Fabrication and Evaluation of Low-Cost CrSi2 Thermoelectric Legs

Sylvain Le Tonquesse,Michihiro Ohta,Yoshitaka Matsushita,Takao Mori,Priyanka Jood,David Berthebaud

doi:10.3390/cryst11091140

Abstract

CrSi2 is a promising thermoelectric material constituted of non-toxic and earth abundant elements that offer good perspectives for the mass production of inexpensive and reliable thermoelectric modules for waste heat recovery. Realization of robust metallic contacts with low electrical and thermal resistances on thermoelectric materials is crucial to maximize the conversion efficiency of such a device. In this article, the metallization of an undoped CrSi2 with Ti and Nb using a conventional Spark Plasma Sintering process is explored and discussed. These contact metals were selected because they have compatible thermal expansion coefficients with those of CrSi2, which were determined in this study by X-ray Diffraction in the temperature range 299–899 K. Ti was found to be a promising contact metal offering both strong adhesion on CrSi2 and negligible electrical contact resistance (<1 μΩ cm2). However, metallization with Nb resulted in the formation of cracks caused by large internal stress inside the sample during the fabrication process and the diffusion of Si in the metallic layer. A maximum conversion efficiency of 0.3% was measured for a sandwiched Ti/CrSi2/Ti thermoelectric leg placed inside a thermal gradient of 427 K. The preliminary results obtained and discussed in this article on a relatively simple case study aim to initiate the development of more reliable and efficient CrSi2 thermoelectric legs with an optimized design.

Highlights

Thermoelectric (TE) generators are all-solid-state and greenhouse gas emission-free devices that enable the direct conversion of heat into electricity via the Seebeck effect
The refinement converged to the lattice parameters a = 4.42692(7) Å and c = 6.3729(2) Å that were in good agreement with literature values [32]
Ti and Nb were selected as contact metals as their thermal expansion coefficients matched closely those of CrSi2, which were determined by a preliminary XRD analysis as αa = 9.93(9) × 10−6 K−1, αc = 7.2(1) × 10−6 K−1, and αV = 26.2(2) × 10−6 K−1 at 299 K

Summary

Introduction

Thermoelectric (TE) generators are all-solid-state and greenhouse gas emission-free devices that enable the direct conversion of heat into electricity via the Seebeck effect. The conversion efficiency is a monotonically increasing function of the figure of merit ZT = α2T/κρ of the TE materials where α is the Seebeck coefficient, ρ the electrical resistivity, κ the thermal conductivity, and T the temperature. The efficiency of current commercial TE modules composed of state-of-the-art materials with ZT > 1 remains below ≈8% which strongly limits their use to niche applications [1,2]. With the aim to increase the applicability of this technology, many attempts to improve the overall performance of TE modules have been reported recently in the literature and consist of using newly developed materials with an improved ZT [3,4,5,6,7], the development of alternative architectures [8,9,10,11,12,13], or fabrication process [14]. Two important challenges remain: (i) the use of less toxic and expensive elements than those comprising the majority of current high-performance materials (Pb, Te, Bi. . . ) and (ii) the realization of good-quality electrical contacts on TE materials that present low thermal and electrical contact resistances, strong mechanical resistance, and a good thermal stability

Methods

Results

Conclusion