Research On The Nonlinear Coarse-grained Models And Mechanical Behaviors Of Carbon Nanotubes Networks

Carbon nanotube(CNT),known as the typical low-dimensional nanomaterial,has been widely used as building blocks for synthesizing novel advanced materials due to its unique atomic structure and fascinating mechanical properties.Many “bottom-up” technologies have been developed to assembly individual CNTs into macroscopically ordered materials,and thus taking full advantage of the superior properties of CNTs in nanoscale.CNT networks,the fundamental amorphous structure for those macroscopic materials,always consist of CNTs with random physical and/or chemical cross-links,depending on the synthesis or assembly method.However,after several decades of intensive work,some key mechanical performances of macroscopic CNT network materials still have huge gap with CNTs.The understanding of the quantitative relationship between macro-mechanics and micro-structure of CNT networks is important of optimizing the manufacturing processes for elevated mechanical performance.However,it is still a tremendous challenge to predict their nonlinear mechanical behaviors by full-atom molecular dynamics(MD)due to the huge computational cost,particularly for CNT networks.In this thesis,novel nonlinear coarse-grained(CG)potentials of CNTs has been completely eatablished to investigate the machanics and microstructures of macroscopic CNT networks materials based on the full-atom Reactive Empirical Bond-Order interatomic potential(REBO potential)of second generation.The main contents are briefly summarized as follows:(1)Based on the REBO potential of second generation,the nonlinear mechanical properties chiral of CNTs under tensile,compressive and bending loads are obtained by a molecular mechanics concept amd developed stick-spiral model.According to the coarsegrained mapping rules and the principle of energy equalization,the explicit expressions of the chirality-dependent higher-order nonlinear CG bonded potentials of(n,m)CNTs are established.Then,considering the area density of the CNTs(the number of carbon atoms per unit volume),explicit solutions for the cohesive energy between two parallel/crossing CNTs are obtained through continuum modeling of the van der Waals interaction between them.In particular,CG non-bonded potentials are improved by using the 18-24 Lennard-Jones potential.The values of equilibrium bond length in the CG model for different SWCNTs are determined by considering the staggered effect.By comparison with static and dynamic properties from full-atom molecular dynamics(MD)simulations as well as analytical results,the present nonlinear CG potentials have high accuracy with 2 ～ 5 orders of magnitude reduction in computing time.(2)Based on the full-atom REBO potential and obtained coarse-grained potential,the nonlinear mechanical behaviors and failure mechanism of ultralong carbon nanotube bundles under transverse tension are obtained by MD simulations,and special attention has been paid to the effects of the CNT radius,chirality,length and the size of indenter.The computational cost of CG MD simulations is found to be around 2 ～ 5 orders of magnitude reduction by comparison with that of full-atom MD simulations.The multi-stage stress-strain features of CNT bundles with different initial strains in experiments are reproduced by CG simulations.Besides their non-uniform initial strains,the diverse chirality in the components of CNTBs is another probable reason to determine the tensile strength.Using the two-parameter Weibull distribution model,the mean tensile strength of CNT bundles constructed by random chiral CNTs can be up to 90 GPa.(3)Samples of 3D complex CNT networks with different crosslink density are built by the semicrystalline lattice method and probability principle of cross-linking formation.The microstructural characteristics,dominated deformation types and fracture modes of CNT network with different crosslink density are obtained by CG MD simulations.Reminiscent of the connectivity problem in the graph theory,the CNTs and crosslinks are regarded as vertices and edges,respectively.The load transfer in CNT networks is found to be determined by the crosslink density via three critical thresholds,namely,percolation,connection,and saturation.An individual path for the successive load transfer through the network is formed at the percolation threshold,and the networks under large strain are dominated by CNT stretching.Then,all CNTs are connected together by crosslinks at the connection threshold,and the bending-dominated regime appears in the schematic phase diagram depicting dominated deformation modes of CNT networks.Above the saturation threshold,the crosslinked CNT network reaches the limit of the load transfer capacity of the CNTs,and the connections are gradually converted into tetrahedrons toward a rigidized connectivity.The strengths of noncrosslinked and crosslinked CNT networks in CG MD simulations are in agreement with that obtained from our experimental results in terms of order of magnitude.The power-law distribution of the number of crosslinks per CNT shows a preferential linking mechanism,i.e.,that the CNTs with high cross-links are more attractive to form new cross-links than the CNTs with low crosslinks.Although the crosslink strength has a remarkable effect on the strength of CNT networks,the three critical thresholds do always exist.