声子共振机理及其对纳米结构热导率的调制研究

Thermoelectric material, which can convert heat energy into electricity and vice versa, is a potential candidate to solve the future energy problems. An efficient thermoelectric material usually features as “electron crystal and phonon glass”. Many materials hold high power factor, i.e., behave as “electron crystal”. However, the high lattice thermal conductivity hinders the improvement of their thermoelectric performance. To realize the “phonon glass” behavior of thermoelectric materials, the phonon scattering mechanism has been widely applied to reduce the thermal conductivity. However, the scattering mechanism has its intrinsic weaknesses that the scattering sources are inside the materials, thus they will also scatter electrons. Besides, the scattering mechanism can only scatter the phonons with wavelengths smaller or comparable to the size of scatterers, thus it is only effective to high frequency phonons rather than the to low frequency modes. To solve the intrinsic limitations of the scattering mechanism, we propose a new material structural design by adopting the phonon resonant mechanism. The resonant mechanism can tailor the propagation of low frequency phonons via the resonant hybridization. Moreover, the resonant structure locates outside the main structure, which has limited impact on the electron transport. As a result, the resonant mechanism can serve as a complementary method to the scattering strategy. Based on the Molecular Dynamics and Lattice Dynamics methods, in this proposal, we aim to study the hybridization mechanism between the resonant and propagating modes, the design and optimization of resonant structure, and the cooperation mechanism between the resonant and scattering effects. With the combination of phonon resonances and scattering, we target to design materials with “phonon glass” property while keeping their “electron crystal” feature. This research would promote the design of new generation thermoelectric materials.

热电材料可将电和热相互转化，是解决能源危机发展清洁能源的关键材料，在废热发电和固态制冷方面具有广阔的应用前景。设计具有“电子晶体，声子玻璃”的材料体系是提高热电效率的关键。研究表明，很多材料具有“电子晶体”特性，但它们的高热导率限制了其热电效率的提高。为实现“声子玻璃”的愿景，目前广泛采用声子散射方法，但由于散射源在材料内部，其在降低热导率的同时也会损害电导率，且其只能散射高频声子。为解决这些弊端，本项目提出声子共振机制以降低材料热导率，其优点在于可有效阻碍低频声子传输，且共振结构在主体材料表面，对电子散射较弱。本项目以Si、Bi2Te3及PbTe几种典型热电材料为研究对象，采用分子动力学和晶格动力学方法探索共振声子与传输声子相互作用机制、共振结构的优化设计及共振与散射机制协同作用机理，确定既具有极低热导率又能维持电输运性能的材料设计方案，为新一代高效热电材料设计提供新的思路及理论支持。

热电材料能将电能和热能相互转化，是解决能源危机发展清洁能源的关键材料，在废热发电和固态制冷方面具有广阔的应用前景。设计具有“电子晶体，声子玻璃”的材料体系是提高热电效率的关键。研究表明，采用声子散射方案降低材料热导率是提高热电效率的有效方法之一。但散射源在降低热导率的同时也会损害电导率，且其只能散射高频声子。为解决这些弊端，本项目提出声子共振机制以降低材料热导率，其优点在于能有效阻碍低频声子传输，且共振结构在主体材料表面，对电子散射较弱。本项目采用分子动力学模拟结合晶格动力学分析等方法对声子共振机制进行了详细研究。首先，本项目以Si薄膜和石墨烯纳米带为研究对象，详细研究了共振声子与传输声子之间的相互作用机理。结果表明，表面共振结构可产生大量低频共振声子并显著降低热导率，这主要得益于共振机制可大幅减小低频声子的弛豫时间，而群速度降低起到次要作用。其次，通过在共振结构中引入合金散射来降低共振结构中的声子平均自由程，我们发现其共振效果被削弱了，这主要是因为共振结构中自由程的减小影响了共振模式的形成，从而减弱共振效果。另外，通过增加共振结构和主体材料之间的界面粗糙度，本项目发现共振效果也会被削弱，这主要归因于粗糙的界面阻碍了共振声子向主体材料中的渗透，从而减弱了共振耦合。最后，我们在硅纳米结构及石墨烯纳米带中引入了点缺陷/螺旋位错，并结合表面共振结构，系统地研究了声子散射和声子共振对热输运的协同效应及其相关机理，以实现对全频段声子输运的高效调控。结果表明，缺陷散射主要降低高频声子热导率（Si：>3THZ；石墨烯纳米带：>8THz），而表面共振结构则主要降低低频声子的热导率，因此二者结合能够抑制整个频率范围内的声子输运，实现极低热导率的设计。本项目分离出了影响声子共振的因素，并通过同声子散射相结合实现了对全频段声子输运的高效调控，为新一代高性能热电材料的开发提供了理论支撑。