Synthesis and Characterization of Thermoelectric Co2XSn (X = Zr, Hf) Heusler Alloys

Alessandro Difalco,Alberto Castellero,Marcello Baricco,Francesco Aversano,Alberto Ferrario,Stefano Boldrini

doi:10.3390/met10050624

Abstract

In this work, we report the results of an experimental investigation on the synthesis, structure, microstructure, mechanical, electrical conductivity, and Seebeck coefficient of Co2XSn (X = Zr, Hf) alloys. In both the alloys, the main constituent is a full Heusler-type compound that coexists with small amounts of secondary phases. Both alloys show a rather high Vickers hardness (around 900 HV) and an indentation fracture toughness typical of ceramics (around 2 MPa·m1/2). The electronic transport properties of the two alloys were measured for the first time. The temperature dependence of both the Seebeck coefficient and the electrical conductivity of the two alloys shows a change in correspondence of the Curie temperature. The Seebeck coefficient reaches a constant plateau, while the electrical conductivities show a transition from metallic to semiconductor behavior. As a consequence, almost constant values of the power factor have been obtained for the power factor above the Curie temperature, which is promising for an efficient exploitation of thermal gradients of several hundreds of degree in waste heat harvesting applications. Finally, on the basis of results from this work and from the literature, the effect of the substitution of the X element on the electronic transport properties in the series Co2XSn (X = Ti, Zr, Hf) is discussed.

Highlights

The Heusler alloys represent an important class of thermoelectric materials since they generally combine good performances with high thermal stability, and, in most cases, ease of synthesis
The structure of the full-Heusler (FH) compounds consists of 4 FCC sublattices with a L21 -type structure, while the one of the half-Heusler (HH) compounds consists of 3 FCC sublattices arranged according to the C1b lattice prototype [1]
Co-based FH alloys are proved to be half-metallic ferromagnets, and they display an electronic density of states (DOS), which leads to the manifestation of spin-Seebeck effects [6,7,8]

Summary

Introduction

The Heusler alloys represent an important class of thermoelectric materials since they generally combine good performances with high thermal stability, and, in most cases, ease of synthesis.The structure of the full-Heusler (FH) compounds consists of 4 FCC sublattices with a L21 -type structure, while the one of the half-Heusler (HH) compounds consists of 3 FCC sublattices arranged according to the C1b lattice prototype [1]. HH alloys show remarkable zT values such as 1.5 for (Zr0.5 Hf0.5 )0.5 Ti0.5 NiSn0.998 Sb0.002 [2] and Nb0.88 Hf0.12 FeSb [3], and about 1.2 for. FH alloys exhibit numerous worthwhile properties related to their particular electronic structures. Co-based FH alloys are proved to be half-metallic ferromagnets, and they display an electronic density of states (DOS), which leads to the manifestation of spin-Seebeck effects [6,7,8].

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Methods

Conclusion