Tomotake Tohei

Bondability of Mg2Si element to Ni electrode using Al for thermoelectric modules

The purpose of this study has been to develop a low cost bonding technique for thermoelectric Mg2Si/Si-Ge modules that provides reliable bonding. Aluminum was chosen as an alternative material to conventional silver alloy braze because of its cost advantage and bondability. The shear strength of an aluminum joint between a Mg2Si element and nickel electrode was 19 MPa. The generation capacity of a prototype Mg2Si/Si-Ge twin couple module was about 20% higher than that of a conventional Si-Ge/Si-Ge twin couple module at 923 K (ΔT = 620 K).

Heat dissipative Pb-free bonding technology using Al-rich Zn/Al/Zn clad solder

Abstract Al-rich Zn/Al/Zn clad solder were developed as Pb-free solder for a die-attachment. The Zn/Al/Zn clad solder was produced by clad rolling of Zn and Al strips in order to prevent Al from oxidation and improve wettability. The Zn/Al/Zn clad solder was melted at 382°C after solid-state interdiffusion of the Zn and Al layers. Bonding was successfully achieved with bonding pressure of a few kilopascals. Thermal cycle life of Invar-to-Cu substrate joint using the Zn/Al/Zn clad solder was longer than that of Pb-Sn-Ag solder. No Kirkendall voids were observed in the vicinity of the bonded interface after ageing at 250 °C for 1000 h.

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.