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GMZ Energy develops new thermoelectric material with lower raw material costs, higher power output; Hafnium-free p-type half-Heusler

Researchers at GMZ Energy, a provider of nano-structured thermoelectric generation (TEG) power solutions for mobile and stationary waste-heat recovery (earlier post), with their colleagues at the University of Houston and Bosch, have developed a new Hafnium-free p-type half-Heusler material which offers substantially lower raw material cost than conventional half-Heusler materials. The material also features enhanced performance and mechanical strength due to GMZ’s patented nanostructuring process.

As presented in a paper published in the RSC journal Energy & Environmental Science, the new material improves thermoelectric power output compared to a conventional Hafnium-based product. Further, by replacing the costly Hafnium element with GMZ’s proprietary formulation, the overall cost-per-watt of the TEG is lowered. Cost reduction is beneficial for vehicle and industrial waste heat recovery applications, the developers noted in their paper.

Thermoelectric (TE) power generators can be used to convert vehicle exhaust heat and industrial waste heat into electrical power. This can improve the gas mileage and reduce greenhouse gas emissions. These heat sources are in the 400–600 ˚C range, which requires TE materials to work in this temperature range. Compared to other TE materials in this temperature range, half-Heusler alloys are more attractive due to their better thermal stability, superior mechanical strength, non-toxicity and competitive price. The potential of half-Heusler alloys for high temperature power generation has been thoroughly discussed in previous literature.

… The performance of the thermoelectric material directly depends on the dimensionless figure-of-merit (ZT) defined by (S2σ/κ)T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. Half-Heusler compounds could be good thermoelectric materials with higher power factors (S2σ).

In recent years, different approaches have been applied to improve the ZT of half-Heusler compounds, and a peak ZT of around 1 has been reported in Hf-based [Hafnium-based] p- and n-type half-Heuslers. However, the Hf-based compositions are expensive due to the high price of Hf and, thus, difficult to be commercialized in large-scale power generation applications. Currently, efforts have been made to reduce the half-Heusler cost by replacing Hf with compounds such as NbFeSb, VCoSb, ZrNiSn, TiCoSb, but so far, the ZT has not been reported higher than 0.5 in Hf-free p-type half-Heuslers. Here we present a new NbFeSb based p-type nanostructured half-Heusler compound Nb0.6Ti0.4FeSb0.95Sn0.05 with a peak ZT of ~1 at 700 ˚C using the cost-effective mass-production nanocomposite approach.

—Joshi et al.

Although peak ZT is not improved in the new material in comparison to the ZT of the Hafnium-based material, the electrical power produced by the new material is enhanced by at least 15%, the team said. The higher electrical power is mainly due to the higher power factor (a 25% improvement over the previous composition). (The ZT value did not increase due to its higher thermal conductivity.)

Furthermore, they noted, the cost of the new composition is significantly reduced compared to that of the previously reported best p-type Hafnium composition due to the elimination of the expensive element Hf.

In addition to power output increase, the raw material cost of Nb0.6Ti0.4FeSb0.95Sn0.05 composition is reduced by six times compared to the best Hf-based composition Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2, which eventually decreases the device cost performance ratio ($ per W) significantly. Moreover, these devices show very good thermal stability under operation up to 600 ˚C. These low cost and thermally stable devices are crucial for vehicle and industrial waste heat recovery applications.

—Joshi et al.

With this invention, the raw material cost of p-type half-Heusler is decreased by more than six fold. This cost reduction, combined with excellent mechanical strength and thermal stability, makes GMZ’s half-Heusler material the best option for high temperature power generation applications when compared to all other available materials, including skutterudites, lead tellurides and more.

—Dr. Giri Joshi, a Material Specialist at GMZ and lead author

The Hafnium-free p-type half-Heusler material was made possible with the help of Professor Zhifeng Ren at the University of Houston and Bosch, both under DOE grant # DE-EE0004840.

Earlier in October, GMZ had introduced the TG16-1.0, a new thermoelectric module capable of producing twice the power of the company’s first product, the TG8-1.0. (Earlier post.) GMZ uses nano-engineered half-Heusler (HH) thermoelectric materials (earlier post) in its current modules, and has filed a broad patent to protect the use of half-Heusler compounds for TE applications.

Resources

  • Giri Joshi, Ran He, Michael Engber, Georgy Samsonidze, Tej Pantha, Ekraj Dahal, Keshab Dahal, Jian Yang, Yucheng Lan, Boris Kozinskyc and Zhifeng Ren (2014) “NbFeSb-based p-type half-Heuslers for power generation applications,” Energy & Environmental Science doi: 10.1039/c4ee02180k

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