Grain-by-Grain Compositional Variations and Interstitial Metals-A New Route toward Achieving High Performance in Half-Heusler Thermoelectrics.

Michael Pollet,Ronald I Smith,Rodolphe Decourt,Sonia A Barczak,John E Halpin,Jan-Willem G Bos,Jim Buckman,Donald A Maclaren

doi:10.1021/acsami.7b14525

Abstract

Half-Heusler alloys based on TiNiSn are promising thermoelectric materials characterized by large power factors and good mechanical and thermal stabilities, but they are limited by large thermal conductivities. A variety of strategies have been used to disrupt their thermal transport, including alloying with heavy, generally expensive, elements and nanostructuring, enabling figures of merit, ZT ≥ 1 at elevated temperatures (>773 K). Here, we demonstrate an alternative strategy that is based around the partial segregation of excess Cu leading to grain-by-grain compositional variations, the formation of extruded Cu "wetting layers" between grains, and-most importantly-the presence of statistically distributed interstitials that reduce the thermal conductivity effectively through point-defect scattering. Our best TiNiCuySn (y ≤ 0.1) compositions have a temperature-averaged ZTdevice = 0.3-0.4 and estimated leg power outputs of 6-7 W cm-2 in the 323-773 K temperature range. This is a significant development as these materials were prepared using a straightforward processing method, do not contain any toxic, expensive, or scarce elements, and are therefore promising candidates for large-scale production.

Highlights

Thermoelectric (TE) generators directly convert waste heat into electricity and could become an important component of a sustainable energy future.[1]
Great progress has been made in improving the efficiency of thermoelectric materials over the past 2 decades, guided by several successful design strategies including the phonon-glass electron-crystal concept, band engineering, and nanostructuring.[3−6] These have led to the design of many materials with peak ZT values above 1, which is generally seen as an indication of viability; in some cases, ZT can exceed 2.6,7 Here, ZT = (S2/ ρκ)T is the thermoelectric figure of merit, where S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the sum of the lattice and electronic thermal conductivities, and T is the absolute temperature
The best n-type materials are based on XNiSn,[11−14] while good p-type performance can be extracted from compositions based on XCoSb and X′FeSb.[15−18] In terms of the individual thermoelectric parameters, the HH materials are characterized by large S2/ρ = 3−6 mW m−1 K−2 and are, in effect, limited by their large κlat, which is typically κlat = 3−4 W m−1 K−1 for optimized compositions

Summary

Introduction

Thermoelectric (TE) generators directly convert waste heat into electricity and could become an important component of a sustainable energy future.[1]. Half-Heusler (HH) materials are highly promising for midtemperature waste heat recovery because they balance the complex commercial trade-off between performance, cost, mechanical strength, and stability.[10] The best n-type materials are based on XNiSn,[11−14] while good p-type performance can be extracted from compositions based on XCoSb and X′FeSb (with X = Ti, Zr, and Hf and X′ = V and Nb).[15−18] In terms of the individual thermoelectric parameters, the HH materials are characterized by large S2/ρ = 3−6 mW m−1 K−2 and are, in effect, limited by their large κlat, which is typically κlat = 3−4 W m−1 K−1 for optimized compositions This has stimulated several approaches to reduce κlat, including alloying at the X-site to increase point scattering of phonons, reduction of grain sizes to enhance boundary scattering of phonons, and segregation of full-Heusler (FH) XNi2Sn phases in metal-rich XNi1+ySn compositions.[19,20] The segregation of FH phases is of interest

Methods

Results

Conclusion