The Theoretical Study Of The Thermoelectric Properties For Several Zintl Compounds And MgAgSb

With the accelerated process of global industrialization, the world energy crisis and environmental pollution has become a problem that can not be ignored in different country, which severely restricts the development of human. Research and development of new clean energy sources become the trend of global energy development. As a new type of renewable energy materials, thermoelectric materials have attracted great attention. Based on Seebeck effect and Peltier effect, thermoelectric materials can perform direct conversion between thermal energy and electrical energy. Due to the low thermoelectric conversion efficiency, the current application of thermoelectric technology still exists only in some special fields. Searching and exploring methods and means for enhancing the thermoelectric conversion efficiency of materials, has great significance for promoting the thermoelectric technology vigorously. The thermoelectric conversion efficiency of a thermoelectric material strongly depends on the dimensionless figure of merit ZT, and the greater the ZT value, the higher the thermoelectric conversion efficiency. In this dissertation, the lattice structure, electronic structure and the thermoelectric properties of Zintl-phase compounds?Ba3Al3P5, Ba₃Ga₃P₅ and Ca₅In₂Sb₆? and MgAgSb are investigated by combining the first-principles method and the semiclassical Boltzmann transport theory, then we explore the methods for enhancing the ZT value. This dissertation is divided into three parts:?1? The electronic structure and the thermoelectric properties of Zintl compounds Ba3M3P5?M = Al, Ga? are investigated by the density functional theory combined with the semiclassical Boltzmann transport theory. It is found that the transport properties of p-type Ba3M3P5 are better than that of n-type one at optimum carrier concentration. By p-type doping, the maximum ZT of Ba3Al3P5 and p-type Ba₃Ga₃P₅ can reach 0.49 at 500 K and 0.65 at 800 K, corresponding to the carrier concentration of 7.1 × 1019 holes per cm3 and 1.3 × 1020 holes per cm3, respectively. The higher thermoelectric performance of p-type Ba3M3P5 than n-type one is mainly due to the large valence band dispersion near the Fermi level. For Ba₃Ga₃P₅, the multiple extrema on the top of valence bands will increase its electrical conductivity. The calculated partial charge density near the Fermi level of Ba3M3P5 shows that there is little charge density around the P1 atoms in Ba3Al3P5. On the contrary, the high charge density appears around all P atoms in Ba₃Ga₃P₅, which may be the reason why Ba₃Ga₃P₅ has multiple extrema on its top of valence bands. Meanwhile, the minimum lattice thermal conductivities of Ba3Al3P5 and Ba₃Ga₃P₅, are small and are comparable to those of Ca5Al2Sb6 and Ca5Ga2Sb6. Compared with p-type Ba3Al3P5, p-type Ba₃Ga₃P₅ shows better thermoelectric properties, which is mainly due to the multiple extrema on its top of the valence bands and its small band gap. Moreover, p-type Ba₃Ga₃P₅ shows nearly isotropic transport behavior. Hence, good thermoelectric performance for p-type Ba₃Ga₃P₅ can be predicted.?2? We have systematically investigated the electronic structure and thermoelectric performances of substitutional doping with Pb on In sites at a doping level of 5%?0.2 e per cell? for Ca₅In₂Sb₆ by using density functional theory combined with semi-classical Boltzmann theory. It is found that in contrast to Zn doping, Pb doping introduces a partially filled intermediate band in the band gap of Ca₅In₂Sb₆, which originates from the Pb s states by weakly hybridizing with the Sb p states. The electrons can be excited not only from the valence band to the conduction but also from the intermediate band to the conduction band and from the valence band to the intermediate band. The intermediate band will dramatically increase the electrical conductivity of Ca₅In₂Sb₆ and it has little detrimental effect on its Seebeck coefficient. Therefore, the ZT value of Ca₅In₂Sb₆ may be greatly enhanced by doping with Pb at In sites with a doping level of 5%. For p-type Ca5In1.9Pb0.1Sb6, the maximum ZT can reach 2.46 at 900 K, with a carrier concentration of 3.85 × 1020 holes per cm3. Thus, the current work will motivate future experimental work for improving the thermoelectric performance of Zintl compounds by Pb doping.?3? The electronic structure and the thermoelectric properties of MgAgSb are studied. a-MgAgSb has been proposed as a promising candidate for room-temperature thermoelectric application. The samples exhibited an intrinsically low thermal conductivity due to large unit cell and distorted structure. For a-MgAgSb, the band below the valence band maximum has 14 conducting carrier pockets. Such intrinsic high band degeneracy attracts our interest in investigating the thermoelectric performance of p-type a-MgAgSb by doping. Then, by using density functional theory combined with semi-classical Boltzmann theory, we studied the electronic structure and thermoelectric performance of substitutional doping with Li on Mg sites for a-MgAgSb. It is found that Fermi level moves down to valence bands and Li-doping induces multiple degenerate valleys near the Fermi level, which will increase the carrier concentration of a-MgAgSb and consequently improve the electrical conductivity, which is responsible for the extraordinary thermoelectric performance of Li-doped a-MgAgSb, and a maximum ZT value of 1.1 may be achieved at 550 K. The present work suggests that increasing band valleys is an effective strategy for enhancing the thermoelectric properties of a-MgAgSb.