Visualizing Strain Evolution and Coordinated Buckling within CNT Arrays by In Situ Digital Image Correlation

Abstract

of CNT arrays in applications such as thermal [ 1-3 ] and electrical [ 4,5 ] interface materials, deformable CNT array strain biosensors, [ 6 ] and electrical brushes [ 7 ] rely intimately on array mechanics for proper operation. Often, optimization for a specifi c application requires consideration of two or more coupled properties, such as stiffness and damping, or stiffness and thermal conductivity. However, the means by which global deformation of a CNT array generates localized deformation of the constituent CNTs, and how this, in turn, determines the collective mechanical properties is not thoroughly understood. Many studies have examined the mechanical properties of CNT arrays via nanoindentation [ 8-11 ] or uniaxial compression, [ 12-15 ] and showed that entangled arrays locally yield by coordinated buckling of CNTs. The buckles propagate in a direction perpendicular to CNT alignment, and strain recovery after unloading can be signifi cantly elastic [ 12 , 15 ] or plastic [ 10,11 , 13,14 ] based on the diameter, alignment, and packing density of the constituent CNTs. Buckle initiation has been observed preferentially near the top [ 11 ] or bottom side of an array, [ 12-14 ] but it is has not been shown how the initiation of buckling relates to the geometry and global deformation of the arrays. More recently, in situ scanning electron microscopy (SEM) mechanical testing has enabled direct observation of buckle evolution and has been utilized for the uniaxial compression of CNT array columns [ 13 ] and the nanoindentation of CNT arrays. [ 11 ] A continuum-based analytical model inspired by the in situ CNT array column compression observations [ 13 ] was recently used to predict the strain distribution that arises during buckling; [ 16 ] however, an experimental technique capable of mapping local strain fi elds within CNT arrays to validate this and other models, and to relate mechanical properties to CNT morphology and array geometry, has not yet been reported. Likewise, a means to quantify the seemingly diverse CNT buckling behavior at a local level is needed to better understand the mechanics driving buckle formation, and to provide a basis for comparing experimental results from different studies. In order to map strain fi elds within CNT arrays, a highresolution, non-contact method is needed, because the small size scale of CNT arrays prevents spatially resolved mechanical measurements via attached hardware such as strain gages. One such

Tailoring the Mechanical Properties of High‐Aspect‐Ratio Carbon Nanotube Arrays using Amorphous Silicon Carbide Coatings

“…These typical buckling characteristics appear to be unique for uncoated CNT arrays. More importantly, the localized periodic buckling events are very reproducible and in excellent agreement with the in situ CNT array compression observations from Hutchens and Maschmann et al [ 5,9 ] Their observations also indicate that buckling events originate at the base of the pillar and the buckling Figure 2 c. The maximum stress that can be applied before the pillar collapses is about 0.85 MPa at a critical compressive strain of about 4.8%. When this stress is exceeded the system transitions from a stable regime (I) towards an unstable regime (II) with rapid strain bursts.…”

“…[1][2][3][4][5] The exceptional properties of CNTs and related materials have triggered tremendous efforts not only to study their intrinsic properties but also to explore their applications in a large variety of fi elds. [6][7][8][9][10][11][12][13] These high-aspect-ratio 3D structures play an important role in the advancement of vertical interconnect technology, [14][15][16][17] fl exible batteries, [ 3 ] stamps for micro/nanoimprint lithography, [ 2,[18][19][20][21] compliant thermal interface materials for low inter-facial resistances, [22][23][24][25] 3D super-capacitors [ 26,27 ] and nano/micro-electromechanical systems (NEMS) and (MEMS). [ 1,[28][29][30] The CNT arrays that we refer to in this work are composed of nominally vertical, interwoven, multi-wall carbon nanotubes.…”

“…[ 31,32 ] A common procedure for growing high-aspect-ratio CNT arrays is via chemical vapor deposition (CVD) on photolithographically defi ned catalyst areas. [ 5,9 ] One of the limitation of this growth process, is the low packing density of the CNTs inside the array. [ 15,33 ] The interwoven CNTs inside the array are held together by a weak van der Waals interaction, allowing tubes to slide along each other.…”

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

“…13 However as seen from Fig. S9, for this current work the shape of the pillar cross-sections can be ruled out as a potential reason for their differences in mechanical behavior and deformation morphology.…”

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

“…Other researchers have reported similar densities in the order of 10 10 toward 10 11 tubes cm -2 . [33][34][35] Furthermore, a representative volume element which consists of multiple interacting CNTs can be defi ned as a unit-cell for the purpose of simulation, see Figure 1 .…”