Bondability of Si-Ge thermoelectric element and molybdenum electrode using aluminum for thermoelectric module

A new method that is low cost and produces highly reliable refractory bonds was developed for bonding Si-Ge thermoelectric devices and Mo electrodes. Aluminum foil was chosen as an alternative material to conventional Ag braze alloy, because of its cost advantages and bonding ability. Good wettability of Al to both the Si-Ge devices and Mo electrodes was achieved at a bonding temperature of 953 K. Molten Al reacted with the Mo electrode and caused partial dissolution of the Si-Ge device. Si-Ge thermoelectric devices could be bonded to Mo electrodes in vacuum, pure nitrogen, and in nitrogen with 4% hydrogen. Mcroscopic observations of cross-sections were conducted to investigate cracking in the bonded joints. The coefficient of thermal expansion (CTE) of Mo (4.5 ppm/K) is similar to that of Si-Ge (4.0 ppm/K), so that the use of a thin Al bonding layer results in sufficiently low thermal stress and allows crack-free bonding. In addition, for 12.5-μm-thick Al foil, almost no degradation of the joint strength occurred after heat treatment at 823 K for 5 h, because the reaction of the thin Al bonding layer to Si-Ge was completed during the bonding process.