| As a typical carbon nanotube(CNT),collapsed nanotubes can be obtained by applying external pressure,an electric field,or metal wrapping to them.Compared to the circular cross-sectional CNTs,the collapsed CNTs have larger interfacial contact area and interfacial strength between CNTs,which makes the CNT can bear higher flexibility,compactness,and fracture strength.These excellent properties make collapsed CNTs ideal to be used in the fields of carbon matrix composites,nanodrives,and nano-optical heat conduction.When its two ends are subjected to specific constraints,CNT can exist in an intermediate state composed of a fully-collapsed state and a fully-circular state,which can be called the semi-collapsed state.Such a state can exist in two configurations: the fully-uplifted and the fully-collapsed configurations.These two configurations can achieve reversible conversion by applying thermal loading or an electric field,which makes the CNT have potential applications in energy storage,thermal rectification,and other fields.Currently,there is a lack of systematic research on how to control the reversible deformation between these two configurations.In this thesis,two typical CNTs with the chiralities of(35,35)and(40,40)were selected as examples to study the reversible deformation control and the thermomechanical coupling effects of the semi-collapsed CNTs using the molecular dynamics method.The effects of thermal loading mode and helium filling rate on their deformation and thermal properties were analyzed.The details are as follows:(1)Firstly,the effects of thermal loading mode on reversible deformation and corresponding thermomechanical coupling effects of semi-collapsed CNTs were systematically studied.The results show that the sink and source temperatures can regulate the reversible deformation of CNTs.Specifically,when the source temperature is kept constant,the fully-uplifted configuration can be transformed into the fully-collapsed configuration by decreasing the sink temperature,and can be reversed by increasing the sink temperature.When the temperature of the cold source falls below the critical interval,the fully-uplifted configuration is transformed into the fully-collapsed configuration;when it exceeds the critical interval,the fully-uplifted configuration remains unchanged.If the cold source temperature falls within the critical interval,the fully-uplifted configuration is in a metastable state that is influenced by the heat flow direction,cooling rate,initial velocity distribution,and number of cyclic thermal loadings.Additionally,the thermal conductivity of the CNT in the fully-uplifted configuration is found to be higher than that of the fullycollapsed configuration.Regardless of the CNT’s configuration,thermal conductivity decreases as the temperature of the cold source increases.The thermal properties of CNT with other cross-sectional shapes have also been calculated,and it is found that the thermal conductivity is in the order of(a)the fully-circular CNT,(b)the fullycollapsed configuration of the semi-collapsed CNT,(c)the fully-collapsed CNT,and(d)the fully-uplifted configuration of the semi-collapsed CNT.There is also a thermomechanical coupling effect for the two typical CNTs: the change of cold source temperature can change their configurations,which would result in the variation of the thermal conductivity of the CNT.(2)The thermomechanical coupling effects of the fully-collapsed configuration of the semi-collapsed CNT with the chirality of(35,35)and the length of 400 (?) under a helium-filled state were further studied,in which the effects of helium filling rate(α)and thermal loading temperature on its deformation and thermal properties were considered.The results show that,for the case of the ambient temperature of 300 K,(a)when α<0.00949,the collapsed length and the thermal conductivity of the fullycollapsed configuration decrease with the increase of α;(b)When 0.00949<α<0.0146,the collapsed length of the fully-collapsed configuration also decreases with the increase of α,but the thermal conductivity increases with the increase of α;(c)When0.0146<α<0.02921,the collapsed length reaches the minimum value,the fullycollapsed configuration is transformed into the fully-uplifted configuration,and the thermal conductivity decreases with the increase of α.(d)When α>0.02921,the collapsed length remains in the minimum,and the thermal conductivity increases with the increase of α.Helium filling can make the regulation of the thermal loading temperature on the degree of collapse of the semi-collapsed CNT more precise.For the case of α=0.0146,the collapsed length decreases with the increase of the thermal loading temperature and increases with the decrease of the thermal loading temperature.In the process of the thermal loading temperature regulating the deformation of helium-filled semi-collapsed CNT,the thermomechanical coupling effect is also found.and its thermal conductivity decreases with the increase of thermal loading temperature.Helium charging and discharging process of releasing part of helium atoms after helium charging can also regulate the semi-collapsed CNT’s configuration,transforming it into fully-uplifted configuration whenα=0.00485.(3)The configuration changes of fully-collapsed CNT with chirality of(40,40)after helium filling at 300 K were studied,and the influence of helium filling rate(α)on its configuration changes and thermal conductivity were analyzed.The results show that:(a)when α<0.0496,the helium-filled CNT keeps in a dumbbell-shaped cross-section configuration,and the thermal conductivity decreases with the increase of α;(b)when 0.0496<α<0.0992,the helium-filled CNT becomes in a peanut-shaped cross-section configuration,and its thermal conductivity also increases with the increase of α;(c)when α>0.0992,the helium-filled CNT becomes in a circular crosssection configuration,but its thermal conductivity decreases with increasing of α. 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