consider the tube waviness (tortuosity) since they rely mostly on predefi ned harmonic force fi elds, nor effects of conformal coating. Furthermore, the discrete CG MD models are also limited to a certain size range and time domain, whereas a continuum model will not be restricted.…”

Crust removal and effective modulus of aligned multi-walled carbon nanotube films

“…Several studies analyzed the nanotube morphology [10], but models that quantify nanotube structures to predict their mechanical responses are still lacking. We recently reported the impact of nanotube morphology on their mechanical responses with a predicted model, by assuming the CNT film is nonhomogeneous [9], [11], [17]. Here we introduced the crust layer, which shows more entanglements and is potentially stiffer than the middle or bottom layer.…”

“…In the third stage, the density decay stage, CNTs grow as the same speed with the previous stage, but the mass density per unit area slowly decreases. Sometimes less aligned and less dense morphology is [17] and MWCNT films [11]. Circles represent the measured modulus applied to the simulation, and stars represent the modulus recalculated from the simulation.…”