Research On The Micro-mechanism Of Mechanical Properties And Regulation Of Carbon Nanotubes Networks

Carbon nanotube(CNT)has received extensive attention and research owing to its unique one-dimensional nanostructure with excellent chemical,electrical and mechanical properties.One of the potential uses of CNTs is to be assembled as building blocks for synthesizing macroscopic materials,thus bringing the excellent mechanical properties of CNTs at the nanoscale to the macroscopic level.Currently,scientists have developed many bottom-up assembly methods to prepare CNTs macroscopic materials,such as CNT fibers,CNT films,CNT sponges,and so forth.Among these CNTs macroscopic materials,CNTs are interconnected by their rich interfacial interactions in a parallel or cross way to form CNT networks with well-defined scale features and diverse structural characteristics.These macroscopic materials with network structures are called CNT network materials.However,the key mechanical properties of CNT network materials synthesized in current experiments are still 1 to 2 orders of magnitude different from their building blocks(CNTs).Understanding the correlation between the microstructural characteristic parameters and the overall mechanical properities of CNT network materals is key to further optimize the design of CNT macroscopic materials.In this thesis,a series of studies were carried out using a combination of theoretical analytical model and coarse-grained molecular dynamics simulations,aiming to investigate the microstructural deformation pattern of CNT network materials under tensile loading and establish the quantitative relationships between their average orientation degree and overall mechanical properties.Meanwhile,pretreatments are conducted to regulate the average orientation degree and mechanical behaviors of CNT networks.The influence mechanism of basic unit CNTS arrangement on mechanical properties of CNT network material is revealed,which provides theoretical support for the optimization design of CNT macrostructure.The main contents of this thesis are as follows:Firstly,the nonlinear mechanical behavior of CNT network under tensile loading is studied by using coarse-grained molecular dynamics simulation method to obtain the stress-strain curves and the micro-structural evolution law of the materials.Then,the structural parameter ξ,which is used to characterize the average orientation degree of CNT network,is proposed,and the analytical formula for the quantitative relationship between the overall strain of CNT networks and ξ was obtained based on the principle of affine deformation.It is shown that ξ increases with the increase of the overall strain of CNT network and eventually converges to a constant value(0.9).The theoretical prediction in good agreement with the simulation results.Secondly,based on the change law of average orientation degree of CNT network under tensile load and CNT inter-tube shear strength a theoretical model was established to predict the load-bearing capacity of CNT network under different strains.Meanwhile,and the modulation effect of mechanical loads on the average orientation degree of CNT network was also studied based on coarse-grained molecular dynamics method.It is shown that the pre-stretching of CNT network materials can effectively regulate their average orientation degree,which is consistent with the prediction of the analytical formula of the quantitative relationship between the overall strain and ξ of the network.In addition,the nonlinear mechanical behaviors of CNT network with different average orientation degrees under large deformation states were studied and it was concluded that the theoretical model developed in this thesis can predict the average orientation degree of CNT network materials based on their overall strain and regulate their mechanical properties.It is shown that the rate of tensile load,compressive pre-strain treatment and cyclic load treatment do not have significant effects on its average orientation degree.Thirdly,the microstructure and mechanical properties of CNT network at different thermal treatment temperatures are studied by using the coarse-grained molecular dynamics simulations.It is shown that compared with the unheat-treated CNT network,the tensile strength of the heat-treated CNT network material was increansed by 50% and the failure strain was diseased by 60%.In addition,the pre-stretching of the heat-treated CNT network materials can enhance the average orientation degree of CNT network materials more effectively.In this thesis,the evolution law of the average orientation degree of CNT network materials under tensile loading and the regulation mechanism of mechanical loading and thermal treatment on the average orientation degree of CNT network materials are systematically studied.The results are of reference value for improving the microstructure of CNT network materials and optimizing their mechanical properties.