Morphology control of aligned carbon nanotube pins formed via patterned capillary densification

Abstract

The exceptional intrinsic properties of aligned nanofibers, such as carbon nanotubes (CNTs), and their ability to be easily densified by capillary forces motivates their use as shape-engineerable materials. While a variety of self-assembled CNT structures, such as cell networks, micropillars, and pins have previously been fabricated via the capillary-mediated densification of patterned CNT arrays, predicting the critical pattern size (s cr) that separates cell versus pin formation and the corresponding process-morphology scaling relations within the micrometer range are outstanding. Here, facile and scalable mechanical patterning and capillary densification techniques are used to establish s cr by elucidating how the effective elastic modulus of aligned CNT arrays during densification governs the resulting pin geometries. Experiments and modeling show that this effective modulus scales with CNT height and is about an order of magnitude smaller for pins as compared to cell networks formed from bulk-scale (i.e. non-patterned) CNT arrays. Patterning therefore results in pins with a lower packing density (commensurate with double the wall thickness) and a larger characteristic length scale than bulk cell networks (i.e. s cr ∼ 5 × cell width). CNT arrays with the initial randomly-oriented carbon 'crust' removed via oxygen plasma etching yield a higher degree of structural uniformity and better agreement with the proposed elasto-capillary model, which enables the use of capillary densification to predictively design hierarchical and shapetunable materials for advanced thermal, electronic, and biomedical devices. RECEIVED

Substrate adhesion evolves non-monotonically with processing time in millimeter-scale aligned carbon nanotube arrays

“…It is noted that this effective array modulus, and not the orders-of-magnitude higher intrinsic CNT axial elastic modulus of 1 TPa 86 governs the bulk CNT array's interfacial behavior, similar to the mechanics governing the capillary densification of CNT arrays, for which CNT-substrate adhesion is an important consideration. 27,87,88 Mode I n(t p ) is therefore quantified by a cubic fit, as detailed in Section S1 of the Supporting Information. The ∼11% lower n value at growth termination (calculated as ∼2.3×10 9 CNTs/cm 2 at t p = 40 min) is kept constant in Mode II, since CNT growth in the vertical direction has stopped, and further information about n scaling after growth termination is not currently available.…”

Selectively Tuning the Substrate Adhesion Strength of Aligned Carbon Nanotube Arrays via Thermal Postgrowth Processing

“…The CNTs were grown on catalyst-coated (CC) wafers, i.e., 1 cm × 1 cm Si wafer substrates with a 500-nm-thick layer of SiO 2 thermal oxide layer, on top of which was deposited a 10-nm-thick Al 2 O 3 layer and then a 1-nm-thick Fe catalyst layer via electron beam physical vapor deposition . The CNTs were grown on the Fe surface via a base-growth mechanism . During the 15 min growth period, the CNTs self-assemble into vertically aligned arrays composed of multiwalled CNTs with an average outer diameter of ∼8 nm (three to seven walls with ∼5 nm inner diameter and intrinsic CNT density of ≈1.6 g/cm 3 ), , inter-CNT spacing of ∼60–80 nm, , and V f of ∼1 vol % CNTs. , …”

“…Thermal processing to tune adhesion is also complementary to prior work that showed a 3 times increase in CNT–substrate adhesion strength achieved by abruptly terminating CNT growth, i.e., abruptly decreasing the precursor gas concentration to zero. The tunability of CNT–substrate strength also enables the capillary-mediated densification of aligned NFs into dense patterned structures, , for which knowledge of substrate interfacial strength effects is still lacking. , …”

Process-Structure-Property Relations in Dense Aligned Carbon Nanotube/Aerospace-grade Epoxy Nanocomposites

“…The processing steps to create A-CNT PNCs from the aerospace-grade epoxy resin (CYCOM 977-3) 23 are as follows, similar to prior work. 18,43 First, a free-standing CNT array (from 1− 30 vol% CNT V f ) was placed in a steel mold matching its dimensions, ensuring that the primary axis of the CNTs was orthogonal to the plane of the mold, and the entangled CNT layer at the top of the as-grown A-CNT array (formed during synthesis) 44 was at the bottom of the mold. 45 Before adding CNTs, the mold was first coated in mold release (Loctite Frekote 700-NC) to ensure that the fabricated PNCs could be removed after processing.…”