Designing Mg3(Sb,Bi)2 and MgAgSb for low- and mid-temperature thermoelectric applications

In response to growing concerns over environmental pollution, global warming, and greenhouse gas emissions, many countries are striving to achieve net-zero emissions by 2050. Given that approximately 60% of the energy produced is lost as waste heat, thermoelectric (TE) materials present a promising solution for converting this waste heat into electricity. The conversion efficiency of thermoelectric materials is quantified by the dimensionless thermoelectric figure of merit, zT = S2σT/(κl+κe), where S, σ, κl, κe and T are the Seebeck coefficient, electrical conductivity, lattice thermal conductivity, electronic thermal conductivity and temperature, respectively. Achieving high zT values is challenging due to the interdependent nature of these transport properties. The first part of this thesis focuses on the synthesis, characterization, and transport property evaluation of Nb incorporated Mg3Sb2-Mg3Bi2 solid solutions for low temperatures. The grain boundary complexion phenomena and its effects on transport properties were investigated. As a result of a space charge region formed in grain boundaries, the conduction barrier in grain boundary region was lowered and scattering of charge carriers was inhibited. The achievement of low thermal conductivity coupled with low resistivity values and elevated carrier mobility, contributed to the attainment of high zT values, particularly in low-temperature regimes. For mid-temperature applications, the effectiveness of MgB2 incorporation on the thermoelectric properties of Mg3(Sb,Bi)2 system was investigated, inspired by the energy filtering effect. Despite negligible changes in carrier concentration, a significant increase in the Seebeck coefficient of composite materials is observed. This phenomenon is attributed to energy filtering, where carriers are selectively scattered. The addition of MgB2 induces a reduction in the electronic component of thermal conductivity, resulting in a low κ value at 673 K. The achievement of a high Seebeck coefficient, coupled with low thermal conductivity, contributes to high zT values. The second part of this thesis focuses on the synthesis, characterization, and thermoelectric performance of polycrystalline MgAg0.97Sbx (x = 1, 0.995, 0.975) materials. DSC analysis revealed phase transitions at approximately 588 K (α- to β-MgAgSb) and 655 K (β- to γ-MgAgSb). An additional annealing step was incorporated to eliminate the β-MgAgSb phase, improving the crystal structure stability and transport properties. The impact of Sb content on electronic transport characteristics was investigated. Further studies on sintering and annealing parameters revealed that Mg-Ag anti-site defects and strain-induced defects influence thermoelectric behavior, with high Seebeck coefficients, high Hall and weighted mobilities, and low lattice thermal conductivity, resulting in a zT of 0.82 at 330 K. Additionally, the effect of various dopants (Y and Nb on the Mg site, Zn and Co on the Ag site, and Ge and Te on the Sb site) was investigated.