Elsevier

Intermetallics

Volume 127, December 2020, 106988
Intermetallics

Role of secondary phases and thermal cycling on thermoelectric properties of TiNiSn half-Heusler alloy prepared by different processing routes

https://doi.org/10.1016/j.intermet.2020.106988Get rights and content

Highlights

  • Rapid solidification inhibits TiNi2Sn primary phase, refines TiNiSn microstructure.

  • Secondary phases affect thermoelectric properties following effective medium theory.

  • Thermal cycling induces surface oxidation, affecting thermoelectric properties.

Abstract

Samples of a TiNiSn half-Heusler thermoelectric alloy were prepared using arc melting and rapid solidification by planar flow casting. On the one hand, arc melted samples show multiple phases and need long annealing time to be homogenized into a single phase. On the other hand, rapidly solidified samples show an almost single phase, with a grain refined microstructure. This result is a consequence of the melt undercooling, that allows to bypass the primary solidification of TiNi2Sn full-Heusler and to hinder grain growth. Rapidly solidified samples were consolidated into dense bulk samples by means of open die pressing. The grain refined microstructure obtained by rapid solidification is maintained after sintering, leading to a reduced lattice thermal conductivity, due to a scattering of phonons at grain boundaries. The role of the residual secondary phases (Ni3Sn4, TiNi2Sn and Sn) on the electrical conductivity and thermopower is discussed in the framework of the effective medium theory and related models, explaining the dispersion of thermoelectric properties observed in the literature for nominally stoichiometric TiNiSn. Thermal cycling above 700 K causes surface oxidation, leading to altered thermoelectric properties.

Introduction

The thermoelectric effect allows to convert temperature gradients directly into an electrical current and can be used for energy harvesting from waste heat source in industrial, domestic and transport environments [1,2]. Despite current thermoelectric materials are limited in conversion efficiency, thermoelectricity has attracted great interest in the last years, due to the compactness, reliability and durability of the devices, together with the absence of fluid and moving parts [3]. The performance of thermoelectric materials can be evaluated by the dimensionless figure of merit ZT=α2σκlat+κelT, where α is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, κlat and κel are the lattice and electronic contributions to thermal conductivity, respectively. An ideal thermoelectric material should have a high power factor, PF=α2σ and a low thermal conductivity in a wide range of temperatures. Various materials were studied for power generation applications, such as GeTe [4,5], PbTe [6], Bi2Te3 [7], silicides [8], skutterudites [9,10] and Heusler-type alloys [[11], [12], [13]]. Half-Heusler compounds constitute a group of ternary intermetallic compounds with the general formula ABX, where A is a highly electropositive transition metal, B is a low electropositive transition metal and X is the main group element [14]. Their crystal structure consists in three interpenetrating face centered cubic (f.c.c.) sublattices and one vacant f. c.c. sublattice. Half-Heusler compounds with 18 valence electrons have been extensively studied as potential thermoelectric materials for power generation in the medium/high temperature range (800–1000 K) [[15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]]. ZT of half-Heusler compounds can be improved by reducing the lattice thermal conductivity, thus several methods were investigated for increasing phonon scattering through mass fluctuation [37], phase separation [11,26] and microstructure refinement [30,31,33,36]. Among the different families of half-Heusler compounds, it is well known that the MNiSn (M = Ti, Zr, Hf) alloys show the best thermoelectric performances [[22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]]. Reported values of Seebeck coefficient and electrical conductivity of TiNiSn alloys prepared with different processing routes are quite scattered, due to the presence of residual secondary phases [24,28]. For example, processing from the melt (arc melting, induction melting) leads to the formation of sample with multiple phases, that need long annealing to be homogenized [32].

The aim of this work is to study the effect of two different pre-processing routes (arc melting and rapid solidification [38]) on the structure and microstructure of TiNiSn half-Heusler compound. The phase selection observed after different preparation routes is described on the basis of thermodynamic arguments. Arc melted ingots and rapidly solidified flakes were post-processed by annealing and powder sintering, respectively, in order to obtain homogeneous and dense massive samples for thermoelectric characterizations. The effect of residual secondary phases on the measured values of the Seebeck coefficient and electrical conductivity is discussed in the framework of models based on the effective medium theory. The role of grain boundary scattering on the lattice thermal conductivity is evaluated.

Section snippets

Material and methods

Polycrystalline samples of TiNiSn and TiNi1·03Sn were synthesized by arc-melting of elemental metals (Titanium 99.99%; Nickel 99.95%; Tin 99.995%) in Argon (5.5) atmosphere. Residual traces of oxygen in the melting atmosphere were removed by previously melting Ti and Zr getters. The ingots were flipped over and re-melted five times to ensure chemical homogeneity. On the one side, in order to obtain a single-phase sample, the arc melted ingot, wrapped in a Ta foil and sealed under vacuum in a

Effect of pre-processing on phase formation and microstructure

Fig. 1(a) shows the XRD pattern of the AM ingot. The relative amount of the phases and the corresponding lattice parameters, obtained by Rietveld refinement, are reported in Table 1. The AM ingot shows the presence of TiNiSn half-Heusler phase, together with TiNi2Sn full-Heusler, Ti6Sn5 and Sn phases.

Fig. 2(a) shows the backscattered electron image of the AM ingot. The phases identified are half-Heusler TiNiSn (1), full-Heusler TiNi2Sn (2), Ti6Sn5 (3), Ni3Sn4 (4), Ti5Sn3 (5), Ti (6) and Sn (7).

Conclusions

In this work, dense TiNiSn bulk samples were obtained using two different processing routes (i.e. arc melting plus annealing and rapid solidification followed by sintering). The solidification of the arc melted (AM) sample can be described by the Scheil model, that assumes complete miscibility in the liquid phase and absence of diffusion in the solid phases. Stable and metastable isopleth Ni–TiSn, obtained maintaining and suspending the TiNi2Sn phase, respectively, showed that a liquid

CRediT authorship contribution statement

Francesco Aversano: Investigation, Writing - original draft. Mauro Palumbo: Software. Alberto Ferrario: Investigation, Writing- Reviewing and Editing, Writing - review & editing. Stefano Boldrini: Investigation, Writing- Reviewing and Editing, Writing - review & editing. Carlo Fanciulli: Investigation, Writing- Reviewing and Editing, Writing - review & editing. Marcello Baricco: Writing- Reviewing and Editing, Writing - review & editing. Alberto Castellero: Conceptualization, Writing- Reviewing

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Alberto Castellero and Francesco Aversano thank University of Turin and Compagnia di Sanpaolo for financial support (Project n. CSTO162398). The authors also thank Dr. G. Fiore (University of Turin) and Mr. E. Bassani (CNR-ICMATE, Unità di Lecco) for the support in the arc melting and ODP processing of the samples, respectively. Prof. Peter Rogl is kindly acknowledged for providing the TDB file for the Ti–Ni–Sn system.

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