Thermal Transport And Modulation Mechanism In Organic Polymers And Small Molecular Crystals Via Molecular Dynamics Simulations

Organic materials are extensively used in many fields from daily life to industrial manufacture,whose thermal transport property is a crucial factor.On one hand,heat dissipation issue is the bottleneck that limits the development of high-performance electronics.On the other hand,thermoelectric effect can realize the conversion between heat and electricity.Therefore,the study on thermal transport in organic materials would not only promote the enhancement of energy utilization efficiency,but also aid the development of high-performance electronics.To pursue high thermal conductivity,we proposed a van der Waals(vd W)confinement effect,and designed a crosswise paved laminate(CPL)structure.Taking polyethylene as an example,through molecular dynamics(MD)simulations,polyethylene CPL is found to have a value of thermal conductivity reaching as high as 181 Wm^-1K^-1,three-order larger than that of bulk amorphous polyethylene.This new mechanism breaks the common point that vd W forces are adverse to thermal transport.Analysis indicates that confined vd W potential barriers endows better lattice order.Furthermore,this idea is also validated for graphene nanoribbon systems.This study would provide new insights into some fields including phononics.Thermal conductivity of cross-linked polymers is usually relatively lower,due to their amorphous network structure.Here,we proposed a bottom-up parallel-linking strategy.Through MD simulations,we reported that the along-chain thermal conductivity of epoxy resin reaches 0.8 Wm^-1K^-1,which is more than twice as larger as that of amorphous cross-linked one.Moreover,thermal conductivity can be further improved by about one order via applying tensile strain.Morphology analysis interprets the underlying mechanism.This structure could be realized by molecular layer deposition method,provides new strategy for enhanceing the thermal conductivity of cross-linked polymers.According to different applications of organic semiconductors,modulating the thermal conductivity is vital for their device performance.Here,we proposed to use doping to accomplish this aim.Taking poly(3,4-ethylenedioxythiophene)(PEDOT)as an instance,through MD simulations about PEDOT systems under different doping concentration,it is found that the room-temperature thermal conductivity along three directions decreases with doping concentration.Analysis shows doping mainly descreases the lifetimes of phonons in low-frequency range.In addition,doped systems possess hybrid thermal transport characteristics.This study offers guides for modulation of thermal transport in organic conjugated polymers.In thermoelectric reseach field,we proposed to employ low-dimensional electric transport in bulk phonon-glass crystals to achieve high thermoelectric performance.The bis-dithienothiophene(BDTT)small molecular crystal is selected as an example.Through MD simulations,the thermal conductivity in inter-plane direction is 0.34 Wm^-1K^-1.Combing with the electrical transport coefficient calculated by cooperators,the p-type thermoelectric figure of merit reaches 1.48 at room temperature.The results validate that?-?stacking molecular crystal is a promising thermoelectric material.This study provides a new idea for thermoelectric research which is also applicable to inorganic materials.Thermal transport through cross interfaces in organic nanostructured electronic deives remains to be investigated.To determine the interfacial thermal conductance(ITC),we derived an analytical model considering two-dimensional heat flow,cross interface thermal model.Taking the copper phthalocyanine nanoribbon as an object,MD simulation results show the ITC is more than two order higher than the experimental value measured by our cooperators.Contact mechanics analysis demonstrate that the ultra-low ITC is mainly caused by the surface roughness and overlapping configuration.This work builds an accurate analytical model for heat conduction in cross interface,provides guidance for thermal management in nanostructured devices.This dissertation undertakes in-depth study on thermal transport and modulation mechanism in organic polymers and small molecular crystals,establishes theoretical basis for heat transport in organic materials from underlying principles to application strategies,promotes the development of electronic devices and thermoelectric applications.