Thermal Properties Of Low-Dimensional Nanostructures By Molecular Dynamics Simulations And Experiments

With the rapid development of nanoscience and nanotechnology, the high-densityheat dissipation problem has become one of the most significant bottlenecks in thefurther miniaturization of nano-devieces. Hence, much attention has been drawn on thelow-dimensional nanostructures due to their excellent thermal properties. This thesis isfirst focused on the heat conduction mechanism of low-dimensional nano-structures byusing molecular dynamics (MD) simulations, and then tries to improve the thermalproperties of low-dimensional composite systems and polyethylene nanowires.The mechanism of heat conduction in low-dimensional materials is studied basedon the MD method. The ballistic-diffusive transport is found to dominate the heatconduction in graphene. The thermal conductivity is proportional to the graphene lengthwith the system length smaller than40nm, which implies that the heat conduction is ina completely ballistic way. The thermal condicutivity exponentially increases with thesystem larger than40nm when the thermal transport is ballistic-diffusive. It is foundthat the thermal conductivity of a carbon nanotube with uniform heat source is only halfof that without heat source because the system is in the ballistic transport. Moreover, forthe graphene thermal nozzle with variable cross-section, the local thermal resistance atthe throat is inversely proportional to the throat width, i.e., the narrowest ribbon width,while independent on the system temperature and graphene shape.The heat conduction in low-dimensional composite systems is studied by the MDmethod. The thermal conductivity of single-wall carbon nanotube (5,5) calculated byequilibrium MD simulations is about1736W/(m·K), while that embedded in argon isreduced by75%, about441W/(m·K). In addition, the thermal transport process ofgraphene supported on substrate has been investigated through nonequlibruim MDsimulations. The calculation results reveal that the thermal conducitivity of graphenedecreases with the increasing interaction between graphene and the substrate, while thethemal properties of the whole composite system is enhanced as the interactionincreases.The heat conduction in polyethylene nanowires is also studied by MD simualtionsand experimental measurements. The MD simulations show that heat condcution in polyethylene chain is in ballistc transport when the chain length is smaller than70nm,while in ballistic-diffusive transport when the chain length is larger than70nm. Thehigh density polyethylene (HDPE) nanowires arrays with diameters of100nm and200nm are fabricated using an improved nanoporous template wetting technique. Thethermal conductivities of these HDPE nanowires arrays, measured by a nanosecondlaser flash method, is about10W/(m·K) at room temperature. And the estimatedthermal conductivity of a single HDPE nanowire is as high as30W/(m·K) whenignoring the phonon scattering effect at the interfaces among nanowires. The orientationof polyethylene chain due to high shear flow and nanoconfinement during fabrication isprobably accounted for their high thermal properties.