Effects of the Nanostructuring of Clathrates M8Ga16Ge30 (M= Ba, Sr) on the Thermoelectric Properties: Synthesis, Characterization and Modelling.

点击翻译按钮获取中文摘要

For the second funding period, we study the influence of the composition (Ga/Ge ratio) on the thermoelectric properties of Ba8Ga16-+xGe30±x with the aim to maximize the power factor (S2σ). We investigate the further reduction of κ by tailoring the size and shape of the nanoparticles. We target spherical nanoparticles smaller than 50 nm. Both aspects will maximize the ZT and will increase the potential applications of such material. The ratio surface/volume atoms is considerable higher in nanoparticles than in bulk materials. Surface atoms might also play an important role in the TE-properties since they can present dangling bonds, capping atoms, re-arrangement. For this reason we will addressed experimental and theoretical scientific activities for understanding their function. In this context, the surface analytical characterization will be performed in collaboration with the TU Ilmenau. In particular photoemission studies (XPS and UPS) will provide information concerning the near surface chemical composition and deviations from the bulk composition and the electronic density of states, which than can be compared with the theoretically predicted DOS and surface density of states including surface states. Moreover, the analytical methods deliver important information concerning the synthesis by analysis of samples obtained by different synthesis parameters and corrosion processes. In bulk-nanostructured materials, the atoms located at the grain boundaries will also represent an important number in relation to the atoms within the grains. Advanced transmission electron microscopy (TEM) techniques, including aberration-corrected high-resolution TEM, aberration-corrected scanning TEM (STEM) and electron energy-loss spectroscopy (EELS), will be used at Forschungszentrum Jülich to determine the atomic scale structures and chemical compositions of the surfaces and in particular the internal grain boundaries of individual and closely-spaced clathrate nanoparticles. Annular dark-field electron tomography will be used to determine the threedimensional morphologies, relative orientations and internal microstructures of the nanoparticles. Structural and chemical measurements obtained using TEM will be correlated with XPS and UPS measurements obtained at TU Ilmenau, as well as with synthesis parameters and thermoelectric properties measured from the same samples.