Correlation Between The Structure And Phsical Properties In Several Typical Oxide Thermoelectric Materials

Thermoelectric materials and thermoelectric effects, which are responsible for the direct conversion of heat into electrical energy and vice versa, have attracted much attention recently driven by their application as clean energy sources and device cooler. The many advantages of thermoelectric devices include solid-state operation, vast scalability, zero-emissions, no maintenance, and long operating lifetime.To date only a few intermetallic compound semiconductors exhibit good thermoelectric performance, and these thermoelectric alloys remain the state-of-the-art high ZT materials even today. However, these thermoelectric alloys are not stable in high temperature, easy oxidation, high-cost, and including heavy metal, so they are not ideal thermoelectric materials. Compared with the conventional thermoelectric alloys, metal oxides are more suitable for high temperature applications because of their structural and chemical stabilities, oxidation resistance, easy manufacture and low-cost. But the thermoelectric performance of earlier oxides is far away from application so that they did not receive enough attention. However, since the discovery large thermoelectric response in some layered cobaltites, perovskite cobaltites, electron-doped manganites, and electron-doped titanates recent years, thermoelectric oxides have received a renewed interesting.In this thesis, we detailedly investigate the structure, morphology, electric transport, magnetic transport, thermal transport, magnetic properties, and thermoelectric properties etc of three typical high-performance thermoelectric oxide systems: Ca3Co4O9 system, CaMnO3 system, and LaCoO3 system. From the study, we have an in-depth understanding of crystalline structure, thermoelectric response, electrical, magnetic, and thermal properties of these three systems, and successfully improve their thermoelectric performance by combining doping, composing, and developing preparation method.In Ca3Co4O9 system, the substitutions of Ag, Y for Ca and Fe, Mn, Cu for Co efficiently improve the thermoelectric properties. Ions doping alters the carrier concentration, induces chemical pressure, influences mobility and electronic correlation, and suppress thermal conductivity of the system. The dopings also have important effects on the transport mechanism, specific heat, and magnetic properties. We also investigate the transport and thermoelectric properties of Ag-added Ca3Co4O9, and find the addition of Ag can efficiently reduce the boundary scattering and thus enhance thermoelectric performance. Moreover, the study indicates that cold high-pressure can obviously improve the texture of such layered materials, increase density, decrease porosity and grain boundary, and thus further facilitate the thermoelectric response. Combining ion doping and cold high-pressure, the largest ZT of Ca3Co4O9 system reaches 0.5 at 1000K, which is a quite high value among ceramic oxides.In electron-doped CaMnO3 system, we study the dominant factors determining the thermoelectric response of this system. After optimizing doping, we get the highest ZT is ~0.2 at 1000K. Although this ZT is large among n-type oxides, it is still far away from application criterion. We demonstrate that a ZT value larger than one in electron-doped CaMnO3 systems seems rather unlikely. Then we analyze the characteristic of high-performance thermoelectric oxides; some strategies for searching new thermoelectric materials with high performance in transition metal oxides are proposed. Subsequently, we systematically investigate the electric, magnetic, thermal, and transport properties etc, and detailedly discuss many rich physical phenomena in this system, such as the correlation between crystalline structure and physical properties, electric and magnetic transport mechanism, metal-insulator transition, magnetic phase separation, percolative transport behavior, magnetoresistance effect, charger ordering phenomenon, thermal transport properties, anomalous thermal conductivity behavior, point-defect scattering mechanism, relationship between spin/orbital degeneracy and electronic configuration and thermopower, unusual magneto-thermopower effect, critical phenomenon, phase transition and fluctuation.In LaCoO3 system, we find hole-doped La1-xCaxCoO3 and La1-xSrxCoO3 have different global distortion and local distortion, which results in their different electric transport, magnetic transport, thermal transport, and magnetic properties. Hole-doping efficiently improve the thermoelectric properties of the system. Then we study the electric and magnetic transport behaviors and magnetic evolution. The glassy ferromagnetism and reentrant spin glass phenomenon are found. Using Arrott plot, we show the magnetic phase diagram of the system. We also investigate the magnetoresistance effect, and find the scaling behavior of magnetoresistance in this system. Similar with manganites, we analyze the correlation between crystalline structure and physical properties. Furthermore, we investigate electron-doped La1-xCexCoO3, and successfully obtain a new promising n-type thermoelectric oxide. The unusual large room-temperature thermoelectric response results from the spin state transition of Co3+. We discuss the relationship between spin blockade effect and thermoelectric properties in this system, and find the asymmetry of electron-doping and hole-doping.On the basis of these studies, we widely and deeply understand the structure, properties, and thermoelectrics of Ca3Co4O9, CaMnO3, and LaCoO3 systems. These results are highly significant for the further study of intrinsic physical mechanism of thermoelectric oxides and the search of new high-performance thermoelectric oxides.