Studies On The Mechanical Properties Of Carbon Nanotubes Systems' Interfaces

Carbon nanotubes(CNTs) have wide application potential due to their excellent physical and mechanical properties. The mechanical properties of the interfaces have important effects on the mechanical properties of multi-walled CNTs and their reinforced composites. In this thesis, continuum models and atomic-scale finite element method are used to investigate the mechanical properties of CNTs systems'interfaces, which are dominated by non-bond van der Waals interactions. The main work and conclusions of this thesis can be summarized as follows:In this thesis, the concept of classical cohesive model has been brought into the wall to wall interaction studies, and a cohesive law for CNT walls in multi-wall CNTs is established based on the van der Waals interactions. For an infinitely long CNT, the shear cohesive stress between CNT walls vanishes, and the tensile cohesive stress depends only on the opening displacement. For a finite CNT, the tensile cohesive stress remains the same, but the shear cohesive stress depends on both opening and sliding displacements, i.e. there is tension/shear coupling.Based on the non-bond van der Waals interactions between the interface atoms, we have established the cohesive law for interfaces between multi-wall CNTs and polymer. We use this model to study the mechanical properties of multi-wall CNTs-reinforced composites under hydrostatic tension. It is found that only the outmost two layers of the multi-wall CNTs make contributions to the reinforcement of composites. The reinforcing effect is improved by increasing the CNTs volume fraction, as well as decreasing the CNTs radii.Many researchers assumed that the pressures on inner wall and the out wall in a double-wall CNTs are inversely proportional to wall radius. In this thesis, a continuum model is obtained analytically from the van der Waals force. We found that the above assumption is incorrect. Buckling of a double-wall CNT under external pressure is studied, the critical buckling pressure of the double-wall CNT is much higher than that of a single-wall CNT due to the wall to wall van der Waals interaction.Probe is one of the most promising application potentials for the CNTs. In order to investigate the effect of interlayer van der Waals interaction on the properties of CNT probes, we simulate the mechanical properties of the multi-stage CNTs, such as axial compression instability and lateral bending modulus, by using the atomic-scale finite element method. The multi-stage CNTs exhibit better axial compression stability and lateral bending modulus than the single wall CNTs, which make them to be a good choice for the probes of scanning microscopy and indentation.