Solder free joining as a highly effective method for making contact between thermoelectric materials and metallic electrodes

Quality of joining and interfacial evolution behavior play a critical role in the performance and reliability of thermoelectric (TE) devices. In this study, different joining methods using Zn−2AlZn−2Al solder alloy (1) and solder-free joining with microlayers of Ti and Cr as interconnecting agents (2) were systematically investigated and demonstrated on the low-cost ZnSb TE system.

ZnSb material, which was chosen to bond with Ag and Ni metallic electrodes, exhibited a maximum zT value of 0.8 at 400∘C. With the joining method (1), Zn from the Zn−2AlZn−2Al solder was found to diffuse/react with both Ag and Ni electrodes, and penetrate into ZnSb legs. SEM-EDX analysis recorded a significant excess of Zn in the ZnSb leg after joining.

We found that, using microlayers of Ti and Cr as interconnecting agent, a very good interfacial contact was obtained, and the starting composition of ZnSb legs was preserved. The interfacial contact of ZnSb/Cr/NiZnSb/Cr/Ni was found to be stable after heat treatment at 400∘C for 30 h, suggesting solder-free joining as an effective method for reliable contacts in TE devices in the medium temperature region (200∘C−400∘C).