纳米线中点缺陷扩散与声子调控

发布时间：2018-04-17 03:07

本文选题：纳米线 + 点缺陷扩散　；参考：《南京航空航天大学》2015年博士论文

【摘要】：纳米线是构建微纳机电系统(MEMS/NEMS)、新型能量收集、转换和存储系统,以及纳尺度超敏传感器件的重要材料。纳米线的力学、电学、热学等性能直接决定了它们所在器件的性能能否达到实际应用要求。人们已经进行了大量纳米线的制备和性能表征研究但其理论还主要基于完美单晶结构。实际应用的纳米线结构是处在一个非常复杂的多场环境中,它们之间可能相互耦合;同时,实际中制备的一维纳米结构通常也并非完美,普遍存在各种各样的缺陷(比如点缺陷、位错等等)。对于纳米结构的多场耦合效应以及含缺陷结构的一些非常规动力学响应,相关研究则略显落后,而这些问题都是实际研究中非常重要且无法回避的问题。本论文通过分子动力学、有限元和理论建模方法研究了纳米线点缺陷扩散导致的特殊力学响应,声子扩散过程调制带来的热电增强等问题。论文的主要贡献有以下三点:1)发现了含点缺陷的Zn O纳米线的超强滞弹性力学响应特性。我们基于理论推导和有限元分析,提出了对应的扩散连续介质模型,揭示出这种超强滞弹性力学行为是由应力梯度导致纳米线内点缺陷长程扩散引起的。纳米线的力学性能随点缺陷的扩散而发生变化,进而呈现出较强的滞弹性现象。相关实验验证了该理论模型的预测结果。同时,我们根据该模型将传统的Gorsky迟豫理论扩展到包含多种点缺陷的非线性滞弹性理论。2)揭示了纳米线力学响应行为与材料性能参数的关系。通过建立和求解耦合点缺陷扩散方程和纳米线的振动方程,论文系统地研究了纳米线尺寸与杨氏模量、密度、点缺陷的扩散系数等性能参数的关系及其对纳米线力学响应行为的影响。解释了纳米线弯曲释放后发生振动或缓慢恢复到伸直平衡位置的不同机理和控制因素,进而定量的研究了纳米线振动受其中点缺陷扩散耦合后的阻尼效应。我们的分子动力学模拟计算验证了该理论模型结果。3)阐明了表面周期结构对纳米线声子扩散的限制及对热电性能的增益。受实验合成的念珠状纳米线启示,通过系统的非平衡分子动力学计算模拟发现了念珠状的纳米线具有较强的声子局域化效应,使其具有比光滑纳米线更低的热导率。进而提出了一个简化的声子散射模型,对该现象进行解释,为设计纳米线表面形貌优化热电性能提供了理论基础。本论文的研究结果为更好地控制一维纳米线结构的力学和热电行为提供了新的途径和思路,并可用于指导实验和中纳米器件的设计。
[Abstract]:Nanowires are important materials for building MEMS / NEMS, new energy collection, conversion and storage systems, and nanoscale hypersensitive sensing devices.The mechanical, electrical and thermal properties of nanowires directly determine whether the performance of their devices can meet the practical application requirements.Many researches have been done on the preparation and characterization of nanowires, but the theory is mainly based on the perfect single crystal structure.The nanowire structures used in practice are in a very complex multi-field environment, and they may be coupled with each other; at the same time, the one-dimensional nanostructures prepared in practice are usually not perfect.There are various defects (such as point defects, dislocations, etc.).For the multi-field coupling effect of nanostructures and some unconventional dynamic responses of structures with defects, the related studies are slightly backward, and these problems are very important and unavoidable in practical research.In this paper, the special mechanical response of nanowires due to defect diffusion and thermoelectric enhancement caused by phonon diffusion are studied by molecular dynamics, finite element method and theoretical modeling.The main contributions of this paper are as follows: 1) the super anelastic response of ZnO nanowires with point defects is found.Based on the theoretical derivation and finite element analysis, a corresponding diffusion continuum model is proposed. It is revealed that the super-strong anelastic mechanical behavior is caused by the long-range diffusion of defects in nanowires due to the stress gradient.The mechanical properties of nanowires change with the diffusion of point defects, and then show a strong anelastic phenomenon.The prediction results of the theoretical model are verified by relevant experiments.At the same time, according to the model, we extend the traditional Gorsky theory to nonlinear anelastic theory. 2) We reveal the relationship between the mechanical response behavior of nanowires and the properties of materials.By establishing and solving the coupling point defect diffusion equation and the vibration equation of nanowires, the dimensions and Young's modulus and density of nanowires are systematically studied in this paper.The relationship between the diffusion coefficient and other properties of point defects and its influence on the mechanical response behavior of nanowires.The different mechanisms and controlling factors of the vibration or slowly restoring to the equilibrium position of the nanowires after bending release are explained, and the damping effect of the vibration of nanowires subjected to the diffusion coupling of the point defects in the nanowires is studied quantitatively.Our molecular dynamics simulations verify the theoretical model. 3) the limitation of surface periodic structure on nanowire phonon diffusion and its gain on thermoelectric properties are illustrated.Inspired by the experimental synthesis of monilide nanowires, it is found that the rosary nanowires have a strong phonon localization effect and have lower thermal conductivity than smooth nanowires through systematic non-equilibrium molecular dynamics simulation.Furthermore, a simplified phonon scattering model is proposed to explain the phenomenon, which provides a theoretical basis for the design of nanowires surface morphology optimization thermoelectric properties.The results of this paper provide a new approach and train of thought for better controlling the mechanical and thermoelectric behavior of one-dimensional nanowire structures, and can be used to guide the design of experimental and mid-nanometer devices.
【学位授予单位】：南京航空航天大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB383.1

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