准一维杂化纳米结构声子输运的分子动力学研究

发布时间：2017-12-28 00:27

本文关键词：准一维杂化纳米结构声子输运的分子动力学研究　出处：《湖南大学》2016年博士论文　论文类型：学位论文

【摘要】：近年来,随着纳米科学技术的快速发展,新型的纳米材料和器件引起了人们的广泛关注。碳纳米管、石墨带和硅纳米线等准一维纳米材料也成为了人们研究的热点,由于其具有优良的光学,电学及磁学性能,被研究人员认为是下一代纳米器件的理想材料。然而当器件的尺寸进入到纳米量级,电子电路将会高度集成化,散热问题正在成为制约该领域发展的主要瓶颈之一。因此研究准一维纳米结构中热传导的规律及底层机制,对于更好的控制热量在器件中传输有着举足轻重的意义。本文采用分子动力学方法研究了准一维杂化纳米结构的声子输运性能,得到了一些重要的结果。首先,我们利用反转非平衡分子动力学方法研究了不同种类的应变对graphene/h-BN杂化纳米带热输运性质的影响。计算结果表明不同的应力对杂化纳米带热导率的影响是不一样的。当拉伸或者剪切应变加载时,随着应变从0增加到0.2,体系在常温下的热导率被减少50%以上。而当压缩或者弯曲应变加载时,体系的热导率对应变的大小不太敏感。此外,我们还发现杂化纳米带的热导率强烈依赖于其尺寸大小和graphene组分比率,并且针对这些结果给出了简要的理论解释。这一研究结果有助于设计基于graphene/h-BN杂化纳米带高性能的热控制器和热电器件。其次,运用非平衡分子动力学的方法,我们研究了热量在graphene/h-BN杂化纳米带界面传输过程。发现热量能够容易地从h-BN传递到graphene,反之却非常困难,这显示出该异质结构具有明显的热整流效应,其原因为graphene和h-BN平面外的声子模在低频区间发生强烈的共振。为了获得最优的热整流效率,我们考虑了环境温度,尺寸,温差和界面密度对体系热整流的影响。比较有趣的是,当温差比较大时,扶手椅型界面的杂化纳米带中出现了负微分热阻现象。声子谱的分析揭示出横向声学波在界面传热的过程中扮演着重要的角色。该研究结果表明graphene/h-BN杂化纳米带可用于设计热整流器和热三极管。接着,运用反转非平衡分子动力学的方法,我们研究了graphene/h-BN超晶格热输运性质。结果表明:通过增加超晶格的周期长度,其热导率先减少后增加,存在一个最小值,显示出声子输运从相干性到非相干性的过渡。此外,我们还发现最小热导率对应的周期长度在高温条件下会减少,并且体系热导率的降低幅度随温度的降低而增加。特别在200K下,一些特定周期长度的graphene/h-BN超晶格的热导率几乎等于其在300K下的热导率。详细的声子谱分析揭示出热导率这种异常的变化主要是源于强烈的声子波动相干。这些结果为纳米结构中声子输运性质的调控提供了一种全新的思路,在热电转换领域具有重要的潜在应用价值。最后,通过谱能密度理论和平衡分子动力学方法,我们系统研究了石墨炔纳米管的声子输运性质。相比于同尺寸的碳纳米管,石墨炔纳米管具有超低的热导率。在室温下,锯齿型石墨炔管的热导率为76W/m K,比碳纳米管的热导率低一个数量级,显示其在热电转换领域的潜在应用价值。通过声子振动模式的分析,发现石墨炔管中sp碳原子和sp2碳原子在低频区域发生了局域共振。这种共振将导致该频率区间内的声子模被局域和呼吸模消失,即声子群速度会大幅度地的降低。其次,声子驰豫时间也由于此类共振会明显降低。根据动力学理论,声子群速度和驰豫时间同时减少将导致体系热导率显著下降。
[Abstract]:In recent years, with the rapid development of nano science and technology, new nanomaterials and devices have attracted wide attention. Carbon nanotubes, graphite ribbons and silicon nanowires and other quasi one dimensional nanomaterials have also become the research focus. Because of their excellent optical, electrical and magnetic properties, researchers believe that they are ideal materials for the next generation of nanodevices. However, when the size of the device enters the nanometer scale, the electronic circuit will be highly integrated, and the heat dissipation is becoming one of the main bottlenecks that restrict the development of this field. Therefore, the study of the law of heat conduction and the underlying mechanism in the quasi one dimensional nanostructure is of great significance for the better control of heat transfer in the device. In this paper, the molecular dynamics method is used to study the phonon transport properties of quasi one dimensional hybrid nanostructures, and some important results are obtained. First, we study the effect of different kinds of strain on the thermal transport properties of graphene/h-BN hybrid nanoribbons by using reverse nonequilibrium molecular dynamics (MD). The results show that the effect of different stress on the thermal conductivity of hybrid nanoscale is different. When the tensile or shear strain is loaded, the thermal conductivity of the system is reduced by more than 50% at room temperature as the strain increases from 0 to 0.2. When the compression or bending strain is loaded, the thermal conductivity of the system is not very sensitive to the size of the strain. In addition, we also found that the thermal conductivity of hybrid nanobelts strongly depended on its size and graphene component ratio, and gave a brief theoretical explanation for these results. The results of this study are helpful for the design of thermal controllers and thermoelectric devices based on the high performance of graphene/h-BN hybrid