Preparation Of Sn - Based Single Crystal Fragment And Study On Thermoelectric Transmission Properties

With the reduction of fossil fuel reserves, energy crisis has become a crucial issue for the sustainable development of society. On the basis of this situation, the proportion of new energy development and utilization in the field of scientific research has enhanced increasingly. As an important novel material which can convert heat to electric energy, thermoelectric(TE) material has drawn significant interest due to the excellent advantanges of non-toxity, regeneration, rich-reserves. In this regard, the type-VIII single crystal clathrates is one of the representative compounds with outstanding performance. Its chemical formula can be summarized as II8III16IV30, among which, group II elements are filled-atoms(II atoms), group III and IV elements are framework-atoms.Each filled-atom provides two electrons for the framework can reach the balance of charge distribution.According to the varied framework atoms, the type-VIII single crystal clathrates that have been intensively studied contain Sn- Ge- and Si- clathrates.Among them,Sn- clathrate has outstanding electrical transmission characteristics than others, which is derived from its large size of framework and lower lattice thermal conductivity. In this paper, we calculated the band structure of Sn- Ge- and Si- clathrates via first principle calculation. The effect of varied framework atoms on the performance of clathrate was also discussed in detail.. In addition, the Ba(Ba/Eu) filled type-VIII single crystal clathrates were prepared by Sn/a-Sn self-flux method. The phase structure, microscopic morphology and optical properties of samples were investigated by a variety of characterized technique. The main conclusions are as follows:The above mentioned Sn- Ge- Si- clathrates are all indirect band gap semiconductors, however, the band structure and the electronic distribution of the clathrates vary according to the framework atoms. The Sn- clathrate with the narrower band gap and higher local density of states indicate it has higher Seebeck coefficient. Moreover, the asymmetric band structure of Sn- clathrate illustrate theenhanced thermoelectric property, which is derived from the different charge distribution in s、p、d of Sn, Ge, Si.a-Sn is the allotrope of white Sn, which is converted by white Sn in the temperature lower than 13℃. The results indicate the as-prepared samples obtained by white Sn self-flux method are p-type single crystal clathrates. Whereas, using a-Sn self-flux method combined with the construction of Ga amount can attain both n/p-type Ba8Ga16Sn30. The samples obtained by this method has lower electronic conductivity resulting in lower ZT value comparied with white Sn self-flux. The Eu atoms tend to replace the Ba atoms, when the samples are the n-type, the Seebeck coefficient of the patterns would decrease with Eu atoms increasing which result in the decreased ZT value. However, when the samples are p-type, its Seebeck coefficient and electric conductivity would increase with Eu atoms increasing leading to increased ZT value.