Thermoelectric materials

Thermoelectric materials

Thermoelectrics have attracted increasing attention as a sustainable and flexible source of electricity able to meet a wide range of power requirements. Their application is wide as they could be used as main source of electricity as in satellites or used to increase efficiency of thermal processes in industries or cars or any other application where temperature gradients exist to produce electricity. The conversion efficiency of thermoelectrics materials is determined by the dimensionless figure of merit, zT, which depends on the thermal and electrical conductivity, the Seebeck coefficient and the temperature. These transport properties are closely related to the micro-/nanostructure of the investigated thermoelectrics materials which can be modified by the processing methods and chemical compositions.

This project is focused on understanding the correlation between the microstructural features and properties of several thermoelectrics materials including half-Heuslers, AgSbTe, and PbTe [1-7]. Since microstructural features such as point defects, dislocations, planar defects, grain boundaries, etc. interact with charge carriers and phonons, their effects on the electrical and thermal conductivities are crucial factors to design thermoelectric materials. In order to understand the impact of these defects, their number densities, structural and chemical characteristics are studied from the micro scale down to the atomic scale. This scale-bridging methodology was applied for the first time to characterize stacking faults and dislocations in thermoelectric materials [7].

Aberration-corrected scanning transmission electron microscopy (STEM) combined with energy-dispersive x-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) helps to reveal the crystal structure, chemical composition, and bonding states on the nanoscale with the near-atomic resolution. In addition, accurate 3D elemental distributions are obtained by using atom probe tomography (APT). Scanning electron microscopy (SEM) correlated with electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) enable the investigation of dislocations and stacking faults in the microscale. Also, local electrical conductivity measurements using the Van-der-Pauw technique is performed in a SEM to correlate electrical conductivity and microstructural features.

An example of a microstructure-property relationship is shown in figure 1 where a high density of dislocations is found by (a) ECCI and (b) bright field TEM. The chemistry of the dislocations is revealed by (c) APT and (d) the effect of the high density of the dislocations, their orientation and their chemistry is calculated using the Debye-Callaway model and compared with experimental values.

Understanding the microstructure evolution at high temperature is crucial as thermoelectric generators operate at high temperature in practical applications. Thus, we perform in situ heating experiments on thin TEM lamellae of thermoelectrics to track their microstructural evolution. The lamellae were prepared by focused ion beam (FIB) and welded onto micro-electromechanical system heating chips.

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