Modeling the Auxetic Transition for Carbon Nanotube Sheets

TL;DR

This paper presents a simple predictive model for the mechanical properties of carbon nanotube sheets, capturing their auxetic behavior and enabling the prediction of how structural modifications affect their Poisson's ratios.

Contribution

The model incorporates key structural features of buckypapers and predicts their complex mechanical responses, including the transition to auxetic behavior, based on nanotube composition and orientation.

Findings

Poisson's ratio can be tuned from positive to negative

Model accurately predicts observed mechanical properties

Structural modifications can induce auxetic behavior

Abstract

A simple model is developed to predict the complex mechanical properties of carbon nanotube sheets (buckypaper) [Hall \textit{et al.}, \textit{Science} \textbf{320} 504 (2008)]. Fabricated using a similar method to that deployed for making writing paper, these buckypapers can have in-plane Poisson's ratios changed from positive to negative, becoming auxetic, as multiwalled carbon nanotubes are increasingly mixed with single-walled carbon nanotubes. Essential structural features of the buckypapers are incorporated into the model: isotropic in-plane mechanical properties, nanotubes preferentially oriented in the sheet plane, and freedom to undergo stress-induced elongation by both angle and length changes. The expressions derived for the Poisson's ratios enabled quantitative prediction of both observed properties and remarkable new properties obtainable by structural modification.