The Molecular Dynamics Simulations Of The Silicon Nanowire Cages' Thermal Conductivity And Graphene's Nano-ratchet Effect

Rapid development in nanotechnologies,including nanostructures and nanomaterials have played indispensable roles in solving energy shortage and environmental pollution.In particular,the great contributions of nanostructures to heat and mass transfer have provided novel ideas on recycling waste heat and water purification.In this work,molecular dynamics simulations were used to investigate the abnormal effect of two nanostructures from the perspectives of nanoscale heat and mass transfer.Also,the practical applications of these nanostructures were demonstrated.First and foremost,this thesis discussed how to construct nanostructures in order to enhance energy conversion from heat to electric power directly by effectively manipulating the transport of phonons,the main carriers of heat.In contemporary society,the energy crisis is serious and a great deal of energy is wasted as useless heat finally.Therefore,how to recycle waste heat is significant to solve the above problem.The development of thermoelectric technologies is essential to handle the crisis.However,the thermoelectric efficiency of traditional bulk materials is ultra low.So people started the research on thermoelectric nanomaterials.In this work,from the phonon transport's perspective,we proposed a special silicon nanowires cage to prohibit heat transfer in order to enhance thermoelectric efficiency.The dependence of thermal conductivity of silicon nanowires cage(SiNWC)on temperature and the size of silicon nanowires were investigeated.It is found that the thermal conductivity is sensitive to the size of silicon nanowires and do not change under different temperature.Then,the effect of structure symmetry and the number of nano-cross-junction on thermal conductivity were discussed.In this work,an ultralow value of thermal conductivity of SiNWC,0.173 W/m-K,which is three order lower than that of bulk silicon.In the next part,boosting water purification through a nano-ratchet is discussed.In recent years,researchers have paid abundant attention to reverse osmosis membrane,thermal distillation and vacuum evaporation in order to mitigate the freshwater shortage.However,limited by the traditional materials,all these methods are not efficient and quick enough.For the first time,this thesis proposed a graphene nano-ratchet to produce vacuum spontaneously to boost the process of water purification.In an asymmetric system,net mass flux by exerting zero-mean periodic force can be obtained.This phenomenon is called nano-ratchet effect.Removing carbon atoms in multilayer graphene,a graphene membrane with cone-shape nanopores was constructed to realize nano-ratchet effect.Through non-equilibrium molecular dynamics simulations,it is found that air molecules spontaneously transport across the graphene membrane and generate remarkable pressure difference.The graphene nano-ratchet mainly consumes low-grade heat.Hertz-Knudsen relation was utilized to calculate water evaporation rate according to the pressure difference.It is found that the significant pressure difference could lead to a 15-fold or greater enhancement of vapor generation.It means that graphene nano-ratchet perfectly combines thermal distillation with vacuum evaporation and remarkable boost water purification.Therefore,it has wide applications.Then the effect of ratchet's geometry and temperature on pressure difference were studied.Through further analysis of the diffusive transport,it is found that pressure difference depends on the competition between ratchet transport and Knudsen diffusion and it was further found that ratchet transport is dominant.