Transport Properties Of Thermoelectric Oxide SrTiO₃: Theoretical Study

Thermoelectric effect refers to a class of phenomena in which a temperature difference creates an electric potential or an electric potential creates a temperature difference. The realization of the above features just need all solid-state thermoelectric devices, so it has the merits of high reliability, non-polluting and so on. For power generation, it has the merit of high reliability compared to traditional mechanical energy generation mode, and has been successfully applied to space probes, such as the thermoelectric power generation devices in the Voyager spacecraft launched in 1977. As for refrigeration, it has the merits of no noise, no pollution, etc., compared with the traditional freon compressor cooling mode, and has been widely applied to the bedroom and the guest house use of noise-free refrigerator. In recent years, due to the two major themes of energy and environment for sustainable development, thermoelectric materials, as a clean, green energy materials, are expected to be widely used asautomobile exhaust heat power generation, home cooling, portable refrigerators, etc.The performance of thermoelectric materials depends on ZT value, higher ZT value gives higher efficiency of energy conversion. ZT= S2σ/k, where S,σandκ, are Seebeck coefficient, electrical conductivity and thermal conductivity, respectively. High-performance thermoelectric material needs to have a high PF value and a low thermal conductivity. From this point of view, looking for high-performance thermoelectric materials can be divided into two directions:First, using low-dimensional technology to improve their thermoelectric properties the traditional thermoelectric materials (such as Bi2Te3, PbTe, SiGe, etc.); second, looking for a new type of bulk thermoelectric materials.In this thesis, environmental friendly materials:thermoelectric oxides are chosen as research directions, and the perovskite oxides SrTiO3 is selected as an object. Combined with density functional theory calculations and molecular dynamics simulation, Boltzmann equation-based multi-band model and the phenomenological phonon transport theory are used to calculate thermal and electrical transport in SrTiO3. Nanocrystallization and doping are both analyzed to find the e possibility of enhancing the thermoelectric performance of SrTiO3. The mainly defects in SrTiO3 ceramics, including oxygen vacancies and grain boundaries are theoretically studied. Main contents of each chapter are as follows:In chapter I, the history and current status of thermoelectric field are introduced, as well as the current status of research about SrTiO3. Starting with the discover of thermoelectric phenomenon in 1823, the development of thermoelectric field for nearly 200 years is introduced. Then from point of view of application, the structure and working principle of thermoelectric device are introduced. Then the most important components:thermoelectric materials are discussed. From two aspects, the low-dimensional thermoelectric materials and new bulk thermoelectric materials, the current research status is described in detail, and two parts are highlighted:the oxide thermoelectric materials and theoretical studies of thermoelectric materials. Finally, the research status of SrTiO3 is introduced.In chapter II, a novel approach was developed to calculate temperature dependent Seebeck coefficient of heavily doped systems. The electronic density of states (DOS) and Fermi energy were determined and then, using these two parameters, the Seebeck coefficient was calculated by using Boltzmann transport theory. This approach is applied to heavily La doped SrTiO3. The calculated Seebeck coefficient agrees well with the experimental data. By analyzing the results, it was shown that Seebeck coefficient is greatly affected by the asymmetry of DOS with respect to Fermi energy.In chapter III, (1) firstly, density functional theory calculations and Boltzmann transport theory are employed to simulate the thermoelectric properties of SrTiO3 ceramics with two-dimensional electron gas (2DEG) grain boundaries (GBs). This material can achieve a large thermoelectric figure of merit (ZT>1 at room temperature) by utilizing quantum confinement and energy filtering at GBs. The latter causes ZT value to reach a maximum before decreasing with increasing GB barrier height. The optimum barrier height was approximately 0.06 eV higher than the Fermi energy of the grain interior. Our results may aid the design of materials with environmentally benign thermoelectric oxides. (2) Secondly, thermoelectric performance of nanocrystalline SrTiO3 ceramics with 2DEG GBs was theoretically investigated. The GBs of STO ceramics consist of 20% Nb-doped STO with thickness of 1,2,4 unit cells, respectively, and the GBs are separated by 10% La-doped STO grain interior (GI) with thickness of 16 unit cells. The calculated transport coefficients indicated that phonon confinement in GI and GBs greatly reduces the lattice thermal conductivity, and