nanoribbons. Secondly, using the method of non-equilibrium molecular dynamics, we studied the transfer process of heat in the graphene/h-BN hybrid nanoribbon interface. It is found that heat can be easily transferred from h-BN to graphene, but vice versa, which shows that the heterostructure has obvious thermal rectification effect. The reason is that the phononic modes of graphene and h-BN are strongly resonating in the low frequency range. In order to obtain the optimal thermal rectifying efficiency, we consider the influence of ambient temperature, size, temperature difference and interface density on the thermal rectifying system. Interestingly, negative differential thermal resistance appeared in the hybrid nanoribbons of the armchair interface when the temperature difference was large. The analysis of phonon spectra shows that the transverse acoustic wave plays an important role in the process of interfacial heat transfer. The results show that the graphene/h-BN hybrid nanoribbons can be used for the design of hot rectifiers and hot triode. Then, the thermal transport properties of graphene/h-BN superlattice are studied by using the inversion nonequilibrium molecular dynamics method. The results show that by increasing the period length of superlattice, the thermal conductivity decreases and then increases, and there is a minimum value, indicating the transition from coherence to non coherence. In addition, we find that the corresponding cycle length of the minimum thermal conductivity decreases under high temperature, and the decrease of the thermal conductivity of the system increases with the decrease of temperature. In particular, under 200K, the thermal conductivity of some graphene/h-BN superlattice with certain period length is almost equal to its thermal conductivity under 300K. The detailed phonon spectrum analysis reveals that the variation of thermal conductivity is mainly due to strong phonon wave coherence. These results provide a new idea for the modulation of the phonon transport properties in nanostructures, and have an important potential application value in the field of thermoelectric conversion. Finally, by means of spectral density theory and equilibrium molecular dynamics, the phonon transport properties of graphite acetylene nanotubes have been studied systematically. Compared with the same size carbon nanotubes, graphite acetylene nanotubes have ultra-low thermal conductivity. At room temperature, the thermal conductivity of zigzag graphite and acetylene tubes is 76W/m K, which is one order of magnitude lower than the thermal conductivity of carbon nanotubes, indicating its potential application in the field of thermoelectric conversion. Through the analysis of the phonon vibration mode, it is found that the SP carbon atom and the SP2 carbon atom in the graphite acetylene tube have local resonance in the low frequency region. This resonance will result in the disappearance of the phonon modes in the frequency range by the local and respiratory modes, that is, the velocity of the sound subgroup will be greatly reduced. Secondly, the time of phonon relaxation also decreases obviously because of this kind of resonance. According to the kinetic theory, the decrease of the velocity and relaxation time of the phonon group will lead to a significant decrease in the thermal conductivity of the system.
【学位授予单位】：湖南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TB383.1;O561

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