Molecular dynamics simulations on enhancement of mechanical properties of clay using carbon nanotubes

Abstract

Clay is one of the most common materials on earth with a long history of use by humankind. It finds its applications in fields such as ceramics, building materials, paper coating and filling, drilling mud, foundry moulds, pharmaceuticals, etc. Nevertheless, due to its low tensile and shear strength, clay has limited use as a construction material. Carbon nanotubes (CNTs), on the other hand, are materials that exhibit superior mechanical properties. Owning to their low density, CNTs have been used as additives and enhancements to various materials (such as polymers, ceramics and cement), and there has been extensive research to explore the properties of these composite materials. However, little research has been done in the field of clay-CNTs composite materials in which clay is used as the matrix. Moreover, the microstructure of the composites at the molecular level and the mechanical behaviours of the clay-CNTs composites are unknown. To address this gap, this thesis explores interactions between 1) CNTs and water; 2) CNTs and clay; and 3) CNTs, clay and water to analyse the mechanical behaviours of the composite materials at a molecular level. The study of CNTs-water interaction can be achieved through molecular dynamics (MD) simulations on water-transport processes in CNTs. The following has been found: 1) the change of driving force on one water molecule in an (8, 0) CNT has three stages that are based on the pre-twist angle; 2) the transportation of different water molecules in a restrained (8, 0) CNT reveals that driving force decreases with water mass; 3) the transport of 20 water molecules is found to be the most efficient way based on the travelling time per water molecule; 4) an (8, 8) CNT with a larger diameter has a higher transportation efficiency when the temperature is above 1500 K; 5) an unrestrained (8, 8) CNT with a length of 19.80 nm is the fastest and most efficient transporter of one water molecule; 6) for an unrestrained (8, 8) CNT, a water molecule is found to be only transportable below 300 K. The study of CNTs-clay interactions is conducted by examining the elastic behaviour of the CNTs-clay model by MD simulations. It is found that the Young’s modulus of the clay model with CNTs is significantly improved compared with the clay model without CNTs. This fact shows that CNTs can indeed enhance the mechanical properties of clay. CNTs-clay-water interaction is investigated by examining the shear behaviour of the CNTs-clay-water model by MD simulations. It is found that the presence of water molecules in the CNTs-clay model greatly increases the shear stress needed to shear the CNTs-clay model compared to the dry CNTs-clay model. This may be due to the frictions generated between a layer of water molecules and the CNTs.