Interfacial Reactions between Sn-Based Solders and n-Type Bi₂(Te,Se)₃ Thermoelectric Material

This study investigated the interfacial reactions between n-type Bi₂(Te,Se)₃ thermoelectric material, characterized by a highly-oriented (110) plane, and pure Sn and Sn-3.0Ag-0.5Cu (wt.%) solders, respectively. At 250 °C, the liquid-state Sn/Bi₂(Te,Se)₃ reactions resulted in the formation of both SnTe and BiTe phases, with Bi-rich particles dispersed within the SnTe phase. The growth of the SnTe phase exhibited diffusion-controlled parabolic behavior over time. In contrast, the growth rate was considerably slower compared to that observed with p-type (Bi,Sb)₂Te₃. Solid-state Sn/Bi₂(Te,Se)₃ reactions conducted between 160 °C and 200 °C exhibited similar interfacial microstructures. The SnTe phase remained the primary reaction product, embedded with tiny Bi-rich particles, revealing a diffusion-controlled growth. However, the BiTe layer had no significant growth. Further investigation into growth kinetics of intermetallic compounds and microstructural evolution was conducted to elucidate the reaction mechanism. The slower growth rates in Bi₂(Te,Se)₃, compared to the reactions with (Bi,Sb)₂Te₃, could be attributed to the strong suppression effect of Se on SnTe growth. Additionally, the interfacial reactions of Bi₂(Te,Se)₃ with Sn-3.0Ag-0.5Cu were also examined, showing similar growth behavior to those observed with Sn solder. Notably, compared with Ag, Cu tends to diffuse towards the interfacial reaction phases, resulting in a high Cu solubility within the SnTe phase.