The devil is in the detail(s): How to get the synthesis of high performance MgAgSb right?

Kurzfassung

88 The devil is in the detail(s): How to get the synthesis of high performance MgAgSb right? Johannes de Boor,1,3,* Amandine Duparchy,1 Theo Renard,1,4 Frederic Kreps,1 Eckhard Müller1,2 1 German Aerospace Center (DLR), Institute of Materials Research, D–51147 Cologne, Germany 2 Justus Liebig University Giessen, Institute of Inorganic and Analytical Chemistry and ZfM/LaMa, D–35392 Giessen, Germany 3 University of Duisburg-Essen, Faculty of Engineering, Institute of Technology for Nanostructures (NST) and CENIDE, D–47057 Duisburg, Germany 4 European Astronaut Center, European Space Agency (ESA), D–51147 Cologne, Germany *johannes.deboor@dlr.de Keywords: MgAgSb, thermoelectric material, composition-processing-microstructure-property relationship, phase purity, sensitivity analysis, nanostructure, transport analysis A thorough understanding and optimization of microstructure are key to enhancing transport properties in thermoelectric (TE) materials. By combining high-resolution microstructural analysis with local and integral transport property measurements of a set of ~50 samples with varying composition and synthesis conditions, we establish an interrelation between the phase constitution and the thermoelectric properties of MgAgSb, a promising p-type TE material near and above room temperature. The samples were synthesized by a combination of ball milling and high- temperature pressure-assisted annealing steps, with MgAg as a precursor for the MgAgSb synthesis. Scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy combined with Selected Area Electron Diffraction was employed for unambigious identification of secondary phases and revealed that those form not only according to the global off- stoichiometry with respect to the MgAgSb single phase but also due to local fluctuations in compositional balance. We deduce several reaction chains that are competing with the intended MgAgSb formation. These reactions may be initiated by remnants of the constituent elements of the initial synthesis steps or local compositional variations in the precursor MgAg, which has a wide homogeneity range. Further analysis using spatially resolved Seebeck microprobe measurements combined with correlating secondary phase type and amount with effective weighted mobility of composite samples containing secondary phases showed that a decrease in weighted mobility can be understood as an overlay of a classical mixing effect and a grain boundary phase effect. Equipped with an understanding on how the secondary phases in MgAgSb are formed and a quantitative assessment of their respective impact on the thermoelectric performance of MgAgSb, we show that fine-tuning the composition of MgAg and its homogenization before the final reaction step with Sb are crucial to reduce the secondary phase content in MgAgSb and with this reproducibly achieve its performance at zTmax ≥ 1.3.