Synthesis Of N-type Bi₂Te_2.7Se_0.3 Thermoelectric Materials And Its Joining Mechanism With Cu

Bismuth telluride is one of the best thermoelectric materials in low temperature zone.It has been well developed in thermoelectric cooling application.However,its application in thermoelectric power generation is still limited.The main reason is that the commercial bismuth telluride is weak in mechanical properties and thermoelectric efficiency.In thermoelectric power generation working period,bismuth telluride needs to be joined with electrode materials,which lacks robust joining methods for mid and low thermoelectric power generation.Aiming at these problems,high figure of merit N-type bismuth telluride is synthesized and robust joining method for low temperature(100?)and mid temperature(250?)thermoelectric power generation is presented.Melting spinning is a rapid cooling method.It brings plenty of nano crystal into materials.Zone melting method to synthesize column polycrystal Bi2Te2.7Se0.3 is used.Then it is made into ribbon by melting spinning.The ribbon is grinded and hot pressed to make Bi2Te2.7Se0.3 bulk material.The effect of hot pressing temperature on the thermoelectric property of Bi2Te2.7Se0.3 bulk material is investigated.It is shown that with the increasing of hot pressing temperature,the thermoelectric property of Bi2Te2.7Se0.3 bulk material rises at first and then drops.The best figure of meirt,0.91,is achieved at 430? hot pressing temperature.Compared with zone melting Bi2Te2.7Se0.3,its lattice thermal conductivity drops by 65%,and its figure of merit rises by 50%.It shows that melting spinning combined with hot pressing is able to decrease the lattice thermal conductivity and increase the figure of merit in bismuth telluride.Soldering Bi2Te2.7Se0.3 and Cu using SAC305 alloy is realized.The effect of 260? soldering temperature on microstructure of the joint is investigated.It shows that Sn will react with Bi2Te2.7Se0.3 severely,and that the joint cannot work robustly above 150?.Then silver nanoparticles are synthesized,of which diameter ranges from 10-14 nm.Then joining Bi2Te2.7Se0.3 and Cu using nano silver paste is achieved.The typical microstructure of the joint is Bi2Te2.7Se0.3/Ag Te+Agx Bi2Te2.7Se0.3/Ag(s,s)/ Cu(s,s)/Cu.The effect of joining parameters on the microstructure and mechanical properties of the joint is investigated.It shows that the increase of joining temperature and holding time will promote the reaction between Bi2Te2.7Se0.3 and Ag.When the interfacial reaction is too severe,the joint will crack.With the increase of joining temperature and holding time,the shear stress of the joint will increase at first and then drop.The best joint is achieved when joining temperature is 290?,holding time is 30 min,pressure is 10 MPa.The best shear sress 4 MPa is achieved.To further improve the microstructure and mechanical properties of the joint,Ni film is electroplated on Bi2Te2.7Se0.3,and the joining of Bi2Te2.7Se0.3 and Cu is achieved.The typical microstructure of the joint is Bi2Te2.7Se0.3/Ni Te/Ni/Ag(s,s)/Cu(s,s)/Cu.The effect of the thickness of the Ni film on the microstrcutre of the joint is investigated.It shows that when the film is too thick,the joint will have too much residual stress and crack.The effect of joining parameters on the microstructure and mechanical properties of the joint is investigated.It shows that the increase of joining temperature and holding time will promote the reaction between Ni and Bi2Te2.7Se0.3.With the increase of joining temperature and holding time,the shear stress of the joint will increase at first and then become stable.The best joint is achieved when joining temperature is 290?,holding time is 30 min,pressure is 10 MPa.The best shear sress 7 MPa is achieved.Compared with directing joining method,the shear stress increases by 75%.