| Energy is the material base of human activity, which is the important safeguard for people’s daily life, the nation’s economic development and social stability. At the moment, Coal, oil and natural gas and other fossil energy are still the main source of energy. Fossil fuels are non-renewable and uneven distribution. The high speed development of economy depends on the continuous supply of energy, and the scramble for energy has become an important goal of competition between countries. Use of fossil fuels produces all kinds of harmful gas, solid waste and waste heat, causing serious pollution to the environment and intensifying the greenhouse effect. The development of new energy and new energy materials research have become the focus of scientists from all over the world. Thermoelectric material is a kind of heat and electricity transformation of new energy materials, and can effectively use a large number of automobile exhaust, industrial waste heat, geothermal and realize no mechanical refrigeration. Thermoelectric materials have no noise, no pollution, small volume and fast response, etc. Thermoelectric materials have a broad application prospect, in the case of environmental pollution and energy crisis increasingly serious, the research of new thermoelectric materials has a very strong practical significance.Thermoelectric materials can directly and reversibly convert heat energy into electrical energy. Thus, waste heat recovery using thermoelectric power generation is attracting great interest over the past few decades. The thermoelectric performance of a material is characterized by the material’s dimensionless figure of merit, ? ?leTSZT ?????2, where ?, S, T, e?, and l? are the electrical conductivity, the Seebeck coefficient, the absolute temperature, the electronic thermal conductivity, and the lattice thermal conductivity, respectively. Therefore, a good thermoelectric material should have a large Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity. Zintl phase compounds consist of electropositive cations which donate electrons to electronegative anions, forming ionic bonds to satisfy valence. They have recently emerged as promising thermoelectric materials. This has been demonstrated by many synthesized Zintl phase compounds, such as Ca5Al2Sb6, Yb14 Al Sb11, and Sr3 Al Sb3. In Zintl compounds, the coexistence of ionic and covalent bonds leads to complex crystal structures with large unit cells, which is helpful to obtaining a low lattice thermal conductivity. The interconnected covalent substructure can form paths for high-mobility charge transport. Because both Seebeck coefficient and electrical conductivity have a strong, but opposite dependence on carrier concentration, it is necessary to find an optimal carrier concentration to achieve the highest thermoelectric performance.In this article, we use first-principles calculations and the semiclassical Boltzmann theory. Based on experimental structural parameters, the structure optimization of Sr Li As and MTl9Te6(M = Bi, Sb) were carried out using the Vienna ab initio simulation package(VASP) based on the projector augmented wave(PAW) method to find the most stable lattice structure. The electronic structures were calculated using the full-potential linearized augmented plane waves(FLAPW) method as implemented in the WIEN2 k. We use the semiclassical Boltzmann theory and the rigid band approach to calculate transport properties in the Boltzmann code.The band structure, the density of states, and the transport properties of Sr Li As were studied using first-principles calculations and the semiclassical Boltzmann theory. The covalent Li-As bonding along the y-direction induces a larger dispersion in the conduction bands along the y-(Γ-Y) direction, which leads to a high electrical conductivity along the y-direction. The transport properties of n-type doping are most likely better than those of p-type doping. Further, the peak value(9.2×1011 W/K2ms) of power factor with respect to relaxation time for n-type Sr Li As appears along the y-direction at 1000 K, with a carrier concentration of 6.5×1020 cm-3. The calculated the minimum lattice thermal conductivity(0.71 W/m K) is comparable to those of other Zintl phase compounds.Through the study of the electronic structures and transport properties of MTl9Te6(M = Bi, Sb), it is found that the transport properties of p-type Sb Tl9Te6 are probably better than those of Bi Tl9Te6 at high temperature. Moreover, the peak of ??2S achieves 4.30×1011 W/K2 ms corresponding to the carrier concentration of 1.92×1020 h+ cm-3. The interaction of different doping atoms and Te atoms results in the difference in band gaps of doping compounds. The reason for p-type Sb Tl9Te6 having good transport properties can be attributed to the large Seebeck coefficient due to its appropriate large band gap. The transport properties of materials are mainly affected by different doping atoms. Therefore, these tellurides can be further improved via partial Tl substitution by other atoms remains to be deeply investigated.Through the study of the electronic structures and transport properties of Ba2 Zn As2, the transport properties of p-type Ba2 Zn As2 are probably better than those of p-type at high temperature. Since the anisotropy of p-type ?? is larger than that of n-type, at the same carrier concentration, the ?? along the z-direction is larger than those of the x-dirrection and z-direction. From the crystal structure, the tetrahedron along the z-direction formed by Zn and As atoms is catenate. The neighbouring tetrahedron is shared edge. The catenate tetrahedron is beneficial to improve conductivity. The maximum value of p-type ??2S along the z-direction is 1.27×1012 WK-2m-1s-1, corresponding to the carrier concentration of 0.48e/uc. |
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