Takahiro Jinushi

Development of the High Performance Thermoelectric Modules for High Temperature Heat Sources

In recent years, power generating systems using thermoelectric elements have become attractive as an effective method of using industrial waste heat, at a temperature of around 773K, to produce energy. However, in order to develop a module usable under such a high temperature, certain concerns have to be overcome, e.g. thermal stress, diffusion of the connecting interfaces, etc. In this research, using an FeSi2 with diffusion barrier layers and a SiGe element produced by a powder metallurgy process, the module structure and installation method were optimized for application in PM sintering furnaces. As a result, from a viewpoint of heat stress at high temperatures and contact thermal resistance, it is confirmed that the optimal structure is the skeleton structure using Cu substrate on the cooling side, which has excellent heat conductivity and the optimal installation method is to adopt a carbon sheet and a mica sheet to the high temperature side, where Si grease is applied to the low temperature side, under pressurized condition. The power of the developed modules indicated 0.5W in an FeSi2 module and 3.8 W with a SiGe module at 827K, respectively. Moreover, neither breakage nor deterioration were observed after 30 heat cycles test simulating sintering furnace.

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).

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.