Ab initio based multiscale simulations of domain structures in thermoelektric materials

降低晶格热导率是发展新型热电材料的重要目标。具有合适纳米结构的材料具有较低的晶格电导。在某些化合物中，由于组分的相分离，这种结构的形成是自发的。在这个项目中，我们使用基于从头算的多尺度模拟来研究部分分离的热电材料的磁畴形成和由此产生的热晶格电导。研究的重点将集中在热电半赫斯勒材料CoTi1-xMnxSb上，它显示出三个亚晶格中的一个上出现了部分偏析。模拟提供了关于磁区的偏析类型和形状的信息，它们是温度和体系化学计量比的函数。此外，我们还研究了不同结构材料的磁畴形成和热晶格电导的动力学。利用蒙特卡罗模拟和相场方法确定了材料的性质。这些模拟是基于我们从量子力学密度泛函计算中得到的组态能的团簇展开。用蒙特卡罗模拟方法分析了合金的共存区。这使得可以预测给定的化合物是通过团簇生长还是通过调幅节点分解来分离。对磁区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衰减的材料。相场法被用来确定较大长度尺度上的三维磁畴结构，这在实验上是非常困难的。计算了纳米和微结构材料的晶格导热系数，揭示了哪些磁区结构导致导热系数特别低。相场方法也被用来研究偏析的动力学，它提供了关于磁畴结构持久性的信息。所有数值研究的目的是确定化合物的结构和热性能随组成和生产参数的函数，以便为开发优化的热电材料提供指导。

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.