Evaluation of Electroplated Co-P Film as Diffusion Barrier Between In-48Sn Solder and SiC-Dispersed Bi₂Te₃ Thermoelectric Material

The diffusion barrier property of Co-P film as a buffer layer between SiC-dispersed Bi₂Te₃ bulk material and In-48Sn solder was investigated. A Co-P film with thickness of ~6 µm was electroplated on SiC-dispersed Bi₂Te₃ substrate, joined with In-48Sn solder by a reflow process, and annealed at 100°C for up to 625 h. The formation and growth kinetics of intermetallic compounds (IMCs) at the interface between the In-48Sn and substrate were studied using transmission electron microscopy and scanning electron microscopy with energy-dispersive x-ray spectroscopy. The results showed that crystalline Co(In,Sn)₃ formed as an irregular layer adjacent to the solder side at the solder/Co-P interface due to diffusion of Co towards the solder, and a small amount of amorphous Co₄₅P₁₃In₁₂Sn₃₀ appeared at the Co-P side because of diffusion of In and Sn into Co-P. The growth of Co(In,Sn)₃ and Co₄₅P₁₃In₁₂Sn₃₀ during solid-state aging was slow, being controlled by interfacial reaction and diffusion, respectively. For comparison, In-48Sn/Bi₂Te₃-SiC joints were prepared and the IMCs in the joints analyzed. Without a diffusion barrier, In penetrated rapidly into the substrate, which led to the formation of amorphous InxBiy phase in crystalline In₄Te₃ matrix. These IMCs grew quickly with prolongation of the annealing time, and their growth was governed by volume diffusion of elements. The experimental data demonstrate that electroplated Co-P film is an effective diffusion barrier for use in Bi₂Te₃-based thermoelectric modules.