Combining previously unpaired materials enables better energy harvesting

The transportation sector accounts for a significant fraction of domestic petroleum usage, with consumption over the next 10 years estimated to range from 14 to 15 million barrels per day.¹ The US Department of Energy estimates a continued increase in petroleum consumption in future decades despite significant efforts to improve fuel economy and develop alternative fuels. In the typical internal combustion engine, only about 25% of the energy produced by burning fuel is used to move the vehicle, with most of the remaining energy (∼2/3) leaving the engine in the form of heat.² Approximately 40% of this waste heat leaves through the engine exhaust, and the rest is ejected to the atmosphere through the engine cooling system. Conversion of this waste heat—which has a high temperature (i.e., high quality)—to electricity using thermoelectric (TE) technology would enable an improvement in automobile fuel economy of up to 5%. However, achieving this goal requires significant advancements in TE technology.

Specifically, those improvements involve using high-efficiency TE materials with a high-TE figure of merit (ZT) that are able to operate reliably at high temperatures. In addition, the TE modules and heat exchangers must be inexpensive enough to meet extremely challenging cost targets. In our work, we have combined two high-performance TE materials that have not previously been paired to form a high-ZT hybrid TE device. We accomplished this with the novel approach of using an enhanced alloy called ‘e-TAGS’^3–5—tellurium, silver, germanium, and antimony (Te₅₀Ag_6.52Ge_36.96Sb_6.52, or TAGS-85) containing 1% of the rare earth element ytterbium (Yb)—as the p-leg material, and combining it with improved half-Heusler (HH) materials containing titanium, hafnium, zirconium, nickel, tin, and antimony—(Ti, Hf, Zr)Ni(Sn,Sb)—as the n-leg material⁶ to raise the thermal-to-electrical conversion efficiency of waste exhaust at 750°C. In semiconductor materials such as these, ‘n-type’ indicates that electrons carry the electrical current, while holes perform that role in the case of ‘p-type’ materials. This hybrid pair takes advantage of recently developed high-ZT modified TAGS alloys and n-type HH alloys to provide a high-ZT, lead-free TE material solution for exhaust gas heat recovery for use in commercial or military vehicle platforms. We developed the n-type HH alloys using a special high-energy milling approach to produce a reduced thermal-conductivity, nanostructured bulk alloy. Our approach included two novel elements to reduce thermal conductivity: high-energy milling and the addition of coherent inclusions.

Nanostructuring of these TE alloys reduces lattice thermal conductivity by enhancing grain-boundary scattering of long-wavelength phonons. For our application, this can be accomplished by processing HH n-type alloys using a special high-energy milling technique, which we have shown in preliminary studies to be effective at producing materials that are ultrafine grained. Typical microstructures produced by high-energy milling contain many grains that are on the order of 100nm. The presence of coherent inclusions within the grains further reduces the lattice thermal conductivity by the scattering acoustic phonons that carry heat. However, in order to decouple the increase in phonon scattering effects from also increasing the carrier scattering effects (which increase parasitic device resistance), these second-phase inclusions must possess a coherent interface with the TE matrix and occupy a volume smaller than the electron wavelength. To address this issue while reducing the thermal conductivity of n-type HH alloys beyond that obtained through grain-size reduction alone, we prepared compositions that naturally phase-segregate during hot pressing to produce coherent, Ni-rich Heusler inclusions within the grains. The combined high-energy milling and nanostructuring approach reduces the thermal conductivity and increased ZT by 30–40%.

Single n-/p-type couples were produced using eTAGS and HH pellets that achieved 9.2% efficiency with a power output of 205mW for a hot-side temperature (T_hot) of 559°C (see Figure 1). This compares to an all-HH (p- and n-type) single couple that achieved a lower 8.4% efficiency at T_hot=750°C (and 7.5% at T_hot=600°C). This proof-of-concept demonstration clearly shows a significant efficiency improvement at a lower hot-side temperature with the hybrid e-TAGS/HH single couple over the baseline HH couple. Because the thermal conductivities of e-TAGS and n-HH are significantly different, an optimized couple will employ elements having different cross-sectional areas to equalize the heat flow. Such an asymmetric couple made from a 1.4×1.4×2mm-tall p-type e-TAGS element and a 1×1×2mm-tall n-HH element achieved 9.7% efficiency with a maximum power output of 284mW at T_hot=565°C.

Figure 1. Comparison of efficiency between a symmetric hybrid e-TAGS/HH single couple, an asymmetric hybrid e-TAGS/HH couple, and an all-HH single couple as a function of hot-side temperature (T_hot). The asymmetric performance reached a maximum efficiency of 9.7%. e-TAGS: Advanced TAGS (tellurium, silver, germanium, antimony) alloy. HH: Half-Heusler.

Based on these results, we assembled an asymmetric hybrid 49-couple module using p-type e-TAGS and n-type HH TE elements. We measured the performance of the hybrid module up to a maximum hot-side temperature of 600°C, and we calculated that the device's efficiency reached a maximum of 10%. Typically, multicouple modules exhibit a noticeably lower efficiency than the corresponding single couples under similar thermal profiles. The improved module efficiency we observed in our work is noteworthy and is believed to be produced by the improved materials and optimized cross-sectional area ratios between the n- and p-type elements.

Our future work will focus on additional chemical and process optimization of the individual thermoelements and on integrating these advanced materials into large-scale power generation modules.