Manipulating Thermal Conductivity Of Low-dimensional System Based On Phonon Wave Nature:A Theoretical And Simulation Study

The decrease of global energy sources and the increase of human demand for energy threaten our sustainable development.Nowadays,the efficiency of energy utilization is quite low because a large portion of energy is wasted as heat.Thermoelectric technology can directly convert heat into electrical energy,and facilitate the reutilization of waste heat to enhance the efficiency of energy utilization.At the same time,the density of device for integrated circuit has been improved rapidly during the past decades,which results in the large enhancement of heating power.Thermal dissipation of integrated circuit becomes the critical factor that limits its further development.Owing to the progress of nanotechnology,people can fabricate materials on microscale and nanoscale,which enlarges the scale to manipulate thermal property of materials.Comparing to macroscale thermal transport in traditional bulk materials,thermal transport in low-dimensional system is quite different and can give rise to many novel phenomena.Exploring laws for thermal transport in low-dimensional system and ways to manipulate their thermal property not only has important scientific significance but also can provide new approaches to solve the above problems.Achieving ultra-low thermal conductivity in isotropic bulk materials,and not reducing electric property is one of the main ideas to increase thermoelectric figure-of-merit of materials.Based on silicon nanowire(SiNW)and silicon nano-cross-junction(SNCJ),we propose a silicon nanowire cage(SiNWC)structure.The SiNWC is a three dimensional isotropic bulk material,which keeps the low-dimensional properties of SiNW and is easy to be applied.Through equilibrium molecular dynamic(MD)simulation,we find that the thermal conductivity of SiNWC is not only three orders of magnitude lower than that of bulk Si,but also one order of magnitude lower than that of SiNW.By further vibrational modal analysis,we identify that the large reduction in thermal conductivity is due to the phonon local resonant hybridization wave effect.Furthermore,we find that the resonant hybridization wave effect does not require the structure to be periodic,which is quite different from the periodic phononic crystal.The SiNWC has ultra-low thermal conductivity which can improve the thermoelectric figure-of-merit and it is easy to fabricate.These both indicate that SiNWC has a good potential for thermoelectric application.To demonstrate that nano-cross-junction(NCJ)effect does not depend on materials,we study phonon transport in graphene nano-cross-junction(GNCJ),which is made up of single layer graphene nanoribbons.Through MD simulation and vibrational modal analysis,we find that phonon local resonant hybridization also occurs in GNCJ.Interestingly,after changing the atomic mass of atoms in pillars by heavier(or lighter)isotopic replacing,the thermal conductivity of GNCJ increases.This is because isotropic replacing changes the frequencies of resonant modes,while,the frequencies of propagating modes keep almost the same.This results in the mismatch between resonant modes and propagating modes,which breaks and decreases the original hybridization.The boundary morphology changes after using nanowire or nanoribbon to form NCJ,which will surely induce extra phonon particle scattering.Understanding the direct individual contributions from phonon particle and wave effects in NCJ will be beneficial to optimally manipulate its thermal conductivity.However,phonon particle and wave effects are coupled together,distinguishing their individual contributions to phonon transport have been a difficult challenge.By combining Monte Carlo and non-equilibrium Green's function methods,we have quantified the particle and wave effects on phonon transport in SNCJ.We find that although the phonon local resonant hybridization wave effect governs phonon transport in SNCJ,the particle effect is quite significant as well and can contribute as much as 0.39 to the total thermal conductivity reduction when the cross section area of SiNW in SNCJ is 17.72 nm~2.The research not only highlights the importance of mutually controlling particle and wave characteristics,but also lays the foundation for precisely manipulating thermal conductivity.Besides local resonant hybridization,phonon wave transport can also give rise to graded thermal conductivity.In general,a functionally graded material is a composite,consisting of two or more phases,which is fabricated such that its composition varies in a certain spatial direction.Through MD simulation,we find that the carbon nanocone(CNC),whose structure is homogeneous and consists of only one phase,is a thermal functionally graded material.The thermal conductivity of CNC increases logarithmically from the top to the bottom of cone.By analyzing the phonon power spectrum and atomic position distribution function,we find that more vibration modes existing on each atom close to the top.These vibration modes destructively interact and result in a confined vibration and smaller amplitude,which leads to a lower thermal conductivity.We propose to use NCJ to manipulate thermal transport in low-dimensional system and discover the graded thermal conductivity.The results possess universality,which can promote the thermoelectric application of low-dimensional system.