Ab initio based multiscale simulations of domain structures in thermoelektric materials

降低晶格热导率是开发新型热电材料的一个重要目标。具有合适纳米结构的材料具有低晶格电导率。在一些化合物中，由于组分的相分离，这种结构的形成自发发生。在这个项目中，我们使用基于从头开始的多尺度模拟来研究显示部分分层的热电材料的域形成和由此产生的热晶格电导率。研究的重点将放在热电半赫斯勒材料 CoTi1-xMnxSb 上，该材料在三个亚晶格之一上显示出部分偏析。模拟提供了有关偏析类型和域形状随温度和系统化学计量变化的信息。此外，我们研究了不同结构材料的域形成动力学和热晶格电导率。材料特性是借助蒙特卡罗模拟和相场方法确定的。模拟基于我们从量子力学密度泛函计算中得出的构型能量的簇展开。蒙特卡罗模拟用于分析合金的共存区域。这允许预测给定的化合物是通过簇生长还是通过旋节线分解来分层。域边界及其规律性的蒙特卡罗模拟有助于确定具有最佳声子阻尼的材料。相场法用于确定较大长度尺度上的三维域结构，这在实验上很难研究。计算纳米和微米结构材料的热晶格传导率，显示哪些域结构导致特别低的热传导。相场方法还用于研究偏析动力学，提供有关域结构持久性的信息。所有数值研究的目的是确定化合物的结构和热性能作为成分和生产参数的函数，以便为优化热电材料的开发提供指导。

The reduction of the thermal lattice conductivity is an important objective in the development of new thermoelectric materials. Low lattice conductivities are found in materials with suitable nanostructures. In some compounds such a structure formation occurs spontaneously due to a phase segregation of components. In this project, we use ab-initio based multiscale simulations to study the domain formation and the resulting thermal lattice conductivity of thermoelectric materials that show partial demixing. The focus of the investigations will be on the thermoelectric half-Heusler material CoTi1-xMnxSb, which shows a partial segregation on one of the three sublattices. The simulations provide information about the segregation type and the shape of the domains as a function of the temperature and the stoichiometry of the system. Furthermore, we investigate the dynamics of the domain formation and the thermal lattice conductivity of differently structured materials. The material properties are determined with the help of Monte Carlo simulations and phase field methods. The simulations are based on a cluster expansion of the configurational energy that we derive from quantum mechanical density functional calculations. The Monte Carlo simulations are used to analyze the coexistence region of the alloy. This allows predicting whether a given compound demixes by cluster growth or by spinodal decomposition. Monte Carlo simulations of the domain boundary and its regularity help to determine a material with an optimum phonon damping. The phase field method is used to determine the three-dimensional domain structure on a larger length scale, which is very difficult to investigate, experimentally. The thermal lattice conductivity is calculated for nano and micro-structured materials, showing which domain structures lead to especially low heat conduction. The phase field method is also used to study the dynamics of segregation, which gives information on the persistence of the domain structures. The aim of all the numerical studies is to determine the structural and thermal properties of the compounds as a function of the composition and the production parameters in order to provide guide lines for the development of optimized thermoelectric materials.