Scale-span Structural Designs And Mechanical Properties Of Super-strong Carbon-based Fibers

It is well-known that the microstructure of traditional carbon fiber is multilayer graphite,thus the theoretical maximum tensile strength of carbon fiber is limited.On the other hand,in recent years,the demand for high-strength fibers increases rapidly,however,traditional carbon fibers couldn't meet the needs of industry and national defense in the near future.Therefore,in order to gain stronger carbon-based fiber,molecular dynamic simulation and molecular structural mechanics were adopted to investigate mechanical properties and reinforcement mechanism of carbon-based fiber in this work.Besides,following theoretical guidance,a super-strong CNT fiber was prepared.Using twisting operation,two different kinds of carbyne fibers(Polyyne and Cumulene)are constructed.Based on the results derived from molecular dynamic simulation,it is found that mechanical properties of such carbyne fibers are highly correlated to the twisting degree,and when cumulene carbyne fibers are under uniaxial tension,the tensile strength and Young's modulus are significantly improved due to the phase transition from Cumulene to Polyyne,maximum up to1.77 TP and 7.28 TPa,respectively.On the other hand,based on the molecular structural mechanics,timoshenko beam model is utilized to simulate carbon covalent bond and Young's modulus,Poission's ratio and diameter of continuum beam are derived.Then based on the geometrical feature of helical carbyne fiber,the continuum model of carbyne fiber is constructed.It's important to note that the analytical model in this work could also predict other similar carbon-based structures.Using full-atom and coarse-grained molecular dynamic simulations,doublehelix carbon nanotube fibers and hierarchical helical CNT fibers are constructed.By uniaxial tensile simulation,it is found that mechanical properties of CNT fibers are related to the arrangement of CNTs and twisting degree which indicates that mechanical behaviors of such CNT fibers could be efficiently tuned by changing related parameters.On the other hand,the self-assembly of parallel double-carbon nanotubes was discovered for the first time at the nanoscale,and the super-strong interfacial interaction between the fibers and the matrix was found,indicating that the helical CNT fibers have a good application prospect in the field of composite materials.At present,CNT fibers are mostly assembled by shot CNTs randomly in experiment,therefore,in order to investigate the mechanical reinforcement mechanisms of CNT fibers,coarse-grained randomly distributed carbon nanotube fiber model are built in this work.Based on the molecular dynamic simulations results,it is revealed that the entanglement in CNT fibers could significantly improve load transfer efficiency between CNTs and thus mechanical properties of CNT fibers are increased.And the entanglement between CNTs are highly correlated with the length and bending degree of CNTs as well as the twisting and compression degree of CNT fiber.On the other hand,following theoretical guidance,a super-strong CNF is designed and prepared by a combined twistingcompression process.It is revealed that then tensile strength of CNT fibers increases as twisting degree increases,besides,when compression operation is applied to CNT fiber,the tensile strength of CNT fibers could be further improved which is exactly same as molecular dynamic results.It is worthy to note that,after a combination of twisting and compression,the maximum tensile strength of CNT fibers can reach 6.8 GPa.In this work,the mechanical reinforcement mechanisms of randomly distributed carbon nanotube fibers was presented,and the influence of twisting and compression operations on mechanical properties of fiber were revealed,which could provide theoretical guidance for experimental preparation of super-strong CNT fibers.Finally,compared with the excellent tensile strength and Young's modulus of carbon nanotubes,their ductility is relatively weak.Therefore,in order to improve such deficiency of carbon nanotubes,we apply the concept of traditional kirigami arts to build three different CNT kirigami structures.Based on the results from molecular dynamic simulation,it is found that the mechanical properties of CNT kirigami structures are highly correlated with the shape and parameters of geometrical holes,and the stretchability could be improved up to265%.This work proposed a new structure design method at nanoscale and extended the application range of carbon nanotubes.