Developing Contacting Solutions for Mg₂Si_1–xSn_x-Based Thermoelectric Generators: Cu and Ni₄₅Cu₅₅ as Potential Contacting Electrodes

Abstract

Magnesium silicides can be used for thermoelectric energy conversion, as high values of figure of merit zT were obtained for n-type (1.4 at 500 °C) and p-type (0.55 at 350 °C) materials. This, however, needs to be complemented by low resistive and stable contacting to ensure long-term thermogenerator operation and minimize losses. In this study, we selected Cu and Ni 45 Cu 55 as contacting electrodes for their high electrical conductivity, similar coefficient of thermal expansion (CTE) and good adhesion to Mg 2 (Si,Sn). Both electrodes were joined to Mg 2 Si 0.3 Sn 0.7 pellets by hot pressing in a current-assisted press. Microstructural changes near the interface were analyzed using SEM/EDX analysis, and the specific electrical contact resistance r c was estimated using a travelling potential probe combined with local Seebeck scanning. Good contacting was observed with both electrode materials. Results show low r c with Cu, suitable for application, for both n-type and p-type silicides (< 10 µΩ•cm 2 ), with the occurrence of wide, highly conductive diffusion regions. Ni 45 Cu 55 joining also showed relatively low r c values (~ 30 µΩcm 2 ) for n-and p-type, but had a less inhomogeneous reaction layer. We also performed annealing experiments with Cu-joined samples at 450 °C for one week to investigate the evolution of the contact regions under working temperatures. r c values increased (up to ~ 100 µΩcm 2 ) for annealed n-type samples, but remained low (< 10 µΩcm 2 ) for p-type. Therefore, Cu is a good contacting solution for p-type Mg 2 (Si,Sn), and a potential one for n-type if the diffusion causing contact property degradation can be prevented.

Overcoming Asymmetric Contact Resistances in Al-Contacted Mg2(Si,Sn) Thermoelectric Legs

“…Nevertheless, a far more important point to ensure long-term TEG performance is the material stability. It has been already shown that the n-type TE properties are very sensitive to Mg evaporation, which greatly alters the carrier concentration [27,30,[55][56][57]. The investigation of suitable coating and annealing conditions should therefore be prioritized for further TEG development, see, e.g., Refs.…”

“…The solid solution Mg 2 (Si,Sn) pellets were prepared similarly to what was reported in previously published papers [27,[30][31][32], with the following nominal stoichiometry: Mg 2.06 Si 0.3 Sn 0.665 Bi 0.035 for n-type and Mg 1.98 Li 0.03 Si 0.3 Sn 0.7 for p-type. The n-type composition contains Mg with an excess of 3% in order to compensate for the Mg evaporation occurring during the sintering, due to its longer duration compared to p-type samples.…”

“…The quality of the joining is estimated by the value of its specific contact resistance rc and the preservation of the Seebeck coefficient of the TE material using a Potential & Seebeck Scanning Microprobe (PSM) [33]; and by its microstructural and chemical composition at the interface using scanning electron microscopy (Zeiss Ultra 55 SEM equipped with an EDX detector). Two r c values are obtained for each contact with two different calculation methods, using the TE material s electrical conductivity to calculate the current density (r c,j(TE) ), or using the current passing through the sample as measured by the PSM (r c,j(PSM) ), as reported in previously published papers [27,30]. The two specific electrical contact resistances are calculated using the following equations:…”

“…A first step in the development of a fully Mg 2 (Si,Sn)-based TEG is the electrode selection. Several attempts were reported with only partially promising results, as the electrodes provided low electrical contact resistance but also led to cracking (Ni, constantan) or altering of the TE material's Seebeck coefficient (Cu, Ag) [27][28][29]. Most recently, aluminum was reported as the best electrode for this TE material system, showing low electrical contact resistance and good thermal and chemical stability of the interface also through annealing [30].…”

The Relationship between Natural Pyrite and Impurity Element Semiconductor Properties: A Case Study of Vein Pyrite from the Zaozigou Gold Deposit in China

“…The Seebeck coefficient measurements of pyrite veinlets were carried out under a potential Seebeck microscope (PSM-II) at the Hangzhou Innovation Institute, Beihang University (China). This instrument was jointly developed by the German Aerospace Center and Panco of Germany; it can measure the spatial distribution of the Seebeck coefficient in a sample and is usually used to check the uniformity of semiconductor materials [24][25][26]. In this study, the instrument was applied to the measurement of natural pyrite veins to observe the in situ Seebeck coefficient changes on the growth profile of pyrite veins (Figure 1).…”