Materials Today Energy
Volume 11, March 2019, Pages 97-105
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Ni and Ag electrodes for magnesium silicide based thermoelectric generators

https://doi.org/10.1016/j.mtener.2018.10.016Get rights and content

Abstract

The development of stable and low resistance contacts between thermoelectric materials and metal bridges is indispensable for highly efficient thermoelectric generators. We have studied suitable electrode materials for n- and p-type Mg2Si1−xSnx (with x ≈ 0.6) as magnesium silicide based solid solutions are among the most promising candidates for large scale waste heat recovery applications. Evaluating in particular Ag and Ni as potential metallization, we find that Ag is a suitable electrode for p-type Mg2Si0.4Sn0.6 with a specific contact resistance of ≈10 μΩcm2 for optimized joining conditions. We also show that separating the material sintering from the joining step can be very advantageous as control over joining temperature allows tailoring of the interface composition and properties. For Ni, a reduction of the joining temperature mitigates cracking of the thermoelectric material due to the mismatch in coefficient of thermal expansion. Locally, relatively low values of 25−50 μΩcm2 for the specific contact resistances between Ni electrode and Mg2Si1−xSnx could be determined for both n- and p-type material.

Introduction

Thermoelectric generators (TEG) can convert heat directly into electricity, and are therefore an effective solution to save energy in any systems that generate waste energy in form of heat. High performance TEG require high conversion efficiency of the thermoelectric (TE) materials and good joining between such materials with electrodes [1]. Development of improved TE materials necessitates the optimization of material's Seebeck coefficient (S), electrical conductivity (σ) and thermal conductivity (κ). The material performance is quantified by the material figure of merit, zT=S2σTκ, which should be as high as possible. Current state-of-the-art TE materials have their zT above 1 and include bismuth telluride, skutterudites, half-Heusler, magnesium silicides [1], [2], [3], [4] and many more.

The efficiency of TEG is governed by the device's figure of merit ZT which is determined by the figure of merit of the employed n- and p-type material and intrinsic passive loss in the device. The device figure of merit is given by the Seebeck coefficient (Sdev), the total electrical resistance (R) and the total thermal conductance (K) of the module according to the following equation [1], [5]:ZT=Sdev2TRK=Sdev2T(Rleg+Rc)(Kleg+Kc)where Rleg and Kleg, Rc and Kc are electrical and thermal resistance of the TE legs and contacts, respectively. Generally, the device performance can be limited if the employed material has poor self-compatibility [6], however, the self-compatibility for Mg2SixSn1−x is good for the optimized material so that is a minor concern [2], [7].

It is clear from Eq. (1) that highly performing TEG require not only good TE materials but also as low as possible contact resistance. Generally, a ratio of lower than 10% between contact resistance and the total device's resistance is considered to be acceptable [8]. In addition, stable interfacial contact is important for the long-term performance and stable operation of TE devices [9].

Among TE materials, magnesium silicide-based materials have been acknowledged as high performing candidates with unique features. This material class consists of elements that are low-cost, abundant, non-toxic and light-weight, making it especially attractive for industrial applications in terrestrial vehicles, aviation, and space. The material can be synthesized with well-known methods (such as mechanical alloying, melting of the elements, self-propagating high temperature synthesis, solid state reaction, etc. with repeatable quality [10], [11], [12], [13]). Therefore, development of magnesium-silicide-based TEG is a straight-forward step towards application. Contact fabrication for Mg2Si devices has achieved promising results. Nickel has been found to be a highly suitable electrode material for n-type Mg2Si due to its match of coefficient of thermal expansion with Mg2Si and very low specific contact resistance, <10 μΩcm−2 [14], [15], [16], [17], [18], [19]. Compared to doped Mg2Si, the solid solutions of Mg2Si and Mg2Sn can have significantly improved thermoelectric properties. In particular, for n-type Mg2Si1−xSnx with 0.6 < x < 0.7, a zT > 1 at ∼500 °C has been repeatedly reported [20], [21], [22], [23]. These solid solutions can thus outperform Mg2Si and would therefore be even more attractive for industrial applications. However, very little has been reported on the contacting of the solid solution Mg2Si1−xSnx. Traditionally, n-type Mg2Si or Mg2Si1−xSnx has been combined with p-type higher manganese silicide for TEG fabrication [19], [24], [25]. As both materials have dissimilar coefficients of thermal expansion this can lead to stability problems of such modules [26], [27], [28]. With recent progress in the development of p-type Mg2Si1−xSnx the fabrication of a Mg2Si1−xSnx only based TEG has become feasible [20]. However, no report has been given on contact development for p-type Mg2Si1−xSnx yet.

