Thermoelectrics - G. Auffermann

Thermoelectrics - G. Auffermann

Thermoelectric (TE) materials yield the direct transformation of waste heat into useful electricity. TE devices are composed on pairs of n-type and p-type materials.

Thermoelectric (TE) materials yield the direct transformation of waste heat into useful electricity. TE devices are composed on pairs of n-type and p-type materials. The efficiency of a TE material is characterized by its dimensionless figure of merit zT:

zT =S2T
ρκ

where S is the Seebeck coefficient (also known as the thermopower), r the electrical resistivity, k the thermal conductivity, and T the absolute working temperature.

Intermetallic half-Heusler compounds with a general formula XYZ (X, Y = transition metals; Z = main group such Sn or Sb) crystallizing in the cubic MgAgAs-type structure (Figure 1) have attracted considerable attention as suitable TE materials because of their very flexible electronic structure.

Figure 1: Half-Heusler structure with general composition XYZ (C1b or MgAgAs structure type) crystallizing in the cubic space group F43m (216)).

Narrow band gap semiconducting C1b Heusler compounds such as TiNiSn and CoTiSb are deal materials for new thermoelectric modules. These compounds show high figure of merit (ZT>1), are available as p- and n-type (Figure 2) semiconductors, cheap and nontoxic. 

Narrow band gap semiconducting C1b half-Heusler compounds with 18 valence electrons in the primitive cell, such as TiNiSn and TiCoSb are deal materials for new thermoelectric modules. These narrow-band-gap compounds were reported to exhibit excellent thermoelectric properties due to wide possibilities of modifying their electronic properties by partial substitution or doping.

Substitution on the main group site Z provides charge carriers, where the substitution on the X or Y site causes mass fluctuation disorder that lead to a reduction in thermal conductivity.

Particularly, isoelectronic partial substitutions of Ti with its heavier homologues Zr or Hf achieve an intrinsic phase separation of the half-Heusler compounds.

There are no currently available thermoelectric materials that fulfil all of these requirements. However, C1b Heusler materials do meet nearly all of the requirements, including a high power factor.

Figure 2: Thermoelectric properties of the p-type half-Heusler compound Ti0.3Zr0.35Hf0.35CoSb1−xSnx as a function of the carrier concentration n at 610°C [1].

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