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A2: Medium temperature materials

Low cost half-Heusler materials for power generation applications

Giri Joshi1, Ran He2, Mike Engber1, Ekraj Dahal1, Jian Yang1, Boris Kozinsky3, Xiaowei Wang1, Martin Cleary1, and Zhifeng Ren2
1 GMZ Energy Inc.2 University of Houston, Department of Physics3 BOSCH Corporation

We report a peak ZT of around 1.0 at 700 oC in nanostructured p-type half-Heusler (HH) composition without hafnium (Hf). The power factor of the new Hf-free composition is 75% higher in comparison to the previously reported Hf-based HH materials Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2, but the ZT value is not improved due to an increase in thermal conductivity. However, there is enough room to increase ZT in the future by reducing the thermal conductivity. The high power factor of the new Hf-free composition led to the increase in power density of a thermoelectric device by 15% in comparison to the device from Hf-based p-type material. The n-type material used to fabricate the device is the best reported nanostructured Hf0.25Zr0.75NiSn0.99Sb0.01 composition. Both the p- and n-type nanostructured samples are prepared by ball milling the arc melted ingot and hot pressing the finely ground powders. Moreover, the raw material cost of these devices is as low as $75/kg due to the elimination of the expensive element Hf in p-type, which is crucial for such materials to be used in large-scale for vehicle and industrial waste heat recovery applications.