Understanding the Working Temperature-Dependent Interfacial Behaviors of Silver Soldering Half-Heusler Thermoelectric Module

The half-Heusler (hH) alloy is deemed a promising thermoelectric (TE) material for medium- and high-temperature applications (300–700 °C) due to its excellent stability and mechanical properties. Despite much effort that has been made recently to improve the performance of hH devices, studies on their long-term aging characteristics are still seldom reported. In this work, we comprehensively analyzed the failure mechanism of aged TE arm samples that were made of p-type Hf0.5Zr0.5CoSb0.8Sn0.2 and n-type Hf0.75Zr0.25NiSn0.99Sb0.01, and soldered with pure Ag. The analysis focused on their performance when operated at various high service temperatures. The tests reveal that the severe diffusion of the Sn element at the hH/Ag interface increases with temperature. Below 550 °C, the TE arm samples can remain stable for up to 672 h (4 weeks). However, above 600 °C, the Sn element diffuses strongly into the silver layer. This diffusion leads to the formation of a three-phase Ag–Cu–Sn composition, which lowers the local melting point. As a result, pores and cracks form on the solder surface, ultimately causing device failure. The molecular dynamics (MD) simulations verify the evolution process of the interface morphology and reveal the underlying principles of element diffusion. Both the simulations and tests conclude that the upper limit temperature for the long-term stable operation of these hH arm samples is around 550 °C. Above this temperature, the solder interface will fail due to Sn diffusion. This finding provides guidance for the design and real applications of hH devices.