Effective mechanical properties in carbon nanotube-silica nanocomposite

Abstract

Carbon nanotube (CNT) based nanocomposites have gained great technological importance in recent years due to their outstanding structural, electronic, optical and thermal properties. However, the effect of nanotube on macroscopic properties of composites remains less understood. The underlying challenge is to characterize the complex interaction between nanotubes and the surrounding matrix which can depend on the interfacial strength of the nanotube and its diameter, as well as various scale-dependent mechanisms. ☐ To address the challenge, this thesis develops a multiscale approach. The approach integrates simulations with density functional theory (DFT), molecular dynamics (MD), and the finite element (FE) method. DFT is used to compute fundamental material properties, MD to study sub-micron scale deformation mechanisms, and FEM to study longer length scale behavior. We also compare the FEM results with the MD results. The entire investigation has two parts: (a) investigation of interfacial strength, and (b) investigation of nanotube diameters, with a focus on their effects on macroscopic properties. For carrying out the investigations, we take silica (SiO2) as an example matrix material (due to its many critical applications) and explore the effect of nanotube reinforcement on stiffness, strength and toughness of CNT-SiO2 nanocomposites. ☐ For the interfacial strength study, the results show that nanotube delaminates when the interfacial strength is low enough and it fractures when the interfacial strength is high. The delamination process can therefore play a significant role in controlling the effective fracture strength and toughness of the CNT-SiO2 nanocomposites. The condition for delimination is governed by an intricate site-dependent interaction between the nanotube and the SiO2 matrix. Nonetheless, increasing interfacial strength improves the effective properties substantially. For example, a two-fold increase in interfacial strength increases effective strength by more than 7% and effective toughness by more than 16%. The overall influence is however non-linear, and there is a corresponding mathematical relationship between the interfacial strength and effective strength or toughness. Additionally, we find that increasing nanotube diameter decreases stiffness and strength, but its effect on toughness is difficult to quantify. ☐ Incorporating the atomistic information in a finite element based continuum framework, it is found that the macroscopic stress-strain response of a hole-SiO2 nanostructure is inconsistent with the corresponding stress-strain response in MD simulation, although the bulk material properties in the finite element calculation are taken from the MD simulation. Therefore, this thesis concludes that developing multiscale computational approach is necessary to determine the stress-strain behavior of composites accurately as well as to predict the implication of nanostructure-reinforcement on macroscopic or effective properties of the nanocomposite.