An Experimental Study on the Thermal Stability of Mg₂Si/Ni Interface under Thermal Cycling

Mg₂Si is a promising eco-friendly thermoelectric material, and Ni is suited for electrical contact on it. In this study, Bi-doped Mg₂Si ingots with Ni contacts were fabricated by co-sintering, and thermal stability was investigated by long-time (500 h, 500 cycles) temperature cycling from 25 °C to a peak temperature (T = 400 and 450 °C) in N₂. The as-sintered Ni/Mg₂Si interfacial region is a multilayer consisting of Mg₃Bi₂, a series of Mg_xSi_yNi_z ternary compounds (ω, ν, ζ, and η-phases), and MgNi₂. In the complex microstructure, the MgNi₂ / η-phase interface was vulnerable to stress-induced voiding at T = 450 °C, which arises from the mismatch of the thermal expansion coefficients. Interfacial voiding was avoided by adding 10 mol% Ag in Ni, which is probably due to the suppression of vacancy migration by the Ag-containing 2nd phase formation at the MgNi₂/η-phase interface.