【作者】 李婷；

【导师】 唐祯安；

【作者基本信息】 大连理工大学 ， 微电子学与固体电子学， 2018， 博士

【摘要】 低维纳米材料因量子限域效应具有众多令人瞩目的优异性能。其中,单层的二维材料不仅利于化学修饰和电子传递,而且具有柔性和透明度高的特点,有望在下一代微纳电子器件中发挥重要作用,是目前国际材料科学研究的前沿焦点。纳米尺度的热调控是这些器件设计中非常关键的环节,高的热导率和低的界面热阻有助于解决微纳器件的散热难题。本文选取了二维材料家族的典型代表石墨烯和六方氮化硼作为研究对象,对其本征的导热性能以及由其所构成的多种纳米尺度界面的热性能进行了深入的理论计算和实验测试研究。为了确保理论计算的方法及其模型选取的合理性,以及计算程序的适用性和可靠性,对已有诸多文献报道的石墨烯、六方氮化硼、碳纳米管和氮化硼纳米管本征材料的热导率进行了计算验证。对于宽度为2.5 nm的扶手椅型石墨烯和氮化硼纳米带以及(10,0)碳纳米管和氮化硼纳米管,当其长度无穷大时的室温热导率分别为1316 Wm-1K-1、526 Wm-1K-1、901 Wm-1K-1和369 Wm-1K-1;在温度范围为100-1200 K,由于声子间散射的加剧热导率随着温度的升高而减小,并且在低温时,氮化硼纳米结构的热导率高于与之对应的碳结构,而随着温度的升高,氮化硼纳米结构的热导率则低于碳纳米结构;当纳米管被压缩时,由于晶格的非简谐振动受到影响,因此总体上其轴向热导率会随着压缩应变的增大而减小,但是与氮化硼纳米管不同的是,碳纳米管的低频声子在小应变下反而会被激发,导致热导率的增大,本文认为纳米管中的低频柔性声子模式对热导率起主导作用。计算了石墨烯/六方氮化硼和碳纳米管/氮化硼纳米管范德华异质结构的界面热阻,并发现后者管间热能传输的整流效应。在温度范围为200-600 K,计算所得界面热阻值在10-7-10-6 Km2W-1数量级,并且随着温度的升高和界面间耦合强度的增大而减小。在二维异质结构中石墨烯和六方氮化硼的声子模式匹配度较高,因此热流传输方向对其界面的导热性能几乎没有影响;但是对于一维异质结构,其声子失配程度较高,热流传输方向的不同会改变纳米管之间的热耦合,进而导致声子在界面传输的非对称性,且不论外管是碳纳米管还是氮化硼纳米管,热能都更倾向于从外管向内管传输。计算了石墨烯/六方氮化硼平面异质结构的界面热导,并且通过改变热流方向以及与衬底间的耦合强度实现对界面热导的调控。在温度范围为200-600 K,界面热导值在1010 Wm-2K-1数量级,比范德华异质结构高出4个数量级,并且界面热导随着温度的升高而增大;在平面异质结构中,石墨烯的声子截止频率高于六方氮化硼,因此当热流传输方向改变时,声子在界面传输的非对称性导致当热流从六方氮化硼流向石墨烯时具有更高的热导值,此异质界面存在热整流效应;二氧化硅衬底的存在会在石墨烯和六方氮化硼中激发出更高频率的声子,促进其界面的导热性能,并且异质结构与衬底间的耦合强度越大,其界面热导值越大;石墨烯/六方氮化硼平面异质结构的有效热导率为116-130 Wm-1K-1,并且有效热导率对温度和衬底耦合强度均表现出较弱的依赖性。计算了石墨烯(六方氮化硼)与二氧化硅衬底间的界面热阻,发现了晶界缺陷可以增强热能从石墨烯向衬底的传输。在温度范围为150-600 K,界面热阻值在10-8-10-7Km2W-1数量级,并且随着温度的升高和衬底耦合强度的增大而减小;相较石墨烯,六方氮化硼界面的热阻值对衬底耦合强度的依赖性较弱,当耦合强度较小时,石墨烯界面的热阻值较大,但是随着耦合强度的增加,石墨烯界面的热阻值反而会小于六方氮化硼界面;石墨烯中晶界缺陷的存在会增大其与衬底之间的摩擦力以及石墨烯与二氧化硅声子模式的几何重叠,进而增大界面间的热耦合,提升界面的导热性能。实现了石墨烯(六方氮化硼)与二氧化硅衬底间界面热阻的测量。搭建了一套用于测量薄膜热导率和界面热阻的3ω测试系统;使用MEMS工艺制备了用于提取二维材料与衬底间界面热阻的测试样品;在温度范围为100-300K,测得单层石墨烯、双层石墨烯、单层六方氮化硼、双层六方氮化硼与二氧化硅衬底间的界面热阻值均在10-8 Km2W-1数量级,且总体上随着温度的升高而衰减;六方氮化硼与二氧化硅之间的界面热阻高于石墨烯界面,并且不论是石墨烯还是氮化硼,其双层原子薄膜与衬底间的热阻均略高于单层;界面热阻的实验测量值与分子动力学计算结果相吻合,为石墨烯(六方氮化硼)器件的热设计和热管理提供了重要参考。更多还原

【Abstract】 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.更多还原