Ti2NiCoSnSb - a new half-Heusler type high-entropy alloy showing simultaneous increase in Seebeck coefficient and electrical conductivity for thermoelectric applications

Anirudha Karati,Ramesh Chandra Mallik,U V Varadaraju,M Nagini,Sanyukta Ghosh,B S Murty,Rajashekhara Shabadi,K G Pradeep

doi:10.1038/s41598-019-41818-6

Abstract

A new single phase high entropy alloy, Ti2NiCoSnSb with half-Heusler (HH) structure is synthesized for the first time by vacuum arc melting (VAM) followed by ball-milling (BM). The BM step is necessary to obtain the single phase. Local electrode atom probe (LEAP) analysis showed that the elements are homogeneously and randomly distributed in the HH phase without any clustering tendency. When the BM was carried out for 1 hour on the VAM alloy, microcrystalline alloy is obtained with traces of Sn as secondary phase. When BM was carried out for 5 h, single HH phase formation is realized in nanocrystalline form. However, when the BM samples were subjected to Spark plasma sintering (SPS), secondary phases were formed by the decomposition of primary phase. Nanostructuring leads to simultaneous increase in S and σ with increasing temperature. The micro (1 h BM-SPS) and nanocrystalline (5 h BM-SPS) alloys exhibited a power factor (S2σ) of 0.57 and 1.02 mWm−1K−2, respectively, at 860 K. The microcrystalline sample had a total thermal conductivity similar to bulk TiNiSn sample. The nanocrystalline alloy exhibited a ZT of 0.047 at 860 K. The microcrystalline alloy showed a ZT to 0.144 at 860 K, in comparison to the nanocrystalline alloy.

Highlights

HH alloys are ternary intermetallic compounds with three interpenetrating sub-lattices
The efficacy of conversion is defined by the figure of merit Z = S2σ/κ where, S is the thermopower or Seebeck coefficient, σ is the electrical conductivity and κ is the total thermal conductivity which comprises of two components namely, the electronic contribution and the lattice contribution
The Rietveld refinement pattern (Fig. 1b) confirmed the HH phase formation crystallizing in the F 4 ̄3m symmetry

Summary

Introduction

HH alloys are ternary intermetallic compounds with three interpenetrating sub-lattices. The latter coined the term “High-Entropy Alloys” owing to their high configurational entropy in comparison to traditional alloys Most of these alloys end up having simple solid solution structures such as BCC, FCC and HCP instead of intermetallic compounds owing to the high mixing entropy in these systems[16]. They exhibit superior mechanical properties[17], grain growth resistance at high temperature[18], high oxidation and corrosion resistance[19,20]. The first report of HEA as TE material was by Shafeie et al in AlCoCrFeNi22 Whilst it had high electrical conductivity, it exhibited low Seebeck coefficient and high thermal conductivity and in turn low ZT. The microcrystalline alloy exhibits higher ZT value than the nanocrystalline alloy

Methods

Results

Discussion

Conclusion