npj: 清华大学-南卡莱纳大学联手模拟—发现位错声子散射

来源：知社学术圈

热电材料作为一种新型的能源转化材料，具有很大的应用价值。通常人们通过降低材料热导率来提高热电转化效率，在材料中引入缺陷是降低热导率主流的做法。位错是材料中最基本的缺陷，通过引入位错降低材料的热导率已经在实验上得到了证实，并且获得了可观的热电转化效率。但是人们对于位错降低热导的原因即位错对于声子（半导体中主要的热载流子）的散射的理解，却只是基于Debey-Callway 模型半经验的公式。

来自清华大学的材料学院的徐贲助理教授和美国南卡罗莱纳大学的胡明教授共同领导的团队，使用非平衡分子动力学方法研究了PbTe中单个位错和声子的散射过程。同之前的研究相比较，他们的研究消除了边界散射和位错之间互相作用两个因素的干扰。通过傅里叶定律计算，在PbTe中，4 ×  1015m−2 密度的位错导致热导率下降了62%；通过截面频谱热流分析，获得了PbTe中被位错散射声子的频率；通过求解位错偶模型的本征模式，发现位错核心部分原子振动局域化增强是导致热导率下降的原因；通过拟合得到PbTe声子的平均自由程并且和传统公式拟合结果对比，发现了传统拟合结果的不足之处。他们的方法不仅可以获得位错对于声子的散射强度，而且其研究方法也可以广泛应用于热电材料中缺陷与声子的相互作用的研究。

该文近期发表于npj Computational Materials 5: 97 (2019)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。

Strong phonon localization in PbTe with dislocations and large

deviation to Matthiessen’s rule 

Yandong Sun, Yanguang Zhou, Jian Han, Wei Liu, Cewen Nan, Yuanhua Lin, Ming Hu and Ben Xu

Dislocations can greatly enhance the figure of merit of thermoelectric materials by prominently reducing thermal conductivity. However, the evolution of phonon modes with different energies when they propagate through a single dislocation is unknown. Here we perform non-equilibrium molecular dynamics simulation to study phonon transport in PbTe crystal with dislocations by excluding boundary scattering and strain coupling effect. The frequency-dependent heat flux, phonon mode analysis, and frequency-dependent phonon mean free paths (MFPs) are presented. The thermal conductivity of PbTe with dislocation density on the order of  1015m−2 is decreased by 62%. We provide solid evidence of strong localization of phonon modes in dislocation sample. Moreover, by comparing the frequency-dependent phonon MFPs between atomistic modeling and traditional theory, it is found that the conventional theories are inadequate to describe the phonon behavior throughout the full phonon spectrum, and large deviation to the well-known semi-classical Matthiessen’s rule is observed. These results provide insightful guidance for the development of PbTe based thermoelectrics and shed light on new routes for enhancing the performance of existing thermoelectrics by incorporating dislocations.