Phase transformation as the mechanism of mechanical deformation of vertically aligned carbon nanotube arrays: Insights from mesoscopic modeling

Abstract

Vertically aligned carbon nanotube (VACNT) arrays or “forests” behave mechanically as foams when compressed, exhibiting a characteristic nonlinear stress – strain response. However, the fiber structure of VACNT forests is unlike that of cellular foams, and the microscopic mechanisms of the deformation are quite different. While numerous studies have addressed the mechanical response of VACNT forests undergoing uniaxial compression, the underlying deformation mechanisms are not yet fully established. In this paper, we report the results of large-scale mesoscopic simulations of the uniaxial compression of a VACNT forest composed of 2-μm-long carbon nanotubes (CNTs) as well as three structurally distinct forests composed of 0.6-μm-long CNTs. The simulations reveal that the compressive deformation proceeds as a phase transformation from an original low-density phase composed of vertically aligned CNT bundles to a densified phase with horizontal alignment of CNTs. The two phases are separated by a well-defined interfacial layer, which advances during the compressive deformation through localized bending and folding of nanotubes. For the 2-μm-tall forest, the folding involves correlated displacements of multiple CNT bundles, hinting on the origin of the collective buckling behavior observed in experiments. The characteristic three-stage stress-strain dependence (an initial "elastic" peak followed by an extended plateau region and a sharp rise of stress in the densification regime), commonly observed in experimental probing of the mechanical properties of VACNT forests, is reproduced in all of the simulations, suggesting that the heterogeneous propagation of densification front may be the general mechanism of the mechanical deformation of VACNT forests.