COMPLEX THERMOELECTRIC MATERIALS

COMPLEX THERMOELECTRIC MATERIALS

The widespread use of thermoelectric generators has been limited by the low material efficiency of the thermoelectric material. A number of strategies for Complex Thermoelectric Materials [1] with higher thermoelectric figure of merit, zT, are being actively studied. Complex electronic band structures provide mechanisms to achieve high zT in thermoelectric materials through band structure engineering.  High zT is obtained p-type PbTe and PbSe which contains both light and heavy valence bands that can be engineered by alloying to achieve high valley degeneracy which leads to an extraordinary peak zT of about 2 at 750K [2].

Figure: A thermoelectric generator directly converts heat into electricity with no moving parts. The long term  reliability of these systems has encouraged NASA to use thermoelectric generators in many space probes since the 1960s (up to 35 years unattended).  Today, thermoelectrics are being considered for terrestrial applications such as automotive and industrial waste heat recovery as well as solar-electricity generation.

Complex crystal structures that enable relatively low thermal conductivity have lead to several new classes of thermoelectric materials. Ca3AlSb3, Ca5Al2Sb6 and Yb14AlSb11 are complex Zintl compounds containing differently connected AlSb4 tetrahedra that obtain zT near 1 at high temperatures. Fast diffusing or ‘liquid-like’ elements in the complex materials Zn4Sb3 [3] and Cu2Se [4] provide additional mechanisms to scatter and otherwise inhibit phonon heat conductivity. The principles of Zintl chemistry facilitates the search for new complex materials and the tuning of known thermoelectric materials with earth abundant, non-toxic elements [5]

Finally, the incorporation of nanometer sized microstructure reduces thermal conductivity from long mean-free-path phonons. This principle has been successfully demonstrated in (Bi,Sb)2Te3 alloys with arrays of dislocations at grain boundaries [6]. The synthesis of nanoscale composites can be controlled with the aid of equilibrium phase diagrams (experimental or theoretically determined) to produce microstructure of varying composition and length scale [7].

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Prof. G. Jeffrey Snyder

2220 Campus Drive
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Evanston, IL 60208-3109
USA

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