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

在第二个资助期，我们研究了组分（Ga/Ge比）对Ba 8 Ga 16-+ xGe 30 ±x热电性能的影响，目的是最大化功率因数（S2σ）。我们研究了通过定制纳米颗粒的尺寸和形状来进一步降低κ。我们的目标是小于50 nm的球形纳米颗粒。这两个方面都将使ZT最大化，并将增加这种材料的潜在应用。纳米颗粒中的表面/体积原子比比在块状材料中高得多。表面原子也可能在TE-性质中起重要作用，因为它们可以呈现悬挂键、封端原子、重新排列。因此，我们将讨论实验和理论科学活动，以了解它们的功能。在这种情况下，将与TU Ilmenau合作进行表面分析表征。特别是光电子能谱研究（XPS和UPS）将提供有关近表面的化学成分和偏差的本体组成和电子态密度，这比可以与理论预测的DOS和表面态密度，包括表面状态的信息。此外，分析方法通过分析由不同合成参数和腐蚀过程获得的样品提供关于合成的重要信息。在体纳米结构材料中，位于晶界处的原子也将代表与晶粒内的原子相关的重要数量。先进的透射电子显微镜（TEM）技术，包括像差校正的高分辨率TEM，像差校正的扫描TEM（STEM）和电子能量损失光谱（EELS），将在Forschungszentrum Jülich用于确定表面的原子尺度结构和化学成分，特别是单个和紧密间隔的笼形纳米颗粒的内部晶界。环形暗场电子断层扫描将用于确定纳米颗粒的三维形态，相对取向和内部微观结构。使用TEM获得的结构和化学测量结果将与在TU Ilmenau获得的XPS和UPS测量结果以及从相同样品测量的合成参数和热电性能相关。

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.