THERMOELECTRIC MATERIALS AND DEVICES
Thermoelectric materials can be used in all-solid-state devices to produce electricity from hot sources. Figure 1 schematically
represents how electricity can be generated for a heat source heated by a solar concentrator. With an appropriate thermal storage
scheme, this could provide a 24-hour source of power. Efficient thermoelectric (TE) materials are usually semiconductors that possess
simultaneously high electronic conductivity (σ), high thermoelectric power, and low thermal conductivity (κ). These properties define the
thermoelectric figure of merit ZT = (S^2 σ/κ)T; where T is the temperature. The S^2 σ product is often called the power factor. The
quantities S^2 σ and κ are transport quantities and therefore are determined by the details of the crystal and electronic structure and
scattering of charge carriers. Generally they cannot be controlled independently, however, the combination of new theories and
experimental results suggests that they may be able to be decoupled to a significant degree. This raises potential new research
opportunities for huge improvements in the figure of merit. State-of-the-art thermoelectric materials have ZT ~ 1. Recent developments
on superlattices and nanostructured materials have led to the demonstration of ZT values of up to 2.4 (Figure 2). These nanostructured
materials possess significantly lower thermal conductivity than their bulk counterparts, while having a power factor comparable to that of
their bulk counterparts. With further research and development on thermoelectric materials and understanding of electron and phonon
transport mechanisms (to achieve ZT>3), thermoelectric converter efficiency up to 35% could be achieved.Figure 1 Illustration of a solar-thermoelectric Figure 2 Progress timeline in thermoelectric materials.
power generator.