Helium Aggregation Of Single-walled Carbon Nanotubes On The Mechanical Properties

In the field of nuclear engineering and industry,the emergence and accumulation of helium atoms in metal,which act as the structural materials of furnace for nuclear fusion or fission,is always a severe safety problem of nuclear device.The aggregation of helium,thus forming helium bubble which causes swelling of materials,usually degrades the mechanical properties of metals.Since the discovery of carbon nanotube(CNT) in 1991,it is found that carbon nanotube with special structure and nano-meter scale has many outstanding properties,such as high rigidity and electronic property.It has high specific surface area,which enables it to be a perfect material of adsorption.To date,scientists have successfully filled various materials into cabon nanotubes,such as metal halid,water,rare gases and hydrogen.Storing helium in carbon nanotube has attracted more and more interests:It can prevent helium from accumulating in metal;Also.It might allay the material swelling and brittle failure caused by helium because of its excellent mechanic properties.In this thesis, molecular dynamics(MD) was used to simulate the helium adsorption and accumulation in single-walled nanotube(SWNT) and calculate the effect of helium on mechanical property of SWNT.The interaction between carbon atoms was described with the combination of Tersoff-Brenner potential and Lennard-Jones(LJ) potential, and the interactions of C-He and He-He,with LJ potentials.First of all,the stable structure of helium atoms filled in single-walled carbon nanotube(SWNT) was studied.It is found that helium atoms of low filling density are inclined to form shell structure in SWNT,and with the increase of filling density helium atoms form a linear chain in the axial line along nanotube surrounded by the previous He shell.On the base of these results,the effect of filling helium on the buckling force of SWNT was further investigated.We find that filling helium in SWNT does not change the Young's modulus of SWNT.However,there is a critical density D_c.When filling density is lower than D_c,buckling force almost remains the same as the hollow SWNT,while when filling density is higher than D_c,buckling force of filled SWNT increases with the increment of filling density.This is attributed to the Van de Walls(vdW) interaction between helium atoms and carbon atoms.We also studied the impact of temperatures on the compression property of filled SWNT. The result shows that raising temperature will reduce the buckling force of empty SWNT,however,for filled SWNT the effect of temperature on buckling force is not as obvious as that of empty SWNT.The reason may be that that the thermal motion of helium atoms increases in high temperature.Then the helium shells swell,which means that the space among helium shell and nanotube wall will become small.This results in the repulsive force of helium atoms to the nanotube wall,thus resists the buckling of filled SWNT under the compressure.In this theis we also calculated the potential distribution of helium in SWNT array.The results show that the lowest point of potential is in the interstitial space,which is the stable point of helium atom outside SWNT.