Brazing Process And Mechanism Of Tetrahedtite Thermoelectric Materials

The thermoelectric material is a solid functional material using carriers(electrons and holes)to achieve movement of the direct conversion of heat and electricity,mainly used in two aspects of thermal power generation and thermoelectric cooling.The energy conversion efficiency of the thermoelectric material and the comprehensive performance are measured by a dimensionless figure of merit,ZT.With the increase of ZT,the performance of thermoelectric materials becomes better and better.Copper-containing sulfide materials have a high ZT value,in which the tetrahedrite has complex and highly symmetrical crystal structure.Moreover,the tetrahedrite has a lower lattice thermal conductivity and stability.Therefore,this paper choosed the tetrahedrite as the main research object,studied the preparation and connection of materials to enhance the thermoelectric properties of the material and realized the high reliability connection with the electrode.The microstructures of the tetrahedrite thermoelectric materials were investigated by means of melting,annealing and hot pressing sintering.The results showed that the Cu12Sb4S13 had a second phase in the thermoelectric materials,which affected the overall thermoelectric properties of the materials.The Cu10.4Ni1.6Sb4S13 thermoelectric materials with single phase composition were obtained by replacing the Cu in Cu12Sb4S13 with the potential of Ni.The thermoelectric properties of the materials after doping elements were compared and found that the thermoelectric properties of the dopant materials were improved.When the temperature was 427?,the ZT value of Cu12Sb4S13 was 0.54,while the Cu10.4Ni1.6Sb4S13 was 0.85.The melting point of Cu10.4Ni1.6Sb4S13 was 604? and the actual use temperature was 420?.So the melting point of solder should be selected between 420?-604? to ensure the service temperature and stability of thermoelectric materials.The Cu10.4Ni1.6Sb4S13 thermoelectric material was brazed with Al-Si-Cu solder and Zn-Al solder.The interface had cracks and dissolution after welding and it could not be eliminated by adjusting the process parameters The brazing connection of Cu10.4Ni1.6Sb4S13 and the Cu electrode was realized by Ag-Cu-Sn solder.When the brazing temperature was 500? and the holding time was 5min,the interfacial structure of the obtained joint was Cu10.4Ni1.6Sb4S13/Cu2S3Sn/Sn S+Ag3Sn/ Cu3Sn/Cu.During the brazing process,the S element in the thermoelectric material produced a serious and long-range diffusion,resulting in the formation of large amounts of sulfide in the brazing seam.With the increase of brazing temperature,the diffusion of S element increased,and the S element diffused on the electrode side was combined with Cu to form Cu2 S intermetallic compound layer.The base material was dissipated,the degree of dissolution became larger and the dissolution width increased exponentially.The maximum shear strength was only 3.1MPa.The Cu10.4Ni1.6Sb4S13 thermoelectric material was brazed with Cu electrode.by using Ag-Cu-Sn solder and adding Ni diffusion barrier in order to slow down the joint diffusion reaction When the brazing temperature was 480? and the holding time was 2.5min,the interfacial structure of the obtained joint was Cu10.4Ni1.6Sb4S13/Ni Sb+Ni Sb S +Cu2S3Sn/Sn S+Ag3Sn/Ni3Sn2/Ni/(Cu,Ni)3Sn2/Ag3Sn+Cu3Sn/Cu3 Sn.It was found that the Ni diffusion barrier layer could effectively hinder the diffusion of S elements generated from the thermoelectric material.The brazing process parameters had important influence on the interface microstructure and mechanical properties.When the brazing temperature was 480? and the holding time was 5min,the reaction layer of the thermoelectric material was clear and the thickness was moderate.The interface of the interface formed a good metallurgical combination and the joint was flat and no obvious defects.The interface structure was the best and the shear strength was 9.3MPa.The shear strength of the joint was 200% higher than that of the non-diffusion barrier.The fracture mechanism of Cu10.4Ni1.6Sb4S13/Cu joint was obtained as fracture of the brittle fracture The fracture location appears on the thermoelectric material and its reaction layer.