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Lamya Abdellaoui1 Siyuan Zhang1 Stefan Zaefferer1 Ruben Bueno Villoro1 Cynthia Rodenkirchen3 Baptiste Gault1 Oana Cojocaru-Mirédin1 Yaron Amouyal2 Dierk Raabe1 Christina Scheu1

1, Max Planck Institut füe Eisenforschung, Düsseldorf, , Germany
3, RWTH Aachen University, Aachen, , Germany
2, Technion–Israel Institute of Technology, Haifa, , Israel

Research on thermoelectric (TE) materials has experienced a considerable increase in interest within the last years. The design of different microstructures and new classes of materials like complex chalcogenides is considered a promising approach for improving the efficiency of potential TE materials [1]. TE materials directly convert heat into electricity through the Seebeck effect. The conversion efficiency is determined by the dimensionless figure of merit, ZT, which depends on the thermal conductivity κ, electrical conductivity σ, the Seebeck coefficient S and the temperature T. Attempts to optimize ZT require reducing κ, while maintaining relatively high values of σ and S.

The AgSbTe2 compound is a promising p-type semiconductor. The δ phase, which is stable between about 600 and 550° C, and exists metastable after quenching to room temperature has a rocksalt cubic crystal structure where Ag and Sb atoms are randomly distributed on the Na+ sites and Te occupying the Cl- sites. It is suitable for thermoelectric power generation in the low-to mid-temperature range (e.g. 600-800 K).
The as-quenched (AQ) δ-phase Ag16.7Sb30Te53.3 synthesized in our study exhibits a good TE performance with a merit of figure value of ZT=0.5 at 300°C. This performance value is related to the various microstructural features, which were investigated down to the atomic scale. We used several methods such as electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) (both are SEM-based techniques), Cs-probe corrected scanning transmission electron microscopy (STEM), focused ion beam (FIB) sectioning, energy dispersive X-ray spectroscopy (EDS) and atom probe tomography (APT) . Notably the AQ material produced as a single crystal, shows a distinct mosaic structure with abundant low angle grain boundaries, where an array of dislocation and stacking faults networks are accumulated. We analyzed the number densities of the different types of structural defects in large scale and studied the atomic structure and chemistry of the defects. We observed a different chemical composition, specifically a change of the Ag and Sb content, within the stacking faults compared to the bulk. The presence of these stacking faults at the low angle grain boundaries seems to play a major role to achieve a low κ, [2].
The AQ bulk samples were subjected to a heat treatment for 8h and 192h at 380°C [3]. The 8h annealed sample showed similar good TE behavior as the AQ samples. Large precipitates (3µm in width) were found within the matrix which contain 1 to 8% Ag and possess a Sb2Te3 crystal structure. We will discuss the impact of this different microstructural features on the performance of our compounds [3].

[1] D. M. Rowe. Thermoelectric handbook: Macro to Nano (Taylor&Francis, New York, 2006, pp51-1)
[2] Roychowdhury, S,et.al. ACS Energy Letters 2 (2): 349-356. (2017)
[3] Cojocaru-Mirédin, O. Abdellaoui, L, et.al. ACS Appl Mater Interfaces 9 (17):14779-14790. (2017)

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