Processing And Mechanism Of Brazing Nano Structure Bi₂Te_2.7Se_0.3 Thermoelectric Materials

Thermoelectric material is a functional material that can convert electricity into heat between each other.At present,Bi₂Te₃-based thermoelectric materials are one of the most mature thermoelectric materials;they are mainly used in thermoelectric power generation in low temperature range.But the mechanical properties and thermoelectric properties of commercial Bi₂Te₃-based thermoelectric materials are poor.For practical applications,thermoelectric materials also need to be connected with the metal electrode in order to make thermoelectric power generation devices,the quality of the joint directly affect the reliability of the entire device.In this paper,the Bi₂Te₃-based thermoelectric materials with high mechanical properties and high thermoelectric properties of nanostructures were prepared by hot-pressing and melt spinning.The brazing process and mechanism of thermoelectric materials and metal electrodes were studied.Melt spinning is a rapid solidification technology,the melt spinning used in this paper can reach a cooling rate of 10⁶-10⁷K/s,through the ultra-fast solidification can inhibit the growth of grain.Therefore,we prepared Bi₂Te_2.7Se_0.3 ribbons by melt spinning.The SEM analysis shows that the microstructures of the free surface were between 100 nm and 300 nm,and the contact surface were amorphous and nanocrystalline mixture.The ribbons were pestle into powder and then hot pressed to form a bulk material.The analysis showed that Bi₂Te_2.7Se_0.3 was still pure phase and the nanostructures in the ribbons were retained in the bulk.The nanoindentation test showed that the elastic modulus of the nanostructure bulk material reached 52.026 GPa,which was 33% higher than that of the molten ingot.The thermal conductivity test showed that the thermal conductivity was 27% lower than that of commercial Bi₂Te₃,and the highest thermoelectricity value ZT = 0.84 at 375 K,which was about 68% higher than that of commercial Bi₂Te₃.This indicates that the introduced nanostructures not only enhance the mechanical properties of the material by fine grain strengthening,but also form effective enhanced scattering to the low frequency phonons,which significantly reduces the thermal conductivity and improves the thermoelectric properties.The SnAgSb filler metal was used to braze the nanostructured thermoelectric material and Cu electrode directly.The results showed that the Te diffused seriously and cracked on the thermoelectric material side.In order to restrain the diffusion and achieve high reliability joints,the use of resistance to vapor deposition in the surface of the metal diffusion barrier layer,the metal diffusion barrier layer was prepared by resistance heated evaporation in the surface of Bi₂Te_2.7Se_0.3.The study showed that only Ni can effectively block the thermoelectric material diffusion reaction.SnAgSb solder is used to realize the reliable connection of Ni-plated Bi₂Te_2.7Se_0.3 thermoelectric material with metal Cu.The typical interface structure of the joint is Bi₂Te_2.7Se_0.3/SnTe+ BiSe/?Sn+Ag₃Sn+?Sb,Bi?+NiTe₂+??Sn+Bi-rich?/Cu₆Sn₅/Cu3Sn/Cu.The effects of brazing temperature and holding time on the microstructure and shear strength of the joints were analyzed.With the increase of brazing temperature and the holding time,the shear strength of the joint increased first and then decreased.When the brazing temperature T=300?,holding time t=5min,the maximun shear strength of the joint was 15 MPa.Finally,through the calculation of the Gibbs free energy of the interface products,combined with the typical structure of the joint,it is found that the evolution process of the interface structure can divide into four stages: solder melting,element diffusion,interfacial reaction and solidification.