Abstract
Finding a solution to energy-related problem is currently one of the greatest challenge faced by people. The power efficiency is low and approximately half of the energy is exhausted as waste heat. Thermoelectric device is regarded as a potential material for its ability of directly converting waste heat into electrical power. Thermoelectric materials are not only reducing the environmental pollution but also improving the power efficiency by recycling the waste heat. The operating temperature of devices in recycling waste heat is high, which may lead to form intermetallic compounds at interface that causes the reduction of thermoelectric performance and the decrease of reliability of the related products. The phase formation and phase transformation in the joints are crucial to the lifetime of products. Ag-24.5at.%Ge alloy is a promising Pb-free solder, Nickel and Cobalt are commonly used as barrier layer material, and lead-tellurium is one mid-temperature thermoelectric material. This study examines the interfacial reactions in the joint PbTe/Ni(or Co)/Ag-24.5at.%Ge, and proposes the phase diagrams of Ag-Ge-Ni and Ag-Ge-Co ternary system. Interfacial reactions between diffusion barrier and solder are investigated. the -Ni5Ge3 phase is formed adjacent to the Ni substrate in the Ni/Ag-24.5at.%Ge couple reacted at 400oC. A region of nearly pure Ag is formed between this reaction phase and the eutectic Ag-24.5at%Ge, and is the result of Ag left over caused by the reactions between Ge and Ni. The -Ni5Ge3 phase is also formed in the Ni/Ag-24.5at.%Ge couple reacted at 750oC, but the microstructures of -Ni5Ge3 phase is different from those at 400oC, which is caused by the diffusion rate of solder in solid state and liquid state. In addition, Co/Ag-24.5at.%Ge couple reacted at 400oC and 750oC are examined as well. The -Co5Ge3 phase and CoGe phase form and grow with longer reaction time. Because diffusion rate of solder in solid state is higher than those in liquid state, it leads to different interfaces adjacent to the solder and reaction path. Phase diagrams including isothermal section of Ag-Ge-Ni ternary system at 400oC and Ag-Ge-Co ternary system at 750oC are experimentally established with CALPHAD. Interfacial reactions between diffusion barrier and thermoelectric material are also investigated. No compound is observed in the Ni/PbTe couple reacted at 400oC, which means thermoelectric materials could maintain high thermoelectric performance. This results imply that Ni may be one of promising candidate of diffusion barrier for PbTe thermoelectric material recycling waste heat at 400oC.