Tuning the thermoelectric properties of skutterudite phase (CoSb3) by melt-centrifugation & incorporation of InSb and carbon-coated nano boron

In today's world, classical energy reserves are being depleted rapidly and unwanted results such as the progression of global warming and air pollution arise with the use of these resources. Recent studies indicate that more than 60% of the energy produced in the world is lost as a form of waste heat. Therefore, harvesting waste heat is considered as one of the promising options for contributing sustainable energy solutions. For that purpose, thermoelectric materials, which directly convert heat to electricity (or vice versa), have gained a significant interest among researchers. The thermoelectric generators use heat energy (temperature gradient) as input to produce electrical energy. They are silent and scalable solid-state devices with no moving parts making them ideal for power generation applications. To generate significant amount of energy by using thermoelectric materials, first, conversion efficiency should be increased, which is evaluated by the dimensionless figure of merit, zT. zT is defined as zT = α2σT/(κlat+κe), where α, σ, κlat, κe and T correspond to Seebeck coefficient, electrical conductivity, lattice thermal conductivity, electronic thermal conductivity, and temperature, respectively. Since these transport properties are mostly interdependent, achieving optimum zT values is challenging. The skutterudites (CoSb3 or Co4Sb12) are promising thermoelectric materials, operating at mid-to-high temperatures, due to their high electronic mobility combined with low thermal conductivity which is associated with the rattling filler atoms in the void sites. In this thesis, the electrical and thermal transport properties of n-type Yb filled Co4Sb12 were investigated after applying microstructure engineering strategies such as melt-centrifugation, carbon-coated nano boron and InSb incorporations. Solid-state synthesis, high energy ball milling (mechanochemical synthesis) and then spark plasma sintering techniques were employed to obtain the materials to be analyzed. To reduce the thermal conductivity, dislocation arrays and porosity as additional phonon scattering centers were intended to be created for the first time in this study by the melt-centrifugation method. Unfortunately, melt-centrifugation experiments did not result in increased zT values for this isotropic system as opposed to other anisotropic materials such as Bi2Te3 and Mg3Sb2. As-obtained porous and thus lightweight materials are still promising for the module applications. To enhance the electronic transport, reduce the thermal conductivity, thus, increase the zT values, incorporations of carbon-coated nano boron (CnB) and InSb to Yb0.4Co4Sb12 phase were then studied. The highest zT values of around 1.3 and 1.5 were obtained for 0.1 wt.% CnB added Yb0.4Co4Sb12 sample and 1 wt.% and 1.5 wt.% InSb added Yb0.4Co4Sb12 samples, respectively. These values are 30 % and 50 % higher than the untreated skutterudite phase, respectively. Besides the enhancement in zT values, mechanical properties were also improved with the additions of CnB and InSb into the Yb0.4Co4Sb12 microstructure. The additions of InSb and CnB were found to be quite successful in boosting up the thermoelectric performance of the skutterudites.