Thermal Effect Analysis Of Carbon Nanotubes And Solution Shear Rheological Characteristics

The carbon nanotubes(CNTs)have been studied and developed for more than 30 years.So far,the preparation of high-purity single structure carbon nanotubes is still a worldwide problem,therefore the application of carbon nanotubes is greatly limited.There have been many studies on the mechanical properties of single structure carbon nanotubes and their assemblies,but more accurate analysis of the thermal effect for CNTs and in-situ observation of the assembled structure of carbon nanotubes are still challenging.In this thesis,firstly,the axial elastic properties of carbon nanotubes are analyzed on the basis of molecular mechanics and molecular structure mechanics,and the mechanical model expression related to temperature is established.The thermodynamic properties of carbon nanotubes with different chirality types under axial tension are also studied.At the same time,the existing molecular structure mechanics methods are modified.The modified model can better predict the elastic properties of carbon nanotubes and greatly reduce the deviation from the continuum method.The modified method simplifies the calculation of the model and maintains the applicability of the thermal environment.In the theoretical study of molecular mechanics,the inversion energy is often ignored,resulting in inaccurate calculation results.Therefore,this paper studies the inversion energy in the method of molecular mechanics,in which it links with the molecular structure mechanics model for the carbon nanotubes.Moreover,a new expression of molecular mechanics energy is presented.The calculation results show that the inversion energy of CNTs has a great influence on the energy of the system under the small diameter,which cannot be ignored.It is also found that the inversion energy is insensitive to temperature and can be ignored.The modification of this energy method makes the calculation results for the mechanical properties of carbon nanotubes more accurate.The thermodynamic analysis of single CNT with different end cap structures has been carried out under hydrostatic pressure,and the effect of end cap structures on the transverse mechanical properties of carbon nanotubes was investigated.The transverse mechanical properties of single-walled carbon nanotubes(SWCNTs)under hydrostatic pressure were obtained,and the effect of temperature was analyzed.The predicted results show that,except for the very small diameter of carbon nanotubes,the bulk modulus and transverse elastic properties are less sensitive to the end cap structure of carbon nanotubes.The research on the interaction behavior of multiple CNTs under hydrodynamic conditions has a very important theoretical and application value.This dissertation focuses on the shear flow behavior of a suspension of multiple dispersed carbon nanotubes under hydrodynamic conditions,in order to explore the motion and interaction mechanism of carbon nanotubes in solution.The full-atom model of a single CNT is simplified to a coarse-grained model,and on this basis,the dissipative particle dynamics(DPD)method is used to simulate the rheological behavior of carbon nanotube suspensions,and a new potential function form is proposed to represent the interaction between surfactant-coated carbon nanotubes.The simulation results show the well applicability of this potential function and the feasibility of the DPD method.The relationship between the shear flow behavior of carbon nanotubes in solution and the length and concentration of carbon nanotubes was further clarified,and the length and concentration of carbon nanotubes in the optimal orientation were obtained.Finally,using the non-equilibrium molecular dynamics(NEMD),the simulations of the shear flow behavior of carbon nanotube suspensions are performed.The viscosity of suspension,shear rate,length and concentration of carbon nanotubes were established.A series of viscosity properties exhibited by the liquid are also analyzed.The results show that there are heterogeneous flow phenomena,namely shear banding and wall slip,in solutions containing multiple dispersed high aspect ratio of carbon nanotubes.By comparing the solutions containing carbon nanotubes with different concentrations and lengths,the heterogeneous flow is closely related to the concentration,length and shear rate of CNTs.