(358h) Interfacial Stabilities in Thermoelectric Devices

Thermoelectric devices play a vital role in energy applications as they have the potential to improve energy efficiency by recovering waste heat into electricity. In conjunction with solar heating panels, they are renewable energy sources. Typically, thermoelectric devices consist of arrays of P-N junctions, which incorporate multiple interfaces. The quality and stability of these interfaces are fundamental factors that influence the performance and reliability of the devices.

The two most critical interfaces in the devices are the thermoelectric substrates/barrier layer and barrier layer/joining materials interfaces. The substrates/barrier layer interfaces are unique to the thermoelectric devices. Co, Ni and (Co,Ni) barrier layers are the most commonly used barrier layer materials. Their interfacial reactions with various thermoelectric substrates, such as PbSe, PbTe, CoSb₃, Bi₂Te₃, Bi₂Se₃, SnSe₂, Cu₂Se and Ag_1-xSn_xTe, are discussed.

Ni₃Pb₂Se₂, is formed in the Ni/PbSe couples reacted at 300℃, 350℃ and 400℃, but no interfacial reaction at 250℃. The reaction path is Ni/Ni₃Pb₂Se₂/PbSe and Ni is the fastest diffusion species in the Ni/PbSe couples. No interfacial reactions are observed in the Ni/PbTe couple heat-treated at 500℃ and 400℃ for a month. Significant reactions are observed when reacted at 650℃. The reaction path is Ni/β-Ni₃Te₂+Liquid (I)/Liquid (I)/PbTe, and Ni is the dominating diffusion specie.

Both Co and Ni have good adhesion and low interfacial reaction rates with CoSb₃. The reaction phases are CoSb₂ and CoSb phases at the Co/CoSb₃ contact, and are Ni₅Sb₂ and (Co,Ni)Sb phases at Ni/CoSb₃. The reaction rate with Ni is much faster than that with Co. The reaction layer thick is 2 mm and 24 mm in the Co/CoSb₃ and Ni/CoSb₃ couples after reaction at 450^oC for one day. Based on these results, it is suggested the Co is a suitable barrier layer for CoSb₃ at 450^oC.

While there are similarities between thermoelectric devices and microelectronic devices, the operating temperatures of thermoelectric devices are typically higher. Consequently, the selection and use of barrier layer materials are even more critical and essential. According to the available studies, Co, Ni and (Co,Ni) do have significant interfacial reactions with most of the thermoelectric substrates and might not be suitable barrier layers. Therefore, there is a need for the development of novel barrier layer materials to meet the requirements and demands of thermoelectric devices.