Specific Transport Properties Relevant To Crystal Structures And Micro Defects Of Thermoelectric Materials

Thermoelectric (TE) materials, which can convert heat and electricity directly and reversely, are a new class of functional materials. There are of great important and potential application values in TE power generators and cooling devices. With the further research of TE materials, their crystal structures become more and more complex and the doped compositions become more and more complicated, which have brought great difficulties in practical research work. The band theory of solids is one of the most successful theories of condensed matter physics. Many basic properties of solids, such as magnetic properties, electrical properties and so on, are all closely related to the electronic structures of solids. Therefore by using the band theory of solids and exploring the electronic structure to study the thermoelectric transport problems of complex systems, the TE behavior could be understood and the evolution laws between the crystal structures and TE performances could be discovered.In this thesis, the first principles calculation method was used based on the density functional theory to study and reveal the relations between the specific crystal defects and the electronic structures (such as density of states, band structure,bonding character,effective mass and so on) of three TE materials (ReSi1.75,β-Zn4Sb3 and Rh3ScSi7). Then the effects of doping to the crystal and electronic structures were analyzed. The transport properties of these materials were also resolved based on the band structure and semi-classical Boltzmann transport theory. The calculation results were compared and analyzed with the experimental data. The reason for the specific anisotropic transport properties was explained in detail, thus the means of improving the TE performance were predicted and proposed.The results show that ReSi1.75 is a narrow gap semiconductor. The valence band maximum is a flat band; whereas the conduction band minimum is a parabolic band. The dangling bonds are formed between the Re d electrons and the Si vacancy defects, which makes ReSi1.75 show semiconductor behavior. The effective mass of holes along [001] direction is comparatively large; whereas the effective masses of electrons along [100] and [010] directions are comparatively large. The TE performance should be much more excellent along [100] direction for p-doped ReSi1.75 and [001] direction for n-doped ReSi1.75.β-Zn4Sb3 is a p-type narrow gap semiconductor, whose electronic structure is not sensitive to its crystal structures. The bond lengths of some Zn-Zn bond are exceptionally short, which are not stable in energy. After fully relaxed, the bond length of Zn-Zn bond increases larger significantly whereas the bond length of Zn-Sb bond increases slightly. The reason is that the covalent Zn-Zn bonds are weak and the Zn-Sb bonds are rather strong. The doping techniques cause contrary effects on the Seebeck coefficient and electrical conductivity ofβ-Zn4Sb3. Therefore, the advantage to improve TE performance ofβ-Zn4Sb3 by element doping was discovered not to be significant.Rh3ScSi7 is a semi-metal, the valence band maximum crosses the Fermi level and enters the conduction bands, which makes the hole carriers play a dominant role in Rh3ScSi7. Rh3ScSi7 is an anisotropic TE material. Al doping can increase the power factors on (0001) plane of Rh3Sc(Si0.98Al0.02)7 obviously; a maximum of 50% increase can be achieved. The figure of merits ZT of both Rh3ScSi7 and Rh3Sc(Si0.98Al0.02)7 increase with the rising temperatures. The thermoelectric performance ZT on (0001) plane of Rh3ScSi7 could be significantly improved Al doping.