A Study On The Interfacial Thermal Performance Of Graphene And Hexagonal Boron Nitride

Due to the quantum confinement effect,low-dimensional nanomaterials possess many remarkable properties.Monolayer two-dimensional(2D)materials are currently the frontier focus of materials science research and are expected to play an important role in the next generation of micro-nano electronics,because they are not only conducive to chemical modification and electron transfer,but also have the characteristics of flexibility and high transparency.Thermal control in the nanometer scale is a key part in device design,and high thermal conductivity and low interfacial thermal resistance(ITR)are helpful to solve the heat dissipation problem in the micro devices.In this paper,graphene and hexagonal boron nitride(h-BN),typical representatives of 2D family,are focused.The intrinsic thermal conductivity and the thermal properties of various nano-scale interfaces are investigated in depth theoretically and experimentally.In order to ensure the reliability of simulated models as well as calculation programs,the intrinsic thermal conductivity of graphene,h-BN,carbon nanotube(CNT)and BN nanotube(BNNT)are explored.For the armchair graphene and BN nanoribbons with width as 2.5 nm and(10,0)CNT and BNNT,the thermal conductivities at room temperature are estimated as 1316 Wm-1K-1,526 Wm-1K-1,901 Wm-1K-1 and 369 Wm-1K-1,respectively,when their lengths are infinite.In the temperature range of 100-1200 K,thermal conductivity decreases with the increase of temperature due to stronger phonon-phonon scattering.At low temperature,the thermal conductivity of BN nanostructure is higher than that of the carbon counterpart,however,with the increase of temperature,it is lower than the latter.Because the nonharmonic lattice vibrations are affected when the nanotubes are under axial compression,the thermal conductivity will decrease with the increase of compressive strains.Specifically,unlike BNNT,low frequency phonons in the CNT can be stimulated with small compression strain,leading to an increase in the thermal conductivity.It is believed that the flexible phonon modes in the low frequency dominate heat conduction in the nanotubes.The ITR of both graphene/h-BN and CNT/BNNT van der Waals(vdW)heterostructures are studied,and thermal rectification effect can be found only in the latter one.In the temperature range of 200-600 K,the calculated ITR is on the order of 10-7-10-6 Km2W-1,and decreases with the increase of temperature as well as interfacial coupling strength.As the phonon modes of graphene and h-BN are well matched in the 2D hetero structure,heat flux direction has no influence on the thermal performance of the interface.However,for the one-dimensional(1D)heterostructure,the phonon modes of CNT and BNNT are mismatched and heat flux direction can affect the intertube thermal coupling,leading to the asymmetry of phonon transmission at the interface.Therefore,thermal energy prefers to transfer from the outer tube to the inner one regardless of whether CNT or BNNT acts as the outer tube.The interfacial thermal conductance(ITC)of in-plane graphene/h-BN heterostructure is studied,which can be controlled by changing the heat flux direction or the coupling strength with substrate.In the temperature range of 200-600 K,the ITC is on the order of 1010 Wm-2K-1,four orders larger than that of the vdW heterostructure,and it increases with the increase of temperature.Thermal rectification occurs because the phonon cutoff frequency of graphene is higher.The asymmetry of phonon transmission at the interface results in larger values of ITC when heat transports from h-BN to graphene.When it is supported by SiO2,higher frequency phonons are stimulated both in graphene and in h-BN,which promotes heat conduction across the interface,and ITC is increased with the increase of substrate coupling.The effective thermal conductivity of in-plane graphene/h-BN is 116-130 Wm-1K-1 and is weakly dependent on both temperature and substrate coupling.The ITR between graphene(h-BN)and SiO2 substrate is studied,and the enhancement of heat conduction is observed by introducing grain boundary defects in graphene.In the temperature range of 150-600 K,the calculated ITR is on the order of 10-8-10-7Km2W-1 and decreases with the increase of temperature and substrate coupling.However,The ITR of h-BN/SiO2 interface is less dependent on the coupling strength compared to that of graphene/SiO2.When the strength is weak,the ITR of graphene interface is larger than that of h-BN interface,whereas it is lower than the latter when the substrate coupling is strengthened.The existence of grain boundary defects in graphene will increase the friction and the geometric overlap of phonon modes between graphene and SiO2,thus increasing the thermal coupling at the interface and improving the interface thermal performance.The ITR between graphene(h-BN)and SiO2 substrate is measured experimentally.The 3co system is built for the thermal conductivity and ITR measurements of thin films.The samples are prepared by MEMS for extracting the ITR between 2D materials and substrate.In the temperature range of 100-300 K,the measured ITR is on the order of 10-8 Km2W-1 for all the samples,and it generally decreases with the increase of temperature.The ITR of h-BN/SiO2 interface is larger than that of graphene/SiO2 and the ITR between bilayer and substrate is slightly higher than that of monolayer for both graphene and h-BN.The experimental values agree well with those from MD calculations,which provide valuable references for the thermal design and thermal management of grapheme(h-BN)devices.