| Summary: | 博士 === 國立中央大學 === 物理學系 === 105 === With increasing concerns regarding environmental protection and the growing demand to use clean energy have stimulated research to develop an alternative and sustainable energy sources. Thermoelectric effect enables direct and reversible conversion of thermal energy into electrical energy, and provides a viable route for power generation from waste heat. Thermoelectric (TE) materials are becoming increasingly important in the field of electricity generation and realize refrigeration without the emission of toxic matters, offer a great opportunity for solving today’s energy and environmental issues. Among all state-of-the-art high temperature thermoelectric materials, SnSe and CaZn2Sb2 have been firmly established as a potential TE material, which exhibit low thermal conductivity and high figure of merit values. SnSe widely identified as a simple layered orthorhombic crystal structure at room temperature, which is similar with a three dimensional distortion of NaCl structure. It has been reported that a single crystal SnSe shows excellent thermoelectric performance, with zT value around 2.6 at 923 K. The single crystal fabrication method and the poor mechanical property, which prevent the practical applications of thermoelectric devices. Therefore, it is quit essential to find its polycrystalline counterparts to fabricate highly dense thermoelectric materials and suitable for an industrial scale up. Doping is an effective way to optimize the thermoelectric properties of p-type SnSe and CaZn2Sb2 by reducing its thermal conductivity and adjusting its carrier concentration.
In this dissertation, it is proposed that highly dense polycrystalline (Sn1-xGex)Se and (Ca1-xEux)Zn2Sb2 bulks could be prepared by melt-quench and spark plasma sintering method to engineer the thermoelectric performance and mechanical properties of specimens. We
successfully fabricated highly dense pure polycrystalline (Sn1-xGex)Se and (Ca1-xEux)Zn2Sb2 bulk samples using the proposed melt-quench and spark plasma sintering approach.
In this study, Sn was substituted with Ge in the p-type polycrystalline (Sn1-xGex)Se system to engineer the transport property. We found that Ge doping increases Seebeck coefficient and simultaneously reduces lattice thermal conductivity by enhancing phonon scattering via lattice defect. All germanium substituted samples show low thermal conductivity, which is mainly attributed to phonon scattering from disordered dopant atoms and the high anharmonic boding nature of SnSe. As a result the highest TE dimensionless figure of merit zT = 0.77 was obtained for (Sn0.99Ge0.01)Se at 800 K, which shows 40% enhancement over the pristine polycrystalline SnSe (zT=0.56). Similarly, Zintl phases are a class of materials that can be used in TE devices because they often possess complex crystal structures necessary for the desired thermoelectric properties (Seebeck, electrical resistivity, thermal conductivity). Ca was substituted with Eu in the Zintl phase p-type polycrystalline (Ca1-xEux)Zn2Sb2 compound to optimize the transport property. After europium substitution it shows that more than 50% decrement in electrical resistivity because of high charge carrier concentration and low thermal conductivity due to high phonon scattering through structural disorder yielded by the incorporation of multiple ions in the cation. Consequently, the cationic substitution of rare earth element Eu in the Ca site significantly increased thermoelectric dimensionless figure of merit. These results indicate that doping with Ge and Eu are an effective approach to enhance thermoelectric performance of SnSe and CaZn2Sb2 compounds respectively.
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