Effect of joining temperature on the interconnection zone and electrical resistance of Ag/n-Mg2Si and Ag/n-Mg2Sn contacts

Mg2X (X = Si, Sn) based compounds are among the most viable materials for waste heat recovery due to their high thermoelectric figure of merit zT, abundance of the constituent elements, low mass density and environmental benignity. Naturally, to establish an improved life expectancy of thermoelectric devices, high zT must be complemented by robust joining between the thermoelectric legs and a metal electrode that causes only small thermal and electrical contact resistance. We have studied Ag as a potential electrode for n-Mg2Si and n-Mg2Sn TE materials and have systematically explored the influence of a variation of the joining temperature on the resulting interconnection zone (IZ) and the electrical contact resistance. With respect to the electrical contact resistance, a reasonably low value of < 20 mu omega cm(2) for Mg2Si/Ag was found at 500 degrees C and a value of < 30 mu omega cm(2) was obtained for Mg2Sn/Ag at 400 degrees C. At 450 degrees C, Mg2Si/Ag also showed a lower contact resistance (approximate to 35 mu omega cm(2)) than Mg2Sn/Ag (approximate to 165 mu omega cm(2)). Analyzing the observed phases, we can deduce that the superior interface quality of Mg2Si/Ag compared to Mg2Sn/Ag is not due to the electrical properties of the IZ itself. Instead the observed differences are presumably governed by microcracking which is less for Mg2Si/Ag, even though the coefficient of thermal expansion mismatch between Mg2Si and Ag is larger than that between Mg2Sn and Ag. We illustrate the use of phase diagrams to correlate IZ intermetallic layers to equilibrium phases to explain experimental observations of microstructures.