Thermoelectric materials are solid state devices having the capability to convert heat to electrical energy and vice versa. These materials are simple, have no moving parts and use no greenhouse gases. But the major drawback of these materials is their low conversion efficiency. Hence enhancement of thermoelectric efficiency is required to make the use of these devices widespread. Thermoelectric efficiency is related to a parameter termed figure of merit, ZT which is associated with the inter-related transport properties such as Seebeck coefficient, electrical and thermal conductivity. Efficient thermoelectric material should possess high Seebeck coefficient (S), high electrical conductivity () and low thermal conductivity (). The present investigation revolves around improvement of ZT of CoSb3 either by chemical doping or through microstructural modifications. These materials possess structural voids, which can be filled with foreign atoms. The rattling motion of these filler atoms reduces the thermal conductivity of these materials, thereby increasing the thermoelectric efficiency. The rattler atoms chosen for the present study are Mn and Yb. Both coarse and fine-grained MnxCo4Sb12 (x = 0. 0.2, 0.4, 0.8, 1.2 and 1.6) and Yb0.19Co4Sb12 have been synthesized and subjected to various structural and functional property characterizations. The structural study based on Rietveld Analysis and the corresponding difference Fourier maps confirms the void occupancy by Mn and Yb in MnxCo4Sb12 (x 0.2, 0.4 and 0.8) and Yb0.19Co4Sb12. In higher Mn content, x=1.2 and 1.6, Mn was found to partially substitute Co site and partially fill the voids and the remaining precipitated out as free particles. A comparative study of coarse and fine-grained CoSb3 has thrown light in to the grain size effect on the thermoelectric properties. Lowering of grain size helped in enhancement of ZT in CoSb3. Seebeck coefficient (thermoelectric power), electrical and thermal conductivity have been measured for different concentrations of the filler Mn atoms between 300K and 673K. A change in sign of the Seebeck coefficient from negative to positive occurs, when Mn concentration exceeds x=0.8. Electrical resistivity values was found to decrease initially with Mn filling with the minimum value at Mn content, x=0.4 and then gradually increase as Mn content increases. The thermal conductivity value decreases with Mn content in the CoSb3 indicating their rattling property which helps in the enhancement of the overall thermoelectric efficiency. There is a reduction in the value of ktotal in Mn filled CoSb3 than that of the unfilled counterpart. This decrease in the ktotal is a clear indication of the rattling motion of the filler Mn atom in the structural void of CoSb3. Highest ZT of 0.36 is achieved by Mn0.4Co4Sb12 at 373K. Higher concentration of Mn (with x= 1.2 and 1.6) proved to be detrimental in terms of improvement of the value of ZT. Grain size reduction helped in improvement of ZT in Mn0.2Co4Sb12. Maximal ZT of 0.06 at 523K is achieved in hot pressed Mn0.2Co4Sb12. The corresponding coarse-grained material is found to possess ZT of 0.01 at the said temperature. The enhancement can be attributed to high / ratio and high density. Similarly, fine grained Yb0.19Co4Sb12 shows higher ZT compared to the coarse-grained sample because high / and high S.

The increasing price of oil, global warming and rapid industrial growth has drawn much attention to renewable energy technologies over the last few decades. The total energy consumption is estimated to increase 1.4% per year globally. About 90% of this energy supply is generated through fossil fuel combustion with a typical efficiency of 30-40%. The remaining 60-70% of the energy is lost to the environment via automotive exhaust or industrial processes. It is highly desired to retrieve wasted heat to improve the overall efficiency of the energy conversion. Developing thermoelectric materials and devices is a potential solution to utilize waste heat as an energy source. Skutterudites are known to be promising thermoelectric materials in the temperature range 600K to 900K. Novel nanoengineering approaches and filling of skutterudites structure can further improve the transport properties of the material. In this work, Cobalt Antimonide (Co4Sb12) based skutterudites were fabricated via mechanical milling and alloying. Rear earth material Ytterbium and Cerium are used as fillers to substitute the cages in the crystal lattice of these materials. Base material is synthesized via thermochemical reduction of the precursors under hydrogen. Further processing of the material is performed with ball milling and Spark Plasma Sintering (SPS). Ball milling parameters were optimized for nanostructuring of Co4Sb12. Grain size was significantly reduced after SPS compaction. Finally, Thermoelectric transport properties of the material is evaluated over the temperature range 300K to 900K for five different composition of the skutterudites materials. Significant reduction in materials thermal conductivity was achieved through nanostructuring.

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.