Capillary densification and adhesion tuning of aligned carbon nanotube arrays for shape-engineerable architectures

Abstract

The advantaged intrinsic and scale-dependent properties of aligned nanofibers (NFs), such as carbon nanotubes (CNTs), and their ability to be self-assembled by capillary forces motivates their use as shape-engineerable materials. In order to achieve facile and accurate patterning and densification of NFs into new architectures, a mechanistic understanding of the parameters governing NF synthesis, densification, and substrate adhesion is needed. Here, parametric experiments and models are developed to evaluate the scaling of NF-substrate adhesion strength (Fa) of mm-scale tall aligned CNT arrays as a function of CNT growth time (tg), and to control the solvent-based capillary densification of sub-mm-scale tall CNT arrays into cell and pin structures, which have tunable geometries based on CNT height, Fa, and, if used, the as-grown CNT array pattern size (s).

One-dimensional scaling relations are presented that accurately predict the morphology of these capillary-densified CNTs exhibiting multiple spatial scales, including long-range cellular networks formed from bulk-scale CNT arrays, and solid, micron-scale pins formed via the densification of patterned CNT arrays below and at the critical s that separates cell vs. pin formation. The effective CNT array elastic modulus (E), and not the orders-of-magnitude higher isolated CNT axial modulus, is found to govern the width, area, wall thickness, and volume fraction of the densified cells and pins. E is about an order of magnitude smaller for pins as compared to cell networks formed from bulk-scale (i.e., non-patterned) CNT arrays, and patterning therefore results in pins with a lower packing density (commensurate with double the wall thickness) and a larger characteristic spacing than bulk cell networks.

Further increasing s recovers the bulk-scale cell scaling relations, as the cell geometry plateaus for s>~~ 1000 [mu]m. Additional tuning of the cell network geometry is possible by altering Fa via a simple post-growth annealing step. However, while controlling this adhesion is needed for bulk-scale manufacturing and application-specific performance, experimental and theoretical approaches to date have neglected to address the scaling of Fa with tg, a crucial process parameter governing CNT synthesis, structure, and properties. Here, the non-monotonic scaling of Fa with tg is measured experimentally via uniform tensile CNT array separation from a flat growth substrate and modeled analytically based on atomic and meso-scale CNT evolution using contact mechanics. CNT growth termination is marked by a reduction in CNT number density and a plateau in array height, signifying the transition between two distinct process-structure-property scaling regimes.

At this growth termination point, experiments and modeling indicate a one- and two-orders-of-magnitude increase in Fa and E, respectively. Here, the observed increase in CNT wall thickness and sp³ bond character with tg shows that the accumulation of turbostratic carbon species in the CNT array contributes to the evolving mechanical response. Future paths of study are recommended to extend this work towards two-dimensional capillary densification of NF arrays and Fa tunability via post-processing, which would allow for the densification of taller NF arrays (towards mm- and cm-scales) to expand the suitability of these materials for a broader range of applications. Collectively, these results enable the use of capillary densification and tunable NF-substrate adhesion for the design and manufacture of bulk nanoengineered materials and emerging nanoscale technologies.