Aggregate-driven reconfigurations of carbon nanotubes in thin networks under strain: in-situ characterization

Laurence Bodelot,Luka Pavić,Simon Hallais,Bérengère Lebental,Jérôme Charliac

doi:10.1038/s41598-019-41989-2

Abstract

This work focuses on the in-situ characterization of multi-walled carbon nanotube (CNT) motions in thin random networks under strain. Many fine-grain models have been devised to account for CNT motions in carbon nanotube networks (CNN). However, the validation of these models relies on mesoscopic or macroscopic data with very little experimental validation of the physical mechanisms actually arising at the CNT scale. In the present paper, we use in-situ scanning electron microscopy imaging and high-resolution digital image correlation to uncover prominent mechanisms of CNT motions in CNNs under strain. Results show that thin and sparse CNNs feature stronger strain heterogeneities than thicker and denser ones. It is attributed to the complex motions of individual CNTs connected to aggregates within thin and sparse CNNs. While the aggregates exhibit a collective homogeneous deformation, individual CNTs connecting them are observed to fold, unwind or buckle, seemingly to accommodate the motion of these aggregates. In addition, looser aggregates feature internal reconfigurations via cell closing, similar to foam materials. Overall, this suggests that models describing thin and sparse CNN deformation should integrate multiphase behaviour (with various densities of aggregates in addition to individual CNTs), heterogeneity across surface, as well as imperfect substrate adhesion.

Highlights

Due to their large specific surface area[1], carbon nanotubes (CNTs) have been of utmost interest for sensing applications since the early days of CNT research[2,3]
Experimental results show that this proposed mechanism may not be fully accurate for denser carbon nanotube networks (CNN): in Yin et al.[26] for example, the authors suggest that the piezoresistivity of a CNN/polymer composite made out of long multi-walled carbon nanotubes (MWCNTs) may be mostly controlled by the presence of aggregates
In order to address this issue, in-situ scanning electron microscope (SEM) imaging of CNNs under strain has started to develop in recent years: in Hutchens et al.[30] and Maschmann et al.[31] very dense pillars of vertically-aligned CNTs show in-situ buckling under compressive strain; in Whitby et al.[32] vortex-like motion of CNTs and pore diameter reduction is observed in a 3D dense CNN/polymer composite under compression; in Gui et al.[33] the microstructural motion of a biphasic CNN is analysed under compression; in Abu Obaid et al.[34] the evolution of the helicoidal organization of the fibres of CNT yarns is observed under tension; and in Stallard et al.[35] straightening and buckling of fibres within direct-spun CNT mats submitted to very large tensile strains are reported

Summary

Introduction

Due to their large specific surface area[1], carbon nanotubes (CNTs) have been of utmost interest for sensing applications since the early days of CNT research[2,3]. Numerical models have evolved to better account for CNT entanglements[27,28] and surface-CNT interactions[29] ( for 3D and self-standing CNNs27, and for supported CNNs29) They appear to predict well the global mechanical and electrical behaviour of the CNNs under study, including even complex phenomena such as buckling[27] and piezoresistivity hysteresis[29]. The limitation of these approaches is that the relevance of the model is usually validated only by fitting global curves such as Young’s modulus or resistance as a function of strain. There is no in-situ SEM study on substrate-supported CNNs even though the microstructural understanding of the CNN-substrate interaction is critical to model these structures appropriately (as shown in Jin et al.[29])

Objectives

Methods

Results

Conclusion