| In the past few decades,thermoelectric materials have attracted great attention due to the shortage of global energy.High-efficiency thermoelectric materials are important for power-generation devices that are designed to convert waste heat into electrical energy.The conversion of waste heat into electrical energy plays an important role in our current challenge to develop alternative energy technologies to reduce our reliance on fossil fuels.In general,the energy conversion efficiency of thermoelectric materials are mainly related to its three parameters of electrical conductivity,thermal conductivity and Seebeck coefficient,which are mutually constrained to limit the further improvement of conversion efficiency.Improving the energy conversion efficiency requires finding ways to weaken or eliminate the coupling among the three parameters in the material.Recently,the discovery of topological material has demonstrated the possibility of decoupling electron and phonon,providing a completely new platform for the study of high-efficiency thermoelectric materials.In this thesis,we conducted a detailed study on the single crystal growth method and transport properties of the LiMgBi,and some preliminary results have been obtained by exploring its congener AMgSb(A = Na,K).Among them,theoretically LiMgBi,which is Half-Heusler structured and crystallizes in the cubic F(?) 3m(No.216)space group,is predicted to be a topological insulator and possess excellent thermoelectric property.The thesis contains the following four main chapters:Chapter 1 gives a brief introduction to the topological insulators,localization effect and three thermoelectric effects.The localization and thermoelectric effect are also explained quantitatively in terms of physical mechanisms.At last,we provide an overview of current thermoelectric advances in several typical topological insulator.Chapter 2 mainly introduces the crystal growth and structure characterization methods,such as solid-phase reaction,flux method,X-ray diffractometer,scanning electron microscope,and transport property measurement methods.Chapter 3 introduces the growth methods and physical properties of AMgX(A =Li,Na,K;X = Sb,Bi)series compounds.We have successfully grown the single crystal samples of LiMgBi by the flux method,and successfully synthesized polycrystalline samples of KMgSb and Na MgSb single crystal by the solid-phase reaction method.The X-ray powder diffraction pattern of those samples are consistent with the previous results.The Hall effects measurement of LiMgBi single crystal indicates that it is a system with multi-band feature,and hole-type carriers dominate the transport process.The electrical resistivity of LiMgBi shows a metal-to-semiconductor-like transition at high temperature(>160 K)and its resistance tends to saturate at low temperature(<50K).Moreover,it is a worth noting thing that a weak antilocalization effect could be observed obviously at the measurement of low-field magnetoresistance of LiMgBi.The magnetoresistance experimental data fits the theoretical formula very well,which implies that there is strong spin-orbit coupling in the LiMgBi,and the spin-orbit interaction is one of the important causes that the material has a non-trivial band structure in topological.The thermoelectric measurement reveals that LiMgBi has a larger Seebeck coefficient and lower thermal conductivity at room temperature,and the positive Seebeck coefficient further confirms that hole-type carriers are dominated in the transport possess.In addition,the electrical transport measurement of Na MgSb single crystal and KMgSb polycrystals exhibit a semiconducting behavior,and the Arrhenius model fit gives the band gap are 0.79 and 0.14 e V for Na MgSb and KMgSb,respectively.In Chapter 4 we summarize the main research of this paper and provide an outlook on the future studies. |
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