Many studies and experiments have been conducted over the years to find solutions to the electricity problem. This issue is not just related to how fossil fuels are dispensed. Also, the environmental concerns associated with using fossil fuels have become a severe issue, which is a major cause of environmental pollution and ozone layer damage. As such, the need for energy becomes one of the essential goals. Therefore, research has begun to revolve around thermoelectrics, which is a straightforward approach for generating energy, by converting heat directly into electricity. Cobalt antimonide (CoSb3) belongs to a broad family of materials with the skutterudite structure, which have been recently identified as potential new thermoelectric materials with high performance. The CoSb3 is one of the numerous promising thermoelectric materials in the intermediate temperature range. The binary CoSb3 is a narrow bandgap semiconductor with a relatively flat band structure and excellent electrical performance. The thermoelectric performance efficiency of binary CoSb3 is measured by its figure of merit. The figure of merit is important for thermoelectric materials and is primarily governed by the Seebeck coefficient because it exhibits a square dependence. The Seebeck coefficient of the CoSb3 can be affected by many factors that can either increase or decrease it. Strain is an important aspect for the transport properties, including the Seebeck coefficient. The goal of this thesis project is to study the effect of point defects and strain on the Seebeck coefficient of skutterudite CoSb3. The binary CoSb3 skutterudite was explored through density functional theory (DFT) to calculate the ground-state properties, in particular the Seebeck coefficient. Two different CoSb3 structures were considered, an ideal one (without any defects) and the other was termed real (containing defects). In both cases, the Seebeck coefficient and its response were studied while strain was applied by changing the volume of the structure. The non-equilibrium Green's function was used within a DFT simulation to get a transmission distribution, where it was essential for calculating the Seebeck coefficient. Moreover, oxygen molecules were placed over the (001) surface of 2 × 2 × 1 CoSb3 supercell to establish if oxidation leads to point defect formation. These simulations were carried out by DFT-based molecular dynamics. It is found that the strain affects the Seebeck coefficient in the ideal structure. At compression, the absolute value of the Seebeck coefficient increases.  By contrast, the Seebeck coefficient changed its sign from negative to positive and increased to 894 μVK−1at tension, which was unexpected. The electron density distribution map was explored to explain the behavior of the Seebeck coefficient at equilibrium, compression, and tension. It can be seen that the electron distribution between Co and Sb is increased at compression, implying an increased orbital overlap (covalent interaction). By contrast, the tension reduces the electron distribution between Sb and Co. The real structure induced by oxidation exhibits Sb vacancies. The See-beck coefficient is affected differently than that of the ideal structure. At equilibrium, the Seebeck coefficient increases to 151 μVK−1. The electron density distribution between Sb and Co is enhanced in the real structure compared to the ideal one. The most drastic change is found at tension, where the Seebeck coefficient reaches−270 μVK−1. It may be speculated that this occurs due to O which increases the orbital overlap. The strategy introduced in this work, an interplay of defects and strain effects, may be beneficial for other thermoelectric materials.

In the present work, the plasma repair for damaged ultra-low-k (ULK) materials, newly developed at the Fraunhofer ENAS, is studied with density functional theory (DFT) and molecular dynamic (MD) methods to obtain new insights into this repair mechanism. The ULK materials owe their low dielectric constant (k-value) to the insertion of k-value lowering methyl groups. During the manufacturing process, the ULK materials are damaged and their k-values increase due to the adsorbtion of hydroxyl groups (OH-damage) and hydrogen atoms (H-damage) that replaced themethyl groups. The first investigation point is the creation of repair fragments. For this purpose the silylation molecules bis(dimethylamino)-dimethylsilane (DMADMS) and octamethylcyclotetrasiloxane (OMCTS) are fragmented. Here, only fragmentation reactions that lead to repair fragments that contain one silicon atom and at least one methyl group were considered. It is shown that the repair fragments that contain three methyl groups are preferred, especially in a methyl rich atmosphere. The effectivity of the obtained repair fragments to cure an OH- and H-damage are investigated with two model systems. The first system consists of an assortment of small ULK-fragments, which is used to scan through the wide array of possible repair reactions. The second system is a silicon oxide cluster that investigates whether the presence of a cluster influences the reaction energies. In both model systems, repair fragments that contain three methyl groups or two oxygen atoms are found to be most effective. Finally, the quantum chemical results are compared to experimental findings to get deeper insight into the repair process.:1. Introduction 2. Theoretical Background 2.1. Ultra-low-k Materials 2.1.1. Definition, Usage and Challenges 2.1.2. k-Restore 2.2. Reaction Theory 2.2.1. Reaction Process 2.2.2. Thermal Influence 3. Computational Methods 3.1. Overview 3.2. Density Functional Theory 3.2.1. Theoretical Background 3.2.1.1. The Schrödinger Equation and the Variational Principle 3.2.1.2. From the Electron Density to the Kohn-Sham Approach 3.2.1.3. Exchange-Correlation Functionals and Basis Sets 3.2.2. Used Program Packages 3.3. ReaxFF 3.3.1. Theoretical Background 3.3.2. Used Program Packages 4. Model System 4.1. Damaged ULK Materials 4.1.1. ULK-Fragments 4.1.2. Silicon Oxide Cluster 4.2. Repair Fragments 4.2.1. Overview 4.2.2. Fragmentation of DMADMS 4.2.3. Fragmentation of OMCTS 4.2.4. Continuing Reactions 5. Results and Discussion 5.1. Reactions between Repair Fragments and ULK-Fragments 5.1.1. Repair of OH-damages 5.1.2. Repair of H-damages 5.1.3. Selected Repair Reactions with Gaussian 5.2. Reactions Between Repair Fragments and Silicon Oxide Cluster 5.2.1. Comparison Between ULK-Fragments and Silicon Oxide Cluster 5.2.2. Comparability of DFT and MD Results 5.3. Comparison with Experimental Results 6. Summary and Outlook A. Appendix A.1. Temperature Influence . A.1.1. Temperature Influence on the DMADMS Fragmentation in Dmol3 A.1.2. Temperature Influence on the OMCTS Fragmentation in Dmol3 . A.2. Tests A.2.1. DMADMS Fragmentation with Gaussian A.2.2. G2 Test Set A.2.3. Calculation Time of the Silicon Oxide Cluster in Dmol3 A.3. Error Analysis A.3.1. Basis Set Superposition Error in Dmol3 A.3.2. Dispersion Correction A.4. Illustration of Defects A.5. Bookmark