Abstract
Thermoelectric applications have attracted increasing interest recently due to its capability of converting waste heat into electricity without hazardous emissions. Compare with the bulk materials, many researches have been able to experimentally reveal that physical properties were different in low dimensional materials. In nanoscale, the enhancement of the thermoelectric efficiency by the reduction of lattice thermal conductivity (κph) due to phonon blocking. In addition, tuning the position of Femi level to achieve the optimum power factor (σα2) by doping concentration effect is another strategy to improve the zT value. In this thesis, we have synthesized the high quality and large-scale nanomaterials based on the mentioned concept and divided into two systems to investigate the thermoelectric properties. In the first part, the thermoelectric properties of a Bi0.8Sb1.2Te2.9 nanowire (NW) were in-situ studied as it was trimmed from 750 down to 490 and 285 nm in diameter by a focused ion beam. While electrical and thermal conductivities both indubitably decrease with the diameter reduction, the two physical properties clearly exhibit different diameter dependent behaviors. The much lower thermal conductivities were observed as compared with the theoretical prediction of Callaway model. The consequence indicates that in addition to the size effect, extra phonon scattering of defects created by Ga ion irradiation was attributed to the reduction of thermal conductivities. The size dependence of Seebeck coefficient and figure of merit (zT) show the maximum at 750 nm, then decrease linearly with size decrease. The study not only provides the thoroughly understanding of the size and defect effects on the thermoelectric properties but also proposes a possible method to manipulate the thermal conductivity of NWs via ion irradiation. In the second part, A two steps, surfactant-free solution growth process was utilized to synthesize p-type Ag doped SnSe nanocrystals in gram quantities. The formation mechanism of SnSe nanocrystals studied by the high resolution transmission electron microscopy. A clear phase transition near 800 K was discovered in the temperature dependence of thermal conductivity. The thermoelectric properties of SnSe pellets prepared by spark plasma sintering, exhibit a significant increase of zTmax (0.8 at 850 K) in the 3 % Ag doped SnSe. The zTmax value is about 40 % higher than that of the prinstine SnSe. The consequence is mainly attributed to the enhancement of carrier concentration and power factor by Ag doping. Our results demonstrate that this facile chemical method is amenable to fabricate high quality SnSe nanocrystals and might also be applied to other anisotropic crystalline materials.