键序理论研究粗糙表面对硅纳米线热导率的影响

发布时间：2018-06-07 18:58

本文选题：硅纳米线 + 热导率　；参考：《湘潭大学》2017年硕士论文

【摘要】：纳米结构热输运的理解和调控将对热电装置以及微米、纳米电子设备的热操作的发展起到非常重要的作用。由于低维材料具有很高的比表面积,源于纳米结构中声子与表面的散射,显著的尺寸效应限制了热输运性能。然而目前人们对声子-表面散射的物理机制仍了解不够透彻,声子-表面散射对热输运的精确影响仍不清楚,需要对此进行深入研究。本文提出了一种新的声子-表面散射机制,即表面键序缺陷对声子的散射。表面原子配位数比体内原子低(称为键序缺陷),表面原子的键长收缩,键能增强,导致力常数增加,表面原子力常数的增加对晶格振动系统的哈密顿量产生微扰,从而对声子产生散射。我们结合键序理论和量子微扰理论,推导出了该机制的散射率公式。结合该散射机制与声子玻尔兹曼输运方程,我们计算了粗糙表面硅纳米线的热导率值,远远低于Casimir极限,计算结果与实验测量很好吻合,并且解释了粗糙硅纳米线热导率显著下降的物理机制是由于表面键序缺陷对高频声子强烈散射,导致高频声子对粗糙硅纳米线热导率的贡献明显下降。我们进一步研究了粗糙表面硅纳米线的直径、粗糙度均方根、粗糙度相关长度和比表面积这四个形貌参数,以及表面自相关函数(高斯表面以及指数表面)对热导率的影响。结果表明比表面积是对粗糙硅纳米线热导率影响最重要的参数。在形貌参数相同的情况下,由于指数表面具有更多的高频小起伏,其比表面积大于高斯表面,所以具有指数表面的粗糙硅纳米线热导率也更低。本文的研究不仅有助于理解声子-表面散射的物理机制,也有助于利用表面工程对纳米结构热导率进行调控。
[Abstract]:The understanding and regulation of thermal transport in nanostructures will play an important role in the development of thermoelectric devices and the thermal operation of micron and nano-electronic devices. Due to the high specific surface area of low-dimensional materials due to the scattering of phonons and surfaces in nanostructures, the thermal transport properties are limited by the significant size effect. However, the physical mechanism of phonon-surface scattering is not well understood, and the exact effect of phonon-surface scattering on thermal transport is still unclear. In this paper, a new phonon-surface scattering mechanism is proposed, which is the scattering of phonons by surface bond order defects. The coordination number of surface atoms is lower than that of internal atoms (called bond order defects). The bond length of surface atoms shrinks and bond energy increases, which results in the increase of force constants, and the increase of surface atomic force constants causes perturbation to the Hamiltonian of lattice vibration systems. Thus the phonon is scattered. Based on bond order theory and quantum perturbation theory, the scattering rate formula of this mechanism is derived. Combining the scattering mechanism with the phonon Boltzmann transport equation, we calculate the thermal conductivity of silicon nanowires on rough surface, which is far below the Casimir limit, and the calculated results are in good agreement with the experimental measurements. It is explained that the physical mechanism of the decrease of thermal conductivity of rough silicon nanowires is that the contribution of high frequency phonons to the thermal conductivity of rough silicon nanowires is obviously decreased due to the strong scattering of high frequency phonons by surface bond defects. We further investigated the influence of surface autocorrelation function (Gao Si surface and exponential surface) on thermal conductivity of silicon nanowires on rough surface, including the diameter of silicon nanowires, the root-mean-square of roughness, roughness correlation length and specific surface area, and the influence of surface autocorrelation function (Gao Si surface and exponential surface) on thermal conductivity. The results show that the specific surface area is the most important parameter affecting the thermal conductivity of rough silicon nanowires. With the same morphologic parameters, the thermal conductivity of the rough silicon nanowires with exponential surface is lower because the exponential surface has more high frequency and small fluctuations and its specific surface area is larger than that of the Gao Si surface. The study in this paper not only helps to understand the physical mechanism of phonon-surface scattering, but also helps to regulate the thermal conductivity of nanostructures by surface engineering.
【学位授予单位】：湘潭大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TB383.1;O551.3

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