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

In recent years, clathrate compounds as thermoelectric materials developing rapidly because of application prospect in business, with superior thermoelectric aplication. At present, Type-VIII Sn-based clathrates is one of the most representative compounds. The charge compensation effect analysis has shown that in II8III16IV30 type-VIII clathrates, each filling atom(as the donor) donates two electrons to the framework(as the acceptor), and the total electrons donated by the filling atom are accepted completely by the framework, and hence it should behave intrinsic semiconductor properties. Adjusting the III/IV atomic ratio to deviate from the 16/30, the conductivity type of clathrates can be changed: the excess III atoms in compounds can lead the p-type conduction and the defect can result n-type conduction.In this paper, type-VIII Ba8Ga16Sn30 single crystal was prepared by doped Ge and Mg atoms. The effects of these two kind of doping elements and different doping amounts on the structure、thermoelectric transport properties were studied, and at the same time we study the electric structure of Mg doped type-VIII Ba8Ga16Sn30 based the first principle. The main conclusions are as follows:Using the Sn self flux method, with Ge as the doping elements, prepare VIII-type Sn based compounds with n-type conduction material. The actual content is small in single crystals of Ge-doped. With the increase of the amount of doping, lattice constant of the samples decreased slightly. X=0.5 sample obtaining the maximum ZT value of 1.25 in the vicinity of 500 K.Mg doped Sn-based Single crystalline samples Ba8Ga16-XMgXSn30(0≤X≤1.5) were grown from Sn flux. The prepared compounds show n-type conduction. With the increase of Mg content, the melting point of the crystalline samples increases, the lattice constant decreases. The actual content of filling atomic Ba in doped samples less than the ideal value of 8.0. Corresponding occupancy of Ba in the dodecahedron, for the sample of X=1.5, is about 0.93. At last, the sample of X=1.5, near 430 K, obtain the maximum power factor value of 1.26×10-3 Wm-1K-2. The first principle calculation results indicated that the reduce of filling atomic Ba in content leads the whole band of Ba8Ga16Sn30 overall shift to higher energy direction. After doping Mg, the band in the high energy direction of the conduction band is more intensive. Density of states near the Femi energy levels is steeper. And the effect of Mg doping on the valence band of is significant, and the effect of s valence electron of filling atoms Ba on conduction band is more obvious.