Thermoelectric materials

Thermoelectric materials can be used to generate electricity from a heat source through the Seebeck effect, whereby a temperature difference leads to a difference in voltage for power generation. The opposite effect, known as the Peltier effect, is exploited for heating and cooling for instance. The efficiency of the conversion can be increased by introducing defects that efficiently scatter phonons, i.e. the carriers of lattice vibrations and hence heat, but do not affect much the movement of electrons so as to maintain good electrical conductivity.

In this project, we will reveal how the microstructure at the near-atomic-scale correlates with the thermoelectric performance to help guide the design of new thermoelectric materials. This will be achieved by revealing and fine-tuning the property-enhancing lattice imperfections, such as grain or phase boundaries, dislocations and secondary phases. The composition and structure of the defects in thermoelectric materials will be well characterized by a combination of electron channeling contrast imaging (ECCI) in a scanning electron microscope [1], atom probe tomography (APT) and high-resolution imaging by scanning transmission electron microscopy (STEM) [2].

As the synthesis and post-treatment of the materials play a crucial role in the development of the microstructure, we compare different synthesis routes, such as melt spinning, arc melting, laser surface remelting for the example of the full-Heusler compound Fe2VAl. First results (Figure 1) show precipitation of vanadium carbides and vanadium nitrides along antiphase boundaries, which are stabilized during the fast quenching process in melt spinning [3].

Figure 1. (a) SEM image showing meltspun Fe2VAl flakes, (b) APT reconstruction indicating VCxNy precipitates at an antiphase boundary. (c) 1D-concentration profile in a 20nm-diameter cylinder perpendicular to the antiphase boundary. [3]

For Pt-doped NbCoSn half-Heusler compound, segregation of Pt was observed at grain boundaries, as shown in Figure 2. By correlating these microstructural observations with their thermoelectric properties, we can conclude that it is inadequate to describe grain boundaries solely as scattering centers of phonons and electrons. The local decoration states at grain boundaries are also crucial for the transport properties and thus for the thermoelectric performance [4].

Figure 2. APT analysis of Pt-doped NbCoSn: (a) distribution map of Pt atoms, indicating the presence of two grains (G1 and G2), (b) 1D-concentration profile in a 20nm-diameter cylinder perpendicular to the grain boundary of G1/G2.
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