Microstructure Of Ge-based Clathrate Thermoelectric Materials Studied By Positron Annihilation

Thermo-electric conversion is a new and green environmental energy conversion technology. It can make the waste heat resuable and has become one of the hot research fields in the energy research and development. Thermoelectric materials have attracted more and more attentions due to their potential applications in the thermoelectric devices. How to improve the thermoelectric properties has become the most important topic. In recent years, clathrates with cage structures have attracted great attention because it exhibits both high electric conductivity and low thermal conductivity. Electron density analysis has shown that in A8B46 type-I clathrates, each guest atom (as donor) provides two electrons to the framework (as acceptor) and the total electrons from the guest atoms are accepted completely by the framework, and hence it should behave intrinsic characteristics of semiconductors. Substitution of the host atoms in the clathrates by doping will gain a more complex structure, which can sufficiently reduce the lattice thermal conductivity through phonon scattering to make clathrates with high performance. In this thesis, we adjusted the microstructure of the clathrates by doping or substitution with guest atoms. A combination of positron annihilation spectroscopy, X-ray diffraction and thermoelectric transport property measurements were used to study the defect properties and their effects on the thermoelectric properties from the microstructural point of view.1. Clathrate Ba6Ge25 is prepared by melt method and spark plasma sintering. XRD measurements confirm that the samples are single phase. Additional diffraction peaks were observed in the temperature rage of 200-250K, suggesting possible structural transition. Variation of the positron lifetime and Doppler broadening S parameter also reveal such transition. First principle calculation of the positron lifetime as a function of temperature indicates that the structure transition might be involved with the movement of Ba atoms in Ge cages, which enhanced the structural disorder and causes change of the local electron density and reconstruction of lattice structure. The specific heat capacity, electrical resistivity and magnetic susceptibility all show abnormal transition in the same temperature range, suggesting that the movement of Ba guest atoms in the cage plays important role on the thermal, electric and magnetic properties in clathrate Ba6Ge25.2. Clathrate Ba8Cu6Ge4o bulk materials have been prepared by combining melt method with spark plasma sintering (SPS). XRD patterns show that the sample obtained after melt and SPS is single phase. Temperature dependence of the positron annihilation lifetime and Doppler Broadening spectra were measured for the BasCu6Ge40 compound in the temperature of 20-300K. Both positron lifetime and Doppler broadening S parameter show slight increase with increasing temperature and there is no pronounced transition in whole temperature range. We can get a conclusion that there is no occurrence of structural transition in Ba8Cu6Ge40 and the change of lifetime and S parameter is most probably due to the lattice thermal expansion. The measured eleclectric resistivity, magnetic susceptibility and specific heat capacity all show no anomalous transiton in the whole temperature range, indicating that clathrate Ba8Cu6Ge40 does not undergo a phase transition below room temperature, which is different from Ba6Ge25.3. The n- tpye Yb/Ba double-atom-filled YbxBa8-xGai6Ge3o (x=0,0.1,0.3,0.5,0.7) clathrates with different Yb contents have been synthesized by combining melting reaction with spark plasma sintering method. The effects of both double-atom filling and Yb doping on the structure, thermal, and electrical properties were studied. XRD patterns of YbxBa8-xGa16Ge3o show single phase of the samples obtained after melt and SPS and the solubility limit of Yb in the Ba-Ga-Ge system is above 0.7 when it is expressed by the formula of YbxBa8-xGa16Ge30. With increasing Yb contents x, the electrical conductivity firstly increses and then decreases. The electrical conductivity decreases monotonously with increasing temperatue in the whole temperature range, indicating that they are intrinsic semiconductor. With increasing Yb contents x, the positron average lifetime first increses and then decreases with a maximum value at x = 0.5, suggesting that concentration of vacancy first increases with increasing Yb contents x (x<0.5) and then decreases (x>0.5). The double-atom filling of Yb/Ba plays a role on the lattice thermal conductivity of the YbxBa8-xGa16Ge30 samples and the lattice thermal conductivity first increases gradually with increasing x and then begins to decrease when x>0.5.