quantum confinement 2DEG at GBs greatly enhances the power factor. The energy filtering effect on electrons and boundary scattering of phonons at GBs further enhances the ZT value, which reaches 0.8 at 300K due to the co-existence of these effects. In addition, The ZT value reaches 1.2 at 300K if the minimum lattice thermal conductivity can be achieved by this nanocrystalline ceramics.In chapter IV, (1) firstly, Boltzmann transport theory and density functional theory calculations were employed to calculate transport coefficients of CaTiO3, SrTiO3 and BaTiO3. The transport coefficients of these materials were compared and analyzed by using'Tight Binding Model'. The band narrowing, caused by different lattice constants of these materials, was the mainly reason for their different transport properties. The calculated electronic conductivity and thermal conductivity in line with the Wiedemann-Franz law and the Lorenz factor was determined to be 1.45 for these wide band gap semiconductors. Finally ZT values were estimated and BaTiO3 had the largest ZT value among the three materials, mainly due to its largest lattice constant. (2) V doping can modify the shape of DOS, and effectively increase the Seebeck coefficient. (3) Thirdly, the possibility of forming cascading energy levels in SrTiO3 is analyzed by using the Density Functional Theory based first principles calculations of the electronic structure by doping Bi and Cu as example. The results show that Bi doping and Cu doping introduce defect level in the forbidden band, respectively, and Co-doping of Bi and Cu can introduced two defect levels in forbidden band. The electrons at the top of valence band can transit to the bottom of conduction band through a'cascade transition'process. With No Radical Transition Model, the analysis points out that the probability of electronic transition from the valence band to the conduction band through a cascade transition is much higher than that of direct transition from the valence band to the conduction band. The cascade transitions can effectively increase the carrier concentration in the conduction band.In chapterⅤ, the main defects in SrTiO3 ceramic, including oxygen vacancies and grain boundary, are theoretically studied. (1) It was found that in SrTiO3 with increasing oxygen vacancy concentration, the lattice constant first increases before reaching the maximum at 5% concentration, then decreases. This is due to the lattice distortion caused by oxygen vacancies. By analyzing the atomic force constant matrix, we can see oxygen vacancies mainly change the Ti-O optical branch. The calculations of the lattice heat capacity show that oxygen vacancies can effectively reduce the lattice heat capacity, indicating that oxygen vacancies not only scatter phonons, but also can reduce the heat capacity, so they can reduce the thermal conductivity in two ways. (2) For grain boundaries, theoretical studies focus on its electronic structure. the possibility of grain boundary supercells are constructed by using a self-compiled small program. By analyzing the atomic projection density of states, it is found that the grain boundary interactions can be neglected when their interval is greater than 11.3A; and the grain boundary consists of three atomic layers. By analysising of the partial density of states of grain boundary and grain, it can be seen SrO grain boundary configuration can form a potential well in the conduction band edge, while the TiO2 configuration can form a potential well at the top of valence band. These are due to of the Ti atom and O atoms at the grain boundaries have different bonding character with those in the grain.ChapterⅥsummarizes this thesis, and points out that the formation of nano-structure in bulk materials is an effective way to improve materials' thermoelectric performance, based on the main conclusions in this thesis. To further illustrate this point, the intercalated structure (SnS)1.2TiS2 is taken as an example: pure TiS2 single crystal is a layered N-type thermoelectric materials, PF value is high, but the thermal conductivity is also high. By intercalating SnS layers between TiS2 layers, the lattice thermal conductivity can be effectively reduced, which its electrical properties maintained. This point is demonstrated by calculating partial density of states of the intercalated TiS2 layered structure. This thesis proposes, for the first time, the concept of "forming two-dimensional electron gas at grain boundaries in SrTiO3 ceramics" and "forming cascade impurity levels in the band gap of SrTiO3 a by double-doping" in order to improve the materials'thermoelectric performance, and tests these concepts by doing calculation under Boltzmann equation and density functional theory. It is found that two-dimensional electron gas grain boundary can effectively increase the power factorof the nano-ceramics, and the formation of cascade impurity levels in the band gap can effectively increase the probablity of electrons transition from the valence band maximum to the conduction band minimum.