Researching and developing clean and efficient TE materials and modules

TE devices are based on a physical phenomenon observed in the early 19th century by scientist Thomas Seebeck, who found that when a temperature difference across the junctions of two dissimilar conductors is applied, electrical current would flow (Fig.1). These devices are actually based on (n- and p-type) semiconductors, because of their better thermoelectric properties when compared to metals.

At the atomic scale, the applied temperature gradient causes charge carriers in the two TE materials to diffuse from the hot side to the cold side, due to the larger thermal speed of the carriers in the hot region, similarly to a classical gas that expands when heated. The net effect of the two materials’ different response to this temperature gradient creates a voltage and produces an electric current in the closed circuit. The ratio of the voltage developed to the temperature gradient is related to an intrinsic property of the materials, called the Seebeck coefficient.

A TE device is built up of an array of these couples, which are arranged electrically in series and thermally in parallel (Figs.2 and 3).

Fig.1. Schematic of basic TE circuit (Seebeck effect).

Fig.2. A TE module combines pairs of n- and p-type semiconductors.

Fig.3. TE commercial module

TEGs have long been relegated to use in space-based or other niche applications, but are now being actively considered for a variety of practical waste heat recovery systems—such as the conversion of car exhaust heat into electricity.

ThermoMag aims at developing novel nanostructured Mg2Si-based TE materials and to integrate them into clean and efficient proof-of-concept TE modules for industrial applications.

Rationale for choosing 3D nanocrystalline Mg2Si-based TE materials

Magnesium and silicon are extremely abundant elements
Fig.4. Magnesium and Silicon are extremely abundant elements in comparison with other TE materials.
Magnesium and Silicon are both extremely abundant and relatively easy-to-extract elements and hence very inexpensive compared to other conventional, rarer elemental constituents used in thermoelectrics (e.g. Bi, Te, Ge, Sb, Se etc), see Fig.4.

The dimensionless "figure of merit" ZT is used to characterize the performance of TE materials where Z is a measure of the material's TE properties and T is the average temperature. Recent results [M. Fedorov et al., Phys. Rev. B , 2006] have shown that the ZT value for n-type microstructured Mg2Si-based TE materials can exceed 1. P-type Mg2Si in bulk form has also shown ZT values on the order of 1 [INPL, 2009]. The ThermoMag partnership seeks to significantly raise the ZT of Mg2Si-based TE materials by refining the material nanostructure and via doping of a wide range of elements.

Potential ZT curve for nanostructured Mg2Si (peak ZT>1.5 for n-type), in comparison with other bulk TE materials

Fig.5. Potential ZT curve for nanostructured Mg2Si (peak ZT>1.5 for n-type), in comparison with other bulk TE materials.

Fig.5 shows the ThermoMag target ZT curve for nanostructured Mg2Si, in comparison with other bulk TE materials. The ZT values for Mg2Si peaks near a temperature of 500°C to a value of 1.5, and the peak temperature of ≈500°C is highly suitable for many areas of industrial waste heat recovery.

New 0-D and 1-D materials with high ZT values (e.g. 2-3) often exhibit irreproducible properties, leading to considerable uncertainties. In ThermoMag, quality control procedures between partners will ensure that nanostructured Mg2Si based TE materials can be reproduced consistently.

Additionaly, the non-toxicity of its processing by-products is another feature of Mg2Si which makes this TE material very attractive for industrial scale-up.

Potential specific ZT for nano-Mg2Si compared to other TE compounds
Fig.6. Potential specific ZT for nano-Mg2Si compared to other TE compounds.

Finally, one of the most appealing properties of Mg2Si is its very low density (i.e. <2g/cm3) which for transport applications will be decisive, especially when energy-harvesting TEGs are being carried along with the vehicle for more than 20 years. The maximum value of ZT divided by density of conventional TE materials and ThermoMag target value for Mg2Si are compared in Fig.6.

If successfully developed, nano-Mg2Si would be in a class of its own, relative to other more dense TE compounds, based on Pb, Bi, Te, Co, Sb and Ag.