Molecular Dynamics Simulation Of Thermal Conductivity Of Si/Ge Superlattice Nanowires

Energy shortages and environmental pollution are becoming increasingly prominent and mankind is facing huge challenges.As one of the latest achievements of modern science and technology,thermoelectric conversion technology can realize the direct conversion from low-quality thermal energy to high-quality electrical energy,which is beneficial to alleviating the energy crisis and reducing environmental pollution.However,the application of thermoelectric devices is limited by low energy conversion efficiency.At the same time,the thermal management of micro-nano electronic devices affects their performance improvement.Nanostructured materials can not only improve thermal conductivity and solve heat dissipation problems;they can also use classic size effects and interface effects to keep their electrical conductivity constant and significantly reduce the thermal conductivity of the material,thereby increasing the thermoelectric figure of merit.The periodic structure of the superlattice can enhance phonon-interface scattering,and nanowires with high surface area to volume ratio can enhance phonon-surface scattering.In this paper,Si/Ge superlattice nanowires are used as the research object,and the method of molecular dynamics simulation is used to explore the heat transport mechanism inside the material and understand the change law of the thermal conductivity of the material.Size effect and surface effect affect the thermal conductivity of Si/Ge superlattice nanowires.The simulation results show that the contradictory effects of phonon-boundary scattering and long-wavelength phonons on the phonon coherence make the thermal conductivity have a non-monotonic dependence on the cross-sectional diameter.The abnormal increase in the thermal conductivity of the ultrafine superlattice nanowires shows that the dimensionality reduction to constrain phonons does not necessarily lead to a decrease in thermal conductivity.Si/Ge superlattice nanowires with triangular cross-sections have higher surface to volume ratio,enhance phonon-surface scattering,and have more obvious phonon localization effect.Compared with rectangular and circular cross-sectional superlattice nanowires,the thermal conductivity of Si/Ge superlattice nanowires with riangular cross-sections is the lowest.When the doping percentage of Ge in Si/Ge superlattice nanowires is between 30% and 50%,the thermal conductivity reaches the minimum value,indicating that the changes of phonon state density,group velocity and mean free path caused by atomic mass change are more dramatic when Ge atoms are treated as impurities.The period length is an important parameter affecting the thermal conductivity of Si/Ge superlattice nanowires.The simulation results show that the thermal conductivity of Si/Ge superlattice nanowires with uniform period lengths changes non-monotonously with the increase of period length,and the minimum thermal conductivity is due to the change of phonons from wave transport to particle transport.The thermal conductivity of Si/Ge superlattice nanowires with gradient distribution and ungradient distribution increases monotonically with the increase of period length.For different period length distributions,the thermal conductivity increases with the increase of the total length of the sample,and then gradually tends to saturation,which is a typical size effect.However,the increasing range of thermal conductivity of both gradient and ungradient distribution superlattice nanowires is shorter than that of uniform distribution,and the thermal conductivity is lower than that of uniform distribution.With the increase of temperature,the thermal conductivity of uniformly distributed superlattice nanowires decreases monotonically,which is due to the more intense lattice vibration and the enhanced phonon-phonon scattering at high temperature.However,the thermal conductivity of gradient distribution and ungradient distribution is less dependent on temperature,which is the result of two competing mechanisms,namely phonon localization weakening and the proportion of incoherent phonons increasing at high temperatures.In addition,the different distribution of period length and the spatial position of these layers can lead to the variation of phonon localization degree.