Influence of Bi2Te3 substrates with different microstructures on the thermal stability and kinetics of Ni/Bi2Te3 joints

Thermoelectric technology enables direct conversion between thermal and electrical energy, and the long-term reliability of its devices depends on the stability of the interface between the thermoelectric legs and the metal electrodes. Bi2Te3-based alloys are thermoelectric materials for near-room-temperature applications, and Ni is often used as a diffusion barrier to prevent reactions between the alloy and electrodes and solder. This study systematically investigated the effect of Bi2Te3 substrates with different microstructures on the thermal stability of the Ni/Bi2Te3 interface. The study selected four Bi2Te3 thermoelectric materials: n-type zone-melted, n-type hot-extruded, p-type hot-pressed, and p-type hot-extruded. Samples were prepared by electroplating a Ni barrier layer and welding it to a Cu electrode. The samples were heat-treated at different temperatures, and the evolution of the interfacial microstructure, diffusion kinetics, and electrical performance degradation was analyzed. The results indicate that, for both n-type and p-type materials, those prepared by the hot extrusion process exhibit superior interfacial thermal stability. Microstructural analysis reveals that the interfacial reactions in n-type materials are predominantly governed by grain-boundary diffusion. In the hot-extruded n-type samples, the refined grain structure and the Bi/Se grain-boundary segregation developed during heat treatment effectively suppress the rapid grain-boundary diffusion of Ni atoms. In contrast, the interfacial reactions in p-type materials are mainly controlled by bulk diffusion. For the hot-extruded p-type samples, Bi enrichment in the region adjacent to the interfacial reaction layer increases the kinetic barrier for Ni diffusion, thereby significantly inhibiting the growth of the interfacial reaction layer.