Mikrayapı Mühendisliği ile n tipi Mg3Sb2 Zintl fazlarının termoelektrik özelliklerinin geliştirilmesi

Environmental concerns have been growing due to dramatic consequences (e.g., air pollution, global warming) of utilization of harmful energy sources such as fossil fuels. Recent studies indicate that more than 60% of energy produced in the world is lost as a form of waste heat. Therefore, harvesting waste heat would be one of the promising options to support the global energy sustainability. Thermoelectric materials, which directly convert heat to electricity or vice-versa, have gained great interests of researchers for both energy generation and refrigeration applications. To generate significant amounts 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/(κl+κe), where α, σ, κl, κe and T correspond to Seebeck coefficient, electrical conductivity, lattice thermal conductivity, electronic thermal conductivity and temperature, respectively. As these transport properties are interdependent, achieving optimum zT values is challenging. Zintl phase thermoelectric materials display high thermoelectric efficiencies stemming from their tunable electronic transport properties along with their inherently low thermal conductivities. Among them, Mg3Sb2 has recently attracted significant attention due to its high zT as an n-type material. In this thesis study, the electrical and thermal transport properties of n-type Mg3Sb2 were investigated through Bi, Te, nano-boron (nB) and carbon-coated nano-boron (CnB) doping as well as microstructure engineering strategies. Pure Mg3.2Sb2 and Mg3.2Sb1.5Bi0.49Te0.01 powders have been synthesized by high energy ball milling and were sintered by spark plasma sintering technique. To decrease the thermal conductivity even further, melt-centrifugation method was applied to create dislocation arrays and porosity in the microstructure as additional phonon scattering centers. According to SEM and TEM analyses, porous microstructure and dislocation arrays were created by centrifugation method. Further, LFA measurements proved that thermal conductivity values were decreased from 0.98 W/Km to 0.47 W/Km. When heat treatment time increased to an hour, the zT value has increased from 1.2 to 1.67 for Mg3.2Sb1.5Bi0.49Te0.01 compound. However, nB, CnB and MgB2 incorporation experiments didn't positively contribute to the enhancement of thermoelectric properties. Overall, the melt-centrifugation technique was applied on n-type Mg3.2Sb2 Zintl phase for the first time and was successful in creating lattice dislocations and a porous structure leading to substantial improvement in both peak and average zTs of Mg3.2Sb1.5Bi0.49Te0.01. This material with porous structure and lower density displays as good thermoelectric efficiency as the fully dense analogues. Since lightweight materials are preferable and advantageous for several thermoelectric applications including energy harvesting from a vehicle's exhaust system, as-prepared Mg3.2Sb1.5Bi0.49Te0.01 material may have a great potential to commercialize.