Developing Contacting Solutions for Mg2Si1-xSnx-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 Ni45Cu55 as contacting electrodes for their high electrical conductivity, similar coefficient of thermal expansion (CTE), and good adhesion to Mg2(Si,Sn). Both electrodes were joined to Mg2Si0.3Sn0.7 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 rc was estimated using a traveling potential probe combined with local Seebeck scanning. Good contacting was observed with both electrode materials. Results show low rc with Cu, suitable for applications, for both n- and p-type silicides (<10 µΩ·cm2), with the occurrence of wide, highly conductive diffusion regions. Ni45Cu55 joining also showed relatively low rc values (∼30 µΩ·cm2) 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 1 week to investigate the evolution of the contact regions under working temperatures. rc values increased (up to ∼100 µΩ·cm2) for annealed n-type samples but remained low (<10 µΩ·cm2) for p-type. Therefore, Cu is a good contacting solution for p-type Mg2(Si,Sn) and a potential one for n-type if the diffusion causing contact property degradation can be prevented.