In this work, for the first time, joining of both p- and n-type Mg2Si1−xSnx with different electrodes was investigated. Ni and Ag are selected as electrode materials due to their excellent electrical and thermal conduction, similar thermal expansion coefficient (CTE) with Mg2Si1−xSnx, corrosion resistance and abundant availability. Furthermore, Ag has good wetting properties and a high shear strength [29], [30]. p- and n-type Mg2Si1−xSnx were directly joined with the electrode material following two methods. In the first method (1-step joining), Mg2Si1−xSnx powder is sandwiched between two electrode layers and sintered. In the second method (2-step joining), Mg2Si1−xSnx was pre-sintered before being joined with the electrode. The influence of the sintering approach and parameters, the differences between the two electrode materials, and the differences between n- and p-type materials are systematically compared with respect to the interface microstructure and the resulting electrical contact resistances.

Section snippets

Materials and methods

We have used p-type material with a composition of Mg1.98Li0.02Si0.4Sn0.6 (Mg2Si1−xSnx with x ≈ 0.6) and n-type material with a composition of Mg2Si0.3Sn0.665Bi0.035 (Mg2Si1−xSnx with x ≈ 0.7) for the contacting experiments with Ag. Moreover, n-type material with a composition of Mg2Si0.385Sn0.6Sb0.015 (Mg2Si1−xSnx with x ≈ 0.6) has been used for the contacting experiments with Ni. We and others have shown previously that around the chosen composition the best thermoelectric properties can be

Results and discussion

The coefficients of thermal expansion (CTE) of p- and n-type Mg2Si1−xSnx from room temperature to 400 °C and of the selected electrode materials at room temperature are shown in Fig. 1. Mg2Si1−xSnx with x = 0.6 and x = 0.7 possesses a CTE of 16.5–17.5 and ∼17.5–18.5 (10−6 K−1), respectively. Ag has the advantage of having a CTE closer to that of Mg2Si1−xSnx. Ni, regardless of having lower CTE than that of Mg2Si1−xSnx, is still a possible candidate because the CTE mismatch can be tolerated in a

Discussion

We have obtained results both for 1-step and 2-step joining. The former one has the advantage of less process steps but also a number of drawbacks. For 1-step joining the joining temperature is more or less determined by the optimum sintering temperature of the material, otherwise the loss in thermoelectric performance will outweigh any gain in optimized contacts. The sintering temperature window is material specific but previous results indicate that it is around 100 K wide for magnesium

Conclusion

We have investigated contacts between both p- and n-type Mg2Si1−xSnx (with x ≈ 0.6) and Ni and Ag electrodes. These contacts were fabricated by co-sintering of thermoelectric material and the metal film in a 1-step (powder of thermoelectric material) and a 2-step (using a pre-compacted thermoelectric pellet) process. In both methods, mechanically good bonds between the metal and Mg2Si1−xSnx could be obtained. We find that 2-step joining is highly advantageous compared to the faster 1-step

Data statement

This is an original research, therefore the raw/processed data will be available on request. The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

Acknowledgements

The authors would like to gratefully acknowledge the endorsement from the DLR Executive Board Member for Space Research and Technology and the financial support from the Young Research Group Leader Program. Also, special thanks to Pawel Ziolkowski (DLR) for his help with contact resistance measurement and Przemek Blaschkewitz (DLR) for his support with the thermoelectric measurements. We are grateful to Reinhard Sottong and Nicole Knoblauch (DLR) for the measurement of thermal expansion. The

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