Soldering Technology And Joining Mechanism Of Bi₂Te_2.7Se_0.3 And Cu Electrode

Bi₂Te₃-based thermoelectric material is one of the best thermoelectric materials in low temperature area,mainly used for power generation and refrigeration near room temperature.Device connection is a key step for thermoelectric conversion technology from"laboratory"to"commercialization".However,when the high-temperature end joint is welded and served for a long time,the intense diffusion reaction of the elements at the interface will deteriorate the joint performance continuously.A reliable diffusion barrier layer is prepared on the surface of thermoelectric materials and a reliable connection between the barrier layer and the electrode is an effective measure to solve this problem.This paper studies the preparation of Ni Mo barrier layer on Bi₂Te_2.7Se_0.3 surface and the method of brazing the barrier layer with Cu electrode.Ni Mo diffusion barrier layer was successfully prepared on Bi₂Te_2.7Se_0.3 surface by electroplating method.The surface morphology and deposition rate of Ni Mo coating were effectively controlled by adjusting the current density in the plating process,and the optimal current density was determined to be 8A·dm^-2.At this current density,the surface particle diameter of the coating is less than 1μm.It was discovered that when Mo content reached about 20at.%,the crystallization property of the coating became worse.The surface morphology and Mo content of the Ni Mo coating were effectively controlled by adjusting the bath temperature and sodium molybdate concentration.The optimum bath temperature is 40℃and the concentration of Mo O₄^2-is 10g·L^-1.Under the optimal plating parameters,the Mo content of the coating is 18at.%,and the coating thickness increases at a rate of0.5μm/min.The brazing of Bi₂Te_2.7Se_0.3 with Cu electrode after Ni Mo metallization was realized by using Sn42Bi58 solder.The diffusion of Sn,Te and Cu at the interface was effectively inhibited by Ni Mo coating.The effect of coating thickness on the interface was studied,and the optimal plating time was determined to be 10min.The effects of brazing process parameters on the interface microstructure and mechanical properties of the joint were studied.The best parameters were soldering temperature of 190℃and seaking time of 3min.The maximum shear strength of the joint is about 12MPa and the contact resistance is less than 3.27μΩ·cm².In order to solve the matter of CTE mismatch during brazing of single Ni Mo coating,the interface structure of the joint was improved and Ni/Ni Mo composite coating was introduced.The influence of brazing process parameters on the interface microstructure and mechanical properties of the joint was studied.It was found that the shear strength of the joint with composite coating was increased by about 1.5times.In order to simulate the service environment of brazed joints,the brazed joints with or without blocked layer were placed to heat aging treatment at 110℃,and the holding time was 50h,100h,150h,200h and 250h,respectively.In the presence of Ni Mo coating,the thickness of the interfacial reaction layer is only 1μm after aging for 250h,which is 1/9 of that in direct brazing.The shear strength is stable at about9MPa and the contact resistance increases to 12.39μΩ·cm².Through data fitting calculation,the life of the joint is predicted.Compared with direct brazing,it is greatly improved,which proves that the coating has a good service stability.