The Electronic Structure And Thermoelectric Properties Of Doped In₂O₃and Two Zintl-phase Compounds

As the worldwide concern over environmental impact and limited resources for fossil fuels intensifies, energy harvesting, and energy conversion from various sources have become very active area of research. Thermoelectric conversion, which has the advantages of not using any moving mechanical parts; being environmental friendly, and being easy to control, can play an important role in a global sustainable energy solution. However, its use has been limited due to its low efficiency. In this dissertation, the first-principles calculation method based on density function theory and the semiclassical Boltzmann theory are employed to study the electronic structure and thermoelectric properties of In2O3and Zintl-phase (Ca5Al2Sb6and Ca5Ga2As6). The main results are shown as following:1. the electronic structure and the thermoelectric properties of Ge doped In2O3are studied By study the formation energies of In32-xGexO48(x=0,1,2,3,4,5,6, and7), we found that the solubility limit of Ge-doping in In32O48is9.375at.%(x=3), so we are only interested in the electronic structure and thermoelectric properties of In32-xGexO48(x=0,1,2, and3). The study results shows that the band gap and electronic structure near the Fermi level can be modified significantly by Ge doping. The largest value of S2σ/τ is about5X1012W/K2ms for x=3at20K, so In29Ge3O48is expected to be a promising thermoelectric materials at low temperature.2. the electronic structure and the thermoelectric properties of Sn-doped ln2O3are studied Research has shown that the solubility limit of Sn in In2O3is under8at.%, so we are interested in the electronic structure and thermoelectric properties of In32-xSnxO48(x=0,1, and2). By study the electronic structure, we found that the Fermi level shifts upward into the conduction band. Further studies show that, for In31SnO48, the anti-bonding states introduced by doping affect the states near the bottom of the conduction band through hybridization and open a small gap along Г-H-N-Г-P direction. At room temperature, S2σ/τ is1.37×1012W/K2ms for x=1at300K, so In29Ge3048is expected to be a promising thermoelectric material at room temperature.3. the electronic structure and the thermoelectric properties of ln24M8O48(M=Ge, Sn, Ti, and Zr) are studied The result shows that doping In2O3with Sn is more conductive to get better thermoelectric properties. Further investigation revealed that:(1) the electronic configuration of Sn is very similar to that of substituted atom;(2) among the four dopant atoms, the atomic number of Sn is the most adjacent with the host atom's;(3) most strikingly, the largest magnitude of S2σ/τ and the absolute values for Seebeck coefficient follow the order of the difference in atomic number between the dopant and the host atom. So, we can say that in order to improve the thermoelectric properties of In2O3by doping, an attractive doping atom should have the similar electronic configuration and an adjacent atomic number to the doped atom.4. the electronic structure and the thermoelectric properties of Ca5Al2Sb6are calculated. There is a combination of heavy and light bands at conduction band edge, which may lead to a combination of high Seebeck coefficient and reasonable conductivity. On the other hand, the top of the valence band shows strongly anisotropic one-dimensional structure, which is favorable for improving the thermoelectric properties. The largest value of S2σ/τ is about2.8×1012W/K2ms for z direction at T=30K, n=-6.3×1021cm3. The combination of heavy and light bands at conduction band edge is favorable for improving the thermoelectric properties of materials.5. the electronic structure and the thermoelectric properties of Ca5Ga2As6are calculated. By study the anisotropy of the transport properties, we found that the transport properties of p-type CasGa2As6are better than that of n-type Ca5Ga2As6. The transport properties along the z direction are much better than the other two directions. At different temperature, the peak value of ZeT is appears at the different carrier concentration, which make it is possible to obtain the highest efficiency under larger temperature gradients. The largest value of ZeT is0.95for z direction at T=300K,5.19x1019cm-3. The anisotropic one-dimensional structure is favorable for improving the thermoelectric properties of Ca5Ga2As6...